Special Issue

The National Conference on Emerging Foodborne Pathogens: Implications 
and Control, March 24-26m 1997, Alexandria, Virginia, USA

Infectious Disease as an Evolutionary Paradigm

Joshua Lederberg
Sackler Foundation Scholar, Rockefeller University, New York, New York, USA
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      The basic principles of genetics and evolution apply equally to
      human hosts and to emerging infections, in which foodborne
      outbreaks play an important and growing role. However, we are
      dealing with a very complicated coevolutionary process in which
      infectious agent outcomes range from mutual annihilation to
      mutual integration and resynthesis of a new species. In our
      race against microbial evolution, new molecular biology tools
      will help us study the past; education and a global public
      health perspective will help us deal better with the future.

Life expectancy in the United States from 1900 to the present (Figure 1)
shows an overall steady rise, reflecting improved health conditions in
general, the result of advances in medical science, hygiene, personal care,
health technologies, and public health administrations. The rise
decelerates asymptotically to a near plateau from the 1950s to the 1970s,
reflecting an epidemic of coronary disease, which we do not yet fully
understand. Improvements in medical care, attention to life style, or
indiscriminate use of aspirin may all be responsible for the subsequent
decrease in deaths from coronary disease. Up to the 1940s, the rising curve
is jagged, reflecting sporadic infectious disease outbreaks, especially the
Spanish influenza outbreak of 1918. Whether the life expectancy curve
continues to rise smoothly or whether it has some jagged declines depends
on what we do about transmission of infectious disease, including foodborne
disease. When plotted another way (Figure 2), both the absolute number of
deaths from infectious disease and the proportion of total deaths
attributable to infectious disease also show steady amelioration from 1900
almost to the present.

[Figure Not Available in ASCII]

Figure 1. Life expectancy in the United States, at birth, 20th century.

[Figure Not Available in ASCII]

Figure 2. Trends in infectious diseases mortality, 1900-1992. Source: CDC, 
unpub. data.

The 1918 Spanish influenza pandemic may be a prototype for future emerging
infections. Although minimized as not much more than a bad cold, influenza
took a terrible toll in 1918, especially on young people (Figure 3).

[Figure Not Available in ASCII]

  Figure 3. Pneumonia and influenza mortality, by age, in certain epidemic
   years. (Reprinted with permission of W. Paul Glezen and Epidemiologic
  Reviews. Emerging Infections: Pandemic Influenza. Epi Rev 1996;18:66).

Somewhat older persons may have been protected by immunity from prior
exposure to related strains of influenza. The disease, with rapid onset of
fulminating pneumonic symptoms, killed 20 to 25 million persons worldwide.
The infectious agent was not available for study at that time. However,
very recently the Armed Forces Institute of Pathology recovered with PCR
technology genetic fragments of the 1918 influenza virus (1). Less than 10%
of the entire genome has been recovered to date, but recovery of complete
sequences is likely. Although the target genes have not yet provided a clue
as to why the 1918 influenza was so devastating, they demonstrate the
enormous potential of today's molecular biology tools.

These tools will enable us to better study paleovirology and
paleomicrobiology. We are accustomed to stereotyping historical disease
outbreaks as if we really knew what they were, but we really know very
little detail about their genetic features. For example, we talk about the
great historic plagues as if they indeed were Yersinia or cholera or
malaria. We should look forward to finding out about the 14th century black
death, if it was indeed Yersinia pestis. Although clinically unmistakable,
that is not to say it was caused by the identical genotype of present
Yersinia strains.

We need to look ahead as well as back. In this century, emerging and
reemerging infections have stimulated flurries of interest, but in general
we have been complacent about infectious diseases ever since the
introduction of antibiotics. The effect of antibiotics on acute infections
and tuberculosis as well as the effect of polio vaccination led to a
national, almost worldwide, redirection of attention to chronic and
constitutional diseases. However, the HIV pandemic in the early 1980s
caught us off guard, reminding us that there are many more infectious
agents in the world. It is fortuitous that retroviruses had already been
studied from the perspective of cancer etiology; otherwise, we would have
had no scientific platform whatsoever for coping with HIV and AIDS.

The Committee on International Science Engineering and Technology provided
an interagency review setting out a policy framework for the United States'
global response to infectious disease (Table 1). The policy provides a
worldwide mantle for surveillance and monitoring, remedial measures,
development of new drugs, vaccines, and treatment modalities. The global
outlook is necessary, even if for purely selfish reasons, because to
infectious agents the world is indivisible, with no national boundaries.
Our thinking has been impoverished in terms of budget allocations for
dealing with health on an international basis.

Table 1. Examples of pathogenic microbes and infectious diseases recognized
since 1973 (2)
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 Year           Microbe              Type               Disease
---------------------------------------------------------------------------

1973  Rotavirus                    Virus    Major cause of infantile
                                            diarrhea worldwide

1975  Parvovirus B19               Virus    Aplastic crisis in chronic
                                            hemolytic anemia

1976  Cryptosporidium              Parasite Acute and chronic diarrhea
                                            parvum
1977  Ebola virus                  Virus    Ebola hemorrhagic fever

1977  Legionella                   Bacteria Legionnaires' disease
                                            pneumophila

1977  Hantaan virus                Virus    Hemorrhagic fever with renal
                                            syndrome (HRFS)

1977  Campylobacter jejuni         Bacteria Enteric pathogens distributed
                                            globally

1980  Human T-lymphotropic virus I Virus    T-cell lymphoma-leukemia
      (HTLV-1)

1981  Toxic producing strains of   Bacteria Toxic shock syndrome(tampon
      Staphylococcus aureus                 use)

1982  Escherichia coli O157:H7     Bacteria Hemorrhagic colitis; hemolytic
                                            uremic syndrome
1982  HTLV-II                      Virus    Hairy cell leukemia
1982  Borrelia burgdorferi         Bacteria Lyme disease

1983  Human immunodeficiency       Virus    Acquired immunodeficiency
      virus(HIV)                            syndrome (AIDS)
1983  Helicobacter pylori          Bacteria Peptic ulcer disease
1985  Enterocytozoon bieneusi      Parasite Persistent diarrhea
1986  Cyclospora cayetanensis      Parasite Persistent diarrhea
1988  Human herpes-virus-6 (HHV-6) Virus    Roseola subitum

1988  Hepatitis E                  Virus    Enterically transmitted non-A,
                                            non-B hepatitis
1989  Ehrlichia chafeensis         Bacteria Human ehrlichiosis

1989  Hepatitis C                  Virus    Parenterally transmitted
                                            non-A, non-B liver infection
1991  Guanarito virus              Virus    Venezuelan hemorrhagic fever

1991  Encephalitozoon hellem       Parasite Conjunctivitis, disseminated
                                            disease
1991  New species of Babesia       Parasite Atypical babesiosis

1992  Vibrio cholerae O139         Bacteria New strain associated with
                                            epidemic cholera

1992  Bartonella henselae          Bacteria Cat-scratch disease; bacillary
                                            angiomatosis

1993  Sin Nombre virus             Virus    Adult respiratory distress
                                            syndrome
1993  Encephalitozoon cuniculi     Parasite Disseminated disease
1994  Sabia virus                  Virus    Brazilian hemorrhagic fever

1995  HHV-8                        Virus    Associated with Kaposi sarcoma
                                            in AIDS patients
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We are engaged in a type of race, enmeshing our ecologic circumstances with
evolutionary changes in our predatory competitors. To our advantage, we
have wonderful new technology; we have rising life expectancy curves. To
our disadvantage, we have crowding; we have social, political, economic,
and hygienic stratification. We have crowded together a hotbed of
opportunity for infectious agents to spread over a significant part of the
population. Affluent and mobile people are ready, willing, and able to
carry afflictions all over the world within 24 hours' notice. This
condensation, stratification, and mobility is unique, defining us as a very
different species from what we were 100 years ago. We are enabled by a
different set of technologies. But despite many potential
defenses-vaccines, antibiotics, diagnostic tools-we are intrinsically more
vulnerable than before, at least in terms of pandemic and communicable
diseases.

We could imaginably adapt in a Darwinian fashion, but the odds are stacked
against us. We cannot compete with microorganisms whose populations are
measured in exponents of 1012, 1014, 1016 over periods of days. Darwinian
natural selection has led to the evolution of our species but at a terrible
cost. If we were to rely strictly on biologic selection to respond to the
selective factors of infectious disease, the population would fluctuate
from billions down to perhaps millions before slowly rising again.
Therefore, our evolutionary capability may be dismissed as almost totally
inconsequential. In the race against microbial genes, our best weapon is
our wits, not natural selection on our genes.

New mechanisms of genetic plasticity of one microbe species or another are
uncovered almost daily. Spontaneous mutation is just the beginning. We are
also dealing with very large populations, living in a sea of mutagenic
influences (e.g., sunlight). Haploid microbes can immediately express their
genetic variations. They have a wide range of repair mechanisms, themselves
subject to genetic control. Some strains are highly mutable by not
repairing their DNA; others are relatively more stable. They are
extraordinarily flexible in responding to environmental stresses (e.g.,
pathogens' responses to antibodies, saprophytes' responses to new
environments). Mechanisms proliferate whereby bacteria and viruses exchange
genetic material quite promiscuously. Plasmids now spread throughout the
microbial world (3). They can cross the boundaries of yeast and bacteria.
Lateral transfer is very important in the evolution of microorganisms.
Their pathogenicity, their toxicity, their antibiotic resistance do not
rely exclusively on evolution within a single clonal proliferation.

We have a very powerful theoretical basis whereby the application of
selective pressure (e.g., antibiotics in food animals) will result in drug
resistance carried by plasmids, or pathogens attacking humans. It is not
easy to get direct and immediate epidemiologic evidence, but the
foundations for these phenomena exist and must be taken into account in the
development of policies. We have barely begun to study the responses of
microorganisms under stress, although we have examples where root
mechanisms of adaptive mutability are themselves responses to stress. In
recent experiments, bacterial restriction systems are more permissive of
the introduction of foreign DNA, possibly letting down their guard in
response to "mutate or die" circumstances. This does not reflect bacterial
intelligence-that they know exactly what mutations they should undergo in
response to environmental situations. Their intrinsic mutability and
capacity to exchange genetic information without knowing what it is going
to be is not a constant; it is certainly under genetic control and in some
circumstances varies with the stress under which the microbes are placed.

Evolution is more or less proportionate to the degree of genetic divergence
among the different branches of the three-tiered tree of life, with the
archaeal branch, the eubacterial branch, and the eukaryotes (Figure 4). The
tree illustrates the small territory occupied by humans in the overall
world of biodiversity. It shows mitochondria right next to Escherichia
coli. Bacterial invasion of a primitive eukaryote 2-1/2 to 3 billion years
ago, synchronized with the development of primitive green oxygen-generating
plants, conferred a selective advantage to complexes that could use oxygen
in respiration. Our ancestors were once invaded by an oxidative-capable
bacterium that we now call a mitochondrium and that is present in every
cell of every body and almost every species of eukaryote. We did not evolve
in a monotonous treelike development; we are also the resynthesis of
components of genetic development that diverged as far as the bacteria and
were reincorporated into the mitochondrial part of our overall genome.
Another example of lateral transfer is the symbiosis that resulted from
chloroplast invasion of green plants.

[Figure Not Available in ASCII]

Figure 4. The three-domain tree of life based on small-subunit rRNA 
sequences. Reprinted with permission of Norman R. Pace and ASM News. ASM 
News 1996;62 (9):464.

The outcome of encounters between mutually antagonistic organisms is
intrinsically unpredictable. The 1918 influenza outbreak killed half
percent of the human population; but because the consequences were to
either kill the host or leave the host immune, the virus died out totally,
leaving no trace in our genomes, as far as we know. Historic serology on
survivors has found memory cells and antibodies against H1N1, the serotype
of the resurrected 1918 virus. Unlike the influenza virus, which left no
known genetic imprint, 400 to 500 retroviruses are integrated into our
human genome. The full phylogeny of these encounters is unknown, but many
of these viruses may precede the separation of homo sapiens from the rest
of the hominid line.

Infectious agent outcomes range from mutual annihilation to mutual
integration and resynthesis of a new species. Much has been made of the
fact that zoonoses are often more lethal to humans than to their original
host, but this phenomenon cannot necessarily be generalized. Most zoonoses
do not affect humans adversely. Some are equally capable in a new host. We
tend to pay most attention, however, to those, such as yellow fever, for
which we have not genetically or serologically adapted and which cause
severe disease.

Canine distemper provides an example of a quasihereditary adaptation. In
the Serengeti, the disease migrated from village dogs to jackals, which
shared prey and had contact with lions. About one-fourth of the preserve's
4,000 lions died of canine distemper (4) but the survivors are immune and
will pass immunoglobulin, to their offspring. The cubs' maternal immunity
will likely mitigate infection and permit a new equilibrium, not because of
genetic adaptation but because of the preimmunized host. This is also the
most plausible explanation for how savage the polio virus has been as a
paralytic infection of young people. It may also apply to hepatitis, where
cleaner is not always better if it means we do not have the "street smarts"
to respond to new infectious
 challenges. These nongenetic adaptations
between parasite and host complicate our outcome expectations.

Short-term shifts in equilibrium can give ferocious but temporary
advantages to a virus. Long-term outcomes are most stable when they involve
some degree of mutual accommodation, with both surviving longer. New
short-term deviants, however, can disrupt this equilibrium. The final
outcome of the HIV pandemic cannot be predicted. More strains with longer
latency may be taking over, mitigating the disease. However, deviant
strains could counteract this effect by overcoming immunity and rapidly
proliferating, with earlier and more lethal consequences.

We should also consider somatic evolution, a Darwinian process that occurs
with every infection. In the clonal selection model of immunogenesis (5),
an apparently random production of immunoglobulin variants, both by
reassortment of parts and by localized mutagenesis, gives rise to candidate
antibodies, which then proliferate in response to matching epitopes. We do
not understand the details of how a given epitope enhances stepwise
improvements in affinity and productivity of antibodies at various stages.
The process may be more complicated than we realize; so may Darwinian
evolution.

Despite the prior arguments against relying on host or genotype evolution
as a response to infection, historically we have done so and now have
"scars of experience." A notable example is malaria, wherein the Duffy
mutation against Plasmodium vivax is the only host defense with no
deleterious consequences. The thalassemias, G6PD deficiency, and hemoglobin
S are all hemopoietic modifications that thwart the plasmodia; but in
homozygotes, they themselves cause disease. In the evolution of our
species, for every child spared an early death because a hemoglobin S
mutation impeded Plasmodium development, another will succumb to sickle
cell disease unless we can intervene. Specific remedies do not exist.
Although somatic gene therapy is an interesting possibility, one that will
probably progress in the next 20 years, it is paradoxical that we know more
about hemoglobin S than any other molecular disease. The entire concept of
genetic determination of protein structure has been based on these early
observations, yet we are still searching with limited success for ways to
put it to therapeutic use.

Biotechnology may enable other forms of genetic intervention through which
homo sapiens could conceivably bypass natural selection and random
variation. In the absence of alternatives, we might speculate about these
kinds of "aversive therapies" as a last resort to save our species.

The ultimate origin of life is still the subject of many theories, as is
the origin of viruses (Table 2). Each virus is different. We know nothing
of virus phylogenies and cannot even substantiate the distinctions of the
several hundred categories. We do not know their origin, only that they
interact with host genomes in many ways. Particles could come out of any
genome, become free-living (i.e., independent, autonomously replicating
units in host cells), reenter a host genome as retroviruses and possibly
others do, and repeat the cycle dozens of times. But no one can give a
single example or claim to have significant knowledge of how any particular
virus evolved, thus presenting a scientific challenge for the next 20 or 30
years.

Table 2. The origin of viruses
---------------------------------------------------------------------------
    Viruses are genomic fragments that can replicate only in the context of
an intact living cell. They cannot therefore be primitive antecedents of
cells.

    Within a given species, viruses may have emerged as genetic fragments
or reduced versions from chromosomes, plasmids, or RNA of

    1) the host or related species
    2) distant species
    3) larger parasites of the same or different hosts
    4) further evolution and genetic interchange among existing viruses

    Once established, they may then cycle back into the genome of the host
as an integrated episome; there they may have genetic functions or in
principle might reemerge as new viruses.

    These cycles have some substantiation in the world of bacterial
viruses; but we have no clear data on the provenience of plant or animal
viruses.
---------------------------------------------------------------------------

We are dealing with more than just predation and competition. We are
dealing with a very complicated coevolutionary process, involving merger,
union, bifurcation, and reemergence of new species (Table 3). Divergent
phenomena can occur in any binary association, with unpredictable outcomes.
We have hundreds of retroviruses in our genome and no knowledge of how they
got there. As to HIV, we have no evidence as yet that it has ever entered
anyone's germ line genome: we really do not know whether it ever enters
germ cells. The outcomes of even that interaction could be much more
complicated than the purely parasite/host relationships we are accustomed
to.

Table 3. Genetic evolution
-------------------------------------------------------------------------
Microbes (bacteria, viruses, fungi, protozoa)
                     Rapid and incessant

                     Huge population sizes 1014+ and generation times in
                     minutes vs. years
Intraclonal process
                     DNA replicationmay be error-pronein sea of mutagens
                     sunlight; unshielded chemicals, incl. natural
                     products

                     RNA replicationintrinsically unedited, >10-3 swarm
                     species

                     Haploid: immediate manifestation, but partial
                     recessives not accumulated contra multicopy plasmids
                     Amplification
                     Site-directed inversions and transpositions:
                     phase variation
                     ?? Other specifically evolved mechanisms:
                     genome quadrant duplication; silencing
Interclonal process

                     Promiscuous recombinationnot all mechanisms are
                     known
                     Conjugationdozens of species
                     Viral transduction and lysogenic integration:
                     universal
                         Classical: phage-borne toxins in C. diphtheriae

                     Plasmid interchange (by any of above) and
                     integration
                         Toxins of B. anthracis

                         Pasteur: heat attenuation: plasmid loss;
                     chemically induced
                     RNA viral reassortment; ?? and recombination?
                     Transgressive-across all boundaries
                         Artificial gene splicing

                         Bacteria and viruses have picked up host genes
                     (antigenic masking?)

                         Interkingdom: P. tumefaciens and plants, E. coli
                     and yeast

                         Vegetable and mineral! oligonucleotides and
                     yeast.
Host-parasite
coevolution

                     Coadaptation to mutualism or accentuation of
                     virulence?

                     Jury is still out (May and Anderson). Many zoonotic
                     convergences.
                     Probably divergent phenomena, with short-term
                     flareups and Pyrrhic
                         victories, atop long-term trend to coadaptation.
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Innovative technologies for dealing with microbial threats have the
potential for fascinating therapeutic opportunities (Table 4). Some, like
bacteriophage, have been set aside as laboratory curiosities. Nothing is
more exciting than unraveling the details of pathogenesis. Having the full
genomes of half a dozen parasitic organisms opens up new opportunities for
therapeutic invention in ways that we could not have dreamed of even 5
years ago, which will lead to many more technologies. In food microbiology,
we should keep in mind the probiotic as well as the adversarial and
pathogenetic opportunities in our alimentary tracts.

Table 4. Technologies to address microbial threats
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Antibacterial chemotherapy
          Potentially unlimited capability; bacterial metabolism and
          genetic structure notably
              different from human genome sequencing pointing to
          bacterial vulnerabilities
          Economic-structural factors public expectation for
          unachievable bargains in safety
              assurance, cost of development, and ultimate pricing
          Dilemmas of regulation of (ab)use

          Resurgent interest in bacteriophage and other biologically
          oriented approaches
Antiviral chemotherapy
          Much more difficult program, inherently
          Gross underinvestment
          New approaches: antisense, ribozymes, targeted D/RNA cleavers
          Problematics of sequence-selective targets
Vaccines
          Gross underinvestment; other structural problems as above
          Liability/indemnification
          Vaccination as service to the herd

          New approaches: hot biotechnology is coming along especially
          live attenuated: but testing dilemmas
          Safety issues about use of human cells lines; adjuvants
Immunoglobulins and their progeny
          Phage display and diversification: biosynthetic antibody
          Passive immunization for therapy
Biologic response modifiers

          New world of interleukins, cell growth factors so far just
          scratching surface
          Interaction with pathogenesis
          Intersection with somatic gene therapy
Technologies for diagnosis and monitoring
          Etiologic agents and control
          Host polymorphisms and sensitivities
Homely technologies needed
          Simple, effective face-masks
          Palatable water-disinfectants
          Home-use diagnostics of contamination
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The Committee on International Science Engineering and Technology report
(2) provides some recommendations (Table 5). We need a global perspective.
We need to invest in public health, especially food microbiology, not just
medical care, in dealing with disease. It is important to prevent foodborne
disease through sensible monitoring, standards of cleanliness, and consumer
and food-handler education and not just care for its victims.

Table 5. CISET* recommendations for addressing global infectious disease
threats
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  1. Concerted global and domestic surveillance and diagnosis of disease
     outbreaks and endemic occurrence. This must entail the installation of
     sophisticated laboratory capabilities at many centers now lacking
     them.
  2. Vector management and monitoring and enforcement of safe water and
     food supplies; and personal hygiene (e.g., Operation Clean Hands).
  3. Public and professional education.
  4. Scientific research on causes of disease, pathogenic mechanisms,
     bodily defenses, vaccines, and antibiotics.
  5. Cultivation of the technical fruits of such research, with the full
     involvement of the pharmaceutical industry and a public understanding
     of the regulatory and incentive structures needed to optimize the
     outcomes.

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*Committee on International Science, Engineering and Technology Policy of
the National Science and Technology Council.

Today we emphasize individual rights over community needs more than we did
50 to 75 years ago. Restraining the rights and freedoms of individuals is a
far greater sin than allowing the infection of others. The restraints
placed on Typhoid Mary might not be acceptable today, when some would
prefer to give her unlimited rein to infect others, with litigation their
only recourse. In the triumph of individual rights, the public health
perspective has had an uphill struggle in recent pandemics.

Education, however, is a universally accepted countermeasure, especially
important in foodborne diseases. Food safety programs should more
specifically target food handlers, examining their hands to determine if
they are carriers, to ensure they are complying with basic sanitation. 

We typically do this only after an outbreak. Perhaps we should have further
debate on the social context for constraints and persuasion to contain the
spread of infectious agents.

Address for correspondence: Joshua Lederberg, Rockefeller University, 1230
York Avenue, New York, NY 10021-6399, USA; fax: 212-327-8651; e-mail:
Lederberg@mail.rockefeller.edu.

References

  1. Taubenberger JK, Reid AH, Frafft AE, Bijwaard KE, Fanning TG. Initial
     genetic characterization of the 1918 "Spanish" influenza virus.
     Science 1997;275:1793-6.
  2. NSTC-CISET Working Group on Emerging and Reemerging Infectious
     Diseases. Infectious disease-a global health threat. Washington (DC):
     The Group;1996.
  3. Lederberg J. Plasmid (1952-1997). Plasmid. In press 1997.
  4. Roelke-Parker ME, Munson L, Packer C, Kock R, Cleaveland S, Carpenter
     M, et al. A canine distemper virus epidemic in Serengeti lions
     (Panthera leo). Nature 1996;379:441-5.
  5. Lederberg J. The ontogeny of the clonal selection theory of antibody
     formation: reflections on Darwin and Ehrlich. Ann N Y Acad Sci
     1988;546:175-87.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/lederber.txt

Emerging Foodborne Diseases: An Evolving Public Health Challenge

Robert V. Tauxe
Centers for Disease Control and Prevention, Atlanta, Georgia, USA
---------------------------------------------------------------------------

      The epidemiology of foodborne disease is changing. New
      pathogens have emerged, and some have spread worldwide. Many,
      including Salmonella, Escherichia coli O157:H7, Campylobacter,
      and Yersinia enterocolitica, have reservoirs in healthy food
      animals, from which they spread to an increasing variety of
      foods. These pathogens cause millions of cases of sporadic
      illness and chronic complications, as well as large and
      challenging outbreaks over many states and nations. Improved
      surveillance that combines rapid subtyping methods, cluster
      identification, and collaborative epidemiologic investigation
      can identify and halt large, dispersed outbreaks. Outbreak
      investigations and case-control studies of sporadic cases can
      identify sources of infection and guide the development of
      specific prevention strategies. Better understanding of how
      pathogens persist in animal reservoirs is also critical to
      successful long-term prevention. In the past, the central
      challenge of foodborne disease lay in preventing the
      contamination of human food with sewage or animal manure. In
      the future, prevention of foodborne disease will increasingly
      depend on controlling contamination of feed and water consumed
      by the animals themselves.

Every year, in the United States foodborne infections cause millions of
illnesses and thousands of deaths; most infections go undiagnosed and
unreported. As the epidemiology of foodborne infections evolves, old
scenarios and solutions need to be updated. This article reviews main
trends in the evolution of foodborne disease epidemiology and their effect
on surveillance and prevention activities.

Preventing foodborne disease is a multifaceted process, without simple and
universal solutions. For most foodborne pathogens, no vaccines are
available. Consumer education about basic principles of food safety, an
important component of prevention, by itself is insufficient. Food reaches
the consumer through long chains of industrial production, in which many
opportunities for contamination exist. The general strategy of prevention
is to understand the mechanisms by which contamination and disease
transmission can occur well enough to interrupt them. An outbreak
investigation or epidemiologic study should go beyond identifying a
suspected food and pulling it from the shelf to defining the chain of
events that allowed contamination with an organism in large enough numbers
to cause illness. We learn from the investigation what went wrong, in order
to devise strategies to prevent similar events in the future. Although
outbreaks make the news, most foodborne infections occur as individual or
sporadic cases. Therefore, the sources of sporadic cases must also be
investigated and understood.

Emerging Foodborne Pathogens

Substantial progress has been made in preventing foodborne diseases. For
example, typhoid fever, extremely common at the beginning of the 20th
century, is now almost forgotten in the United States. It was conquered in
the preantibiotic era by disinfection of drinking water, sewage treatment,
milk sanitation and pasteurization, and shellfish bed sanitation (Figure
1). Similarly, cholera, bovine tuberculosis, and trichinosis have also been
controlled in the United States. However, new foodborne pathogens have
emerged. Among the first of these were infections caused by nontyphoid
strains of Salmonella, which have increased decade by decade since World
War II (Figure 1). In the last 20 years, other infectious agents have been
either newly described or newly associated with foodborne transmission
(Table 1). Vibrio vulnificus, Escherichia coli O157:H7, and Cyclospora
cayetanensis are examples of newly described pathogens that often are
foodborne. V. vulnificus was identified in the bloodstream of persons with
underlying liver disease who had fulminant infections after eating raw
oysters or being exposed to seawater; this organism lives in the sea and
can be a natural summertime commensal organism in shellfish (1). E. coli
O157:H7 was first identified as a pathogen in 1982 in an outbreak of bloody
diarrhea traced to hamburgers from a fast-food chain (2); it was
subsequently shown to have a reservoir in healthy cattle (3). Cyclospora,
known previously as a cyanobacterialike organism, received its current
taxonomic designation in 1992 and emerged as a foodborne pathogen in
outbreaks traced to imported Guatemalan raspberries in 1996 (4,5). The
similarity of Cyclospora to Eimeria coccidian pathogens of birds suggests
an avian reservoir (4,5).
 [Figure Not Available in ASCII]
 Figure 1. Reported incidence of typhoid fever and nontyphoidal
 salmonellosis in the United States, 1920-1995.

-----------------------------------------------------
Table 1. New pathogens that are foodborne and pathogens newly recognized as
predominatly foodborne in the United States in the last 20 years
-----------------------------------------------------
Campylobacter jejuni
Campylobacter fetus ssp. fetus
Cryptosporidium cayetanensis
Escherichia coli O157:H7 and related E. coli (e.g., O111:NM, O104:H21)
Listeria monocytogenes
Norwalk-like viruses
Nitzschia pungens (cause of amnesic shellfish poisoning)
Salmonella Enteritidis
Salmonella Typhimurium DT 104
Vibrio cholerae 01
Vibrio vulnificus
Vibrio parahaemolyticus
Yersinia enterocolitica
-----------------------------------------------------

Some known pathogens have only recently been shown to be predominantly
foodborne. For example, Listeria monocytogenes was long known as a cause of
meningitis and other invasive infections in immunocompromised hosts. How
these hosts became infected remained unknown until a series of
investigations identified food as the most common source (6). Similarly,
Campylobacter jejuni was known as a rare opportunistic bloodstream
infection until veterinary diagnostic methods used on specimens from humans
showed it was a common cause of diarrheal illness (7). Subsequent
epidemiologic investigations implicated poultry and raw milk as the most
common sources of sporadic cases and outbreaks, respectively (8). Yersinia
enterocolitica, rare in the United States but a common cause of diarrheal
illness and pseudoappendicitis in northern Europe and elsewhere, is now
known to be most frequently associated with undercooked pork (9).

These foodborne pathogens share a number of characteristics. Virtually all
have an animal reservoir from which they spread to humans; that is, they
are foodborne zoonoses. In marked contrast to many established zoonoses,
these new zoonoses do not often cause illness in the infected host animal.
The chicken with lifelong ovarian infection with Salmonella serotype
Enteritidis, the calf carrying E. coli O157:H7, and the oyster carrying
Norwalk virus or V. vulnificus appear healthy; therefore, public health
concerns must now include apparently healthy animals. Limited existing
research on how animals acquire and transmit emerging pathogens among
themselves often implicates contaminated fodder and water; therefore,
public health concerns must now include the safety of what food animals
themselves eat and drink.

For reasons that remain unclear, these pathogens can rapidly spread
globally. For example, Y. enterocolitica spread globally among pigs in the
1970s (10); Salmonella serotype Enteritidis appeared simultaneously around
the world in the 1980s (11); and Salmonella Typhimurium Definitive Type
(DT) 104 is now appearing in North America, Europe, and perhaps elsewhere
(12); therefore, public health concerns must now include events happening
around the world, as harbingers of what may appear here.

Many emerging zoonotic pathogens are becoming increasingly resistant to
antimicrobial agents, largely because of the widespread use of antibiotics
in the animal reservoir. For example, Campylobacter isolated from human
patients in Europe is now increasingly resistant to fluoroquinolones, after
these agents were introduced for use in animals (13). Salmonellae have
become increasing resistant to a variety of antimicrobial agents in the
United States (14); therefore, public health concerns must include the
patterns of antimicrobial use in agriculture as well as in human medicine.

The foods contaminated with emerging pathogens usually look, smell, and
taste normal, and the pathogen often survives traditional preparation
techniques: E. coli O157:H7 in meat can survive the gentle heating that a
rare hamburger gets (15); Salmonella Enteritidis in eggs survives in an
omelette (16); and Norwalk virus in oysters survives gentle steaming (17).
Following standard and traditional recipes can cause illness and outbreaks.
Contamination with the new foodborne zoonoses eludes traditional food
inspection, which relies on visual identification of foodborne hazards.
These pathogens demand new control strategies, which would minimize the
likelihood of contamination in the first place. The rate at which new
pathogens have been identified suggests that many more remain to be
discovered. Many of the foodborne infections of the future are likely to
arise from the animal reservoirs from which we draw our food supply.

Once a new foodborne disease is identified, a number of critical questions
need to be answered to develop a rational approach to prevention: What is
the nature of the disease? What is the nature of the pathogen? What are
simple ways to easily identify the pathogen and diagnose the disease? What
is the incidence of the infection? How can the disease be treated? Which
foods transmit the infection? How does the pathogen get into the food, and
how well does it persist there? Is there is an animal reservoir? How do the
animals themselves become infected? How can the disease be prevented? Does
the prevention strategy work?

The answers to these questions do not come rapidly. Knowledge accumulates
gradually, as a result of detailed scientific investigations, often
conducted during outbreaks (18). After 15 years of research, we know a
great deal about infections with E. coli O157:H7, but we still do not know
how best to treat the infection, nor how the cattle (the principal source
of infection for humans) themselves become infected. Better slaughter
procedures and pasteurization of milk are useful control strategies for
this pathogen in meat and milk, as irradiation of meat may be in the
future. More needs to be learned: for example, it remains unclear how best
to prevent this organism from contaminating lettuce or apple juice. For
more recently identified agents, even less is known.

New Food Vehicles of Transmission

Along with new pathogens, an array of new food vehicles of transmission
have been implicated in recent years. Traditionally, the food implicated in
a foodborne outbreak was undercooked meat, poultry or seafood, or
unpasteurized milk. Now, additional foods previously thought safe are
considered hazardous. For example, for centuries, the internal contents of
an egg were presumed safe to eat raw. However, epidemic Salmonella
Enteritidis infection among egg-laying flocks indicates that intact eggs
may have internal contamination with this Salmonella serotype. Many
outbreaks are caused by contaminated shell eggs, including eggs used in
such traditional recipes as eggnog and Caesar salad, lightly cooked eggs in
omelettes and French toast, and even foods one would presume thoroughly
cooked, such as lasagna and meringue pie (19,20). E. coli O157:H7 has
caused illness through an ever-broadening spectrum of foods, beyond the
beef and raw milk that are directly related to the bovine reservoir. In
1992, an outbreak caused by apple cider showed that this organism could be
transmitted through a food with a pH level of less than 4.0, possibly after
contact of fresh produce with manure (21). A recent outbreak traced to
venison jerky suggests a wild deer reservoir, so both cattle and feral deer
manure are of concern (22). Imported raspberries contaminated with
Cyclospora caused an epidemic in the United States in 1996, possibly
because contaminated surface water was used to spray the berries with
fungicide before harvest (5). Norwalklike viruses, which appear to have a
human reservoir, have contaminated oysters harvested from pristine waters
by oyster catchers who did not use toilets with holding tanks on their
boats and were themselves the likely source of the virus (23).

The new food vehicles of disease share several features. Contamination
typically occurs early in the production process, rather than just before
consumption. Because of consumer demand and the global food market,
ingredients from many countries may be combined in a single dish, which
makes the specific source of contamination difficult to trace. These foods
have fewer barriers to microbial growth, such as salt, sugar, or
preservatives; therefore, simple transgressions can make the food unsafe.
Because the food has a short shelf life, it may often be gone by the time
the outbreak is recognized; therefore, efforts to prevent contamination at
the source are very important.

An increasing, though still limited, proportion of reported foodborne
outbreaks are being traced to fresh produce (24). A series of outbreaks
recently investigated by the Centers for Disease Control and Prevention
(CDC) has linked a variety of pathogens to fresh fruits and vegetables
harvested in the United States and elsewhere (Table 2). The investigations
have often been triggered by detection of more cases than expected of a
rare serotype of Salmonella or Shigella or by diagnosis of a rare infection
like cyclosporiasis. Outbreaks caused by common serotypes are more likely
to be missed. Various possible points of contamination have been identified
during these investigations, including contamination during production and
harvest, initial processing and packing, distribution, and final processing
(Table 3). For example, fresh or inadequately composted manure is used
sometimes, although E. coli O157:H7 has been shown to survive for up to 70
days in bovine feces (25). Untreated or contaminated water seems to be a
particularly likely source of contamination. Water used for spraying,
washing, and maintaining the appearance of produce must be
microbiologically safe. After two large outbreaks of salmonellosis were
traced to imported cantaloupe, the melon industry considered a "Melon
Safety Plan," focusing particularly on the chlorination of water used to
wash melons and to make ice for shipping them. Although the extent to which
the plan was implemented is unknown, no further large outbreaks have
occurred. After two large outbreaks of salmonellosis were traced to a
single tomato packer in the Southeast, an automated chlorination system was
developed for the packing plant wash tank. Because tomatoes absorb water
(and associated bacteria) if washed in water colder than they are,
particular attention was also focused on the temperature of the water bath
(26,27). No further outbreaks have been linked to southeastern tomatoes.
Similar attention is warranted for water used to rinse lettuce heads in
packing sheds and to crisp them in grocery stores as well as for water used
in processing other fresh produce.

Table 2. Foodborne outbreaks traced to fresh produce 1990-1996
----------------------------------------------------------------
Yr.  Pathogen           Vehicle         Cases  States  Source
                                        (No.)  (No.)
---------------------------------------------------------------
'90  S. Chester         Cantaloupe      245    30      C.A. (superscript a)
'90  S. Javiana         Tomatoes        174    4       U.S. (superscript b)
'90  Hepatitis A        Strawberries    18     2       U.S.

'91  S. Poona           Cantaloupe      >400   23      U.S./C.A
'93  E. coli O157:H7    Apple cider     23     1       U.S.
'93  S. Montevideo      Tomatoes        84     3       U.S.
'94  Shigella flexneri  Scallions       72     2       C.A.
'95  S. Stanley         Alfalfa sprouts 242    17      N.K. (superscript c)
'95  S. Hartford        Orange juice    63     21      U.S.
'95  E. coli O157:H7    Leaf lettuce    70     1       U.S.
'96  E. coli O157:H7    Leaf lettuce    49     2       U.S.
'96  Cyclospora         Raspberries     978    20      C.A.
'96  E. coli O157:H7    Apple juice     71     3       U.S.
---------------------------------------------------------------------------
(superscript a) Central America
(superscript b) United States
(superscript c) Source not known

Table 3. Events and potential contamination sources during produce
processing
---------------------------------------------------------
Event                      Contamination sources
--------------------------------------------------------
Production and harvest
    Growing, picking,      Irrigation water, manure,
    bundling               lack of field sanitation

Initial processing
    Washing, waxing,       Wash water, handling
    sorting, boxing

Distribution
    Trucking               Ice, dirty trucks

Final processing
    Slicing, squeezing,    Wash water, handling,
    shredding, peeling     cross-contamination
---------------------------------------------------------

A New Outbreak Scenario

Because of changes in the way food is produced and distributed, a new kind
of outbreak has appeared. The traditional foodborne outbreak scenario often
follows a church supper, family picnic, wedding reception, or other social
event. This scenario involves an acute and highly local outbreak, with a
high inoculum dose and a high attack rate. The outbreak is typically
immediately apparent to those in the local group, who promptly involve
medical and public health authorities. The investigation identifies a
food-handling error in a small kitchen that occurs shortly before
consumption. The solution is also local. Such outbreaks still occur, and
handling them remains an important function of a local health department.

However, diffuse and widespread outbreaks, involving many counties, states,
and even nations (28), are identified more frequently and follow an
entirely different scenario. The new scenario is the result of low-level
contamination of a widely distributed commercial food product. In most
jurisdictions, the increase in cases may be inapparent against the
background illness. The outbreak is detected only because of a fortuitous
concentration of cases in one location, because the pathogen causing the
outbreak is unusual, or because laboratory-based subtyping of strains
collected over a wide area identifies a diffuse surge in one subtype. In
such outbreaks, investigation can require coordinated efforts of a large
team to clarify the extent of the outbreak, implicate a specific food, and
determine the source of contamination. Often, no obvious terminal
food-handling error is found. Instead, contamination is the result of an
event in the industrial chain of food production. Investigating,
controlling, and preventing such outbreaks can have industrywide
implications.

These diffuse outbreaks can be caused by a variety of foods. Because fresh
produce is usually widely distributed, most of the produce-related
outbreaks listed in Table 2 were multistate events. Some of the largest
outbreaks affected most states at once. For example, a recent outbreak of
Salmonella Enteritidis infections caused by a nationally distributed brand
of ice cream affected the entire nation (29). Although it caused an
estimated 250,000 illnesses, it was detected only when vigorous routine
surveillance identified a surge in reported infections with S. Enteritidis
in one area of southern Minnesota. The consumers affected did not make
food-handling errors with their ice cream, so food safety instruction could
not have prevented this outbreak. The ice cream premix was transported
after pasteurization to the ice cream factory in tanker trucks that had
been used to haul raw eggs. The huge epidemic was the result of a basic
failure on an industrial scale to separate the raw from the cooked.

S. Enteritidis infections also illustrate why surveillance and
investigation of sporadic cases are needed. A diffuse increase in sporadic
cases can occur well before a local or large outbreak focuses attention on
the emergence of a pathogen. The isolation rate for S. Enteritidis began to
increase sharply in the New England region in 1978 (Figure 2); all cases
were sporadic. In 1982, an outbreak in a New England nursing home was
traced to eggs from a local supplier. However, the egg connection was not
really appreciated until 1986, when a large multistate outbreak of S.
Enteritidis infections was traced to stuffed pasta made with raw eggs and
labeled "fully cooked." This outbreak, affecting an estimated 3,000 persons
in seven states, led to the documentation that S. Enteritidis was present
on egg-laying farms and to the subsequent demonstration that both outbreaks
and sporadic cases of infections were associated with shell eggs (19,30).
Since then, Enteritidis has become the most common serotype of Salmonella
isolated in the United States, accounting for 25% of all Salmonella
reported in the country and causing outbreaks coast to coast. Eggs remain
the dominant source of these infections, causing large outbreaks when they
are pooled and undercooked and individual sporadic cases among consumers
who eat individual eggs (20,31). Perhaps focused investigation and control
measures taken when the localized increase in sporadic Salmonella cases was
just beginning might have prevented the subsequent spread.

 [Figure Not Available in Ascii]
 Figure 2. Salmonella Enteritidis isolation rates from humans by region,
 United States, 1970-1996.

Changing Surveillance Strategies

In the United States, surveillance for diseases of major public health
importance has been conducted for many years. The legal framework for
surveillance resides in the state public health epidemiology offices, which
share data with CDC. The first surveillance systems depended on physician
or coroner notification of specific diseases and conditions, with reports
going first to the local health department, then to state and federal
offices. Now electronic, this form of surveillance is still used for many
specific conditions (32). In 1962, a second channel was developed
specifically for Salmonella, to take advantage of the added public health
information provided by subtyping the strains of bacteria (33). Clinical
laboratories that isolated Salmonella from humans were requested or
required to send the strains to the state public health laboratory for
serotyping. Although knowing the serotype is usually of little benefit to
the individual patient, it has been critical to protecting and improving
the health of the public at large. Serotyping allows cases that might
otherwise appear unrelated to be included in an investigation because they
are of the same serotype. Moreover, infections that are close in time and
space to an outbreak but are caused by nonoutbreak serotypes and are
probably unrelated can be discounted. Results of serotyping are now sent
electronically from public health laboratories and can be rapidly analyzed
and summarized. Salmonella serotyping was the first subtype-based
surveillance system and is a model for similar systems (34). Yet another
source of surveillance data involves summary reports of foodborne disease
outbreak investigations from local and state health departments (35). About
400 such outbreaks are reported annually, by a system that remains
paper-based, labor-intensive, and slow.

Existing surveillance systems provide a limited and relatively inexpensive
net for tracing large-scale trends in foodborne diseases under surveillance
and for detecting outbreaks of established pathogens in the United States.
However, they are less sensitive to diffuse outbreaks of common pathogens,
provide little detail on sporadic cases, and are not easy to extend to
emerging pathogens. In the future, changes in health delivery may impinge
on the way that diagnoses are made and reported, leading to artifactual
changes in reported disease incidence.

Therefore, CDC, in collaboration with state health departments and federal
food regulatory agencies, is enhancing national surveillance for foodborne
diseases in several ways. First, the role of subtyping in public health
laboratories is being expanded to encompass new molecular subtyping
methods. Beginning in 1997, a national subtyping network for E. coli
O157:H7 of participating state public health department laboratories and
CDC will use a single standardized laboratory protocol to subtype strains
of this important pathogen. The standard method, pulsed-field gel
electrophoresis, can be easily adapted to other bacterial pathogens. In
this network, each participating laboratory will be able to routinely
compare the genetic gel patterns of strains of E. coli O157:H7 with the
patterns in a national pattern bank. This will enable rapid detection of
clusters of related cases within the state and will focus investigative
resources on the cases most likely to be linked. It will also enable
related cases scattered across several states to be linked so that a common
source can be sought.

Another surveillance strategy, now implemented, is active surveillance in
sentinel populations. Since January 1996, at five U.S. sentinel sites,
additional surveillance resources make it possible to contact laboratories
directly for regular reporting of bacterial infections likely to be
foodborne (36; Figure 3). In addition, surveys of the population,
physicians, and laboratories measure the proportion of diarrheal diseases
that are undiagnosed and unreported so that the true disease incidence can
be estimated. This surveillance, known as FoodNet, is the platform on which
more detailed investigations, including case-control studies of sporadic
cases of common foodborne infections, are being conducted.

 [Figure Not Available in ASCII]
Figure 3. Incidence of three infections in FoodNet surveillance areas,
1996.

Yet another new surveillance initiative is the routine monitoring of
antimicrobial resistance among a sample of Salmonella and E. coli O157:H7
bacteria isolated from humans (37). A new cluster detection algorithm is
being applied routinely to surveillance data for Salmonella at the national
level, making it possible to detect and flag possible outbreaks as soon as
the data are reported (38). Implementation of such algorithms for other
infections and at the state level will further increase the usefulness of
routine surveillance.

Further enhancements are possible as active surveillance through FoodNet is
extended to a wider spectrum of infections, including foodborne parasitic
and viral infections. In 1997, active surveillance for Cyclospora began in
FoodNet, which quickly resulted in the detection of a diffuse outbreak
among persons who had been on a Caribbean cruise ship that made stops in
Mexico and Central America (CDC, unpub. data). Application of standardized
molecular subtyping methods to other foodborne pathogens will provide a
more sensitive warning system for diffuse outbreaks of a variety of
pathogens. To handle outbreaks in areas not covered by FoodNet, standard
surveillance and investigative capacities in state health department
epidemiology offices and laboratories should be strengthened. In addition,
enhanced international consultation will be critical to better detect and
investigate international or global outbreaks (28).

Implications of the New Outbreak Scenario for Public Health Activities

Our public health infrastructure is tiered, both in surveillance
responsibilities and in response to emergency situations (39). At the local
level, the county or city health department, first developed in response to
epidemic cholera and other challenges in the 19th century, is responsible
for most basic surveillance, investigation, and prevention activities. At
the state level, epidemiologists, public health laboratorians, sanitarians,
and educators conduct statewide surveillance and prevention activities and
consult with and support local authorities. At the national level, CDC is
the primary risk-assessment agency for public health hazards and conducts
the primary national surveillance as well as epidemic response in support
of state health departments. The Food and Drug Administration, Department
of Agriculture, and Environmental Protection Agency are the primary
regulatory agencies, charged with specific responsibilities regarding the
nation's food and water supplies that interlock and are not always
predictable. The Food and Drug Administration regulates low-acid canned
foods, imported foods, pasteurized milk, many seafoods, rabbits raised for
meat, and food and water provided on aircraft and trains. The Department of
Agriculture regulates meat and poultry, including primary slaughter and
further processing, and pasteurized eggs; investigates animal and plant
diseases; and maintains the county extension outreach program. Shell eggs
do not have a clear regulatory home, as the Department of Agriculture
regulates the grading of shell eggs for quality, but the Food and Drug
Administration, since 1995, has responsibility for the microbiologic safety
of shell eggs.

The new outbreak scenario has several implications for the practice of
public health, starting at the local level. One is that when diffuse
outbreaks are detected, a local health department may need to investigate a
few cases that are part of a larger outbreak despite their apparently small
local impact. Second, an apparently local outbreak may herald the first
recognized manifestation of a national or even international event.

When a diffuse outbreak of a potentially foodborne pathogen is detected,
rapid investigation is needed to determine whether the outbreak is
foodborne, and if possible, identify a specific food vehicle. These
investigations, which typically include case-control studies, may need to
be conducted in several locations at once. While all cases or all affected
states may not need to be included in such an investigation, combining
cases from several locations in one investigation and repeating the
investigation in more than one location can be helpful. For example, in a
recent international outbreak of Salmonella Stanley infections traced to
alfalfa sprouts, concentrations of cases in Arizona, Michigan, and Finland
led to case-control studies in each location, each of which linked illness
to eating sprouts grown from the same batch of alfalfa seeds. This proved
that the seeds were contaminated at the source (40). Parallel
investigations can also lead to new twists. In the large West Coast
outbreak of E. coli O157:H7 infections in 1993, a parallel investigation
conducted in Nevada identified a type of hamburger other than the one
implicated in the initial case-control investigation in Washington, leading
to a broader recall and a more complete investigation of the circumstances
of contamination (15,41). Because well-conducted investigations may lead to
major product recalls, industrial review, and overhaul, and even
international embargoes, it is essential that they be of the highest
scientific quality.

Foodborne outbreaks are investigated for two main reasons. The first is to
identify and control an ongoing source by emergency action: product recall,
restaurant closure, or other temporary but definitive solutions. The second
reason is to learn how to prevent future similar outbreaks from occurring.
In the long run this second purpose will have an even greater impact on
public health than simply identifying and halting the outbreaks. Because
all the answers are not available and existing regulations may not be
sufficient to prevent outbreaks, the scientific investigation often
requires a careful evaluation of the chain of production. This traceback is
an integral part of the outbreak investigation. It is not a search for
regulatory violations, but rather an effort to determine where and how
contamination occurred. Often, the contamination scenario reveals that a
critical point has been lost. Therefore, epidemiologists must participate
in traceback investigations.

Intervention during outbreaks often depends on having enough good
epidemiologic data to act with confidence, without waiting for a definitive
laboratory test, particularly if potentially lethal illnesses are involved.
For example, if five persons with classic clinical botulism ate at the same
restaurant the preceding day (but have nothing else apparent in common),
prudence dictates closing the restaurant quickly while the outbreak is
sorted out-that is, before a specific food is identified or confirmatory
cultures are made, which may take several days or even weeks. Good
epidemiologic data, including evidence of a clear statistical association
with a specific exposure, biologic plausibility of the illness syndrome,
the potential hazard of that food, and the logical consistency of
distribution of the suspect food and cases are essential.

The role of the regulatory agency laboratory is also affected by the new
scenario. Because of the short shelf life and broad distribution of many of
the new foods responsible for infection, by the time the outbreak is
recognized and investigated the relevant food may no longer be available
for culture. Because contamination may be restricted to a single production
lot, blind sampling of similar foods that does not include the implicated
lot can give a false sense of security. Good epidemiologic information
pointing to contamination of a specific food or production lot should guide
the microbiologic sampling and the interpretation of the results. Available
methods may be insufficient to detect low-level contamination, even of
well-established pathogens.

New Approaches to the Prevention of Foodborne Disease

Meeting the complex challenge of foodborne disease prevention will require
the collaboration of regulatory agencies and industry to make food safely
and keep it safe throughout the industrial chain of production. Prevention
can be "built in" to the industry by identifying and controlling the key
points-from field, farm, or fishing ground to the dinner table-at which
contamination can either occur or be eliminated. The general strategy known
as Hazard Analysis and Critical Control Points (HACCP) replaces the
strategy of final product inspection. Some simple control strategies are
self-evident, once the reality of microbial contamination is recognized.
For example, shipping fruit from Central America with clean ice or in
closed refrigerator trucks, rather than with ice made from untreated river
water, is common sense. Similarly, requiring oyster harvesters to use
toilets with holding tanks on their oyster boats is an obvious way to
reduce fecal contamination of shallow oyster beds. Pasteurization provides
the extra barrier that will prevent E. coli O157:H7 and other pathogens
from contaminating a large batch of freshly squeezed juice.

For many foodborne diseases, multiple choices for prevention are available,
and the best answer may be to apply several steps simultaneously. For E.
coli O157:H7 infections related to the cattle reservoir, pasteurizing milk
and cooking meat thoroughly provide an important measure of protection but
are insufficient by themselves. Options for better control include
continued improvements in slaughter plant hygiene and control measures
under HACCP, developing additives to cattle feed that alter the microbial
growth either in the feed or in the bovine rumen to make cows less
hospitable hosts for E. coli O157, immunizing or otherwise protecting the
cows so that they do not become infected in the first place, and
irradiating beef after slaughter. For C. jejuni infections related to the
poultry reservoir, future control options may include modification of the
slaughter process to reduce contamination of chicken carcasses by bile or
by water baths, freezing chicken carcasses to reduce Campylobacter counts,
chlorinating the water that chickens drink to prevent them from getting
infected, vaccinating chickens, and irradiating poultry carcasses after
slaughter.

Outbreaks are often fertile sources of new research questions. Translating
these questions into research agendas is an important part of the overall
prevention effort. Applied research is needed to improve strategies of
subtyping and surveillance. Veterinary and agricultural research on the
farm is needed to answer the questions about whether and how a pathogen
such as E. coli O157:H7 persists in the bovine reservoir, to establish the
size and dynamics of a reservoir for this organism in wild deer, and to
look at potential routes of contamination connecting animal manure and
lettuce fields. More research is needed regarding foods defined as sources
in large outbreaks to develop better control strategies and better barriers
to contamination and microbial growth and to understand the behavior of new
pathogens in specific foods. Research is also needed to improve the
diagnosis, clinical management, and treatment of severe foodborne
infections and to improve our understanding of the pathogenesis of new and
emerging pathogens. To assess and evaluate potential prevention strategies,
applied research is needed into the costs and potential benefits of each or
of combinations.

To prepare for the 21st century, we will enhance our public health food
safety infrastructure by adding new surveillance and subtyping strategies
and strengthening the ability of public health practitioners to investigate
and respond quickly. We need to encourage the prudent use of antibiotics in
animal and human medicine to limit antimicrobial resistance. We need to
continue basic and applied research into the microbes that cause foodborne
disease and into the mechanisms by which they contaminate our foods and
cause outbreaks and sporadic cases. Better understanding of foodborne
pathogens is the foundation for new approaches to disease prevention and
control.

Address for correspondence: Robert V. Tauxe, Centers for Disease Control
and Prevention, 1600 Clifton Road, MS A38, Atlanta, GA 30333 USA; fax:
404-639-2205.

References

  1. Blake PA, Merson MH, Weaver RE, Hollis DG, Heublein PC. Disease caused
     by a marine vibrio: clinical characteristics and epidemiology. N Engl
     J Med 1979;300:1-5.
  2. Riley LW, Remis RS, Helgerson SD, McGee HB, Wells JG, Davis BR, et al.
     Hemorrhagic colitis associated with a rare Escherichia coli serotype.
     N Engl J Med 1983;308:681.
  3. Martin ML, Shipman LD, Wells JG, Potter ME, Hedberg K, Wachsmuth IK,
     et al. Isolation of Escherichia coli O157:H7 from dairy cattle
     associated with two cases of hemolytic uremic syndrome. Lancet
     1986;2:1043.
  4. Ortega YR, Sterling CR, Gilman RH, Cama VA, Diaz F. Cyclospora
     species-a new protozoan pathogen of humans. N Engl J Med
     1993;328:1308-12.
  5. Herwaldt BL, Ackers M-L, the Cyclospora Working Group. International
     outbreak of cyclosporiasis associated with imported raspberries. N
     Engl J Med. In press 1997.
  6. Jackson LA, Wenger JD. Listeriosis: a foodborne disease. Infections in
     Medicine 1993;10:61-6.
  7. Dekeyser PJ, Gossin-Detrain M, Butzler JP, Sternon J. Acute enteritis
     due to related Vibrio; first positive stool cultures. J Infect Dis
     1972;125:390-2.
  8. Tauxe RV. Epidemiology of Campylobacter jejuni infections in the
     United States and other industrialized nations. In: Nachamkin I,
     Blaser MJ, Tompkins L, editors. Campylobacter jejuni: current status
     and future trends, eds. Washington (DC): American Society of
     Microbiology, 1992. pp 9-19.
  9. Tauxe RV, Vandepitte J, Wauters G, Martin SM, Goosens V, DeMol P, et
     al. Yersinia enterocolitica infections and pork: the missing link.
     Lancet 1987;1:1129-32.
 10. World Health Organization. Worldwide spread of infections with
     Yersinia enterocolitica. WHO Chronicle 1976;30:494-6.
 11. Rodrigue DC, Tauxe RV, Rowe B. International increase in Salmonella
     enteritidis: a new pandemic? Epidemiol Infect 1990;105:21-7.
 12. Centers for Disease Control and Prevention. Multidrug-resistant
     Salmonella serotype Typhimurium-United States, 1996. MMWR Morb Mortal
     Wkly Rep 1997;46:308-10.
 13. Endt HP, Ruijs GJ, van Klingeren B, Jansen WH, van der Reyden T,
     Mouton RP. Quinolone resistance in Campylobacter isolated from man and
     poultry following the introduction of fluoroquinolones in veterinary
     medicine. J Antimicrob Chemother 1991;27:199-208.
 14. Lee LA, Puhr ND, Maloney K, Bean NH, Tauxe RV. Increase in
     antimicrobial-resistant Salmonella infections in the United States,
     1989-1990. J Infect Dis 1994;170:128-34.
 15. Cieslak PR, Noble SJ, Maxson DJ, Empey LC, Ravenholt O, Legarza G, et
     al. Hamburger-associated Escherichia coli O157:H7 in Las Vegas: a
     hidden epidemic. Am J Public Health 1997;87:176-80.
 16. Humphrey TJ, Greenwood M, Gilbert RJ, Rowe B, Chapman PA. The survival
     of salmonellas in shell eggs cooked under simulated domestic
     conditions. Epidemiol Infect 1989;103:35-45.
 17. Kirkland KB, Meriwether RA, Leiss JK, MacKenzie WR. Steaming oysters
     does not prevent Norwalk-like gastroenteritis. Public Health Rep
     1996;111:527-30.
 18. Holmberg SD, Feldman RA. New and newer enteric pathogens: stages in
     our knowledge. Am J Public Health 1984;74:205-7.
 19. St. Louis ME, Morse DL, Potter ME, DeMelfi TM, Guzewich JJ, Tauxe RV,
     et al. The emergence of Grade A eggs as a major source of Salmonella
     enteritidis infections: implications for the control of salmonellosis.
     JAMA 1988;259:2103-7.
 20. Mishu B, J Koehler, Lee LA, Rodrigue D, Brenner FH, Blake P, Tauxe RV.
     Outbreaks of Salmonella enteritidis infections in the United States,
     1985-1991. J Infect Dis 1994;169:547-52.
 21. Besser RE, Lett SM, Weber JT, Doyle MP, Barrett TJ, Wells JG, Griffin
     PM. An outbreak of diarrhea and hemolytic uremic syndrome from
     Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA
     1993;269:2217-20.
 22. Keene WE, Sazie E, Kok J, Rice DH, Hancock DD, Balan VK, et al. An
     outbreak of Escherichia coli O157:H7 infections traced to jerky made
     from deer meat. JAMA 1997;277:1229-31.
 23. Kohn MA, Farley TA, Ando T, Curtis M, Wilson SA, Jin Q, et al. An
     outbreak of Norwalk virus gastroenteritis associated with eating raw
     oysters: implications for maintaining safe oyster beds. JAMA
     1995;273:466-71.
 24. Tauxe R, Kruse H, Hedberg C, Potter M, Madden J, Wachsmuth K.
     Microbial hazards and emerging issues associated with produce; a
     preliminary report to the National Advisory Committee on Microbiologic
     Criteria for Foods. Journal of Food Protection. In press 1997.
 25. Wang G, Zhao T, Doyle MP. Fate of enterohemorrhagic Escherichia coli
     O157:H7 in bovine feces. Appl Environ Microbiol 1996;62:2567-70.
 26. Zhuang R-Y, Beuchat LR, Angulo FJ. Fate of Salmonella montevideo on
     and in raw tomatoes as affected by temperature and treatment with
     chlorine. Appl Environ Microbiol 1995;61:2127-31.
 27. Rushing JW, Angulo FJ, Beuchat LR. Implementation of a HACCP program
     in a commercial fresh-market tomato packinghouse: a model for the
     industry. Dairy, Food and Environmental Sanitation 1996;16:549-53.
 28. Tauxe RV, Hughes JM. International investigations of outbreaks of
     foodborne disease: public health responds to the globalization of
     food. BMJ 1996;313:1093-4.
 29. Hennessey TW, Hedberg CW, Slutsker L, White KE, Besser-Wiek JM, Moen
     ME, et al. A national outbreak of Salmonella enteritidis infections
     from ice cream. N Engl J Med 1996;334:1281-6.
 30. Passarro DJ, Reporter R, Mascola L, Kilman L, Malcolm GB, Rolka H, et
     al. Epidemic Salmonella Enteritidis infection in Los Angeles County,
     California: the predominance of phage type 4. West J Med
     1996;165:126-30.
 31. Centers for Disease Control and Prevention. Outbreaks of Salmonella
     serotype Enteritidis infection associated with consumption of raw
     shell eggsUnited States, 1994-1995. MMWR Morb Mortal Wkly Rep
     1996;45:737-42.
 32. Centers for Disease Control and Prevention. Summary of notifiable
     diseases, United States, 1995. MMWR Morb Mortal Wkly Rep 1995;44(53).
 33. Centers for Disease Control and Prevention. Proceedings of a national
     conference on salmonellosis, March 11-13, 1964. U.S. Public Health
     Service Publication No 1262. Washington (DC): U.S. Government Printing
     Office; 1965.
 34. Bean NH, Morris SM, Bradford H. PHLIS: an electronic system for
     reporting public health data from remote sites. Am J Public Health
     1992;82:1273-6.
 35. Bean NH, Goulding JS, Lao C, Angulo FJ. Surveillance for
     foodborne-disease outbreaksUnited States, 1988-1992. CDC Surveillance
     Summaries, October 25, 1996. MMWR Morb Mortal Wkly Rep 1996;45(SS-5).
 36. Centers for Disease Control and Prevention. Foodborne Diseases Active
     Surveillance Network, 1996. MMWR Morb Mortal Wkly Rep 1997;46:258-61.
 37. Centers for Disease Control and Prevention. Establishment of a
     national surveillance program for antimicrobial resistance in
     Salmonella. MMWR Morb Mortal Wkly Rep 1996;45:110-1.
 38. Hutwagner LC, Maloney EK, Bean NH, Slutsker L, Martin SM. Using
     laboratory-based surveillance data for prevention: an algorithm for
     detecting Salmonella outbreaks. Emerg Infect Dis 1997;3:395-400.
 39. Meriwether RA. Blueprint for a national public health surveillance
     system for the 21st Century. Journal of Public Health Management and
     Practice 1996;2:16-23.
 40. Mahon BE, Pnk A, Hall WN, Komatsu K, Dietrich SE, Siitonen A, et al.
     An international outbreak of Salmonella infections caused by alfalfa
     sprouts grown from contaminated seed. J Infect Dis 1997;175:876-82.
 41. Bell BP, Goldoft M, Griffin PM, Davis MA, Gordon DC, Tarr PI, et al. A
     multistate outbreak of Escherichia coli O157:H7-associated bloody
     diarrhea and hemolytic uremic syndrome from hamburgers: the Washington
     experience. JAMA 1994;272:1349-53.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/tauxe.txt

Emergence of New Pathogens as a Function of Changes in Host Susceptibility

J. Glenn Morris, Jr.* and Morris Potter
*University of Maryland School of Medicine, Baltimore, Maryland, USA; and
Centers for Disease Control and Prevention, Atlanta, Georgia, USA
---------------------------------------------------------------------------

      A pathogen may emerge as an important public health problem
      because of changes in itself or its transmission pathways.
      Alternatively, a microorganism may emerge as a pathogen or
      acquire new public health importance because of changes in host
      susceptibility to infection. Factors influencing host
      susceptibility within the population as a whole include
      increases in the number of immunocompromised patients;
      increased use of immunosuppressive agents, particularly among
      persons receiving cancer chemotherapy or undergoing organ
      transplantation; aging of the population; and malnutrition. In
      considering the emergence of foodborne pathogens and designing
      interventions to limit their spread, the susceptibility of
      these population subgroups to specific infections should be
      taken into account.

Occurrence of disease is a function of several major variables: the
virulence of the microorganism (i.e., its possession of factors that allow
it to cause illness), its mode of transmission (i.e., how it gets to the
host), and host susceptibility (i.e., how well the host can defend itself
against the microorganism). Increased susceptibility to infection may be
measured in terms of infectious dose (the number of microorganisms it takes
to cause illness) and of the ability of the host to limit spread of the
microorganism (e.g., the ability to limit spread of microorganisms from the
intestinal tract to the bloodstream). These same variables apply to
emerging pathogens. Microorganisms may emerge as pathogens because they
have developed new virulence genes or resistance to standard therapeutic
methods. Emergence may be related to changes in transmission pathways,
which permit a known pathogen to move into new, previously unexposed
populations. Finally, increases in the number of persons susceptible to a
specific microorganism may result in its emergence as an important public
health problem; at the same time, when attention is focused on populations
with increased susceptibility to infection, organisms that might not
otherwise be recognized as pathogens may be identified.

Many factors influence the susceptibility of populations to infection,
including increases in diseases that cause immunosuppression, increased use
of immunosuppressive agents, aging of the population, and malnutrition. In
considering these categories, it should be recognized that "host
susceptibility" is not a single entity. Changes in host susceptibility may
be due to various mechanisms, with each mechanism having a greater or
lesser impact on the ability of the host to defend itself against infection
with specific pathogens or classes of pathogens. These are very complex
biologic systems; nonetheless, the general categories outlined below may be
of value in identifying groups or populations for further study.

Increases in Diseases That Cause Immunosuppression

Hereditary diseases associated with immunosuppression are present in a
small but relatively constant proportion of the population. The most common
of these diseases, a selective immunoglobulin A deficiency, has been found
in as many as 0.3% of some blood donor populations (1) and may be
associated with recurrent diarrhea; infections with giardia, in particular,
have been noted to be more common among such immunocompromised patients.

In contrast to patients with hereditary immunodeficiencies, the population
of patients with acquired immunodeficiencies is rapidly increasing. As of
June 1996 in the United States, an estimated 223,000 persons > 13 years of
age had AIDS (Figure 1); this represents increases of 10% and 65% since
mid-1995 and January 1993, respectively. During 1995, human
immunodeficiency virus (HIV) infection remained the leading cause of death
among persons 25 to 44 years of age (Figure 2), accounting for 19% of
deaths from all causes in this age group (2).

[Figure Not Available in ASCII]

 Figure 1. Cases among persons aged > 13 years, adjusted for delays in
 reporting, by quarter year, United States, 1988-June 1996. (Points
 represent quarterly prevalence; the line represents "smoothed"
 prevalence. Estimates are not adjusted for incomplete reporting of
 diagnosed AIDS cases or AIDS deaths. From the Centers for Disease Control
 and Prevention. Update: Trends in AIDS incidence, deaths, and
 prevalence-United States, 1996. MMWR Morb Mortal Wkly Rep 1997;46:165-73).

[Figure Not Available in ASCII]

 Figure 2. Death rates per 100,000 population for leading causes of death
 among persons ages 25 to 44 years, by year, United States, 1982-1995.
 (Based on underlying causes of death reported on death certificates,
 using final data for 1982-1994 and preliminary data for 1995. From the
 Centers for Disease Control and Prevention. Update: Trends in AIDS
 incidence, deaths, and prevalence, United States, 1996. MMWR Morb Mortal
 Wkly Rep 1997;46:165-73).

Persons with AIDS show a clear increase in susceptibility to infection with
Salmonella species. Data suggest that risk for nontyphi Salmonella
infections is increased 20- to 100-fold among AIDS patients (3-7). Among
persons infected with Salmonella, AIDS results in a several-fold increase
in the risk for septicemia (3,5,6); AIDS also results in increases in
infections at other extraintestinal sites, compatible with an overall
increase in risk for dissemination of the organism. This increase in risk
is reflected in increases in the percentage of total Salmonella isolated
from blood. For example, for persons ages 25 to 49 years in states with
high AIDS incidence, the percentage of Salmonella isolates from blood
increased from 2.3% in 1978 to 17.8% in 1987 among men and from 3.1% to
8.1% among women; in contrast, no changes in blood-isolate percentages
occurred for either sex in states with low AIDS incidence (8). These latter
studies further suggest that serotype is an important risk determinant,
with increases in bacteremia in states with high AIDS incidence associated
primarily with infections due to Salmonella Enteritidis and Salmonella
Typhimurium. While these data are for nontyphoidal Salmonella, studies
outside the United States suggest that AIDS patients have a similar
increase in risk for infection with Salmonella typhi in areas endemic for
typhoid fever. For example, in Lima, Peru, the risk for typhoid increased
25-fold in HIV-infected persons 15 to 35 years of age (9); HIV infection
also appeared to influence the clinical presentation of typhoid fever, with
severe diarrhea and gastrointestinal symptoms seen more often than
expected.

While the above data suggest a continuing climb in salmonellosis and, in
particular, Salmonella bacteremia in conjunction with an increasing AIDS
prevalence, anecdotal data from AIDS clinicians do not support this view.
The standard of care for AIDS patients now includes routine prophylactic
therapy with trimethoprim/sulfamethoxazole to prevent Pneumocystis carinii
pneumonia. Trimethoprim/sulfamethoxazole is also one of the first-line
therapies for salmonellosis; widespread prophylactic use of this drug in
the AIDS population may have reduced the incidence of serious Salmonella
infections, although this protective effect could be diminished in the face
of increasing resistance to this antimicrobial agent among clinical
isolates of Salmonella (10).

Fewer data are available on susceptibility of AIDS patients to other acute
bacterial foodborne infections. While the initial impression was that the
risk for Campylobacter jejuni infections was not higher among AIDS
patients, a 35-fold increase in the Campylobacter case rate among persons
with AIDS was noted in one study from Los Angeles (11). Other data indicate
that HIV-positive patients can contract persistent C. jejuni infections,
with chronic diarrhea, fever, and fecal leukocytes (12). In studies in the
San Francisco area, AIDS patients were estimated to have a 280-fold
increase in incidence of listeriosis, as compared with the general
population (13). Of 98 nonpregnant adults with invasive Listeria infection
identified between November 1988 and December 1990 in selected counties in
California, Tennessee, Georgia, and Oklahoma, 20% were HIV-positive (13).

Before AIDS, Toxoplasma gondii was of concern primarily because of the risk
for congenital infection in infants of mothers who had acute illness during
pregnancy. T. gondii is now the leading cause of space-occupying cranial
lesions in persons with AIDS (14,15); data from the 1980s suggest that 5%
to 10% of AIDS patients get toxoplasmic encephalitis (16). In an estimated
50% of cases, Toxoplasma is transmitted by food (17). In this context,
Toxoplasma must be regarded as an important emerging pathogen in this
patient population.

The AIDS epidemic has also drawn attention to microorganisms not previously
recognized as pathogens. Perhaps the most important of these is
Cryptosporidium. In early investigations of AIDS-associated diarrhea, it
rapidly became apparent that most patients were not infected with
traditional enteric pathogens. Many of these patients were infected with
Cryptosporidium; an estimated 10% to 20% of cases of AIDS-associated
diarrhea are due to this microorganism (18). In subsequent years,
Cryptosporidium was also recognized as the cause of intestinal infections
in healthy hosts (19); and, most recently, it has been recognized as a
major waterborne pathogen (20). Isospora belli has also been implicated as
a cause of diarrhea in AIDS patients, as have the microsporidia (18). More
recently, enteroaggregative Escherichia coli and nonpathogenic bioserovars
of Yersinia enterocolitica were associated with diarrheal disease in AIDS
cases (21,22). Further work will determine if these latter agents are
indeed emerging pathogens in this patient population.

Increased Use of Immunosuppressive Agents

Advances in medical treatment have resulted in increasing numbers of
immunosuppressed patients (including patients undergoing organ
transplantation or cancer chemotherapy) and patients with serious
underlying chronic diseases; these patients, too, may be at increased risk
for infection with microorganisms that might otherwise not be recognized or
associated with serious illness. The number of new cancer cases has
steadily increased over the past 20 years. For white males, cancers at all
sites have increased from an estimated 364 new cases/100,000 population in
1973 to 462 new cases/100,000 population in 1994; white females show an
increase from 295 new cases/100,000 population in 1973 to 347 new
cases/100,000 population in 1994 (23). Patients are also surviving longer.
For white males, the 5-year relative cancer survival rate was 56.6% in 1986
to 1993, compared with 42% in 1974 to 1976; for white females, the 1986 to
1993 rate was 62.3%, compared with 57.6% in 1974 to 1976. While specific
data are lacking, these increases appear to have been accompanied by
increased use of chemotherapeutic regimens and regimens that may have more
toxicity than those used in the past. The number of solid organ transplants
has also increased substantively (Figure 3). In particular, more complex
procedures such as liver, heart, and lung transplants have increased. This,
in turn, has resulted in larger numbers of chronically immunocompromised
persons in the general population.

[Figure Not Available in ASCII]

 Figure 3. Number of organ transplants, by year and by site, 1988-1996
 (data obtained from the United Network for Organ Sharing).

 Note: Heart-lung transplants too few to be visible.

Aside from the direct immunosuppressive effect of the agents administered
to these patients, other associated factors may contribute to
susceptibility to infection. Many, if not most, patients receiving
chemotherapy or immunosuppressive agents are also treated with
antimicrobial drugs, which can have profound effects on the bacterial flora
of the intestinal tract. These disturbances of gut microbial ecology may
predispose to colonization and infection with other microorganisms, some of
which may have increased virulence. Many chemotherapeutic agents have
direct toxicity for the gut mucosa; the resulting mucositis increases the
susceptibility of these patients to bloodstream infections with whatever
microorganisms are present in gut. The concentration of these highly
susceptible patients on certain wards or units in a hospital may also
increase the risk for nosocomial transfer of specific microorganisms. For
example, vancomycin-resistant enterococci (VRE) (which, at least in Europe,
have been associated with food [24]) have emerged as a substantive problem
in cancer centers and transplant units (25-27). Persons who are
immunosuppressed or have serious underlying illness are much more
susceptible to colonization and infection with the organism. In some
oncology and transplant units, more than 10% of patients are colonized or
infected with VRE (26,27), providing well-documented opportunities for
transfer of the organism to other immunosuppressed hosts in the same unit.

Cancer patients who have just undergone chemotherapy are often profoundly
neutropenic. In this setting, especially in conjunction with the mucositis
mentioned above, virtually any microorganism in the intestinal tract can
enter the bloodstream and cause potentially fatal illness. For example, it
has been found that raw produce in salads may be an important route by
which patients acquire Pseudomonas (28); as a result, severely neutropenic
patients are generally restricted to cooked food. Salmonella infections are
reported among cancer patients, although the relative risk for infection in
this population is not well characterized. Patients with neoplastic disease
do appear to have a substantively increased risk for Salmonella septicemia,
with 35% patients in one series having septicemia (29), versus fewer than
1% in healthy hosts. Cancer patients appear to be at increased risk for
invasive Listeria infections (13). Toxoplasmosis tends to be of particular
concern among transplant patients (30,31).

Aging of the Population

The absolute number of the elderly in the United States is rapidly
increasing, as is the proportion of the U.S. population they comprise. In
1950, 3.8 million persons (2.6% of the population) were over the age of 74,
as opposed to 14.7 million (5.6% of the population) in 1995 (23; Figures 4
and 5). The elderly appear to be at a clearly increased risk for death from
foodborne and diarrheal disease. Between 1979 and 1987, 28,538 persons in
the United State had diarrhea as an immediate or underlying cause of death;
51% of these persons were more than 74 years of age, 27% were adults age 55
to 74, and 11% were children under the age of 5 (32).
[Figure Not Available in ASCII]
 Figure 4. Number of persons >74 years of age, U.S. population, for
 selected years, 1950-1990. From the National Center for Health
 Statistics. Health, United States, 1996-97 and Injury Chartbook.
 Hyattsville, Maryland, 1997.

[Figure Not Available in ASCII]

Figure 5. Percentage of U.S. population >74 years of age, for selected
years, 1950-1990. From National Center for Health Statistics. Health,
United States, 1996-97 and Injury Chartbook. Hyattsville, Maryland, 1997.

The increased susceptibility of the elderly to infection and death may be
due to a number of factors. Aging results in senescence of the
gut-associated lymphoid tissue, increasing susceptibility to infection.
Aging may also result in a decrease in gastric acid secretion: in one
study, stimulated acid output was reduced approximately 30% in persons aged
65 to 98, with a 40% reduction in pepsin output (33). As a low pH of the

stomach is a major barrier to entry of enteric pathogens, reductions in
gastric acidity can clearly increase the susceptibility to infection.
Social factors may also influence susceptibility. For the elderly,
residence in a long-term care facility was a major independent risk factor
for diarrheal death (32). While a number of factors contribute to the
increased risk, the communal living environment, combined with problems of
fecal incontinence, create an environment in which enteric and foodborne
pathogens are easily spread (32,34).

Incidence of salmonellosis and Campylobacter diarrhea appears to be higher
among the elderly (35,36). More striking, however, is the increase in
frequency of Salmonella bacteremia as compared with isolations from other
sites: Salmonella infections in the elderly are more likely to cause
bacteremia (37; Figure 6), which, in turn, substantively increases the risk
for death. For example, in a recent nursing home outbreak in Maryland, 50
(35%) of 141 residents became ill, seven had bacteremia, nine were
hospitalized (with a median length of hospitalization of 22 days), and four
died (38). E. coli O157:H7 is also a common pathogen in nursing homes and
among the elderly. In one reported nursing home outbreak, 55 of 169
residents were affected; overall, 19 (35%) of the affected residents died
(39).

[Figure Not Available in ASCII]

 Figure 6. Ratio of blood isolates to total isolates of Salmonella by age
 group of the person from whom the isolate was obtained as reported to the
 Centers for Disease Control, Atlanta, GA, in 1968-79. From Blaser MJ,
 Feldman RA. Salmonella bacteremia: reports to the Centers for Disease
 Control and Prevention, 1968-1979. J Infect Dis 1981;143:743-6.

Malnutrition

The above discussions have focused on issues most relevant to the United
States and other industrialized countries. However, on a global scale,
probably the leading cause of increased host susceptibility to infection is
malnutrition. While accurate data on the prevalence of malnutrition are
difficult to obtain, problems are accentuated in developing countries, in
areas of political unrest, and among marginalized populations in the United
States and other affluent nations. In Mexico, according to a probabilistic
survey in 1990, 42.3% of children under 5 years of age had some degree of
malnutrition (40).

Malnutrition increases host susceptibility through a number of mechanisms.
It weakens epithelial integrity and may have a profound effect on
cell-mediated immunity, with functional deficiencies in immunoglobulins and
defects in phagocytosis. Malnutrition also may initiate a "vicious cycle"
of infection predisposing to malnutrition and growth faltering, which in
turn may lead to an increased risk for further infection (40,41). In
studies in Bangladesh, malnourished and well-nourished children had the
same number of infections with diarrheal pathogens such as enterotoxigenic
E. coli; however, diarrhea in malnourished children was of longer duration
and had greater potential long-term nutritional consequences (42). Overall,
malnutrition appears to result in a 30-fold increase in the risk for
diarrhea-associated death (40).

Conclusions

Host susceptibility (and changes in the susceptibility to infection of
groups within the general population) is a critical variable in assessing
the public health effects and understanding the emergence and spread of
pathogenic microorganisms. Surveillance within populations with increased
susceptibility to infection may allow identification of new pathogens
before they are recognized within the general population. Studies designed
to identify the reasons for the increased susceptibility of a specific
population to a specific agent may reveal how a microorganism is able (or
not able) to breach normal host defense mechanisms. Finally, from a public
health standpoint, risk management strategies for emergent foodborne
pathogens must clearly identify and focus on populations with increased
susceptibility to infection.

Address for correspondence: Glenn Morris, 934 MSTF, University of Maryland
School of Medicine, 10 S. Pine St., Baltimore, MD, 21201, USA; fax:
410-706-4581; e-mail: jmorris@umppa1.ab.umd.edu.

References

  1. Buckley RH. Primary immunodeficiency diseases. In: Bennett JC, Plum F,
     editors. Cecil textbook of medicine. Philadelphia: W.B. Saunders
     Company; 1996 .p.1401-8.
  2. Centers for Disease Control and Prevention. Update: trends in AIDS
     incidence, deaths, and prevalenceUnited States, 1996. MMWR Morb Mortal
     Wkly Rep 1997;46:165-73.
  3. Celum CL, Chaisson RE, Rutherford GW, Barnhart JL, Echenberg DF.
     Incidence of Salmonellosis in patients with AIDS. J Infect Dis
     1987;156:996-1002.
  4. Smith PD, Macher AM, Bookman MA, Boccia RV, Steis RG, Gill V, et al.
     Salmonella typhimurium enteritis and bacteremia in the acquired
     immunodeficiency syndrome. Ann Intern Med 1985;102:207-9.
  5. Profeta S, Forrester C, Eng RHK, Liu R, Johnson E, Palinkas R, Smith
     SM. Salmonella infections in patients with acquired immunodeficiency
     syndrome. Arch Intern Med 1985;145:670-2.
  6. Gruenewald R, Blum S, Chan J. Relationship between human
     immunodeficiency virus infection and Salmonellosis in 20- to
     59-year-old residents of New York City. Clin Infect Dis
     1994;18:358-63.
  7. Angulo FJ, Swerdlow DL. Bacterial enteric infections in persons
     infected with human immunodeficiency virus. Clin Infect Dis
     1995;21:S8493.
  8. Levine WC, Buehler JW, Bean NH, Tauxe RV. Epidemiology of nontyphoidal
     Salmonella bacteremia during the human immunodeficiency virus
     epidemic. J Infect Dis 1991;164:81-7.
  9. Gotuzzo E, Frisancho O, Sanchez J, Liendo G, Carrillo C, Black RE,
     Morris JG Jr. Association between the acquired immunodeficiency
     syndrome and infection with Salmonella typhi or Salmonella paratyphi
     in an endemic typhoid area. Arch Intern Med 1991;151:381-2.
 10. Lee LA, Puhr ND, Maloney EK, Bean NH, Tauxe RV. Increase in
     antimicrobial-resistant Salmonella infections in the United States,
     19891990. J Infect Dis 1994;170:12834.
 11. Sorvillo FJ, Lieb LE, Waterman SH. Incidence of campylobacteriosis
     among patients with AIDS in Los Angeles County. J Acquir Immune Defic
     Syndr Hum Retrovirol 1991;4:598-602.
 12. Perlman DM, Ampel NM, Schifman RB, Cohn DL, Patton CM, Aguirre ML, et
     al. Persistent Campylobacter jejuni infections in patients infected
     with human immunodeficiency virus (HIV). Ann Intern Med
     1988;108:540-6.
 13. Schuchat A, Deaver KA, Wenger JD, Plikaytis BD, Mascola L, Pinner RW,
     et al. Role of foods in sporadic listeriosis. 1. Case-control study of
     dietary risk factors. JAMA 1992;267:2041-50.
 14. Luft BJ, Brooks RG, Conley FK, McCabe RE, Remington JS. Toxoplasmid
     encephalitis in patients with acquired immune deficiency syndrome.
     JAMA 1984;252:913-7.
 15. Porter SB, Sande MA. Toxoplasmosis of the central nervous system in
     the acquired immunodeficiency syndrome. N Engl J Med 1992;327:1643-8.
 16. McCabe RE, Remington JS. Toxoplasma gondii. In: Mandell GL, Douglas
     RG, Bennett JE, editors. Principles and practice of infectious
     diseases. 3rd ed. New York: Churchill Livingston; 1990. p. 2090-103.
 17. United States Department of Agriculture. Pathogen reduction; hazard
     analysis and critical control point (HACCP) systems; proposed rule.
     Washington (DC): Federal Register 1995;60:6774-889.
 18. Smith PD, Quinn TC, Strober W, Janoff EN, Masur H. Gastrointestinal
     infections in AIDS. Ann Intern Med 1992;116:63-77.
 19. Wolfson JS, Richter JM, Waldron MA, Weber DJ, McCarthy DM, Hopkins CC.
     Cryptosporidiosis in immunocompetent patients. N Engl J Med
     1985;312:1278-82.
 20. Centers for Disease Control and Prevention. Assessing the public
     health threat associated with waterborne cryptosporidiosis: report of
     a workshop. MMWR Morb Mortal Wkly Rep 1995;44(RR-6):19.
 21. Mayer HB, Acheson D, Wanke CA. Enteroaggregative Escherichia coli are
     a potential cause of persistent diarrhea in adult HIV patients in the
     United States. In: Program and abstracts of the 31st US-Japan Cholera
     and Related Diarrheal Diseases Conference; 1995 Dec 1-3; Kiawah
     Island, South Carolina.
 22. Saillour M, de Truchis P, Risbourg M, Nordman P, Nauciel C, Perronne
     C. Yersinia enterocolitica gastroenteritis in HIV infected patients.
     In: Abstracts of the 35th Interscience Conference on Antimicrobial
     Agents and Chemotherapy; 1995 Sept 17-20; San Francisco, California.
 23. National Center for Health Statistics. Health, United States, 1996-97
     and Injury Chartbook. Hyattsville (MD): The Center; 1997.
 24. Bates J, Jordens JZ, Griffiths DT. Farm animals as a putative
     reservoir for vancomycin-resistant enterococcal infection in man. J
     Antimicrob Chemother 1994;34:507-16.
 25. Morris JG Jr, Shay DK, Hebden JN, McCarter RJ, Perdue BE, Jarvis W, et
     al. Enterococci resistant to multiple antimicrobial agents, including
     vancomycin: establishment of endemicity in a university medical
     center. Ann Intern Med 1995;123:250-9.
 26. Papanicolaou GA, Meyers BR, Meyers J, Mendelson MH, Lou W, Emre S, et
     al. Nosocomial infections with vancomycin-resistant Enterococcus
     faecium in liver transplant recipients: risk factors for acquisition
     and mortality. Clin Infect Dis 1996;23:760-6.
 27. Montecalvo MA, Horowitz H, Gedris C, Carbonaro C, Tenover FC, Issah A,
     et al. Outbreak of vancomycin-, ampicillin-, and
     aminoglycoside-resistant Enterococcus faecium bacteremia in an adult
     oncology unit. Antimicrob Agents Chemother 1994;38:1363-7.
 28. Remington JS, Schimpff SC. Please don't eat the salads. N Engl J Med
     1981;304:433-5.
 29. Wolfe MS, Armstrong D, Louria DB, Blevins A. Salmonellosis in patients
     with neoplastic disease. Arch Intern Med 1971;128:546-54.
 30. Luft BJ, Naot Y, Araujo FG, Stinson EB, Remington JS. Primary and
     reactivated toxoplasma infection in patients with cardiac transplants.
     Ann Intern Med 1983;99:27-31.
 31. Hofflin JM, Potasman I, Baldwin JC, Oyer PE, Stinson EB, Remington JS.
     Infectious complications in heart transplant recipients receiving
     cyclosporine and corticosteroids. Ann Intern Med 1987;106:209-16.
 32. Lew JF, Glass RI, Gangarosa RE, Cohen IP, Bern C, Moe CL. Diarrheal
     deaths in the United States, 1979-87. JAMA 1991;265:3280-4.
 33. Feldman M, Cryer B, McArthur KE, Huet BA, Lee E. Effects of aging and
     gastritis on gastric acid and pepsin secretion in humans: a
     prospective study. Gastroenterology 1996;110:1043-52.
 34. Bennett RG. Diarrhea among residents of long-term care facilities.
     Infect Control Hosp Epidemiol 1993;14:397-404.
 35. Hargrett-Bean N, Pavia AT, Tauxe RV. Salmonella isolates from humans
     in the United States, 1982-1986. MMWR Morb Mortal Wkly Rep
     1988;30(SS-2):25-31.
 36. Tauxe RV, Hargrett-Bean N, Patton CM, Wachsmuth K. Campylobacter
     isolates in the United States, 1982-1986. MMWR Morb Mortal Wkly Rep
     1988;37(SS-2):1-13.
 37. Blaser MJ, Feldman RA. Salmonella bacteremia: reports to the Centers
     for Disease Control, 1968-1979. J Infect Dis 1981;143:743-6.
 38. Taylor JL, Dwyer DM, Groves C, Bailowitz A, Tilghman D, Kim V, et al.
     Simultaneous outbreak of Salmonella enteritidis and Salmonella
     schwarzengrund in a nursing home: association of S. enteritidis with
     bacteremia and hospitalization. J Infect Dis 1993;167:781-2.
 39. Carter AO, Borczyk AA, Carlson JAK, Harvey B, Hogkin JC, Karmali MA,
     et al. A severe outbreak of Escherichia coli O157:H7-associated
     hemorrhagic colitis in a nursing home. N Engl J Med 1987;317:1496-500.
 40. Santos JI. Nutrition, infection, and immunocompetence. Inf Dis Clin
     North Am 1994;8:243-67.
 41. Black RE, Brown KH, Becker S. Effects of diarrhea associated with
     specific enteropathogens on the growth of children in rural
     Bangladesh. Pediatrics 1984;73:799-805.
 42. Black RE, Brown KH, Becker S. Malnutrition is a determining factor in
     diarrheal duration, but not incidence, among young children in a
     longitudinal study in rural Bangladesh. Am J Clin Nutr 1984;37:87-94.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/morris.txt

Chronic Sequelae of Foodborne Disease

James A. Lindsay
University of Florida, Gainesville, Florida, USA
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      In the past decade, the complexity of foodborne pathogens, as
      well as their adaptability and ability to cause acute illness,
      and in some cases chronic (secondary) complications, have been
      newly appreciated. This overview examines long-term
      consequences of foodborne infections and intoxications to
      emphasize the need for more research and education.

The term foodborne disease encompasses a variety of clinical and etiologic
conditions and describes a subset of enteric disease (1-4), which in the
United States ranks second in prevalence to respiratory disease (2). In
foodborne disease, the food may act as a vehicle for the transmission of
actively growing microorganisms or products of metabolism (toxins), or it
may have a passive role as a vehicle for the transmission of nonreplicating
bacteria, viruses or protozoa, or stable biologic toxins. In most cases,
the clinical conditions usually associated with foodborne disease are
acute: diarrhea, vomiting, or other gastrointestinal manifestations such as
dysentery. However, other pathophysiologic responses may occur
independently or accompany acute-phase responses (1-4). A number of chronic
sequelae may result from foodborne infections, including ankylosing
spondylitis, arthropathies, renal disease, cardiac and neurologic
disorders, and nutritional and other malabsorptive disorders
(incapacitating diarrhea). The evidence that microorganisms or their
products are either directly or indirectly associated with these long-term
sequelae ranges from convincing to circumstantial (1-4). The reason for
this disparity is that, except in rare circumstances, chronic complications
are unlikely to be identified or epidemiologically linked to a foodborne
cause because these data are not systematically collected. Moreover, host
symptoms induced by a specific pathogen or product of a pathogen are often
wide-ranging and overlapping and therefore difficult to link temporally to
a specific incident. These impediments manifest themselves because the
problems associated with chronic disease can result from an infection
without overt illness. Alternatively, the chronic sequelae may be unrelated
to the acute illness and may occur even if the immune system successfully
eliminates the primary infection; therefore, activation of the immune
system may initiate the chronic condition as a result of an autoimmune
response (2-4). The variability of the human response-from overt illness to
chronic carrier status-is perhaps the most confounding issue.

Cost of Chronic Sequelae

As the incidence of foodborne disease increases, the incidence of chronic
sequelae may also rise. Several authors have estimated that chronic
sequelae may occur in 2% to 3% of foodborne disease cases and suggest that
the long-term consequences to human health and the economy may be more
detrimental than the acute disease. An estimated 80 million cases of
foodborne disease occur annually in the United States, which suggests
significant morbidity figures and costs to society in the billions of
dollars per year (2,4).

Infection: The Microbe/Toxin versus the Host

Several microbial pathogens are highly adapted to parasitization,
exhibiting environmentally responsive and adaptive traits that allow
attachment, invasion, and replication in the host (2,4). Microbial
pathogenicity can be viewed solely from the perspective of the microbe;
however, this would be not only unidimensional, but also wrong (2,4). A
major selective force that regulates the phenotype of an infecting
microbial pathogen population is the host's immune system, which is also
highly adaptive, especially in discriminating self and nonself antigens
(2,4). When the host-parasite relationship is examined holistically,
mechanisms that successful pathogens have apparently evolved to elude the
immune system include antigenic heterogeneity or variation; sequestration,
either intracellularly or in certain specific host sites; molecular
mimicry, through either imitation (cross-reaction) or adsorption of host
protein; and direct immune stimulation and/or suppression (2-5).

Rheumatoid Disease

Several bacteria, including salmonellae, induce septic arthritis by
hematogenous spread to the synovial space, causing inflammation. Viable
organisms are recoverable from synovial fluid, and treatment usually
involves antibiotic therapy. Prognosis depends on host factors and
virulence of the organism; either complete resolution or permanent joint
damage can occur (1-4,6,7).

Yersinia enterocolitica, Y. pseudotuberculosis, Shigella flexneri, Sh.
dysenteriae, Salmonella spp., Campylobacter jejuni, and Escherichia coli
initiate aseptic or reactive arthritis, an acute, nonpurulent joint
inflammation following infection elsewhere in the body, for example the
bowel. Klebsiella pneumoniae has been implicated, although it appears now
that the bacterium is connected with fecal carriage by ankylosing
spondylitis probands (4). Although a distinct clinical disease, reactive
arthritis also occurs in the Reiter syndrome triad with conjunctivitis and
uveitis. A subset of patients with symptoms of reactive arthritis and
Reiter syndrome get ankylosing spondylitis, a rheumatoid inflammation of
synovial joints and entheses within and distal to the spine (8).

The relative risk of developing these seronegative spondyloarthropathies
after a gram-negative enterobacterial infection is high for persons
positive for the major histocompatibility class (MHC) antigen B27 and the
cross-reacting MHC B7 group. Indeed, persons who are human leukocyte
antigen (HLA)-B27 positive have an 18-fold greater risk for reactive
arthritis, a 37-fold greater risk for Reiter syndrome, and up to a 126-fold
greater risk for ankylosing spondylitis than persons who are HLA-B27
negative and have the same enteric infections. Other genes that may be
related or act in concert appear to determine which disease is acquired
(2,5,7,9). These chronic complications are related to a genetically
determined host risk factor in combination with an environmental trigger.
No cause-and-effect relationship of enteric pathogens in ankylosing
spondylitis has been established (4); however, a low but consistent
incidence (0.2% to 2.4%) of reactive arthritis occurs after outbreaks of S.
Typhimurium, Sh. flexneri, and C. jejuni. Biotypes and phage types of Y.
enterocolitica O:3 and O:9, endemic to Scandinavia, are either highly
arthritogenic or affect a more genetically predisposed population with
persistent and debilitating symptoms that may last for years (2,3).

The sharing of antigenic determinants by a microbe and its host is a
frequent natural occurrence, and bacterial antigens from the pathogens that
directly cross-react with MHC B27 have been demonstrated (6,9).
Additionally, the plasmid-mediated synthesis of bacterial B27 "modifying
factor," a protein that binds to and subsequently alters the conformation
of B27, has been reported (10). In both of these models, immune recognition
of the foreign antigen leads to an autoimmune anti-B27 response.
Alternatively, B27 may act nonimmunologically because dissemination of
bacterial antigens to infected joints stimulates a local T-cell
inflammatory response. Here, B27 may act as a receptor for bacteria or
antigens thereof, facilitating invasiveness from mucosal surfaces in the
gut (9). Indeed, transfected B27 on the surface of mouse L cells reportedly
can alter bacterial invasion capability (11).

Despite the strong familial association related to the MHC B27 gene,
B27-negative persons are known to become ill, albeit less often, but with
apparently equal severity, as shown by an epidemiologic investigation of
rheumatoid arthritis following the 1985 S. Typhimurium gastroenteritis
outbreak due to contaminated milk (4).

Autoimmune Thyroid Disease

Graves disease is an autoimmune disease mediated by autoantibodies to the
thyrotropin receptor (12,13). The first indication that the disease may be
linked to infection was finding antibody titers to Y. enterocolitica
serotype O:3 suggestive of molecular mimicry in a majority of patients with
Graves disease. Several studies have shown that two low molecular weight
envelope proteins of Yersinia contain epitopes cross-reactive with the
thyrotropin receptor. These proteins are chromosomally encoded, exposed to
the surface of the bacterium, and produced by both virulent and avirulent
strains of Yersinia (Y. pestis, Y. pseudotuberculosis, Y. enterocolitica
VW+ and WV-). In addition to autoantibody, a suppressor cell dysfunction
may be involved in Graves disease (12,13). Severe hypothyroidism may also
result from chronic intestinal giardiasis due to infection by Giardia
lamblia; treatment with metronidazole can result in complete elimination of
the parasite and recovery of regular intestinal thyroid hormone absorption
(14).

Inflammatory Bowel Disease

Inflammatory bowel disease is the collective term for Crohn disease and
ulcerative colitis. While both infections are chronic inflammatory diseases
with histologic infiltrates of macrophages and lymphocytes and a prolonged
clinical course, the primary clinical and pathologic effects are
gastrointestinal. The infections can be difficult to differentiate because
the symptoms are often similar (15). The acute clinical characteristics are
diarrhea, abdominal pain, fever, and weight loss; and the acute pathologic
features include a constant flux of neutrophils into inflamed mucosa,
eventually penetrating the epithelium into the intestinal lumen. The
chronic spontaneously relapsing disorder exhibits many of the symptoms of
the acute state; however, this phase has an average symptom duration of 3.2
years before correct diagnosis. Abdominal abscesses are a common and
dangerous complication of Crohn disease, while in ulcerative colitis,
abdominal perforations may lead to peritonitis. Crohn disease involves the
ileum or colon (anaerobes are important), while ulcerative colitis appears
restricted to the colon (aerobes are important). Nationality and familial
associations suggest a genetic predisposition for the disease (4,15).

Although the cause of inflammatory bowel disease and the mechanism(s) for
spontaneous exacerbations and remissions are unknown, much research has
focused on transmissible agents, including foodborne pathogens. An
association between bacterial L-forms and inflammatory bowel disease has
been sporadically reported, with isolation of Pseudomonas, Mycobacterium,
Enterococcus fecalis, and E. coli from affected tissue but not from
appropriate controls. There is considerable debate as to whether L-forms
are pathogenic in humans or persist in affected tissue.

Mycobacterium paratuberculosis, the causative agent of Johne disease in
ruminants, may be associated with Crohn disease through the production of
L-forms of the bacterium. Subclinically infected cows shed M.
paratuberculosis, and the organism has been identified in pasteurized milk
by polymerase chain reaction specific for the M. paratuberculosis insertion
sequence IS900. The pathogen model suggests that a susceptible human
neonate first contracts the organism after ingesting commercial dairy
products. This invokes an antigen-poor (lacking a cell wall) L-form that
grows slowly and persists in the lamina propria, stimulating a chronic
low-grade inflammation. The immune response increases in severity over
years without bacterial replication, ultimately producing the pathologic
features of Crohn disease (15,16). Another model proposes an autoimmune
phenomenon mediated by alterations in inflammatory cytokine profiles,
possibly as a result of infection (4).

Recent immunocytochemical techniques demonstrated antigens to Listeria
monocytogenes, E. coli, and Streptococcus spp. in Crohn disease tissues.
Macrophages and giant cells immunolabelled for antigen specific to these
organisms were found beneath ulcers, around abscesses, along fissures,
within the lamina propria, in granulomas, and in germinal centers of
mesenteric lymph nodes (17).

Superantigens and Autoimmunity

In contrast to conventional antigens, superantigens interact with the
variable side of the V chain of the T-cell receptor by recognizing
elements shared by a subset of T cells. Depending on the type of
interaction, recognition can have different consequences, including
proliferation and expansion, suppression (clonal deletion), or,
alternatively, induction of prolonged unresponsiveness (anergy) or cell
death (apoptosis) (18-21). Superantigens from several foodborne bacteria
(e.g., Staphylococcus, Streptococcus, Yersinia, and Clostridium) have been
isolated and characterized. Many are thought to be associated with several
autoimmune disorders, for example, rheumatic heart disease, rheumatoid
arthritis, multiple sclerosis, Graves disease, Sjogren syndrome, autoimmune
thyroiditis, psoriasis, Kawasaki disease, Crohn disease, and
insulin-dependent diabetes mellitus (6,18-24).

Although it is accepted that superantigens have a role in autoimmune
disorders, the acceptance is based on extensive animal model studies
(6,18-24) but limited human clinical studies. In human diseases where
superantigens have been clearly demonstrated as the cause, for example,
toxic shock syndrome, initial T-cell proliferation and T-cell receptor-mRNA
up-regulation have been observed, but the long-term sequelae in terms of
T-cell function are unknown (22). Recent studies suggest that superantigens
may also cause an acute flare of a disease within patients in remission
from a preexisting autoimmune disorder.

Renal Disease

After colitis caused by E. coli O157:H7 and other enterohemorrhagic strains
of E. coli, hemolytic uremic syndrome develops in some patients (1,2,25).
The syndrome is characterized by a triad of symptoms: acute renal failure,
thrombocytopenia, and microangiopathic hemolytic anemia. Acute renal
failure is the leading cause of death in children, and thrombocytopenia is
the leading cause of death in adults. Hemolytic uremic syndrome is a
worldwide problem that mirrors the distribution of E. coli O157:H7 and
other Shiga and Shiga-like toxin-producing microorganisms. Outbreaks of
hemorrhagic colitis and subsequent cases of hemolytic uremic syndrome have
developed as a result of various food vehicles. Besides O157:H7, other
Shiga-like toxin-producing E. coli, Citrobacter, Campylobacter, Shigella,
Salmonella, and Yersinia have been linked to the disorder (1,2,25,26).

The toxin-mediated damage to the kidneys may not be limited to the
glomerular endothelial cells as once thought but may include the tubular
epithelial cells (26-28). Binding studies showed the toxins to be specific
for the glycosphingolipid globotriaosylceramide (Gb3), which is present on
renal but not umbilical endothelial cells. This may account for the
differential sensitivity of renal cells to toxin-induced damage, since Gb3
was present in the glomeruli of infants under 2 years of age but not in the
glomeruli of adults. Thus, the presence of Gb3 in the pediatric renal
glomerulus may be a risk factor for development of hemolytic uremic
syndrome (28). Characterization of the Shiga toxin receptor has led to a
potential preventive treatment (4).

Neural and Neuromuscular Disorders

Guillain-Barr syndrome is a subacute, acquired, inflammatory demyelinating
polyradiculoneuropathy that frequently occurs after acute gastrointestinal
infection. The disease is characterized by alexia, motor paralysis with
mild sensory disturbances, and an acellular increase in the total protein
content in the cerebrospinal fluid. The disease occurs worldwide and is the
most common cause of neuromuscular paralysis. Cases have three dominant
characteristics: the predilection to nerve roots, mononuclear infiltration
of peripheral nerves, and eventual demyelination (primary a
xonal
degeneration) (29). Severe cases tend to occur in the summer and have been
linked to previous infection with C. jejuni, although other enteric
pathogens may trigger the syndrome.

Some controversy exists regarding whether Guillain-Barr syndrome is an
autoimmune disease. Although adequate data exist to classify the syndrome
as an autoimmune disease (four major Rose-Witebsky criteria are almost
completely met), the immunologic mechanisms at work in Guillain-Barr
syndrome triggered by C. jejuni are likely to be complex (29-31). Studies
of the relationship between Guillain-Barr syndrome and C. jejuni support
the hypothesis of molecular mimicry, since peripheral nerves may share
epitopes with surface antigens of C. jejuni (32). This has been supported
by studies in which anti-GM1 IgG antibodies recognized surface epitopes on
intact C. jejuni, and the reaction was strain-specific for certain Penner
serotypes. There are inconclusive data with regard to Guillain-Barr
syndrome and HLA, although some studies have shown a predilection for the
HLA-B35 haplotype (29-31). Cytokines may be responsible for inducing the
inflammatory process and probably play a role in the response leading to
nerve demyelination. Complement has a role in the process leading to nerve
damage, possibly through the production of activation products, which lead
to an increase in the permeability of the blood nerve barrier, which
perpetuates the inflammation. Although Guillain-Barr syndrome might be
considered an autoimmune response, it also serves as an example of a
disease with an infectious origin, a disease that entails the integrated
actions of both humoral and cellular immunity.

Ciguatera poisoning is the most common foodborne disease related to the
consumption of fin fish; this distinctive clinical syndrome is
characterized by a plethora of gastrointestinal, neurologic, and sometimes
cardiovascular features (33,34). Two toxins are involved in toxicosis.
Ciguatoxin-1 (cig-1), the principal toxin, is a heat stable, lipid-soluble
polyether that is not inactivated by heat, cold, or gastric juices, nor
eliminated by drying, salting, smoking, or marinating. Cig-1 induces
membrane depolarization in nerve and muscle tissue by opening
voltage-dependent sodium channels. A second toxin, maitotoxin, is
water-soluble and opens calcium channels. The role of this second toxin in
the pathophysiology of the disease is less well understood. The acute
symptoms of the toxicosis are varied and include paresthesia of the
extremities, circumoral paresthesia, reversal of hot and cold sensations,
dental pain, myalgias, arthralgias, generalized pruritus, cranial nerve
dysfunction, and dysuria. Severe acute symptoms require urgent care with
parenteral atropine for bradyarrhythmias. Mannitol is often administered to
counter the effect of the toxin on the sodium channels; however, the
mechanism of action is unknown, and the therapy is useless after 24 hours.
Many of these symptoms may remain chronic and are often misdiagnosed as
chronic fatigue syndrome, brain tumors, or multiple sclerosis. The
management of the chronic symptoms is frustrating for the patient and
clinician. Interventions include amitriptyline, tocaidine, or mexilitine to
modulate sodium channels in conjunction with calcium channel blockers such
as nifedipine. Antidepressants such as Prozac also appear to be useful.
Patients with chronic symptoms frequently report waxing and waning of
symptoms. Activities such as sexual intercourse and drinking alcohol
significantly exacerbate expression. Some women with chronic symptoms
report worsening during menses. Mood levels, weather conditions, and
dietary constituents often exacerbate symptoms. Some clinicians advocate a
strict diet that avoids all seafood, fish byproducts, nuts, and alcohol,
and in some cases, patients are asked to abstain from sex. One distinctive
feature in this toxicosis is that one episode of ciguatera poisoning does
not confer immunity. In fact, it is likely to sensitize the patient to
otherwise subthreshold doses of toxin (33,34).

Amnesic shellfish poisoning is caused by domic acid, a conjugate of kainic
and glutamic acid (35). In small quantities domic acid has an excitatory
effect, but in large amounts it is neurotoxic. The toxicosis is first
characterized by gastrointestinal symptoms followed by neurologic
dysfunction. Severe cases may be prolonged and chronic; sequelae include
confusion with disorientation, paucity of speech, lack of response to deep
pain due to blocking of receptors in the spinal cord, autonomic nervous
system dysfunction, seizures, abnormal ocular movements, grimacing posture,
myoclonus, loss of reflexes, and coma. Other prominent chronic sequelae
include loss of visual-spacial recall and mononeuropathies without
dementia, mimicking Alzheimer's disease. The toxicosis is particularly
serious in the elderly, and any deaths usually occur within this
population. Valium, calcium channel blockers, phenobarbital, diazapam,
thiobarbiturates, and hypothermia are treatments for patients with severe
and chronic cases.

General Immunity, Organ Impairment, and Neurologic Disorders

Toxoplasmosis due to Toxoplasma gondii is a chronic latent parasitic
infection (36,37). In humans the parasite exists in two forms: the
tachyzoite, the rapidly multiplying stage that actively invades host cells
and represents the principal form of the acute phase of the disease; and
the bradyzoite, the form that multiplies very slowly in host cells,
resulting in the formation of cysts that persist in tissues. Toxoplasma
infection in humans is usually asymptomatic because of effective immunity
involving antibodies, T cells, and cytokines. Activated macrophages, CD4
and CD8 lymphocytes, and the cytokines IFN-g, TNFa, and IL-6 play a major
role in control of both the acute infection and maintenance or prevention
of the chronic stage (37,38). Indeed, treatment with IFN-g is used to
control passage into the chronic stage, and treatment with anti-IFN-g
reactivates chronic infection (39). The production of nitric oxide may have
opposing effects. Nitric oxide production protects against T. gondii and at
the same time limits the immune response, probably contributing to the
establishment of the chronic state (40).

The incidence of congenital toxoplasmosis is uncertain but may be as high
as 9,500 cases a year (1). The percentage attributable to food is
uncertain; however, consumption or contact with contaminated meat is more
important as a cause than is contact with cats (1). Congenital impairments
associated with maternal toxoplasmosis infection passed to the fetus
include hearing loss, visual impairment (retinal lesions, strabismus), and
slight to severe mental retardation. These impairments are still present in
80% of persons who reach the age of 20 years (1). Chronic toxoplasmic
encephalitis may occur when a person's immune system is impaired. Indeed,
toxoplasmic encephalitis marked by dementia and seizures has become the
most commonly recognized cause of central nervous system opportunistic
infections in AIDS patients. Additionally, it appears that certain cancer
treatments weaken the immune system, and old infections in the muscles can
become reactivated, causing severe complications or death (1,41,42).

Helminth parasites can cause serious disease in infected persons (42). The
impact of helminth infections is due less to the severity of the diseases
they cause, than to the vast number of persons infected. For example, more
than one billion people are infected with the largest intestinal nematode,
Ascaris lumbricoides. Although there is usually no overt clinical sign of
infection, disease can arise from an overwhelming infection or an
inappropriate immune response. Additionally, infected persons frequently
harbor more than one parasite for years. Most intestinal helminth parasites
have direct life cycles, with no intermediate host or vector, and are
transferred by contaminated food. Some species, such as Trichuris
(whipworm) and Enterobius (pinworm), are restricted to the gut, but others,
such as Ascaris, have tissue-migrating phases. All, however, induce a
dramatic expansion of the Th2 lymphocyte subset. It remains unclear whether
these Th2-derived responses (induction of interleukin-4 [IL-4] and
down-regulation of IFNg), resulting in stimulation of IgG1 and IgE
isotypes, eosinophilia, and mastocytosis are responsible for the
immune-mediated pathologic response. Immunologic lesions may occur where
early infection is associated with a strong T-cell proliferative response
that becomes down-regulated in established chronic disease (evidence of a
Th1 defect in the chronic disease). In ascariasis, an allergic response
generated by the lung migratory phase (chronic immune sensitization) can
cause pneumonia and, in animal models, spontaneous development of
idiopathic bronchial asthma. A formative influence on the response of the
immune system is the antigenic environment during pregnancy. Children born
to infected mothers may have significantly higher susceptibility to the
same infection later in life (42).

Viral agents induce autoimmune disorders, and one potential mechanism of
induction is molecular mimicry (43,44). Hepatitis A virus infection is a
well-recognized cause of acute hepatitis with jaundice in adults. In most
affected persons, the course is usually relatively short-lived and benign,
and symptoms are usually resolved within weeks. Occasionally, relapses
occur after initial recovery, or recovery is marked by severe or prolonged
cholestasis. However, even in these cases recovery is usual. Chronic
sequelae of hepatitis A virus infections are rare and poorly defined;
however, several recent studies suggest that hepatitis A virus infection
triggers the onset of (idiopathic) autoimmune chronic active hepatitis
within a genetically predisposed subgroup. Apparently, the chronic disorder
may develop despite normal serologic response to hepatitis A virus
infection. The triggering factors and mechanism of action remain
ill-defined; however, in the most recent study, the authors concluded that
hepatitis A virus infection may be the precipitating event in the
pathogenesis of this disorder (45).

Metabolic activation and detoxification play a crucial role in determining
the toxic response of humans to mycotoxin exposure. These highly toxic
secondary metabolites are produced by a wide variety of molds including
Aspergillus, Fusarium, and Penicillium. Mycotoxins exhibit properties of
acute, subacute, and chronic toxicities with some molecules being
carcinogenic, mutagenic, and teratogenic. Because mycotoxins are resistant
to food processing and do not degrade at high temperatures, they enter the
human food supply (46-49).

In many cases, the relationship between mycotoxins as the causative agent
of disease in humans is difficult to determine. Acute effects of
gastroenteritis may be easily identified; however, chronic effects often
result from ingestion of low to moderate levels and can be difficult to
recognize (46-50). The most threatening effects of ochratoxin A are its
nephrotoxicity and carcinogenicity. Ochratoxin A is increasingly involved
in an endemic nephropathy, a human chronic interstitial neuropathy that is
usually associated with urinary tract tumors. Aflatoxins have been
implicated in both acute and chronic liver disease in humans; however,
other organs (kidney, spleen, pancreas) may also be affected (51). The best
studied chronic effects are those induced by the fumonisins, zearalenone,
and trichothecene mycotoxins produced by Fusarium sp. (49). Fumonisin
levels in corn-based foods have been statistically associated with an
increased risk of human esophageal cancer. Zearalenone is an estrogenic
mycotoxin. Ingestion by animals, especially swine, causes hyperestrogenism
with symptoms of enlargement and prolapse of the uterus, atrophy of ovaries
and testicles, enlargement of mammary glands, and infertility. This
mycotoxin might add to the estrogen load of humans. Human consumption of
trichothecene-contaminated foods causes acute symptoms of headaches,
chills, severe nausea, vomiting, and visual disturbances, which may last 7
to 10 days. Since trichothecenes modulate immune function, over time
mycotoxicosis could reduce immune resistance to infectious diseases,
facilitate tumor growth through reduced immune function, and cause
autoimmune disease (48).

Heart and Vascular Diseases

Several foodborne pathogens have been either directly or indirectly
associated with endocarditis and myocarditis, and any heart damage appears
to be permanent (2,52). Persons with ankylosing spondylitis linked to
enteric pathogens as the trigger show a high incidence of cardiac
conduction abnormalities, which may be sequelae to other seronegative
arthropathies. A possible connection between foodborne gram-negative
bacteria and atherosclerosis has been proposed, suggesting that the
bacteria gain access to the lymphoid and general circulation with relative
frequency. Endotoxin from degrading bacteria in macrophages may act in
concert with the inflammatory factors (cytokines) induced by endotoxin from
endothelium and smooth muscle cells. Although the process of atherogenesis
is complex and involves many factors, the hypothesis is attractive and
provides a model system for further study. Oxidative stress responses by E.
coli and S. Typhimurium and the induction of the peroxide stimulon and the
superoxide stimulon have also been recently implicated in atherosclerosis,
rheumatoid arthritis, and inflammatory bowel disease (53).

Nutritional and Gastrointestinal Disturbances

Enteric pathogen-induced diarrhea may lead to a variety of conditions
including loss of fluids, anorexia, and malabsorption of nutrients, all
forms of malnutrition. The enteric pathogens that cause malabsorption and
nutrient loss vary and include Enterobacteriaceae, Rotavirus, Amoeba,
Cryptosporidium, and Giardia (2,54-58). Unless treated with antimicrobial
drugs, many diarrheal episodes become chronic; however, the stress of even
short periods of diarrhea may result in subtle changes in immunologic
status. The extent of the diseases depends mostly on the immune status of
the person and may last for several years or for life. Death due to
diarrheal illness in the immunosuppressed and in persons with AIDS is
nearly 80%. No effective treatment is available, although treatment with
several antibiotics in combination shows promising results. However, AIDS
patients may also develop further sequelae. Cryptosporidium are
host-adapted, which may lead to pulmonary or tracheal cryptosporidiosis
accompanied by coughing and frequent low-grade fever. In these cases there
is no effective treatment. Similarly, in cyclosporidiosis, AIDS patients'
enteric infection is chronic. Long-term prophylaxis with trimoxazole is
required, and discontinuation of the treatment causes severe relapse
(2,54-58).

Severe cases of diarrhea lasting months or years and characterized by
dysentery, with foul-smelling, mucous bloody stools accompanied by
flatulence and abdominal distention, may result from C. jejuni,
Citrobacter, Enterobacter, or Klebsiella enteric infections. These
infections always require extensive antibiotic therapy and usually result
in failure to thrive. Enteric infections may alter bowel permeability which
allows absorption of otherwise excluded food components. Proteins that can
modulate the immune system can be absorbed possibly with deleterious
consequences such as the induction of autoimmunity and atopy. Several
studies of both human and porcine models indicate that significant
quantities of unwanted proteins can be absorbed by damaged gut tissue and
that maximum expression of diarrhea corresponds with peak protein uptake
(2).

Helicobacter pylori is the undisputed cause of chronic gastritis.
Environmental sources indicate that H. pylori can survive in water, chilled
foods, milk, and fresh vegetables for several days because of fecal
contamination. The species has never been isolated from these sources;
however, infectious, via
ble but nonculturable (nonspiral coccoid) bodies
may survive in fresh water for more than a year. H. pylori can be found in
human feces and can be transmitted directly from person to person by the
fecal-oral or oral-oral route. H. pylori can be found in several animal
reservoirs; however, the possibility of animal-to-animal or zoonotic
transmission is unknown (59,60). Ingestion of H. pylori leads to acute
gastritis, and colonization of the stomach is virtually always accompanied
by chronic inflammation that disappears within 6 to 12 months after
eradication of the infection. Infections are generally acquired during
childhood or adolescence and result in chronic gastritis lasting for
decades or life. On the basis of histologic and serologic follow-up
studies, this chronic gastritis has been suggested as an important risk
factor (odds ratio 9.0; p = 0.001) or first stage in a multistep process
leading to gastric mucosal atrophy, intestinal metaplasia, and eventually
gastric cancer (61,62).

Chronic Sequelae and Personality Changes

One area that has received scant interest is the effect of a chronic
infection on human personality factors. Personality changes might be
predicted: continual pain from arthritis, an irritable bowel, or chronic
diarrhea would be enough to make anyone temperamental, moody, or depressed.
Studies using Cattell's 16 Personality Factor questionnaire showed highly
significant correlations (p < 0.01 - 0.002) between chronic toxoplasmosis
and several personality factors (63,64). Men and women showed distinct
differences in behavioral states. For men, low superego strength,
protension, guilt-proneness, and group dependency were positively
associated, whereas in women the related factors were affectothymia,
alexia, untroubled adequacy, and self-sufficiency. A correlation of the
intensity of the personality factor-shifts with the duration of the
infection suggested that the infection per se induced the shift in
personality, not vice versa.

An exploratory study using the 16 Personality Factor questionnaire and the
Holmes and Rahe Social Readjustment Rating Scale, which measures stressful
life events, was made of patients with rheumatoid arthritis (65). As a
group, patients with rheumatoid arthritis exhibited higher stress at
disease onset (p < 0.01); a large high-stress-at-onset subgroup of
rheumatoid arthritis patients had a worse prognosis. Although there were
important personality changes in the high-stress-at-onset-rheumatoid
arthritis patients, the study concluded that the interaction between the
variables that determined personality changes were very complex and could
not simply be referred to as the "rheumatoid arthritis personality"
complex.

Conclusion

Foodborne diseases are for the most part preventable; however, there is an
inherent risk associated with the consumption of certain types of uncooked
foods. Recognition by the public health community and the public that many
foodborne illnesses may have serious chronic sequelae would help eliminate
many illnesses and reduce health-care cost. Public health authorities could
make a substantial impact by reducing poor or unhygienic food production or
food-handling practices and by educating the public about how harmful
microorganisms enter the food chain and how they can be avoided.

Address for correspondence: James A. Lindsay, Food Science and Human
Nutrition Department, University of Florida, Gainesville, FL 32611 USA; fax
352-392-9467; e-mail: jali@gnv.ifas.ufl.edu.

References

  1. Foegeding PM. Foodborne pathogens: risks and consequences. Council for
     Agricultural Science and Technology. Task Force Report 1221; 1994.
  2. Archer DL, Young FE. Contemporary issues: diseases with a food vector.
     Clin Microbiol Rev 1988;1:377-98.
  3. Bunning VK. Immunopathogenic aspects of foodborne microbial disease.
     Food Microbiology 1994;11:89-95.
  4. Bunning VK, Lindsay JA, Archer DL. Chronic health effects of foodborne
     microbial disease. World Health Stat Quart 1997;50:51-6.
  5. Miller FW. Genetics of autoimmune diseases. Exp Clin Immunogenet
     1995;12:182-90.
  6. Behar SM, Porcelli SA. Mechanisms of autoimmune disease induction: the
     role of the immune response to microbial pathogens. Arthritis Rheum
     1995;38:458-76.
  7. Yu DT, Thompson GT. Clinical, epidemiological and pathogenic aspects
     of reactive arthritis. Food Microbiology 1994;11:97-108.
  8. Sieper J, Braun J. Pathogenesis of spondylarthropathies. Arthritis
     Rheum 1995;38:1547-54.
  9. Gaston JH. How does HLA-B27 confer susceptibility to inflammatory
     arthritis? Clin Exp Immunol 1990;82:1-2.
 10. McGuigan LE, Prendergast JK, Geczy AF, Edmonds JP, Bashir HV.
     Significance on non-pathogenic cross-reactive bowel flora in patients
     with anklosing spondylitis. Ann Rheum Dis 1986;45:566-71.
 11. Kapasi K, Inman RD. HLA-B27 expression modulates Gram-negative
     bacterial invasion into transfected L-cells. J Immunol
     1992;148:3554-9.
 12. Wolf MW, Misaki T, Bech K, Tvede M, Silva JE, Ingbar SH.
     Immunoglobulins of patients recovering from Yersinia enterocolitica
     infections exhibit Graves' disease-like activity in human thyroid
     membranes. Thyroid 1991;1:315-20.
 13. Luo G, Seetharamaiah GS, Niesel DW, Zhang H, Peterson JW, Prabhakar
     BS, Klimpel GR. Purification and characterization of Yersinia
     enterocolitica envelope proteins which induce antibodies that react
     with human thyrotropin receptor. J Immunol 1994;152:2555-61.
 14. Seppel T, Rose F, Schlaghecke R. Chronic intestinal giardiasis with
     isolated levothyroxine malabsorption as reason for severe
     hypothyroidism: implications for localization of thyroid hormone
     absorption in the gut. Exp Clin Endocrinol Diabetes 1996;104:180-2.
 15. Chiodini RJ. Crohn's disease and the mycobacteriosis: a review and
     comparison of two disease entities. Clin Microbiol Rev 1989;2:90-117.
 16. Millar D, Ford J, Sanderson J, Withey S, Tizard M, Doran T,
     Hermon-Taylor J. IS900 PCR to detect Mycobacterium paratuberculosis in
     retail supplies of whole pasteurized cow's milk in England and Wales.
     Appl Environ Microbiol 1996;62:3446-52.
 17. Liu Y, van Kruiningen HJ, West AB, Cartun RW, Cortot A, Colombel JF.
     Immunocytochemical evidence of Listeria, Escherichia coli, and
     Streptococcus antigens in Crohn's disease. Gastroenterology
     1995;108:1396-401.
 18. Goodacre JA, Brownlee CED, Ross DA. Bacterial superantigens in
     autoimmune arthritis. Br J Rheumatol 1994;33:413-9.
 19. Kotb M. Bacterial pyrogenic exotoxins as superantigens. Clin Microbiol
     Rev 1995;8:411-26.
 20. Kotb M. Infection and autoimmunity: a story of the host, the pathogen
     and the copathogen. Clin Immunol Immunopathol 1995;74:10-22.
 21. Johnson HM, Torres BA, Soos JM. Superantigens: structure and relevance
     to human disease. Proc Soc Exp Biol Med 1996;212:99-109.
 22. Leung DY, Meissner HC, Fulton DR, Murray DL, Kotzin BL, Schlievert PM.
     Toxic shock syndrome toxin secreting Staphylococcus aureus in Kawasaki
     syndrome. Lancet 1993;342:1385-8.
 23. Kay RA. The potential role of superantigens in inflammatory bowel
     disease. Clin Exp Immunol 1995;100:4-6.
 24. Conrad B, Weidmann E, Trucco G, Rudert WA, Behboo R, Ricordi C, et al.
     Evidence for superantigen involvment in insulin dependent diabetes
     mellitis aetiology. Nature 1994;371:351-5.
 25. Tarr PI. Escherichia coli O157:H7: clinical, diagnostic, and
     epidemiological aspects of human infection. Clin Infect Dis
     1995;20:1-10.
 26. Tesh VL, O'Brien AD. The pathogenic mechanisms of Shiga toxin and the
     Shiga-like toxins. Mol Microbiol 1991;5:1817-22.
 27. Pudymaitis A, Armstrong G, Lingwood CA. Verotoxin-resistant cell
     clones are deficient in the glycolipid globotriosylceramide:
     differential basis of phenotype. Arch Biochem Biophys 1991;286:448-52.
 28. Lingwood CA. Verotoxin-binding in human renal sections. Nephron
     1994;66:21-8.
 29. Shoenfeld Y, George J, Peter JB. Guillain-Barr as an autoimmune
     disease. Int Arch Allergy Immunol 1996;109:318-26.
 30. Peterson MC. Clinical aspects of Campylobacter jejuni infections in
     adults. West J Med 1994;161:148-52.
 31. Bolton CF. The changing concepts of Guillain-Barr Syndrome. New Engl
     J Med 1995;333:1415-7.
 32. Oomes PG, Jacobs BC, Hazenberg MP, Banffer JR, van der Meche FG.
     Anti-GM1 IgG antibodies and Campylobacter bacteria in Guillain-Barr
     Syndrome: evidence of molecular mimicry. Ann Neurol 1995;38:170-5.
 33. Swift AE, Swift TR. Ciguatera. J Toxicol Clin Toxicol 1993;31:1-29.
 34. Lange WR. Ciguatera fish poisoning. Am Fam Physican 1994;50:579-84.
 35. Todd ECD. Domic acid and amnesic shellfish poisoning: a review.
     Journal of Food Protection 1993;56:69-83.
 36. Smith JL. Foodborne toxoplasmosis. Journal of Food Safety
     1991;12:17-57.
 37. Derouin F. Pathology and immunological control of toxoplasmosis. Braz
     J Med Biol Res 1992;25:1163-9.
 38. Khalil SS, Rashwan EA. Tumor necrosis factor-alpha (TNF-alpha) in
     human toxoplasmosis. J Egypt Soc Parisitol 1996;26:53-61.
 39. Olle P, Bessieres MH, Malecaze F, Seguela JP. The evolution of ocular
     toxoplasmosis in anti-interferon gamma treated mice. Curr Eye Res
     1996;15:701-7.
 40. Hayashi S, Chan CC, Gazzinelli R, Roberge FG. Contribution of nitric
     oxide to the host parasite equilibrium in toxoplasmosis. J Immunol
     1996;156:1476-81.
 41. Shrittematter C, Lang W, Wiestler OD, Kleihues P. The changing patter
     of HIV associated cerebral toxoplasmosis. Acta Neuropathol (Berl)
     1992;8:475-81.
 42. Allen JE, Maizels JM. Immunology of human helminth infection. Int Arch
     Allergy Immunol 1996;109:3-10.
 43. Moyer L, Warwick M. Mahoney FJ. Prevention of hepatitis A virus
     infection. Am Fam Physician 1996;54:107-14.
 44. von Herrath MG, Oldstone MBA. Virus-induced autoimmune disease. Curr
     Opin Immunol 1996;8:878-85.
 45. Rahyaman SM, Chira P, Koff RS. Idiopathic autoimmune chronic hepatitis
     triggered by Hepatitis A. Am J Gastroenterol 1994;89:106-8.
 46. Robens JF, Richard JL. Aflatoxins in animal and human health. Rev
     Environ Contam Toxicol 1992;127:69-94.
 47. Coulombe RA. Biological actions of mycotoxins. J Dairy Sci
     1993;76:880-91.
 48. Pestka JJ, Bondy GS. Immunologic effects of mycotoxins. In: Miller JD,
     Trenholm HL, editors. Mycotoxins in grain. St Paul (MN): Eagan Press;
     1994. pp. 339-58.
 49. Jackson LS, De Vries JW, Bullerman LB, editors. Fumonisins in food.
     New York: Plenum Press; 1996.
 50. Neal GE. Genetic implication in the metabolism and toxicity of
     mycotoxins. Toxicol Lett 1995;82-83:861-7.
 51. Creppy EE, Baudrimont I, Betbeder AM. Prevention of nephrotoxicity of
     ochratoxin A, a food contaminant. Toxicol Lett 1995;83-83:869-77.
 52. Archer DL. Foodborne gram-negative bacteria and atherosclerosis: is
     there a connection? Journal of Food Protection 1987;50:783-7.
 53. Farr SB, Kogoma Y. Oxidative stress response in Escherichia coli and
     Salmonella typhimurium. Microbiol Rev 1991;55:561-85.
 54. Bruckner DA. Amebiasis. Clin Rev Microbiol 1992;5:356-69.
 55. Wolfe MS. Giardiasis. Clin Microbiol Rev 1992;5:93-100.
 56. Chappell CL, Matson CC. Giardia antigen detection in patients with
     chronic gastrointestinal disturbances. J Fam Pract 1992;35:49-53.
 57. Smith JL. Cryptosporidium and Giardia as agents of foodborne disease.
     Journal of Food Protection 1994;56:451-61.
 58. Keusch GT, Hamer D, Joe A, Kelley M, Griffiths J, Ward H.
     Cryptosporidia-who is at risk. Schweiz Med Wochenschr
     1995;125:899-908.
 59. Owen RJ. Bacteriology of Helicobacter pylori. Bailliere's Clin
     Microbiol 1995;9:415-46.
 60. Fox JG. Non-human reservoirs of Helicobacter pylori. Aliment Pharmacol
     Ther 1995;9(Suppl 2):93-103.
 61. Kuipers EJ, Uyterlinde AM, Pena AS, Roosendaal R, Pals G, Nelis GF, et
     al. Long term sequelae of Helicobacter pylori gastritis. Lancet
     1995;345:1525-8.
 62. Sipponen P, Kekki M, Seppala K, Siurala M. The relationship between
     chronic gastritis and gastric secretion. Aliment Pharmacol Ther
     1996;10(Suppl 1):103-18.
 63. Flegr J, Hrdy I. Influence of chronic toxoplasmosis on some human
     personality factors. Folia Parasitol (Praha) 1994;41:122-6.
 64. Flegr J, Zitkova S, Kodym P, Frynta D. Induction of changes in human
     behaviour by the parasitic protozoan Toxoplasma gondii. Parasitology
     1996;113:49-54.
 65. Latman NS, Walls R. Personality and stress: an exploratory comparison
     of rheumatoid arthritis and osteoarthritis. Arch Phys Med Rehabil
     1996;77:796-800.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/lindsay.txt

Public, Animal, and Environmental Health Implications of Aquaculture

E. Spencer Garrett,* Carlos Lima dos Santos, and Michael L. Jahncke
*National Seafood Inspection Laboratory, Pascagoula, Mississippi, USA;
Food and Agriculture Organization of the United Nations, Rome, Italy;
Virginia Tech Seafood Research and Extension Center, Hampton, Virginia,
USA
------------------------------------------------------------------------

      Aquaculture is important to the United States and the world's
      fishery system. Both import and export markets for aquaculture
      products will expand and increase as research begins to remove
      physiologic and other animal husbandry barriers. Overfishing of
      wild stock will necessitate supplementation and replenishment
      through aquaculture. The aquaculture industry must have a
      better understanding of the impact of the "shrouded" public and
      animal health issues: technology ignorance, abuse, and neglect.
      Cross-pollination and cross-training of public health and
      aquaculture personnel in the effect of public health, animal
      health, and environmental health on aquaculture are also
      needed. Future aquaculture development programs require an
      integrated Gestalt public health approach to ensure that
      aquaculture does not cause unacceptable risks to public or
      environmental health and negate the potential economic
      and nutritional benefits of aquaculture.

U.S. Fisheries System

Coastal estuaries serve as a breeding ground and provide habitats for more
than 75% of commercial landings and 80% to 90% of the recreational catch of
fish and shellfish. From these habitats, hundreds of species of seafood are
produced. Aquacultured species now contribute up to 15% of the U.S. supply
(1,2). Wild species are harvested by 17,000,000 recreational anglers and
nearly 300,000 commercial harvesters. Commercial harvesters deploy 93,000
vessels, while recreational fishermen have millions of recreational fishing
boats. Nearly 5,000 domestic plants are located in every state throughout
the United States, not just in the coastal areas (3). Current per capita
consumption of commercially harvested species averages 15 pounds; it is
estimated that per capita consumption of recreationally harvested seafood
approaches an additional 3 to 4 pounds per person (4).

The seafood business community--in the United States and in other
industrialized countries--cannot rely solely on domestically produced stock.
For a number of years, more than half of U.S. seafood consumption has
relied on imported stock. Currently, the United States imports more than
50% of the consumed seafood, which originates in 172 countries around the
world (3). This trend toward economic reliance on imported stock has
steadily increased over the past 10 years so that now the United States is
the world's second largest importer of seafood. The principal seafood
imports are tuna, shrimp, salmon, lobster, and groundfish (3).

It is difficult to determine where imported fish was harvested. For
example, the United States imports salmon from Switzerland and Panama,
although neither Switzerland nor Panama is noted for vast salmon resources.
U.S. participation in the international seafood trade is very complex,
since in addition to being the world's second largest importer, the United
States is also the world's second largest exporter of seafood (3). This
dichotomy requires that U.S. marketing and import/export food control
inspection strategies be carefully planned. For example, the United States
exports seafood to 162 countries, which has come about with the full
development of northwest and Alaska fisheries and improved efficiency in
processing techniques. Major U.S. exports are salmon, crab, surimi, fish
blocks, groundfish, flatfish, shrimp, and lobster (3).

Current Aquaculture Status

In 1996, U.S. aquaculture production of nearly 227,000 metric tons
consisted of baitfish, catfish, salmon, trout, clams, crawfish, mussels,
oysters, fresh and saltwater shrimp, and miscellaneous species such as
ornamental fish, alligators, algae, aquatic plants, tilapia, and hybrid
striped bass. The United States exported principally rainbow trout,
Atlantic salmon, tilapia, catfish, freshwater crawfish, and live mussels to
19 countries in Europe, North and South America, and Asia. Freshwater
crawfish led the export seafood market at slightly over $8 million, with
the other species accounting for less than $1 million each (3). The United
States also imports large volumes of aquacultured products, approximately
$2.5 billion in cultured products, primarily shrimp and salmon. Imported
cultured seafood accounts for most of the current U.S. trade deficit for
edible fishery products, which was approximately $3.5 billion in 1995.

The Safety of Seafood

Most seafood is safe; however, like all foods, it carries some risk. The
food safety issues for seafood are highly focused, well-defined, and
limited to a very few species. For seafood-borne illnesses (in which the
cause was known) reported to the Centers for Disease Control and
Prevention, more than 90% of the outbreaks and 75% of the individual cases
were associated with ciguatoxin (from a few reef fish species) and
scombrotoxin (from tuna, mackerel, bluefish, and a few other species) and
the consumption of mollusks (mostly raw) (5-12).

Hazards associated with the consumption of all food (including seafood) can
be categorized into three areas: product safety; food hygiene (clean vs.
dirty plants, wholesome vs. unwholesome products); and mislabeling or
economic fraud. Traditionally, the food safety risks of seafood products
(aquacultured and wild-caught) have been subcategorized by environment,
process, distribution, and consumer-induced risk; the environmental risk
category is further subdivided into natural hazards (e.g., biotoxins) and
anthropogenic contaminants (e.g., polychlorinated biphenyls) (13).

"Shrouded" Aquaculture Hazards

The future of aquaculture is bright; aquaculture products are as safe and
wholesome as wild-caught species. However, in addition to the consumer
hazards listed above, there are some less obvious "shrouded" public health
hazards associated with ignorance, abuse, and neglect of aquaculture
technology.

Technology Ignorance

A common practice in many developing countries is the creation of numerous
small fish pond impoundments. However, this approach may have a greater
adverse effect on human health than the creation of a single large
impoundment (14). Small impoundments greatly increase the overall aggregate
shoreline of ponds, causing higher densities of mosquito larvae and
cercaria, which can increase the incidence and prevalence of diseases such
as lymphatic filariasis and schistosomiasis, respectively. Centralized
planning approaches for new freshwater and marine aquaculture sites should
include discussions of the potential effect of large or small impoundments
on such issues as disease transmission, water supply, irrigation, and power
generation (14).

Ignorance of the microbial profile of aquaculture products can also affect
human health as evidenced by the recent transmission of streptococcal
infections from tilapia to humans, which resulted in several meningitis
cases in Canadian fish processors (15). A change in marketing strategies to
sell live fish in small containers, instead of ice-packs, resulted in human
Vibrio infections from live tilapia in Israel in 1996. Such bacteria can be
present in other aquacultured and wild-caught species in addition to
tilapia.

Ignorance of the hazards associated with the use of untreated animal or
human waste in aquaculture ponds to increase production also has tremendous
human health implications (16). For centuries, food growers have cultured
species in waste-water-fed ponds and grown secondary vegetable crops in
waste water and sediment material in integrated aquaculture operations.
However, the potential for transmission of human pathogens to cultured
species and secondary vegetable crops is rarely considered by fishery
aquaculturists. For example, of more than 250 presentations at the 1997
World Aquaculture Society meeting held in Seattle, Washington, few referred
to the potential human health implications of aquaculture (17).

The potential transmission of animal pathogens from exotic aquacultured
species to wild-stock species also affects animal health. Recent outbreaks
of taura, yellow spot, and white head viruses have occurred in aquaculture
shrimp in South Carolina and Texas. Recent studies indicate that native
wild white shrimp may also be susceptible to these exotic viruses (18).

Technology Abuse

Technology abuse includes the willful misuse of therapeutic drugs,
chemicals, fertilizers, and natural fishery habitat areas. The widespread
use and misuse of antibiotics to control diseases in aquaculture species is
worldwide and will probably increase as aquaculturists move towards more
intensive animal husbandry- rearing techniques and stocking densities. For
example, the illegal use of chloramphenicol in shrimp culture to control
diseases may result in violative levels in the harvested product.
Similarly, the improper or illegal use of chemicals (e.g., tributyl tin) to
control pond pests such as snails can also result in human health hazards.
The abuse and misuse of raw chicken manure as pond fertilizer may result in
the transmission of Salmonella from manure to the cultured product (16).

The destruction of mangrove areas to build aquaculture ponds can have a
drastic impact on the survival of wild aquatic species through the
degradation of essential fish habitats and nurseries. In Brazil,
destruction of mangrove areas for shrimp ponds affected climatic changes to
such an extent that the aquaculture operations have been terminated because
consequent reduced rainfall resulted in excessive pond salinity (19).

Technology Neglect

The final "shrouded" hazard associated with aquaculture involves technology
neglect, which includes such events as the abandonment of small aquaculture
ponds in tropical countries, leading to increased mosquito habitats and
concomitant increases in malaria (14). Facility management can be
responsible for technology neglect if employees are not trained in the
proper use and application of therapeutics and chemicals, for example.
Finally, from an animal health perspective, ignorance or willful neglect of
the International Council for Explorations of the Sea/European Inland
Fisheries Advisory Commission Code of Practice for the Introduction and
Transfer of Marine Organisms can result in the escape of exotic species and
animal pathogens into the environment with a potential tragic impact on
native aquatic species (20).

Health Control Considerations

Human Health

Procedures to help protect humans from aquaculture-associated risks include
better education and training of aquaculture personnel on the proper use
and storage of therapeutics and chemical compounds. Additional research on
new more effective and, we hope, safer, antibiotic and vaccine treatment of
aquaculture species is under way. Likewise, certain extralabel use
applications for selected antibiotics are under consideration. Streamlined
enforcement efforts are being developed to ensure compliance with new food
safety regulations and new regulatory control procedures such as Hazard
Analysis and Critical Control Points (HACCP) and the application of HACCP
principles to animal and environmental control procedures (21,22).

The Food and Agriculture Organization and World Health Organization
recommend that the HACCP concept be applied to fresh water aquaculture
programs to control foodborne digenetic trematode infections in humans.
Experiments are being carried out in Asia by a multidisciplinary team of
experts in public health, parasitology, aquaculture, fisheries extension,
and fish inspection (22). In one study in Vietnam, experimental activities
were conducted in two side-by-side fish ponds. In the experimental ponds,
fish were cultured in conjunction with HACCP principles, and control pond
fish were cultured according to conventional local aquaculture practices
(22). Water supply, fish fry, fish feed, and pond conditions in the
experimental pond were identified as critical control points. The HACCP
principles of hazard analysis, preventative measures, critical limits,
monitoring, recordkeeping, and verification procedures relating to the
critical control points were applied; study results showed Clonorchis
sinensis eggs and fish infected with the parasite metacercaria and the
first intermediate host (Melanoides tuberculata) in the experimental ponds
(22). Forty-five percent of control pond fish were infected with C.
sinensis metacercaria, while white fish from the experimental pond
monitored according to HACCP principles were completely free of trematode
infection (22). Preliminary results indicate that application of
HACCP-based principles to silver carp culture in North Vietnam is an
effective way to prevent and control C. sinensis. Similarly, the
application of these principles to fresh water aquaculture ponds in
Thailand and Laos to control Opisthorchis viverrini infections has also
been successful. Additional studies are recommended to confirm these
preliminary results (22).

Animal Health

Procedures to safeguard animal health are set out in the International
Council for Explorations of the Sea and the European Inland Fisheries
Advisory Commission Codes of Practice, which describe how to prevent the
adverse effects of introducing new and exotic species and emerging animal
pathogens. Education and on-site training programs for aquaculture
employees will help them understand the detrimental impact of introduction
of exotic species and animal pathogens, misuse and abuse of therapeutics
and chemicals, and willful habitat destruction. High priority issues also
include implementation of biosecurity procedures in aquaculture operations
to prevent the escape and spread of exotic species and pathogens into the
facility and surrounding natural environment and the use of the HACCP
principles to help control the spread of exotic pathogens to wild aquatic
populations (17,23).

The application of HACCP principles to control transmission of exotic
shrimp viruses from cultured to wild shrimp was proposed at a shrimp
pathogen workshop held in June 1996 in New Orleans, Louisiana (21). Natural
resource regulatory agencies are concerned about the possible transmission
of exotic shrimp pathogenic viruses, recently found in shrimp aquaculture
ponds in Texas and South Carolina, to wild native shrimp populations. The
principles of HACCP, in conjunction with International Council for
Explorations of the Sea and the European Inland Fisheries Advisory
Commission Codes of Practice, were proposed to control the spread of exotic
animal viruses into the environment. Shrimp aquaculture has the following
proposed critical control points: pond site selection; water supply
quality; pond management techniques; and transportation, especially as it
relates to the live transport of aquaculture shrimp species (21).

Approximately 600 million pounds of shrimp are also imported for further
processing into the United States on a yearly basis, half of which are
aquacultured species (3). Natural resource managers, particularly at the
state level, are concerned about the possible transmission of exotic shrimp
pathogens into the environment from shrimp processing plant wastewater
discharge and solid waste material landfill leakage. Proposed HACCP shrimp
processing plant critical control points include unload/receive;
de-ice/wash; thaw; dehead/peel/devein; wash; re-ice; de-ice/wash; re-ice;
and dip/glaze (21).

Application of HACCP principles at aquaculture site and processing plant
locations has the potential to control transmission of exotic human and
animal pathogens. However, to our knowledge, except for the application of
HACCP principl
es to control of human pathogens in Asia (17), no research
has been conducted on this issue.

Address for correspondence: Michael Jahncke, Virginia Tech Seafood Research
and Extension Center, 102 King Street, P.O. Box 369, Hampton, VA 23669 USA;
fax: 757-727-4871; e-mail: mjahncke@vt.edu.

References

  1. Ratafia M. Aquaculture today: a worldwide status report. Aquaculture
     News 1994;3:12-13,18-19.
  2. Rhodes RJ. Status of world aquaculture. Aquaculture Magazine 17th
     Annual Buyers Guide 1987;4-18.
  3. National Marine Fisheries Service. Fisheries of the United States
     1995. Current Fisheries Statistics No. 9500. Washington (DC): U.S.
     Department of Commerce; 1996.
  4. Krebs-Smith SM. Report on per capita consumption and intake of fishery
     products [letter]. In: National Academy of Sciences Committee on the
     Evaluation of the Safety of Fishery Products 1991 Seafood Safety
     Report. Washington (DC): National Academy Press; 1991.
  5. Centers for Disease Control. Annual summary of foodborne diseases
     (1978). Atlanta (GA):CDC;1979.
  6. Centers for Disease Control. Annual summary of foodborne diseases
     (1979). Atlanta (GA):CDC;1981.
  7. Centers for Disease Control. Annual summary of foodborne diseases
     (1981). Atlanta (GA):CDC;1983.
  8. Centers for Disease Control. Annual summary of foodborne diseases
     (1980 and 1981). Atlanta (GA):CDC;1983.
  9. Centers for Disease Control. Annual summary of foodborne diseases
     (1982). Atlanta (GA):CDC;1985.
 10. Centers for Disease Control. Annual summary of foodborne diseases,
     unpublished data. Atlanta (GA):CDC;1989.
 11. Centers for Disease Control. Foodborne disease outbreaks, 1983-87.
     MMWR CDC Surveill Summ 1990;39(SS-1):1-45.
 12. National Marine Fisheries Service. The draft report of the model
     seafood surveillance project. A report to Congress. Pascagoula (MS):
     Office of Trade and Industry Services, National Seafood Inspection
     Laboratory;1995.
 13. Garrett ES. Role of diseases in marine fisheries management.
     Transactions of the 50th North American Wildlife and Natural
     Resources. NOAA Conference 1986. Tech. Memo. NMFS, F/NWR-16.
     Washington (DC): National Marine Fisheries Service;1986.
 14. Mott KE. A proposed national plan of action for schistosomiasis
     control in the United Republic of Cameroon. Geneva: World Health
     Organization (unpublished document WHO/SCHISTO/86,88); 1996.
 15. Centers for Disease Control and Prevention. Invasive infection with
     Streptococcus iniae-Ontario, 1995-1996. MMWR Morb Mort Wkly Rep 1996;
     45(30):650-3.
 16. Buras N. Microbial safety of produce from wastewater fed aquaculture.
     In: Pullin RVC, Rosenthal H, Maclean JL, editors. Environment and
     aquaculture in developing countries. Proceedings of the 31st
     International Center for Living Aquatic Resources Management
     Conference; 1993; Manila, Philippines. Manila: The Center; 1993. p.
     285-95.
 17. World Aquaculture Society. Abstracts of the Annual International
     Conference and Exposition of the World Aquaculture Society and The
     National Aquaculture Conference and Exposition of National Aquaculture
     Association; 1997 Feb 19-23; Seattle, WA.
 18. Virus re-emerges at shrimp farm. The Aquaculture News 1996;4(11):1-15.
 19. Pullin RSV. Discussion and recommendations on aquaculture and the
     environment in developing countries. In: Pullin RSV, Rosenthal H,
     Maclean JL, editors. Environment and aquaculture in developing
     countries. Proceedings of the 31st ICLARM Conference 1993. p. 312-338.
 20. Turner GE, editor. Codes of practice and manual of procedures for
     consideration of introductions and transfers of marine and freshwater
     organisms. EIFAC Occasional Paper 23. Rome: FAO; 1988.
 21. Jahncke JL. The application of the HACCP concept to control exotic
     shrimp viruses. Proceedings of the NMFS Workshop on Exotic Shrimp
     Viruses. 1996 June; New Orleans, LA.
 22. Son TQ, Hoi VS, Dan TV, Nga C, Toan TQ, Chau LV, et al. Application of
     hazard analysis critical control point (HACCP) as a possible control
     measure against Clonorchis sinensis in cultured silver carp
     Hypophthalmichthys molitrix. Paper presented at 2nd Seminar on
     Food-Borne Zoonoses: Current Problems, Epidemiology and Food Safety;
     1995 Dec 6-9; Khon Kaen, Thailand.
 23. Austin B. Environmental issues in the control of bacterial diseases of
     farmed fish. In: Pullin RVC, Rosenthal H, Maclean JL, editors.
     Environmental and aquaculture in developing countries. Proceedings of
     the 31st International Center for Living Aquatic Resources Management
     Conference; 1993; Manila, Philippines. Manila: The Center; 1993. p.
     237-51.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

Produce Handling and Processing Practices

Larry R. Beuchat and Jee-Hoon Ryu
University of Georgia, Griffin, Georgia, USA
---------------------------------------------------------------------------

      In the past decade, outbreaks of human illness associated with
      the consumption of raw vegetables and fruits (or unpasteurized
      products produced from them) have increased in the United
      States. Changes in agronomic, harvesting, distribution,
      processing, and consumption patterns and practices have
      undoubtedly contributed to this increase. Pathogens such as
      Listeria monocytogenes, Clostridium botulinum, and Bacillus
      cereus are naturally present in some soil, and their presence
      on fresh produce is not rare. Salmonella, Escherichia coli
      O157:H7, Campylobacter jejuni, Vibrio cholerae, parasites, and
      viruses are more likely to contaminate fresh produce through
      vehicles such as raw or improperly composted manure, irrigation
      water containing untreated sewage, or contaminated wash water.
      Contact with mammals, reptiles, fowl, insects, and
      unpasteurized products of animal origin offers another avenue
      through which pathogens can access produce. Surfaces, including
      human hands, which come in contact with whole or cut produce
      represent potential points of contamination throughout the
      total system of growing, harvesting, packing, processing,
      shipping, and preparing produce for consumption. Treatment of
      produce with chlorinated water reduces populations of
      pathogenic and other microorganisms on fresh produce but cannot
      eliminate them. Reduction of risk for human illness associated
      with raw produce can be better achieved through controlling
      points of potential contamination in the field; during
      harvesting; during processing or distribution; or in retail
      markets, food-service facilities, or the home.

Advances in agronomic, processing, preservation, packaging, shipping, and
marketing technologies on a global scale have enabled the fresh fruit and
vegetable industry to supply consumers with a wide range of high-quality
produce year round. Some of the same technologies and practices have also
introduced an increased risk for human illness associated with pathogenic
bacteria, mycotoxigenic molds, viruses, and parasites. The use of manure
rather than chemical fertilizer, as well as the use of untreated sewage or
irrigation water containing pathogens, viruses, or parasites, undoubtedly
contributes to this increased risk. Changes in the produce industry, social
demographics, food consumption patterns, and awareness of fresh fruits and
vegetables as potential vehicles of infection may also be contributing to
an increase in documented produce-associated outbreaks of human illness.

Changing factors that contribute to the epidemiology of diseases that may
be associated with fresh fruits and vegetables were discussed by Hedberg et
al. (1). Increases in foodborne illness during the summer are not fully
understood, although fresh produce is likely to play a role since it is
consumed in higher quantities during the summer. The per capita consumption
of fresh produce has increased in the United States in recent years (Figure
1), not only in the summer but also in other seasons, partly because of
increased importation. Knowledge of the presence and numbers of specific
pathogens on produce imported to the United States from countries that may
have lower sanitation standards is minimal. However, produce from a single
grower, packinghouse, or shipper, whether located outside or within the
United States, may be routinely distributed throughout the country, thus
facilitating widespread dissemination of potential pathogens. The
epidemiology of foodborne diseases is greatly influenced by these global
changes. Control or elimination of pathogenic microorganisms from fresh
fruits and vegetables can be achieved only by addressing the entire system,
from the field, orchard, or vineyard to the point of consumption.

[Figure not available in ASCII.]

We reviewed some of the practices (particularly preharvest practices) used
by the fresh fruit and vegetable industry that may promote contamination of
produce with pathogenic microorganisms.

Sources of Contamination

The presence of pathogenic bacteria, viruses, and parasites on fresh fruits
and vegetables has been extensively documented (3). Contamination of
produce can occur in the field or orchard; during harvesting, postharvest
handling, processing, shipping, or marketing; or in the home (Table).

Table. Sources of pathogenic microorganisms on fresh fruits vegetables*
-------------------------------------------------

Preharvest

     Feces
     Soil
     Irrigation water
     Water used to apply fungicides, insecticides
     Green or inadequately composted manure
     Air (dust)
     Wild and domestic animals (including fowl and reptiles)
     Insects
     Human handling

Postharvest

     Feces
     Human handling (workers, consumers)
     Harvesting equipment
     Transport containers (field to packing shed)
     Wild and domestic animals (including fowl and reptiles)
     Insects
     Air (dust)
     Wash and rinse water
     Sorting, packing, cutting, and further processing equipment
     Ice
     Transport vehicles
     Improper storage (temperature, physical environment)
     Improper packaging (including new packaging technologies)
     Cross-contamination (order foods in storage, preparation, and display
     areas)
     Improper display temperature
     Improper handling after wholesale or retail purchase

-------------------------------------------------

*Adapted from Beuchat

Preharvest Sources

Spores of Clostridium species, including C. botulinum and C. perfringens,
as well as spores of enterotoxigenic Bacillus cereus, are commonly found in
soil, so their occasional presence on fruits and vegetables should not be
unexpected. Numbers of clostridial spores on some types of vegetables
appear to increase during the summer (4). Perhaps the most prevalent
disease-causing microorganism in soil is Listeria monocytogenes (5,6).
Twenty-seven strains were isolated from soil and vegetation taken from 19
sites in the Netherlands (7). Plant materials from which the organism was
isolated included dead and decayed corn and soybean plants and wild
grasses, indicating its preference to exist in nature as a saprophyte. A
study of soil and domestic animal feces has shown that Listeria is more
often present during July to September than other months (8). L.
monocytogenes and L. innocua were predominant in feces, whereas L. ivanovi
and L. seeligeri were most common in soil.

Vegetation in a rural area in Virginia where clinical listeriosis is rare
was analyzed for L. monocytogenes (9). Dead soybean plant material and
stalks, leaves, and tassels of corn were collected in April following the
previous planting year. Eight of twelve sampling sites yielded plant
materials positive for L. monocytogenes. Only 25% of the strains were
pathogenic for mice, a low frequency compared with the percentage of
pathogenic strains isolated from Listeria-positive humans and animals in
Virginia and the United States as a whole. These observations suggest that
the predominance of certain serotypes of L. monocytogenes may be influenced
by the environment and that some strains indigenous to decaying plant
vegetation are incapable of causing human illness.

Weiss and Seeliger (10) isolated 154 strains of L. monocytogenes in Germany
from soil and plants, 16 from feces of deer and stag, nine from moldy
fodder and wildlife feeding grounds, and eight from birds. Corn, wheat,
oats, barley, and potato plants and soils from the fields in which they
were growing were among the materials analyzed. Nearly 10% of the corn
plants and 13% of the grain plants were infected with L. monocytogenes.
Plants from cultivated fields had a lower incidence (12.5%) than plants
from uncultivated fields (44%). Twenty-three percent of samples collected
from wildlife feeding grounds were positive for L. monocytogenes. It was
suggested that L. monocytogenes is a saprophyte that lives in a plant-soil
environment and could therefore be contracted by humans and animals through
many possible routes from many sources. Birds and animals are unlikely to
be the only sources responsible for the distribution of L. monocytogenes in
nature and its presence on fruits and vegetables.

The presence of other pathogenic bacteria, viruses, and parasites in soil
likely results largely from application of feces or untreated sewage,
either by chance or design. Whatever the case, soil on the surface of
fruits and vegetables may harbor pathogenic microorganisms that remain
viable through subsequent handling to the point of consumption unless
effective sanitizing procedures are administered.

Irrigation and surface run-off waters can be sources of pathogenic
microorganisms that contaminate fruits and vegetables in the field.
Irrigation water containing raw sewage or improperly treated effluents from
sewage treatment plants may contain hepatitis A, Norwalk viruses, or
enteroviruses (poliomyelitis, echoviruses, and Coxsackie viruses) (11).
Rotaviruses are known to retain viability on the surface of vegetables held
at 4C for up to 30 days (12).

Listeria and other potentially pathogenic bacteria have been reported in
sewage. Watkins and Sleath (13) analyzed 52 sewage, river water, and
industrial effluents for pathogens. Effluents were from abattoirs, cattle
markets, and poultry packing plants. L. monocytogenes was isolated from all
samples. In many instances, populations of L. monocytogenes were higher
than those for salmonellae and, in some instances, L. monocytogenes was
isolated when no salmonellae were detected. Application of sludge
containing L. monocytogenes and salmonellae to soil showed that L.
monocytogenes could survive longer. Populations of L. monocytogenes in soil
remained essentially unchanged during 7 weeks after application.

Treatment of sewage does not always yield a sewage sludge cake or a final
discharge free of Listeria (14). The use of sewage as a fertilizer could
contaminate vegetation destined for human consumption. MacGowan et al. (8)
examined sewage at 2-month intervals in 1991 to 1992 and found 84% to 100%
contained L. monocytogenes or L. innocua.

Application of sewage sludge or irrigation water to soil is one avenue
through which parasites can contaminate fruits and vegetables. Ascaris ova
sprayed onto tomatoes and lettuce remain viable for up to 1 month, while
Endamoeba histolytica could not be recovered 1 week after spraying (15,16).
If sewage irrigation or night soil application is stopped 1 month before
harvest, the produce would not likely be vectors for transmission of
diseases caused by these parasites.

Wang and Dunlop (17,18) recovered Salmonella, Ascaris ova, and Endamoeba
coli cysts from more than half of irrigation water samples contaminated
with either raw sewage or primary-treated, chlorinated effluents. Only one
of 97 samples of vegetables irrigated with this water yielded Salmonella,
but Ascaris ova were recovered from two of 34 vegetable samples. Barbier et
al. (19) concluded that application of sewage sludge containing Taenia
saginata eggs offers a serious risk for cattle even after a 3-week
no-grazing period.

Feces have been suspected as sources of pathogens on contaminated fruits,
vegetables, or minimally processed produce that have subsequently been
associated or confirmed as causes of human disease outbreaks (3). Among the
more recent outbreaks are those linking unpasteurized apple juice to
Escherichia coli O157:H7 infections. This pathogen can remain viable in
bovine feces for up to 70 days, depending on inoculum level and temperature
(17). Cryptosporidium infection linked to consumption of unpasteurized
apple juice was hypothesized to have been caused by contamination of apples
by calf feces (20). Contact of fruits and vegetables by pickers and
handlers at the time of harvest also offers a mechanism by which pathogens
in feces can contaminate raw produce.

Wild birds are known to disseminate Campylobacter (21,22), Salmonella
(22,23), Vibrio cholerae (24), and Listeria species (25). More recently, E.
coli O157:H7 has been isolated from wild bird feces. In a survey of wild
birds (mainly gulls), 0.9% of the bacterial isolates from fecal samples at
an urban landfill and 2.9% of bacterial isolates from fecal samples on
intertidal sediments were Vero cytotoxin-producing E. coli O157:H7 (26).
Pathogenic bacteria are apparently picked up as a result of birds feeding
on garbage, sewage, fish, or lands that are grazed with cattle or have had
applications of fresh manure. Control of preharvest contamination of fruits
and vegetables with pathogenic bacteria by wild birds would be
exceptionally difficult.

Postharvest Sources

Some of the possible preharvest sources of pathogenic microorganisms may
also be postharvest sources (Table). The fecal-oral route of transmission
of pathogens broadens to include workers handling fruits and vegetables
from the point of removal from the plant through all stages of handling,
including preparation at the retail and food service levels and in the
home. Changes in eating habits, particularly the increased consumption of
meals away from home, must be considered when attempting to provide reasons
for increased frequency of outbreaks associated with fresh produce. Proper
training of food-service workers in hygienic practices is essential. One
cannot assume that newly hired personnel have even rudimentary knowledge of
food microbiology. This is particularly critical among teenagers who,
partly because they and their parents are eating more meals away from home,
have had minimal or no exposure to proper food-handling practices.
Instruction in elementary principles of food hygiene at the high school or
middle school levels has diminished greatly in the past two decades.

Traditionally recognized postharvest control points for access of pathogens
to whole or cut produce include transport containers and vehicles and
sorting, packing, cutting, and further processing equipment. The
development of new processing equipment and technologies should include a
team of experts in food microbiology as well as engineering. Too often,
aspects of sanitizing equipment are not considered or are an afterthought
and can increase the risk for contaminated end products. Temperature
control is absolutely critical at every stage of postharvest handling if
any success is to be achieved in minimizing the growth of pathogens.

Removal of Pathogens

Sanitizers that can be used to wash or to assist in lye peeling of fruits
and vegetables are regulated by the U.S. Food and Drug Administration in
accordance with the Federal Food, Drug and Cosmetic Act as outlined in the
Code of Federal Regulations, Title 21, Ch. 1, Section 173.315. As noted by
Barmore (27), no chlorine substitute effective for washing fruits and
vegetables is available. Numerous alternatives for sanitizing equipment
(28) can be used in a total sanitation program, but none has as broad a
spectrum of activity as chlorine.

Chlorine is routinely used as a sanitizer in wash, spray, and flume waters
used in the fresh fruit and vegetable industry. Antimicrobial activity
depends on the amount of free available chlorine (as hypochlorous acid) in
water that comes in contact with microbial cells. The efficacy of chlorine
in killing pathogenic microorganisms has been extensively studied. Possible
uses in packinghouses and during washing, cooling, and transport to control
postharvest diseases of whole produce have been reviewed by Eckert and
Ogawa (29). The effect of chlorine concentration on aerobic microorganisms
and fecal coliforms on leafy salad greens was studied by Mazollier (30).
Total counts were markedly reduced with 
increased concentrations of
chlorine up to 50 ppm, but a further increase in concentration up to 200
ppm did not have an additional substantial effect. A standard procedure for
washing lettuce leaves in tap water was reported to remove 92.4% of the
microflora (31). Including 100 ppm available free chlorine in wash water
reduced the count by 97.8%. Adjusting the pH from 9 to 4.5 to 5.0 with
inorganic and organic acids resulted in a 1.5- to 4.0-fold increase in
microbicidal effect. Increasing the washing time in hypochlorite solution
from 5 to 30 minutes did not decrease microbial levels further, whereas
extended washing in tap water produced a reduction comparable to
hypochlorite. The addition of 100 ppm of a surfactant (Tween 80) to a
hypochlorite washing solution enhanced lethality but adversely affected
sensory qualities of lettuce.

Dipping Brussels sprouts into chlorine solution (200 ppm) for 10 seconds
decreased the number of viable L. monocytogenes cells by about 2 log
(subscript 10) CFU/g (32). The maximum log(subscript 10) reduction of L.
monocytogenes on shredded lettuce and cabbage treated with 200 ppm
chlorine for 10 minutes was 1.3 to 1.7 log(subscript 10) CFU/g and 0.9 to
1.2 log(subscript 10) CFU/g, respectively (12). Numbers
decreased only marginally with increased exposure time from 1 to 10
minutes, which agrees with observations made by Brackett (32) that the
action of chlorine against L. monocytogenes occurs primarily during the
first 30 seconds of exposure. Nguyen-the and Carlin (33) concluded that the
elimination of L. monocytogenes from the surface of vegetables by chlorine
is unpredictable and limited.

Populations of Salmonella Montevideo on the surface and in the stem core
tissue of tomatoes can be substantially reduced by dipping fruits 2 minutes
in a solution containing 60 or 110 ppm chlorine, respectively; however,
treatment in a solution containing 320 ppm chlorine does not result in
complete inactivation (34). The ineffectiveness of 100 ppm chlorine against
S. Montevideo injected into cracks in the skin of mature green tomatoes was
demonstrated by Wei et al. (35). Treatment of alfalfa seeds injected with
Salmonella Stanley (10(superscript 2) to 10(superscript 3) CFU/g) in
100 ppm chlorine solution for 10 minutes has been reported to cause
a substantial reduction in population, and treatment in 290
ppm chlorine solution resulted in a substantial
reduction compared with treatment with 100 ppm chlorine (36). Initial free
chlorine concentrations up to 1,000 ppm, however, did not result in further
reductions. Treatment of seeds containing 10(superscript 1) to
10(superscript 2) CFU/g of S. Stanley for 5 minutes in a solution
containing 2,040 ppm chlorine reduced the
population to less than 1 CFU/g.

We have studied the efficacy of chlorine, hydrogen peroxide, and ethanol in
removing Salmonella from injected alfalfa sprouts. Sprouts were dipped in
solutions containing 200, 500, or 2,000 ppm chlorine for 2 minutes. The
pathogen was reduced by about 2 log(subscript 10) CFU/g after treatment
with 500 ppm chlorine, compared with the control, and to an undetectable
level (<1 CFU/g) after treatment with 2,000 ppm chlorine (Figure 2).
Chlorine treatment (2,000 ppm) of cantaloupe cubes injected with the same
five-serotype cocktail of Salmonella resulted in less than 1 log10
reduction in viable cells (Figure 2). The very high level of organic matter
in the juice released from cut cantaloupe tissue apparently neutralizes the
chlorine before its lethality can be manifested.

[Figure not available in ASCII.]

As noted by Lund (37), the inaccessibility of chlorine to microbial cells
in crevices, creases, pockets, and natural openings in the skin also
undoubtedly contributes to the overall lack of effectiveness of chlorine in
killing pathogens. The hydrophobic nature of the waxy cuticle on tissue
surfaces protects surface contaminants from exposure to chlorine and other
produce sanitizers that do not penetrate or dissolve these waxes.
Surface-active agents lessen the hydrophobicity of fruit and vegetable
skins as well as the surfaces of edible leaves, stems, and flowers, but
they may also cause deterioration of sensory qualities (31,38). Sanitizers
that contain a solvent that would remove the waxy cuticle layer, and with
it enmeshed contaminants, without adversely affecting sensory
characteristics would hold greater potential than chlorinated water in
reducing microbial populations on whole raw produce. Such sanitizers may be
limited to use on produce that will be further processed into juice or cut
products, or on whole fruits, vegetables, or plant parts destined for
immediate consumption, since their application could adversely affect
visual appearance. Clearly, chlorine, at concentrations currently permitted
for use by the industry to wash fresh fruits and vegetables, cannot be
relied upon to eliminate pathogens.

Address for correspondence: Larry R. Beuchat, Center for Food Safety and
Quality Enhancement, University of Georgia, Griffin, Georgia 30223 USA;
fax: 770-229-3216; e-mail: lbeucha@cfsqe.griffin.peachnet.edu.

References

  1. Hedberg CW, MacDonald KL, Osterholm MT. Changing epidemiology of
     food-borne disease: a Minnesota perspective. Clin Infect Dis
     1994;18:671-82.
  2. United States Department of Agriculture. Vegetables and
     specialties/VGS-269/July. Fruits and tree nuts/FTS-278/October.
     Washington (DC): USDA Economic Research Service; 1996. p. 22,81.
  3. Beuchat LR. Pathogenic microorganisms associated with fresh produce.
     Journal of Food Protection 1996;59:204-6.
  4. Ercolani GL. Occurrence and persistence of culturable clostridial
     spores on the leaves of horticultural plants. Journal of Applied
     Microbiology 1997;82:137-40.
  5. Beuchat LR. Listeria monocytogenes: incidence in vegetables. Food
     Control 1996;7:223-8.
  6. Welshimer HJ. Survival of Listeria monocytogenes in soil. J Bacteriol
     1960;80:316-20.
  7. Welshimer HJ, Donker-Voet J. Listeria monocytogenes in nature. Appl
     Environ Microbiol 1971;21:516-9.
  8. MacGowan AP, Bowker K, McLauchlin J, Bennett PM, Reeves DS. The
     occurrence and seasonal changes in the isolation of Listeria spp. in
     shop bought food stuffs, human feces, sewage and soil from urban
     sources. Int J Food Microbiol 1994;21:325-34.
  9. Welshimer HJ. Isolation of Listeria monocytogenes from vegetation. J
     Bacteriol 1968;95:300-3.
 10. Weiss J, Seeliger HPR. Incidence of Listeria monocytogenes in nature.
     Appl Microbiol 1975; 29:29-32.
 11. Bagdasargan GA. Survival of viruses of the enterovirus group
     (poliomyelitis, ECHO, Coxsackie) in soil and on vegetables. Journal of
     Hygiene, Epidemiology, Microbiology and Immunology 1964;8:497-505.
 12. Badaway AS, Gerba CP, Kelly LM. Survival of rotavirus SA-11 on
     vegetables. Food Microbiology 1985;2:199-205.
 13. Watkins J, Sleath KP. Isolation and enumeration of Listeria
     monocytogenes from sewage sludge and river water. J Appl Bacteriol
     1981;50:1-9.
 14. Al-Ghazali MR, Al-Azawi SK. Detection and enumeration of Listeria
     monocytogenes in a sewage treatment plant in Iraq. J Appl Bacteriol
     1986;60:251-4.
 15. Rudolfs W, Falk LL, Ragotzkie RA. Contamination of vegetables grown in
     polluted soil. III. Field studies on Ascaris eggs. Sewage and
     Industrial Waste 1951;23:656-60.
 16. Rudolfs W, Falk LL, Ragotzkie RA. Contamination of vegetables grown in
     polluted soil. II. Field and water studies on Endamoeba cysts. Sewage
     and Industrial Waste 1951;23:478-85.
 17. Wang G, Zhao R, Doyle MP. Fate of enterohemorrhagic Escherichia coli
     O157:H7 in bovine feces. Appl Environ Microbiol 1996;62:2567-70.
 18. Dunlop SG, Wang W-LL. Studies on the use of sewage effluent for
     irrigation of truck crops. Journal of Food Protection 1961;24:44-7.
 19. Barbier D, Perrine D, Duhamel C, Doublet R, Georges P. Parasitic
     hazard with sewage sludge applied to land. Appl Environ Microbiol
     1990;56:1420-2.
 20. Millard PG, Gensheimer KF, Addiss DG, Sosin DM, Beckett GA,
     Houck-Jankoski A, Hudson A. An outbreak of cryptosporidiosis from
     fresh-pressed apple cider. JAMA 1994;272:1592-6.
 21. Luechtefeld N, Blaser M, Reller L, Wang W. Isolation of Campylobacter
     fetus subsp. jejuni from migratory wildfowl. J Clin Microbiol
     1980;12:406-8.
 22. Jones F, Smith P, Watson DC. Pollution of a water supply catchment by
     breeding gulls and the potential environmental health implications.
     Journal of the Institute of Water and Engineering Science
     1978;32:469-82.
 23. Lee JV, Basford D, Donovan T, Furniss A, West D. The incidence of
     Vibrio cholerae in water, animals and birds in Kent, England. J Appl
     Bacteriol 1982;52:281-91.
 24. Fenlow DR. Wild birds and silage as reservoirs of Listeria in the
     agricultural environment. J Appl Bacteriol 1985;59:537-44.
 25. Wallace JS, Cheasty T, Jones K. Isolation of Vero cytotoxin-producing
     Escherichia coli O157:H7 from wild birds. J Appl Microbiol
     1997;82:399-404.
 26. Barmore CR. Chlorineare there alternatives? Cutting Edge 1995; Spr
     1995; 4-5.
 27. Cords BR, Dychdala GR. Sanitizers: halogens, surface-active agents and
     peroxides. In: Davidson PM, Branen AL, editors. Antimicrobials in
     Foods. 2nd ed. New York: Marcel Dekker, Inc.; 1993. p. 469-537.
 28. Eckert JW, Ogawa JM. The chemical control of postharvest diseases:
     deciduous fruits, berries, vegetables and root/tubers crops. Annu Rev
     Phytopathol 1988;26:433-63.
 29. Mazollier J. Iv gamme. Lavage-desinfection des salades. Infros-Crifl
     1988;41:19.
 30. Adams MR, Hartley AD, Cox LJ. Factors affecting the efficiency of
     washing procedures used in the production of prepared salads. Food
     Microbiology 1989;6:69-77.
 31. Brackett RE. Antimicrobial effect of chlorine on Listeria
     monocytogenes. Journal of Food Protection 1987;50:999-1003.
 32. Nguyen-the C, Carlin F. The microbiology of minimally processed fresh
     fruits and vegetables. Crit Rev Food Sci Nutr 1994;34:371-401.
 33. Zhuang R-Y, Beuchat LR, Angulo FJ. Fate of Salmonella montevideo on
     and in raw tomatoes as affected by temperature and treatment with
     chlorine. Appl Environ Microbiol 1995;61:2127-31.
 34. Wei CI, Huang TS, Kim JM, Lin WF, Tamplin ML, Bartz JA. Growth and
     survival of Salmonella montevideo on tomatoes and disinfection with
     chlorinated water. Journal of Food Protection 1995;58:829-36.
 35. Jaquette CB, Beuchat LR, Mahon BE. Efficacy of chlorine and heat
     treatment in killing Salmonella stanley inoculated onto alfalfa seeds
     and growth and survival of the pathogen during sprouting and storage.
     Appl Environ Microbiol 1996;62:2212-5.
 36. Lund BM. Bacterial spoilage. In: Dennis C, editor. Post-harvest
     pathology of fruits and vegetables. London: Academic Press; 1983. p.
     219.
 37. Zhang S, Farber JM. The effects of various disinfectants against
     Listeria monocytogenes on fresh-cut vegetables. Food Microbiology
     1996;13:311-21.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/beuchat.txt

The Impact of Consumer Demands and Trends on Food Processing

Don L. Zink
Nestl USA, Inc., Glendale, California, USA
---------------------------------------------------------------------------
      In the United States, consumer demand for new foods and changes
      in eating habits and food safety risks are affecting the food
      processing industry. The population is becoming older on
      average; moreover, consumers want fresh and minimally processed
      foods without synthetic chemical preservatives. To address the
      need for safer food and compete for consumer acceptance,
      manufacturers are exploring new food processing and
      preservation methods.

Fresh, Preservative-Free Foods That Promote Health

Food industry marketers perceive that consumers want foods that are
convenient; fresh (less-processed and less-packaged); all natural-with no
preservatives (a so-called "clean label"); without a perceived negative
(i.e., foods without high fat, high salt, and high sugar); and healthy. The
industry perception is that consumers want foods that not only cause no
harm but also remedy ailments from heart disease, osteoporosis, and fatigue
to memory loss. Categories of foods that promote health are fortified
foods, performance-enhancing food additives, probiotics, and prebiotics.

Food fortification is an old process. Milk (with vitamins A and D), bread
(with iron and niacin), or salt (with iodine) have long been fortified to
replace nutrients thought to be lost during processing. Newer foods
fortified with nutrients needed by the body to stave off the progression of
diseases associated with aging or enhance physical performance attract the
consumer's attention and sell well in today's marketplace. For example,
marketers are promoting all sorts of foods fortified with calcium to women
concerned about osteoporosis. Performance-enhancing foods are popular. Such
foods range from beverages to replace electrolytes and prolong physical
endurance to amino acids and fatty acids to improve alertness and memory.
Probiotics and prebiotics are two paths to the same result. Research
studies suggest that a desirable intestinal microflora causes the host to
be less susceptible to intestinal pathogens. Probiotics create this
desirable state by incorporating the microorganism directly into the food,
either as a stable culture or as part of food fermentation. This process is
costly, and the microorganisms often do not survive well in the food. Thus,
manufacturers must add 10 to 100 times the needed number of microorganisms
to account for a loss of viability during the product's normal shelf life.
Prebiotics overcomes the limitations of probiotics by adding specific
nutrients, usually a particular carbohydrate, to the food. When ingested as
part of the diet, these specific nutrients "select" for a beneficial
microflora in the intestinal tract.

Food Processing and Food Product Development

The consumer's quest for health is having a great impact on the food
processor. Compared with the marketplace of 25 years ago, today's
marketplace has more perishable products, including fruits and vegetables,
and more innovative packaging. In addition, consumer aversion to
traditional chemical preservatives has left food processors with less
flexibility in choosing preservation methods. To find a technologic edge in
the marketplace, food processors are exploring new processing and
preservation technologies. Some of these technologies include ohmic
heating, high-pressure, pulsed electric field, bright light, and aseptic
processing. Ohmic heating involves passing an electric current through the
food to create heat due to electrical resistance within the food. With
ohmic heating, food particles heat at the same rate as the carrier medium
or sauce. Ohmic heating can enhance food quality by limiting heat damage to
the sauce and food particles. High-pressure processing uses very high
pressure, often thousands of atmospheres, to pasteurize foods without heat.
This technology is ideal for heat-sensitive foods, but some enzymes are
difficult to inactivate with high-pressure processing. Pulsed electric
field processing uses a very strong pulsed electric current to disrupt
microbial cells and pasteurize foods with little or no heating. Bright
light processing uses an intense white light to kill bacteria on the
surface of foods; this light does not penetrate deeply into foods and can
only be used for surface pasteurization.

Aseptic processing dates back to at least the mid-1940s but has yet to
realize its full potential. The most widely used of these new technologies,
aseptic processing involves sterilizing a food product in a continuous
process through a heat exchanger and then filling that food in an aseptic
filler. The aseptic filler is a highly specialized piece of equipment
designed to sterilize the packaging material, fill the sterile product into
its container in a sterile environment, and then seal the package.

Food processors have also explored novel food preservation systems. An
ideal food preservative would come from a natural source and preserve food
without being labeled a synthetic chemical preservative. Such preservatives
include bacteriocins, dimethyl dicarbonate (Velcorin), competitive
microbial inhibition, controlled and modified atmospheres, and irradiation.
Bacteriocins are not new; however, like nisin, they are now being used to
extend shelf life and enhance the safety of a variety of food products. The
use of bacteriocins is likely to be expanded in the future. Dimethyl
dicarbonate, a relatively new preservative used in beverages such as wine,
tea, and juices, is particularly effective in preventing spoilage caused by
yeasts. Competitive microbial inhibition relies on the fact that many
harmless bacteria, notably lactic acid bacteria, can inhibit the growth of
both spoilage bacteria and pathogens. Inhibitory strains of lactic acid
bacteria can be selected for use in dairy cultures or be added to
refrigerated foods to extend shelf life and enhance safety. Modified and
controlled atmosphere packaging are already widely used by the food
industry. They have the potential for even wider use, particularly with
fresh fruits and vegetables sold at retail. These methods rely on
inhibiting microbial growth by excluding oxygen or by inhibitory
concentrations of carbon dioxide. Carefully selected gas mixtures can also
delay the ripening of certain fruits and vegetables and extend the shelf
life of fresh meats. Finally, irradiation, also not a new technology, is
poised for widespread use to enhance the safety and shelf life of many
foods. With proper controls, irradiation could be a valuable means of
reducing Salmonella contamination of poultry and Escherichia coli O157:H7
contamination of ground beef.

A Scientist's View of Consumer Trends

One of the most obvious consumer trends is a dramatic increase in the
consumption of fresh foods, particularly fruits and vegetables. This
increase is the result of the well-publicized value of a high-fiber diet
and betacarotenes as an aid in preventing colon cancer. The number of meals
eaten away from home has increased dramatically. The trend toward dining
outside the home is likely rooted in lifestyle changes such as households
with two working parents. The number of home-delivered meals, the ultimate
convenience food, has also increased, even though the most popular foods
consumed today (pizza and hamburgers) are generally the same as those of 20
years ago. This indicates that the types of foods consumed do not change
rapidly, but the way these foods are consumed has changed. Finally, the
population is getting older on average. Aging may not be a consumer trend,
but it has a profound effect on food safety considerations. An older
population means a more susceptible population.

New food processing and preservation technologies and wider applications of
older technologies have, for the most part, had little impact on most
processed foods. Adoption of new technologies will likely continue at a
slow pace. Consumers consistently buy foods on the basis of value and
taste, not processing technology. Technologies that add value will be the
first to gain consumer acceptance. The demand for convenience foods will
probably increase. Demands on our time are increasing, and we have less
time to spend on food preparation, and more meals will be eaten away from
home, in part because of convenience but also because of a trend for new
tastes and variety in the diet. Finally, the trend toward foods that claim
to enhance performance, rooted in an aging population's need for better
health during longer life-spans, will continue. With increased demand, the
pressure on the food industry for better processing and preservation
methods will also increase and may result in safer food.

Address for correspondence: Don L. Zink, Nestl USA, Inc. 800 N. Brand
Blvd., Glendale, CA 91203, USA; fax: 818-549-6908; e-mail:
donald.zink@us.nestle.com.
---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/zink.txt

Impact of Changing Consumer Lifestyles on the Emergence/Reemergence of
Foodborne Pathogens

Janet E. Collins
American Meat Institute, Arlington, Virginia, USA
---------------------------------------------------------------------------

      Foodborne illness of microbial origin is the most serious food
      safety problem in the United States. The Centers for Disease
      Control and Prevention reports that 79% of outbreaks between
      1987 and 1992 were bacterial; improper holding temperature and
      poor personal hygiene of food handlers contributed most to
      disease incidence. Some microbes have demonstrated resistance
      to standard methods of preparation and storage of foods.
      Nonetheless, food safety and public health officials attribute
      a rise in incidence of foodborne illness to changes in
      demographics and consumer lifestyles that affect the way food
      is prepared and stored. Food editors report that fewer than 50%
      of consumers are concerned about food safety. An American Meat
      Institute (1996) study details lifestyle changes affecting food
      behavior, including an increasing number of women in the
      workforce, limited commitment to food preparation, and a
      greater number of single heads of households. Consumers appear
      to be more interested in convenience and saving time than in
      proper food handling and preparation.

Reporting of foodborne and waterborne diseases in the United States began
more than 50 years ago (1). At that time, state and territorial health
offices were concerned about the levels of morbidity and mortality caused
by typhoid fever and infantile diarrhea; cases were to be investigated and
reported. The underlying purpose of reporting was to obtain information
regarding the role of food, milk, and water in outbreaks of intestinal
illness to provide a basis for public health action.

In 1923, the Public Health Service began publishing summaries of outbreaks
of gastrointestinal illness attributed to milk; in 1938, it added summaries
of outbreaks due to any foods. In 1966, the present system of surveillance
of foodborne and waterborne diseases began to incorporate into an annual
summary all reports of enteric disease outbreaks attributed to microbial or
chemical contamination of food or water. Comprehensive surveillance should
result in greater awareness of the most important food-protection methods.

Between 1983 and 1987, the etiologic agent in foodborne disease outbreaks
was not determined in 62% of the outbreaks (2); between 1988 and 1992, the
foodborne disease was of unknown etiology in 59% of the outbreaks (1).
Bacterial pathogens caused the largest percentage of outbreaks (79%) when
etiology was known-Salmonella caused 69% of bacterial outbreaks. For each
year from 1983 through 1992, the most commonly reported food preparation
practice that contributed to foodborne disease concerned improper holding
or storage temperatures. The second most common practice was poor personal
hygiene of the food handler. Food from unsafe sources was the least
commonly reported factor in each of the 10 years of reporting. It is now
time to examine food handling and determine how to reverse the trend.

Foodborne disease surveillance has traditionally served three purposes. The
first is disease prevention and control. Prevention and control measures
include early identification and removal of contaminated products from the
commercial market and correction of faulty food-preparation practices in
both food-service establishments and the home. Surveillance also provides
knowledge of disease causation. The responsible pathogen is not identified
in more than half of the foodborne disease outbreaks for various reasons,
including late initiation of laboratory investigation, inability of
available technology to identify the pathogen, and lack of identification
of the pathogen with a particular food. Finally, surveillance assists in
administrative guidance. Information enables assessment of trends in
prevalence of outbreaks caused by specific etiologic agents and in vehicles
of transmission. This information assists in identifying common errors in
food handling. In July 1995, the Centers for Disease Control and Prevention
(CDC), Food and Drug Administration (FDA), and Food Safety and Inspection
Service (FSIS) began a comprehensive effort to track major bacterial
pathogens that cause foodborne illnesses (3). CDC provides the overall
management and coordination with state health departments in the five
survey sites of the FSIS/CDC/FDA Sentinel Site Study. The program actively
seeks out specific cases of foodborne illness to identify whether a food
was of concern and to better establish frequency and source of foodborne
disease outbreaks and cases. CDC will use the data to identify emerging
foodborne pathogens and monitor incidence of foodborne illness; FSIS will
use the data to evaluate the effectiveness of new food-safety programs and
regulations to reduce foodborne pathogens in meat and poultry; FDA will use
the data to evaluate its efforts to reduce foodborne pathogens in seafood,
dairy products, fruits, and vegetables.

According to a recent report to Congressional committees (4), experts
believe that the risk for foodborne illness is increasing. The food supply
is changing in ways that can promote foodborne illness, and there are no
comprehensive data to explain at what point pathogens are introduced into
food. Further, because of demographic changes, more people are at a greater
risk of contracting a foodborne illness.

According to Ollinger-Snyder and Matthews (5), changes in agricultural
practices, a growing population susceptible to infectious diseases,
lifestyle changes, the emergence of new foodborne pathogens, and the high
turnover rate reported for workers in the food-service industry indicate
that new approaches are needed to allay consumers' fears and to prevent the
spread of foodborne disease in the United States. They recommend
implementation of Hazard Analysis and Critical Control Points (HACCP)
systems and certification of food-service managers.

The food processing industries are developing and implementing HACCP
systems; the meat and poultry industries are mandated to do so beginning in
January 1998 (6). Hazard analysis has been defined as the identification of
sensitive ingredients, critical processing points, and human factors that
affect product safety. Critical control points have been described as
processing determinants whose loss of control would result in an
unacceptable food-safety risk.

Most contend that the HACCP system approach must be implemented at each
stage of the farm-to-family continuum. Where are the critical control
points and the HACCP system development in the home, food-service or retail
establishments, or the car when food is carried from one location to
another? The consumer is a complex and critical control point in the
process.

Take the case of the barbecued chicken served to 260 guests at an outdoor
barbecue in 1983. Guests were served chicken that was parboiled in the
morning by one set of cooks and then placed in a large container and
refrigerated. The evening cooks assumed the chicken had been adequately
cooked, so they basted it in barbecue sauce and warmed it over the fire.
Some 71% of the guests got sick from the chicken that was insufficiently
cooked and improperly held (5). What of the infected bakery worker who
stirred a vat full of buttercream frosting with a bare hand and arm? Some
5,000 cases of viral gastroenteritis were caused by the infected worker who
claimed he had washed his hands. Other more recent outbreaks (7) appear in
Table 1.

Table 1. Foodborne illness reports from restaurants, 1996
------------------------------------------------------------
     Date      Description                  Cause

------------------------------------------------------------
                                            Employees
                                            did not
     6/96      Salmonella-contaminated      wash hands
               food, 38 cases               before
                                            handling
                                            food

               Escherichia                  Raw food
     9/95      coli O157:H7                  cross-contaminated
               "beef," 11 cases             other

               Salmonella Newport           Raw meat on
     8/95      "chicken," >850              cuting
               cases                        board with
                                            vegetables

               Hepatitis A,                 Human fecal
     1/95      contaminated food,           matter from
               95 cases                     handling-handwashing

               Salmonella,                  Holding
     8/94      hollandaise sauce,           temperature
               56 cases                     too low for
                                            9 hours

               Clostridium                  Unrefrigerated
     1/93      botulinum, canned            storage of
               cheese sauce, 7              opened
               cases, 1 death               container

------------------------------------------------------------

Source: Center for Science in the Public Interest, 1996

Recent data (1) indicate that 80% of reported foodborne illness outbreaks
occur outside the home. Even though illnesses would be expected to be
reported more often when they occur as a result of eating in restaurants,
the numbers are large. National standards for restaurant safety are
contained in the Food Code (8). FDA has the legal authority to impose the
standards on state and local jurisdictions. The Food Code, which is updated
every 2 years, includes temperatures for cooking, cooling, refrigeration,
reheating, and holding food in food-service establishments. County or city
employees are generally charged with responsibility for inspecting
restaurants; each state or locality has its own laws governing restaurant
safety.

Food service outside the home is big business, with sales of more than $300
billion (9) and nearly 10 million employees. The restaurant industry's
share of the food dollar is 43%, and the typical consumer more than 8 years
of age had more than four meals per week away from home in 1996. Given
those statistics, it is clear that food-service establishments play a
critical role in food safety. The Center for Science in the Public Interest
(7) conducted a survey of 45 agencies across the country to determine if
state and local agencies were enforcing 12 key food-safety standards in the
FDA Food Code. The standards chosen for the study affect consumer health
and safety and include such areas as food cooking and refrigeration
temperatures, frequency of inspections, and consumer warnings for raw
foods. Not one of the 45 agencies surveyed was following all of the Food
Code recommendations.

In the survey, only 13% of agencies enforced the Food Code and recommended
cooking temperatures for pork, eggs, fish, and poultry; only 64% of
agencies required hamburgers to be cooked to 155F. Recommendations for
cooling cooked food were followed by only 20% of the agencies, and only 11%
required refrigeration of food at FDA-recommended temperatures.

Every restaurant can take steps to ensure the safety of the food it
prepares and serves to its customers. Continuous employee training and
institution of HACCP-type systems should assist restaurants and other
food-service institutions in improving their food-safety records. Programs
available through the national restaurant trade organization could assist
even the smallest establishments in achieving food-safety goals.

For more than 25 years, the Food Marketing Institute (10) has surveyed
consumers about their changing needs and priorities in food attitudes and
behavior. The 1996 trends report has an expanded focus on the primary
grocery store or supermarket, including questions to help retailers learn
more about take-out foods (Table 2). In 1996, nearly 40% of the 2,000
shoppers surveyed purchased fresh deli items from their primary supermarket
at least once per week, and more than 10% reported purchasing ready-to-eat
take-out foods as frequently. Three-fourths of these shoppers purchased
food from the deli at least once per month, and half bought take-out food
from the supermarket as often.

Table 2. Sources of take-out food (%)

---------------------------------------------------------------------------
Source                            86 87  88  89 90 91  92 93 94 95 96

---------------------------------------------------------------------------
Fast-food restaurant              43 44  41  41 46 51  55 46 46 41 48
Restaurant                        38 33  38  33 27 23  24 27 25 22 25
Supermarket                       10 9   11  12 14 14  12 15 15 17 12
Deli/pizza parlor/bagel
shop/coffee shop/ donut shop      *  *   *   *  *  *   *  *  *  8  4
Gourmet or specialty shop         *  *   *   *  *  *   *  *  *  *  3
Convenience store                 *  *   *   *  2  2   2  2  2  2  1
Some other place                  2  7   3   6  2  1   *  1  1  1  2
It varies                         1  1   1   3  4  5   3  5  3  2  0
Don't eat out                     *  *   *   7  6  4   4  4  4  3  2
Not sure                          3  3   4   1  *  1   1  1  2  3  2
---------------------------------------------------------------------------

*Data not collected for this year.
Source: Food Marketing Institute, 1996

According to the survey, fast-food restaurants dominate (48%) all food
outlets as the primary source of take-out food; only 12% purchase take-out
foods from the supermarket. A recent article in Food Processing Magazine
(11) states, "Somewhere on their way to the supermarket, consumers have
been getting lost." Home-meal replacement, ready-made meals approximating
what Mom used to make, have begun to rapidly compete for the food dollars
of time-pressed consumers. According to Hollingsworth (12), consumers are
eating more meals at home, but they are not cooking more. Consumers want to
get food in a take-out location and go home to eat it (Figure).

[Figure not available in ASCII]

These take-out or eat-at-home foods have built-in food-safety hazards.
Consumers are time-pressed, and they are buying these foods. Are they
treating them as perishable? The U.S. Department of Agriculture (13) has
expressed concerns about these foods; they say that take-out foods need to
be handled with care. Hot foods need to be picked up or received hot and
eaten within 2 hours. If eaten later, hot foods should be divided into
shallow containers, covered loosely, and refrigerated immediately.

Are consumers ready for all of this food handling? Most consumers are
confident that the food they purchase is safe to eat (10). Spoilage of
foods is considered the greatest threat to food safety by the largest group
(49%) of respondents. They count on freshness and expiration dates (22%)
and increasingly see bacteria and contamination as threats (17%). It is
interesting to note, however, that between 1992 and 1996 these shoppers
were less likely (15% vs. 7%) to see spoilage as a threat; similarly,
processing and preparation of foods was less an issue in 1996 than in 1992
(8% vs. 10%).

Consumers are concerned about handling of foods by other shoppers and by
supermarket employees. Consumers rely increasingly on food stores (16%),
manufacturers (21%), government (21%), and themselves (25%) for food-safety
protection. Consumers apparently are willing to share responsibility for
food safety with others, but they want to know that steps are taken during
the processing and distribution of foods to reduce the likelihood of
pathogen or other bacterial contamination.

According to Technomics (14), these supermarket issues noted in the Food
Market Institute trends data (10) will be shared with food-servic

e
operators as the share of consumer food expenditures changes from 51% vs.
49%, 48% vs. 52%, and 45% vs. 55% (projected) for retail expenditures
versus food-service expenditures in 1991, 1996, and 2001, respectively.

The number of households earning more than $75,000 annually continues to
grow, and these households exhibit the highest levels of spending on food
service. Consumer demands are changing the way that food-service operators
and suppliers of food services must react. The area of convenience, highly
prized by consumers today, has profound implications for food. Consumers
want fast service with easy-to-eat foods and no stress, which means a far
greater emphasis on portable foods.

Technology has conditioned us to demand and receive near-immediate
satisfaction. There will be even greater emphasis on faster service,
meaning more emphasis on convenient food formats to expedite preparation.
Packaging and storage will greatly affect product quality and safety.
According to Technomics (14) packaging will need to be temperature-tolerant
and breathable. Preparation and processing technologies will need to have
greater ability to rapidly cool and chill. And then there is the
food-safety concern associated with dispensing equipment. Food will be
required to have an extended shelflife. The safety factors associated with
these new formats will also change.

Consumers want easy access to portable foods. Accessibility to variety in
food options translates to a proliferation in nontraditional locations.
These smaller sites may include back-of-house preparation facilities. This
easy access to smaller operations also suggests a need for more of such
operations and more variety in menu options. While to the consumer this may
translate to upscale menus with indulgence foods such as new and different
bakery items, microbrewery beverages, and gourmet coffees, to the
food-service operator it may mean greater cross-contamination with cream
fillings, unpasteurized fermented drinks, and spoiled milk. New menu
options create new challenges for service and for safety.

According to Steve Harrison, brewmaster of the Sierra Nevada Brewing
Company (Chico, CA), "The concept that a beer will automatically go bad in
`X' number of days is a very untrue one." Consumers do not know that. What
is "skunky beer"? Starting in late 1996, Anheuser-Busch began a freshness
strategy in their advertising. Other large brewers are catching on, so
freshness is associated with quality and safety. Imagine freshness dating,
"born-on dating," as a quality parameter in brewing.

Consumers' increased emphasis on food-safety issues directly affects food
service. The perceived healthfulness and quality of foods affects food
sales; the increasing considerations of cleanliness as healthfulness and
quality as safety become even greater shared responsibilities as
food-service operators take over the roles historically associated with
home kitchens. "On-the-spot exhibition" cooking is of increasing interest
to today's consumers.

In June 1996, the Food Marketing Institute (15) published a review of
foodborne illness. They note that the organisms that cause foodborne
illnesses are found throughout nature and that mishandling and poor
refrigeration are responsible for most contamination. The most common
causes are cross-contamination of cooked foods with raw foods, contaminated
utensils or serving plates, poor hygiene of food handlers, and time or
temperature abuse.

Agreement is widespread that the most serious food-safety problem is
foodborne illness of microbial origin (Table 3). Foodborne pathogens
include a wide array of microorganisms, which have various physiologic
effects on people, ranging from mild to severe, and are associated with a
wide array of foods. Cross-contamination and association of foods within
mixed dishes complicate environmental control. Further, some of the
microbes have evolved and become more resistant to food preparation and
storage techniques. Several industry and government publications
(1,2,8,15,16) summarize biologic hazards associated with foodborne illness.

Table 3. Sources of reported outbreaks with confirmed causes (%)
----------------------------------------------------
                      Restaurant   Other Known  Place
                      1983-  1988- 1983- 1988-  1996
                      87     92    87    92
----------------------------------------------------
Salmonella            50     60    46    58     30
Escherichia coli      <1     <1    2     1      5
Hepatitis A virus     6      7     3     4
Staphyloccus aureus   2      3     10    5
Campylobacter         1      2     6     3      45
Shigella              8      2     6     2      17
----------------------------------------------------

Sources: Centers for Disease Control and Prevention and U.S. Department of
Agriculture for 1996, the 1996 Sentinel Site Study

Mishandling can occur at any point in the food chain-in processing, at
supermarkets or restaurants, or in homes. Many food manufacturers and
retailers have HACCP plans in place, and over the next few years that
number will increase. Consumers, however, must assume responsibility for
the safety of food in the home. Proper preparation and sanitation methods
are key to preventing foodborne illness in the home as in other areas of
food handling. The messages for each of the segments of the food chain are
the same-keep it clean (e.g., wash your hands) and control the temperatures
(keep hot things hot and cold things cold) (Table 4).

Table 4. Pathogen control in foods to reduce foodborne illness
----------------------------------------------------
Pathogen                  Control mechanism
----------------------------------------------------

Campylobacter             Heat foods >= 140F
                          Proper handling

                          Rapid chilling <40F
Salmonella                Hot storage >140F
                          Cooking >165F

Escherichia coli O157:H7  Heat foods >155F
                          Avoid cross-contamination

Staphylococcus aureus     Rapid cooling <40F
                          Personal hygiene

Clostridium botulinum     Boil food 10-15 minutes

Clostridium perfringens   Refrigerate <40F
                          Proper handling

Listeria                  Pasteurization of milk
                          Adequate cooking

----------------------------------------------------

Source: Food Marketing Institute, 1996

For the food-service industry, a number of programs have been developed to
educate food handlers about food-related and personal behaviors that affect
the safety of foods. For example, the Food Marketing Institute (17) has a
Food Protection Certification Program for supermarket personnel to learn
about the FDA Food Code requirements regarding food handling and hygiene.
Similarly, the National Restaurant Association has developed a food-safety
program called Serve Safe, intended to educate food-service workers about
safe food handling.

Who or what teaches the average consumer about food safety? Common sense?
Family? Health and fitness magazines? In May 1996, the Food Marketing
Institute (17) conducted a series of consumer focus groups to establish the
importance of food safety to consumers and to identify barriers to
consumers' safe food purchase, handling, and preparation. They report that
how consumers manage food safety reflects years of conditioning,
observation, and reinforcement from mothers and grandmothers. In some
cases, the more often consumers shop, the less concerned they seem to be
about food safety when it comes to shopping, storage, and handling.
Consumers link safety to fresh food, and they assume that when they shop
more often, they purchase food in smaller quantities and food safety is
less an issue. Respondents in the study also tended to think that cooked
food was generally "safer" than raw food. For example, they believed that
recontamination of unrefrigerated food was less a problem with cooked than
with raw food.

Some safe food practices are observed for convenience, esthetics, or taste
rather than for food safety. Thawing meat is messy; covering food prevents
it from drying out; separating foods in the refrigerator is tidier. These
kinds of behavior improve safety, but consumers may not understand the
food-safety implications.

Overall, the consumers in the Food Marketing Institute study (17) find
food-safety messages generally are "common sense," "basic," "practical,"
and "believable." Messages about such subjects as the order in which to
choose foods in the supermarket, sell-by dates, storage and freezing of
products, ways of keeping hot foods hot and cold foods cold are not
considered too elementary. They also believe that storage times for food
safety do not apply equally across food groups; they do not understand
hazards from vegetables or fruits. Barriers to safe food-handling behavior
in this study included historical (and cultural) practices, feeling of
invulnerability, taste preferences, timing and planning, and space and
convenience.

A 1992 survey conducted at Cornell University and designed to assess
consumer food-safety awareness documented a substantial lack of knowledge
about safe home food preparation practices. Seventy-five percent of those
surveyed knew that Salmonella is associated with meat, poultry, and eggs,
but only 65% would refrigerate a roasted chicken breast immediately; 29%
would leave it on the kitchen counter until it reached room temperature.
Further, 18% said they would not be concerned or were not sure about the
safety of cooked meat left unrefrigerated for more than 4 hours; 14% said
the same for cooked poultry.

In April 1996, the American Meat Institute (16) commissioned a study of
1,000 adults in the United States. Compared with 98% of respondents in the
study who know that harmful bacteria can be present on meat and poultry
products, only 74% made the link to dairy products and eggs; two in five
respondents (43%) recognized that fruits and vegetables may contain harmful
bacteria. These conclusions could be drawn for consumers who responded to
the American Meat Institute (1996) questions. While the U.S. population is
growing (up 10% since 1980), households are becoming smaller. In the 1980s,
the number of households grew 17%, while the average household size
decreased from 2.8 to 2.6 persons. This shift in family size and the
increase in single heads of households has resulted in increased stress in
the family with less time for shopping and food preparation. In addition,
more women are in the workforce. Today, 70% of women ages 25 to 44 years
are in the workforce; 75% work full time. Therefore, no adult is likely to
be in the home for 70% of American households, and many children are
preparing food for themselves. Finally, consumers spend less time on food
preparation. More than 85% of employed women shop and cook, but most spend
less than 30 minutes preparing every meal and 20% spend less than 15
minutes. Consumers are using convenience foods and quick methods of food
preparation, including partially cooked foods that may require special
handling.

The study results provided further documentation that the risk for
foodborne illness is increasing, largely because of societal changes that
affect the way consumers purchase and prepare food. Contributing to this
are changes in the family structure, more women in the workforce, and less
available time for food preparation. Consumers in this study were not able
to correctly separate home preparation issues from food service, nor did
they know correct cooking temperatures to use in their own homes.

The ways in which consumers spread microorganisms to one another and to
themselves include more than just coughing and sneezing. Not washing hands
before, during, and after handling foods clearly contributes to the spread
of foodborne infections and intoxication. Hands can spread disease-causing
microbes to foods from other foods and from infected persons.

In a comprehensive review of 91 scientific articles published after 1986,
Bryan et al.(18) attempted to link hand washing and infections. They report
that hand washing has become an integral component of the tradition and
ritual of prevention practice for the spread of infection, but several
factors confound the ability to establish the effectiveness of hand washing
for reducing infectious disease. Hand-washing practices were shown to
significantly reduce infections transmitted by the fecal-oral route and in
situations of poor personal hygiene. Hand washing is clearly a critical
step in reducing personal contamination of food and cross-contamination
between foods. Hand washing is but one practice that could dramatically
affect risk, if not incidence, of foodborne disease.

According to data provided by the American Society for Microbiology (19),
people do not wash their hands as often as they think they do (Table 5). In
telephone surveys, 94% of respondents claim they always wash up after using
the rest room; however, researchers contend that almost one-third of people
do not wash their hands after using the bathroom. Of the more than 7,000
people nationwide who participated in the study, 81% said they wash their
hands before handling or eating food. However, most say they do not wash up
after petting an animal (48%), coughing or sneezing (33%), or handling
money (22%).

Table 5. American Society for Microbiology/Bayer handwashing survey, 1996
--------------------------------------------------------
                               What they    What they
Behavior/Location:             say(a) (%)     do(b) (%)

--------------------------------------------------------
Wash hands:
After using public restroom         94           68
     Women
                                                 74

     Men
                                                 61

New York (Penn Station)                          60
Chicago (Navy Pier)                              78
New Orleans (casino)                             71
San Francisco (Golden Gate
Park)                                            69
Atlanta (Braves game)                            64
     Women
                                                 89

     Men
                                                 46

--------------------------------------------------------

(a)1,004 adults; (b)6,330 adults
Source: American Society for Microbiology

In early 1997 (8), the U.S. Departments of Agriculture and Health and Human
Services and the U.S. Environmental Protection Agency developed a program
intended to coordinate a food-safety initiative among federal agencies,
immediately after an announcement by U.S. President Clinton (January 1997)
to promote an initiative designed to improve the safety of the nation's
food supply. The president charged the federal agencies to work with
consumers, producers, industry, states, tribes, universities, and the
public to identify ways to improve food safety through government and
private sector action, including public-private partnerships. The
interagency response is a multifaceted program designed to include
surveillance, coordination of activities within the various programs and
agencies, risk assessment, research, inspections, and education. The
underlying premise upon which this program was developed is that foodborne
infections remain a major public health problem. Further, sources of food
contamination are said to be almost as numerous and varied as the
contaminants; bacteria and other infectious organisms are pervasive in the
environment.

The current systems for protecting food in the United States include a
broad range of government agencies and industries, many of which have been
discussed in this paper. Responsibilities are shared among the U.S.
Department of Agriculture (Food Safety and Inspection Service and Animal
and Plant Health Inspection Service), the U.S. Department of Health and
Human Services (FDA and CDC), and the U.S. Environmental Protection Agency.
These respo
n
sibilities include oversight on the farm, in the processing
facilities, during transportation and distribution (including food from
foreign countries), and in food marketing channels including restaurants,
supermarkets, and institutional food services (such as schools and
hospitals).

Surveillance of foodborne illness outbreaks and their causes is a
responsibility of FDA and CDC. Education is shared among the agencies and
is not the primary concern or responsibility of any one of the agencies.
Pivotal to this new initiative is the element of education. Specifically,
the program is intended to reinvigorate education of all those involved in
food preparation, focusing on the use of safe practices. According to USDA
et al. (8), educating people about steps they must take to prevent and
control foodborne illness is a vital link in the food preparation chain. In
spite of the education efforts of the government, both state and federal,
consumer groups, and industry, which have occurred historically, foodborne
illness occurs from a lack of knowledge of the risks involved at all stages
of food preparation. Choices consumers make about how they handle food at
home and about eating food that increases the risk for illness can have an
important effect on foodborne disease incidence.

USDA et al. (8) will develop a program to improve consumer education;
retail, food service, and institutional education; veterinary and producer
education; and industry education in the transportation area. They propose
developing an alliance among industry, consumer groups, and governmental
agencies to mount a comprehensive food-safety awareness campaign for
consumers. Highly focused messages and tactics for the general public and
consumers at high risk will be developed. This thrust is in perfect harmony
with the strategies and tactics proposed by the American Meat Institute
(16) as an outcome to a series of studies and roundtable discussions held
with medical doctors, dietitians, educators, and others. The ability of
industry and consumer groups to work with the government in a program with
common themes and elements is critical to the positive outcome of the
effort. As one of the focus group members in the Food Marketing Institute
(17) said, the more often the message is repeated, the more likely is the
listener to hear it.

The broad-based approach to education, which includes data from
surveillance and inspections, should provide the foundation for changes in
consumer behavior. It is critical that consumers not only take
responsibility for their actions regarding food safety, but that they also
take seriously the learning that must occur for consumers of all ages to
prevent contamination, cross-contamination, and mishandling of foods at
home and in restaurants. Convenience, taste, and variety are welcome
qualities in foods that we enjoy; safety in foods is critical to the public
health and safety of consumers and to the government and businesses that
support those consumers.

Address for correspondence: Janet E. Collins, American Meat Institute 1700
N. Moore Street, Suite 1600, Arlington, VA 22209, USA; fax: 703-527-0938;
e-mail: ami@interramp.com.

References

  1. Centers for Disease Control and Prevention. Surveillance for foodborne
     disease outbreaks, United States, 1988-1992. MMWR CDC Surveill Summ
     1996;45:1-66.
  2. Centers for Disease Control. Foodborne disease outbreaks, 5-year
     summary, 1983-1987. MMWR CDC Surveill Summ 1990;39:15-57.
  3. Food Safety and Inspection Service. FSIS/CDC/FDA sentinel site study:
     the establishment and implementation of an active surveillance system
     for bacterial foodborne diseases in the United States. Washington
     (DC): USDA Report to Congress; 1997.
  4. United States General Accounting Office. Food safety: information on
     foodborne illnesses. Report to Congressional Committees. Washington
     (DC): GAO/RCED-96-96; 1996.
  5. Ollinger-Snyder P, Matthews ME. Food safety issues: press reports
     heighten consumer awareness of microbiological safety. Dairy, Food and
     Environmental Sanitation 1994;14:580.
  6. FSIS Pathogen Reduction/HACCP. Washington (DC): Federal Register 1996
     Jul 6.
  7. Center for Science in the Public Interest. Dine at your own risk: the
     failure of local agencies to adopt and enforce national food safety
     standards for restaurants. Washington (DC): The Center; 1996.
  8. United States Department of Agriculture, United States Department of
     Health and Human Services, United States Environmental Protection
     Agency. Food safety from farm to table: a new strategy for the 21st
     century. Discussion Draft. Washington (DC): United States Department
     of Agriculture; 1997.
  9. National Restaurant Association. In: 1997 Restaurant Industry Pocket
     Factbook. Washington (DC): The Association; 1997.
 10. Food Marketing Institute. Trends in the United States: consumer
     attitudes and the supermarket. Washington (DC): The Institute; 1996.
 11. Neff J. Will home meal replacement replace packaged foods? Food
     Processing 1996;57:35.
 12. Hollingsworth P. The changing role of fast food. Food Technology
     1997;51:24.
 13. United States Department of Agriculture, Food Safety and Inspection
     Service. Take out foods-handle with care. The Food Safety Educator
     1996;1:2.
 14. Management summary update: critical strategic issues 1996-2001.
     Chicago: Technomics, Inc.; 1997.
 15. Food Marketing Institute. Backgrounder: foodborne illness. Washington
     (DC): The Institute; 1996.
 16. American Meat Institute. Putting the food-handling issue on the table:
     the pressing need for food safety education. Washington (DC): American
     Meat Institute and Food Marketing Institute; 1996.
 17. Food Marketing Institute. Food safety: a qualitative analysis.
     Washington (DC): The Institute; 1996.
 18. Bryan JCL, Chorine J, Larson EL. Handwashing: a ritual revisited.
     Infection and Control in Critical Care 1995;7:617-24.
 19. American Society for Microbiology. Americans get caught dirty handed.
     Fact Sheets. Washington

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/collins.txt

Historical Overview of Key Issues in Food Safety

E. M. Foster
Madison, Wisconsin, USA
------------------------------------------------------------------------

      Foodborne transmission of pathogenic and toxigenic
      microorganisms has been a recognized hazard for decades. Even
      half a century ago we knew about the dangers of botulism from
      underprocessed canned foods; staphylococcal poisoning from
      unrefrigerated cream-filled pastries, sliced ham, meat, and
      poultry salads; and salmonellosis from infected animal
      products. Despite new protective measures, changes in
      preservation techniques and failure to follow recognized
      procedures have created new dangers. Moreover, we now recognize
      new organisms that can cause foodborne illness--Listeria
      monocytogenes, Escherichia coli O157:H7, Campylobacter jejuni,
      Vibrio parahaemolyticus, Yersinia enterocolitica, and others.
      Controlling these organisms will require widespread education
      and possibly new regulatory initiatives.

When I was growing up on my parents' farm in East Texas, we never thought
about food poisoning or unsafe food. The only foods we bought were sugar,
salt, flour, and oatmeal; everything else we produced and preserved on the
farm. My mother spent all summer canning fruits and vegetables for winter.
We had no refrigeration; we cured our own meat and drank raw milk. But I
never heard of botulism, staph poisoning, or salmonellosis or perfringens
poisoning until I studied bacteriology in college. Only then did I wonder
how we survived with no refrigeration in a hot climate. Finally, the answer
came to me. We just did not give the bacteria time enough to develop so
they could hurt us. Leftovers from breakfast--hot biscuits, eggs, ham, bacon
or sausage, oatmeal, coffee or milk--went right out to the chickens. Lunch
leftovers--biscuits, cornbread, vegetables, or fried chicken--were saved for
a cold supper 4 or 5 hours later. Any food left went to the pigs. The
bacteria had only a maximum of 3 or 4 hours to grow, and that usually is
not enough. I survived and went on to study food microbiology, which
included what was known then about food poisoning. The guru of food
poisoning in those days was professor Gail M. Dack at the University of
Chicago. Dr. Dack was a protege of Professor E.O. Jordan, who in 1917
published a 107-page book entitled Food Poisoning. Dr. Dack took over the
book and published his first version of Food Poisoning in 1943. In 1949 and
1956, subsequent editions appeared in which certain truisms became
apparent.

Botulism was considered a problem of canners, both home and commercial.
Thus, adequate heat processing would seem to solve the problem. Perhaps it
did for the canner, but now we know that heating will not eliminate all
botulism. Many foods, including salmon eggs, smoked fish, garlic in oil,
vacuum packaged lotus roots, and baked potatoes, can support growth and
botulinum toxin formation if the storage temperature is suitable.
Similarly, we thought staphylococcal poisoning was limited to cream-filled
pastries and cured ham. In recent years, outbreaks of staphylococcal
poisoning have been traced to cheese, whipped butter, ham salad, fermented
sausages, and canned corned beef. We now know how to prevent staphylococcal
poisoning, but not all food handlers understand and fully comply with the
appropriate control measures.

Salmonellosis was once thought a problem with meat from infected animals.
Now we know that a variety of food products can serve as vehicles of this
disease. As early as World War II, we found that dried eggs from the United
States could transmit this disease to our British allies. Thousands of
cases of human salmonellosis in the United States and other industrialized
countries have been transmitted by ice cream, chocolate, potato salad,
cheddar cheese, raw milk, black pepper, pate, aspic, ham, pasteurized milk,
and drinking water.

Clostridium perfringens, known since the 1940s, causes a problem only when
there is gross temperature abuse of cooked food. Clostridium botulinum,
Staphylococcus aureus, C. perfringens, and the salmonellae were well known
in Dr. Dack's day, although the food vehicles might have changed. Not so
well known were many of the organisms that preoccupy us today. For example,
we used to think of Escherichia coli as merely an indicator organism that
suggested insanitary handling. Now we know forms of E. coli can kill.
Thirty years ago, Listeria monocytogenes, Campylobacter jejuni, Aeromonas
hydrophila, Plesiomonas shigelloides, Vibrio parahaemolyticus, and Yersinia
enterocolitica were not known; now these are well-established foodborne
pathogens that we must control.

Although not part of a historical overview, other key issues deserve
attention during this meeting. For example, we once thought that fresh,
uncracked eggs were essentially sterile and safe to eat. We did not
recognize the ability of Salmonella Enteritidis to invade the laying hen
and thereby the yolk of an egg. An outbreak of S. Enteritidis at a Chicago
hotel taught us not to rely on the safety of eggs merely because the shell
was intact. S. Enteritidis in shell eggs is still a serious health problem
and a growing concern to egg and poultry producers.

Of equal, if not greater, concern is Salmonella Typhimurium strain DT 104.
Widely distributed in cattle herds of England, Scotland, and Wales, this
organism is resistant to several antibiotics, including ampicillin,
chloramphenicol, streptomycin, sulfonamides, and tetracycline. Between 1990
and 1995, the number of S. Typhimurium DT 104 isolated from humans in
Britain increased from 259 to 3,837 per year--a 15-fold increase. Moreover,
the percentage of drug-resistant isolates increased from 39% in 1990 to 97%
in 1995. S. Typhimurium DT 104 has been isolated in the United States from
sheep, pigs, horses, goats, emus, cats, dogs, elk, mice, coyotes, ground
squirrels, raccoons, chipmunks, and birds. American egg and poultry
producers are concerned about its entry into U.S. poultry flocks. S.
Typhimurium DT 104 infection in humans has been associated with the
consumption of chicken, sausage, and meat paste as well as with the
handling of sick animals. More than one-third of the patients have required
hospitalization, and 3% have died; these figures are very unusual for
ordinary Salmonella infections and indicate serious problems ahead.

Address for correspondence: E. M. Foster, Professor Emeritus, Food Research
Institute, University of Wisconsin, Madison, WI 53706 USA; fax:
608-263-1114.
------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

Quantitative Risk Assessment: An Emerging Tool for Emerging Foodborne
Pathogens

Anna M. Lammerding* and Greg M. Paoli
*Health Canada, Guelph, Ontario, Canada; and Decisionalysis Risk
Consultants, Ottawa, Ontario, Canada
---------------------------------------------------------------------------

      New challenges to the safety of the food supply require new
      strategies for evaluating and managing food safety risks.
      Changes in pathogens, food preparation, distribution, and
      consumption, and population immunity have the potential to
      adversely affect human health. Risk assessment offers a
      framework for predicting the impact of changes and trends on
      the provision of safe food. Risk assessment models facilitate
      the evaluation of active or passive changes in how foods are
      produced, processed, distributed, and consumed.

The changing epidemiology of foodborne diseases is a result of complex
interactions and changes in pathogens, foods, food distribution, food
consumption, and population immunity (1-3). Predicting the impact of a
trend in one part of the food continuum presupposes understanding of the
whole system. Aspects of the food processing and distribution system can
amplify or attenuate the trend as it grows into a potential health hazard.
While a full understanding of pathogen contamination, infection, and
survival is difficult, a systematic approach to assessing the impact of the
pathogen on health may improve the quality of public health decisions
(4,5).

Quantitative risk assessment is a possible approach for designing programs
to address emerging foodborne diseases. The use of risk assessment in
environmental toxicology illustrates the potential advantages of applying
quantitative risk assessment in a new field.

Risk Assessment Defined

The essence of microbial risk assessment is describing a system in which a
microbial hazard reaches its host and causes harm. Risk assessment consists
of four steps: hazard identification, exposure assessment, dose-response
assessment, and risk characterization (6). The knowledge in each step is
combined to represent a cause-and-effect chain from the prevalence and
concentration of the pathogen to the probability and magnitude of health
effects. In risk assessment, risk consists of both the probability and
impact of disease. In this way, risk reduction can be achieved in either
dimension-by reducing the probability of disease or by reducing its
severity.

Hazard Identification

In hazard identification, an association between disease and the presence
of a pathogen in a food is documented. The information may describe
conditions under which the pathogen survives, grows, causes infection, and
dies. Epidemiologic and surveillance data, challenge testing, and
scientific studies of pathogenicity also contribute information. Data
collected during hazard identification are later used in exposure
assessment, where the impact of processing, distribution, preparation, and
consumption of the food are incorporated.

Exposure Assessment

Exposure assessment describes the pathways through which a pathogen
population is introduced, distributed, and challenged in the production,
distribution, and consumption of food. This step differs from hazard
identification in that it describes a particular food-processing pathway.
Depending on the scope of the risk assessment, exposure assessment can
begin with pathogen prevalence in raw materials (e.g., a "farm-to-fork"
risk assessment), or it can begin with the description of the pathogen
population at subsequent steps (e.g., as input to a food-processing step).
In any case, the intent of risk assessment is to track the pathogen
population and estimate the likelihood of its being ingested by the
consumer. By completing the pathway to the consumer, we incorporate the
important issues of dose-response assessment.

Dose-Response Assessment

Dose-response assessment is used to translate the final exposure to a
pathogen population into a health response in the population of consumers.
This step is very difficult because of the shortage of data on
pathogen-specific responses and because those responses depend on the
immune status of the host (consumer). However, even limited knowledge of
the shape and boundaries of a dose-response function can be informative in
comparing the efficacy of alternate controls. The differences in response
among various susceptible populations are important features in this step
(7).

Risk Characterization

Risk characterization involves integrating the information gathered in the
previous steps to estimate the risk to a population, or in some cases, to a
particular type of consumer. In this step, by modifying the assumptions in
the parameters of previous steps, we can study the effects of these
alternate assumptions on ultimate health risk. Assumptions can be changed
to study the impact of lack of knowledge and the potential gains through
further research or to suggest the impact of a suspected trend. For this
type of analysis, risk assessments are typically done in a computer
environment to ease the computational burden and provide rapid responses to
"what-if" questions using alternate assumptions and situations. Current
spreadsheet applications and available "add-ins" allow generation of
complicated probabilistic models that had previously only been available
through expensive custom software.

Risk Assessment in Environmental Toxicology

In environmental toxicology, quantitative risk assessment has emerged as
the predominant paradigm for describing the public health consequences of
human exposure to environmental contaminants (8). Within this paradigm,
existing situations are measured and compared according to a measure of
population health risk. Similarly, proposed interventions are compared
according to the reduction in population health risk that each intervention
confers.

The adoption of risk assessment was primarily a result of legal and
administrative challenges to regulatory authority during the 1970s (6).
Regulatory agencies were required to provide a clear connection between an
imposed regulation and an expected health benefit. If the expected health
benefit could be quantified, the regulatory agencies were required to
demonstrate that it was substantive. Quantitative risk assessment has since
become widespread for different reasons. It is now used proactively to
support decisions such as selection of waste treatment technologies,
contaminated site cleanup operations, and state and municipal priority
setting for public health initiatives.

The shift of environmental health issues into a framework of risk reduction
opened the field to a broader set of analytic tools and prompted a broader
spectrum of professionals to examine the complex problems in the field.
Scientific societies have emerged with the sole mission of focusing on the
general techniques of risk assessment and their role in public health
decisions. In addition, environmental health risks can be compared with
concurrent public health risks from other sources through the use of common
measures. While this type of comparison is not always performed, scrutiny
of the cost-effectiveness of various regulatory programs is increasingly
required on the basis of risk reduction. Microbial food safety, as a
relative latecomer to the field of risk assessment, can take advantage of
its successes and failures and the wealth of constructive criticisms of
frameworks, decisions, and methods for addressing pervasive uncertainties
(4,8).

Opportunity for Technology Transfer to Microbial Health Risks

Quantitative risk assessment is an emerging tool in the field of microbial
food and water safety (9-12). Recognizing the deficiencies of current
approaches to evaluating the risk for human illness from pathogens in food,
the Council for Agricultural Science and Technology recommended that risk
assessment provide the basis for establishing food safety priorities and
policies (5). Because of recent initiatives advocating the widespread
implementation of Hazard Analysis and Critical Control Points (HACCP)
systems, quantitative risk assessment has been proposed as a means of
providing health-outcome-based specification of microbial criteria for
HACCP plans (12-14). Concurrently, international trade agreements have
advocated that demonstration of increased domestic health risk (in a risk
assessment) is the only acceptable basis for barriers to international
trade in food (15-18). However, one of the most important benefits in the
adoption of quantative risk assessment is improved understanding of the
many factors that determine the safety of the food supply.

Some resistance to the adoption of risk assessment is likely. Good
manufacturing practices and standard operating procedures carry a long
history of reasonably safe production when properly applied. The return on
investment in producing a quantitative risk assessment may not be high for
an individual food company with a very conservative production process.
However, good manufacturing practices and outbreak data are not
particularly useful in predicting the impact of new products, newly
recognized pathogens, and changes in food processing or in comparing
international food systems. Whether changes in the food supply are planned
(as in refocused inspection systems and minimally processed foods) or are
occurring passively (as in changed pathogens, demographics, and consumer
behavior), tools are required to assemble the information that describes
the impact. Quantitative risk assessment may provide the only systematic
means to interpret the impact of changes or trends before they become a
source of epidemiologic data.

In a quantitative risk assessment of broad scope, there is a place for all
the data from diverse information gathering activities relevant to
microbial food safety. Recent analyses of pasteurized liquid egg (19) and
ground beef contamination (20) incorporated evidence from farm-based
studies of pathogen prevalence, technology assessments comparing
decontamination methods, process-specific parameters of lot size and raw
material mixing, growth and death models from predictive microbiology,
monitoring studies of transportation and retail temperature control, and
studies of consumption amounts and cooking preference.

By designing the quantitative risk assessment process as an intelligent
information bank, we can develop a model to accommodate the breadth of
available information. The model provides a focus for discussions among
workers from diverse disciplines: farmers, veterinarians, food-processing
experts, microbiologists, and consumer behavior experts. The model also
allows for consideration and comparison of control strategies for which
experimentation would be very difficult in a "live" environment. The
impact, for example, of an aging population or a shift in cooking practices
can be simulated by a variety of assumptions that reflect the extent of the
change. By placing all of the information together, we can delineate gaps
in knowledge and provide estimates of the benefits of proposed research.

The most obvious users for quantitative risk assessment as applied to
microbial food safety are agencies responsible for food inspection, disease
surveillance, and food standards. These agencies have the most to gain from
models that incorporate existing and new data, capture knowledge of the
relevant features of the food processing and distribution continuum, and
capture knowledge of the variability in consumer behavior and immune system
responses. If models are constantly updated and improved, decisions made to
research, monitor, and control foodborne pathogens can be made with
information that lends itself to multidisciplinary discussion and best
describes what is currently known and unknown. Without such a model, there
is little common ground for the type of collaboration often advocated for
addressing the inherent complexity of foodborne disease.

Risk Assessment Case Examples

Two case examples illustrate the prospects of using risk assessment to
support decisions regarding emerging foodborne diseases.

Escherichia coli O157:H7 in Ground Beef

A model of E. coli O157:H7 in ground beef has been developed to support
comparative assessment of control strategies (20). The model describes the
pathogen population from the production of ground beef (including carcass
processing) to consumer cooking and consumption. The variability and
uncertainty in the model are accommodated through the use of probabilistic
representations for many of the parameters. To generate a representative
distribution of risk, the model is simulated many times with different
values selected from the probability distributions. This is a technique
known as Monte Carlo simulation (20-22).

While the direct output of the model is a distribution of health risk from
eating ground beef hamburger patties, a more important use of the model is
to describe the changes in health risk associated with changes in various
parameters. By changing parameters describing, for example, pathogen
prevalence and concentration in raw material, temperature abuse in
transportation and retail, consumer cooking preference, infectious dose,
and size of susceptible populations, we can study the impact of trends in
disease risk factors. Because this model includes the farm-to-fork
continuum, it is possible to assess the efficacy of interventions that
would otherwise not be compared in the same analysis. In addition, the
importance of improved data at different points in the process can be
estimated.

Toxoplasmosis

A probabilistic model describing the incidence of toxoplasmosis was
generated (23). While this model did not begin at the raw material level,
valuable insights were gained in studying the impact of trends in exposure
to Toxoplasma gondii. In congenital toxoplasmosis, the impact of maternal
exposure to T. gondii depends on whether the mother has previously been
infected (24). If this is the first exposure, the impact further depends on
the trimester of pregnancy. If detected at an early stage and treated with
certain drug therapies, the infection may have a smaller impact.

With such a model, the impact of varying risk factors can be studied. Since
the most serious consequences of toxoplasmosis occur during pregnancy, a
key variable is seroprevalence as a function of age. The protection offered
by prior infection complicates disease therapy; a reduction in exposure to
T. gondii could increase incidence of congenital toxoplasmosis by reducing
the prevalence of immune women of childbearing age. This may be further
complicated by changes in the age profile of pregnancy since younger women
are less likely to have been exposed. In addition to the complexities of
the population immunity profile, various trends in risk factors can be
simulated, such as trends in cat ownership, consumption of implicated
products, and the age distribution of pregnancy. The emergence of
toxoplasmosis as one of the leading causes of death in the human
immunodeficiency virus-positive population can be studied concurrently. The
effectiveness of mitigation strategies (e.g., education and screening
programs designed for pregnant women) can be compared to food-processing
strategies intended to reduce overall exposure.

The model of T. gondii infection provides insight into the importance of
detailed hazard identification to understand the complex mechanisms of
disease, exposure modeling to understand the time-dependent nature of
exposure, and intervention modeling to understand the potential negative
consequences of a reduction in overall exposure. Moreover, the results
underline the importance of performing all of the above tasks in the same
overall exercise if the implications of trends and interventions are to be
fully understood. It is unlikely that a sound decision could be made
without a full microbial risk assessment involving model
ing of the complex
nature of population immunity and exposure.

Conclusions

One of the key benefits of quantitative risk assessment is the development
of models describing the complex nature of pathogen populations in the food
supply. Improved understanding of the efficacy of pathogen reduction is the
most important side effect of this approach. Studies assessing the health
impact of a foodborne pathogen often include extensive documentation of
pathogen levels at unconnected points in the food and consumer pathway. In
contrast, a microbial risk assessment based on a model provides a
repository of knowledge describing health risk outcomes and control
strategies. The model improves with each new related study and each
critical review as more and more relevant data are uncovered. Furthermore,
when a decision is required, a description of the system is already
available in which assumptions and proposed interventions can be tested.

Initially, models can be expected to be crude. However, as a base for
discussion, a model can be very effective at soliciting input from experts
in the food industry and the public health community. Input from
epidemiologists, microbiologists, and industry safety managers can be
merged into the model until it represents the best available understanding
of the interacting features of the food supply and their effect on the
distribution of health risk. Once the model has been developed, the impact
of various control strategies and trends can be simulated. Our current
inability to compare control strategies at different points of the food
supply chain is evidence of the need for a system-level understanding that
will improve decision-making capacity.

Decisions to address foodborne pathogens cannot wait for scientific
certainty. Large degrees of uncertainty require that decisions be made with
great caution; however, there is no excuse for not making the best decision
on the basis of available information. Model-based quantitative risk
assessment can provide the decision-maker additional insights not typically
evident in "piece-meal" considerations of data. The ability to represent
the essentially probabilistic nature of emerging foodborne disease is
another risk assessment attribute not typically achieved by traditional
approaches.

Many gains in decision support can be achieved through model-based risk
assessment. Given that many current concerns are focused on emerging
pathogens, it may be timely to adopt risk assessment as a tool that is well
equipped for studying changes and interventions in the race against
pathogens.

Address for correspondence: A. M. Lammerding, Health Canada, Health of
Animals Laboratory, 110 Stone Road West, Guelph, Ontario, Canada N1G 3W4;
fax: 519-822-2280; e-mail: anna_lammerding@inet.hwc.ca.

References

  1. Altekruse SF, Cohen ML, Swerdlow DL. Emerging foodborne diseases.
     Emerg Infect Dis 1997;3:285-93.
  2. Hedberg CW, MacDonald KL, Osterholm MT. Changing epidemiology of
     food-borne disease: a Minnesota perspective. Clin Infect Dis
     1994;18:671-82.
  3. Smith JL, Fratamico PM. Factors involved in the emergence and
     persistence of food-borne diseases. Journal of Food Protection
     1995;58:696-708.
  4. Rodricks JV. Risk assessment, the environment, and public health.
     Environ Health Perspect 1994;102:258-64.
  5. Foegeding PM, Roberts T, Bennett JM, Bryan FL, Cliver DO, Doyle MP, et
     al. Foodborne pathogens: risks and consequences. Ames (IA): Council
     for Agricultural Science and Technology (CAST) 1994; Task Force Report
     No.:122.
  6. National Research Council. Risk assessment in the federal government:
     managing the process. Washington (DC): National Academy Press; 1983.
  7. Gerba CP, Rose JB, Haas CN. Sensitive populations: who is at the
     greatest risk? Int J Food Microbiol 1996;30:113-23.
  8. Commission on Risk Assessment and Risk Management. Framework for
     environmental health risk management. Washington (DC): The
     Commission;1997.
  9. Jaykus L-A. The application of quantitative risk assessment to
     microbial food safety risks. Crit Rev Microbiol 1996;22:279-93.
 10. Rose JB, Sobsey MD. Quantitative risk assessment for viral
     contamination of shellfish and coastal waters. Journal of Food
     Protection 1993;56:1043-50.
 11. ILSI Risk Science Institute Pathogen Risk Assessment Working Group. A
     conceptual framework to assess the risks of human disease following
     exposure to pathogens. Risk Anal 1996;16;841-8.
 12. Rose JB, Haas CN, Gerba CP. Linking microbial criteria for foods with
     quantitative risk assessment. In: Sheridan JJ, Buchanan RL, Montville
     TJ, editors. HACCP: an integrated approach to assuring the
     microbiological safety of meat and poultry. Trumbull (CT): Food
     Nutrition Press;1996.p.159-70.
 13. Notermans S, Gallhoff G, Zwietering MH, Mead GC. The HACCP concept:
     specification of criteria using quantitative risk assessment. Food
     Microbiol 1995;12:81-90.
 14. Buchanan RL. The role of microbiological criteria and risk assessment
     in HACCP. Food Microbiol 1995;12:421-4.
 15. World Health Organization. Application of risk analysis to food
     standards issues. Report of the Joint FAO/WHO Expert Consultation
     1995.WHO/FNU/FOS/Report No.95.3.
 16. Hathaway S. Harmonization requirements under HACCP-based control
     systems. Food Control 1995;6:267-76.
 17. Dawson RJ. The role of the Codex Alimentarius Commission in setting
     food standards and the SPS agreement implementation. Food Control
     1995;6:261-5.
 18. Hathaway SC, Cook RL. A regulatory perspective on the potential uses
     of microbial risk assessment in international trade. Int J Food
     Microbiol 1997;36:127-33.
 19. Whiting RC, Buchanan RL. Development of a quantitative risk assessment
     model for Salmonella enteritidis in pasteurized eggs. Int J Food
     Microbiol 1997;36:111-26.
 20. Cassin MH, Lammerding AM, Todd ECD, Ross W, McColl S. Quantitative
     risk assessment of Escherichia coli O157:H7 in ground beef hamburgers.
     Int J Food Microbiol. In press 1997.
 21. Thompson KM, Burmaster DE, Crouch EAC. Monte Carlo techniques for
     quantitative uncertainty analysis in public health risk assessments.
     Risk Anal 1992;12:53-63.
 22. Vose D. Quantitative risk analysis: a guide to Monte Carlo simulation
     modelling. New York: John Wiley & Sons, Inc.;1996.
 23. Cassin MH, Lammerding AM, Paoli GM, McColl RS. Population response and
     immunity to Toxoplasma gondii [abstract]. In: Proceedings of the
     Annual Conference of the Society for Risk Analysis and International
     Society for Exposure Analysis; 1996 Dec 8-12; New Orleans, LA. MacLean
     (VA):Society for Risk Analysis. p. 129.
 24. Dixon BR. Prevalence and control of toxoplasmosis - a Canadian
     perspective. Food Control 1992;3:68-75.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/lammer.txt

Communicating Foodborne Disease Risk

Baruch Fischhoff and Julie S. Downs
Carnegie Mellon University, Pittsburgh, Pennsylvania, USA
---------------------------------------------------------------------------

Text Version

      The food industry, like many others, has a risk communication
      problem. That problem is manifested in the public's desire to
      know the truth about outbreaks of foodborne diseases; ongoing
      concern about the safety of foods, additives, and
      food-processing procedures; and continued apathy regarding
      aspects of routine food hygiene. If these concerns are
      addressed in a coherent and trustworthy way, the public will
      have better and cheaper food. However, sloppy risk
      communication can itself cause public health damage. Because
      citizens are ill-equipped to discriminate among information
      sources, the food industry as a whole bears responsibility for
      the successes and failures of its individual members. We review
      risk communication research and practice for their application
      to the food industry.

The guardians of the world's food supply face a communication challenge of
extraordinary complexity. They need to be ready at short notice to deal
with various crises, often involving baffling combinations of foods,
pathogens, handling and distribution practices, dietary norms, and
interactions with medical conditions and medications. Their response to
this challenge may have important health, economic, and even political
implications. Conflicting pressures may come from groups that bear the cost
when the public health response is too swift or too slow. Quick, confident
explanations are expected after outbreaks that may never be fully
understood. When consumers (and producers) need information, they cannot
wait for more research. Consumers can read between the lines, especially
when they perceive their lives or livelihoods at risk. If they misread
messages, the communicators may still be held responsible. Moreover,
consumers know that silence is also a form of communication.

At the same time, the guardians of the food supply must wage a continuing
struggle to improve the handling of food. In the United States, campaigns
are under way for cooking beef more thoroughly, separating raw meat from
salad ingredients, and improving the sanitation of food handlers (e.g.,
Operation Clean Hands). To some extent, these campaigns are the
incarnations of old messages that have not been communicated effectively.
At the same time, the campaigns are responses to changes in the food supply
that have increased the risks associated with conventional practices. For
example, as the incidence or severity of foodborne disease pathogens
increases, the effectiveness of customary food-handling practices
decreases.

This article briefly reviews risk perception and communication research as
a possible resource for better understanding (and perhaps meeting) the
public's needs (1-3). Communication research provides a set of general
tools and theories, as well as a body of results, showing a complex picture
of strengths and weaknesses in lay understanding of risk. We explore here
the implications for anticipating public response to emerging foodborne
pathogens and offer a proposal for how an effective communication campaign
might be organized.

Although risk communication research does not directly address emerging
foodborne pathogens, it is compatible with the model of risk assessment
that the food industry seems to be adopting (4). Drawn from the National
Research Council's (5) volume, Improving Risk Communication, the model
involves overlapping processes of assessing the magnitude of risks (through
analytical procedures), managing their level (through practical measures),
and communicating with the public about them.

Like many other risks, emerging foodborne pathogens are of primary concern
to some specialists but one more thing to worry about for ordinary
citizens. The thought processes that people rely on for making decisions
are the focus of much research (Table; 6).

Table. Thought processes involved in decision-making
---------------------------------------------------------------------------

People simplify. Many decisions require people to deal with more details
than they can readily handle at any one time. To cope with the overload,
people simplify. People want to know if foods are "safe," rather than
treating safety as a continuous variable; they demand proof from scientists
who can provide only tentative findings; and they divide the participants
in risk disputes into good guys and bad guys. Such simplifications help
people cope, yet also lead to predictable biases (7).

Once people's minds are made up, it's hard to change them. People are adept
at maintaining faith in their beliefs unless confronted with overwhelming
evidence to the contrary. One psychologic process that helps people to
maintain their current beliefs is underestimating the need to seek contrary
evidence. Another process is exploiting the uncertainty surrounding
negative information to interpret it as consistent with existing beliefs
(8).

People remember what they see. People are good at keeping track of events
that come to their attention (9),(10). As a result, if the appropriate
facts reach people in a credible way before their minds are made up, their
first impression is likely to be the correct one. Unfortunately, it is hard
for people to gain firsthand knowledge of many risks, leaving them to
decipher the incomplete reports they get.

People cannot readily detect omissions in the evidence they receive. It is
unusual both to realize that one's observations may be biased and to undo
the effects of such biases. Thus people's risk perceptions can be
manipulated in the short run by selective presentations. People will not
know and may not sense how much has been left out (11). What happens in the
long run depends on whether the missing information is revealed by other
experiences or sources.

People may disagree more about what "risk" is than about how large it is.
One obstacle to determining what people know about specific risks is
disagreement about the definition of "risk" (12-15). For some risk experts,
the natural unit of risk is an increase in probability of death; for
others, it is reduced life expectancy; for still others, it is the
probability of death per unit of exposure. If lay people and risk managers
use the term "risk" differently, they may agree on the facts of a hazard,
but disagree about its riskiness.

How much does the public know and understand? The answer to this question
depends on the risks consumers face and the opportunities they have to
learn about them. The next section discusses strategies for improving those
opportunities.
---------------------------------------------------------------------------

Communicating Risk

An overarching theme of risk communication is that people understand risks
that draw their attention and are presented comprehensibly. Whether the
public's attention is aroused spontaneously or as a result of a message,
the opportunity must be seized. The right information must be selected and
communicated appropriately (1,16,17).

The hallmarks of effective communication should be used. Match the
audience's level of technical sophistication. Do not talk down. Clarify
terms (e.g., virus) that are used in everyday speech but not very precisely
(e.g., risk). Organize information. Provide the audience with a quick
logical overview. Make the desired level of detail easy to read. Use
numbers to communicate quantities. Avoid ambiguous verbal quantifiers, such
as "rare" or "likely". Ensure source credibility. Realize that messengers
are a part of the message and essential to its interpretation. Use
knowledgeable sources that will not misrepresent the message. Avoid risk
comparisons with rhetorical implications. Comparing one uncontrollable
accident risk with another, more familiar one (e.g., half as likely as
being injured by lightning) can be useful; however, people dislike
comparisons that imply they should accept one risk because they accept
another, e.g., comparing the risks of nuclear power with those of eating
peanut butter (from aflatoxin).

However useful communication research may be, there is no substitute for
empirical testing of messages. With heterogeneous audiences, any fixed
message will work better for some people than for others. In such cases,
universal understanding may require providing the opportunity for the
public to ask questions through public information sessions, agricultural
extension services, science teachers, or toll-free numbers.

What To Say

The effort to communicate is wasted if the information is not worth
communicating, either because people already know it or because it makes no
difference to them. Indeed, communication can backfire if consumers think
that their time is being wasted with useless messages while they are being
denied pertinent information. An analytical effort to determine what is
worth knowing and a coordinated empirical effort to determine what people
know already are required. These efforts take different forms in situations
where consumers face well-formulated decisions and need only a few
quantitative estimates before making choices, and in situations where
consumers are trying to understand the processes creating and controlling a
risk, in order to follow public discussion, devise decision options, or
understand quantitative estimates.

Identifying Relevant Estimates

The tools of decision analysis provide ways to determine how sensitive
well-structured choices are to uncertainty in different decision parameters
(18,19). The more sensitive parameters should receive more attention,
unless consumers know them already (and need no reminder). If conditions do
not permit sensitivity analyses for individual decision makers, one can
model the information needs of a population similar to the intended
audience. Merz et al. (20) demonstrated this approach for communicating to
carotid endarterectomy candidates. Scraping out the main artery to the
brain reduces the probability of stroke for patients with arteriosclerosis.
However, the procedure can cause many problems, including strokes. Decision
analysis computed the attractiveness of surgery for a hypothetical
population of patients, with a distribution of physical states (e.g.,
stroke risks) and values (e.g., time horizons). The analysis found that
three of the potential complications (stroke, facial paralysis, and
persistent headaches) posed sufficient risk that learning about them should
dissuade about 30% of candidates from surgery. Learning about the other
side effects should affect few additional patients. Therefore, physicians
trying to secure informed consent should (while not hiding other
information) make sure that patients understand the risks of these three
complications.

Identifying Relevant Processes

Risk analysis provides one way to identify the critical processes in
creating and controlling risks. Figure 1 shows a simple model for the risks
of foodborne pathogens. It uses the formalism of the influence diagram
(21,22). Such a model can be used both to assess risks and to characterize
the comprehensiveness of lay understanding. In this model, people incur
food-related risks as a result of decisions, which possibly lead to actions
or exposures. These decisions concern such actions as eating a bite of
suspicious food, choosing a particular diet, or opting for school (or home)
lunch. Those decisions depend, in part, on the perceived risks of those
actions as well as other nonrisk factors (i.e., other costs and benefits).
Exposure may follow, if a pathogen is actually present; it can lead, in
turn, to transmission of the pathogen and to changed health states,
depending on the resistance to disease that the person's health provides.
                                  [Image]
                   Figure 1. Diagram of the general model

Figure 2 elaborates on this model. It shows that food pathogenicity depends
on both the prevalence of pathogens in the environment and the quality of
food handling. A person's own health influences risk perceptions through
the intermediate variable of perceptions of health, which in turn is
influenced by the person's history of food consumption (or avoidance).
Actual pathogenicity influences risk perceptions through awareness, a
variable that communicators might affect. Nonrisk factors include visceral
factors (e.g., hunger), external social factors (e.g., social pressure to
eat any food offered by a host), norms (e.g., not eating dog), and the
expected benefits of consumption (e.g., taste, texture, and other
gustatorial pleasures). Computing risks with this model would require
specifying each variable and estimating the contingencies by using
statistical sources or expert judgment. For risk communication purposes,
even a qualitative model can define the universe of discourse and allow
approximate estimates of the most important relationships (23).
                                  [Image]
                 Figure 2. Diagram of the foodborne model.

Identifying Current Knowledge

Determining what people already know about quantitative estimates is
relatively straightforward, although there are various pitfalls (24,25).
Eliciting knowledge of processes is more difficult. Respondents should be
given the focus of the problem and maximum freedom to express their ideas
and reveal which of the processes in Figure 2 are on their minds. Studies
using open-ended techniques often find that people speak the language of
risk without understanding its terms. For example, in a study about radon,
we found that respondents often knew that it was a colorless, odorless,
radioactive gas that caused lung cancer. However, when pressed, respondents
often revealed inappropriate notions of radioactivity, believing that
anything radioactive would permanently contaminate their homes. Some told
us that they would not test for radon because there was nothing that they
could do if they found a problem (26). In studies with adolescents, we
found other forms of false fluency; for example, teens used terms such as
"safe sex" and "clean needles" without understanding them (23). Without
open-ended probing, we miss misconceptions that a technical expert never
would have imagined, or we use language that does not communicate
effectively with our audience (27).

Food Industry Communication Strategies

Technical experts (in any industry) generally want to get the facts in
order before saying anything. Although that is an appropriate norm within
the scientific community, refusing to address a concerned public can evoke
mistrust and anger, as can failing to arouse an apathetic public. To steer
an appropriate course, communicators need an explicit policy that balances
the risks of saying too much with the risks of saying too little. The
policy must consider both what to say and when to say it. From a decision
theory perspective, citizens need information critical to identifying
actions that will help them achieve personal goals. As a result, any
recommendations should reflect both scientific knowledge and citizens'
values. That is, consumers need to know what is the best gamble, given the
trade-offs between, for example, the risks of throwing out good food and
the risks of eating food that might make them sick.

At times, there may be a temptation not to tell it like it is. For example,
one might argue that risks should be exaggerated when a frank report would
leave people unduly apathetic, as judged by their own standards. That is,
people should say "Thanks for getting my attention" once the grounds for
the overstatement were made clear. Such gratitude requires a public that
not only recognizes the limits of its own understanding, but also accepts
paternalistic and manipulative authorities. That acceptance seems more
likely for 
misrepresentations intended to get a complacent public moving
than for ones intended to allay a hysterical public's fears. In either
case, once the secret is out, all future communications may be subjected to
second guessing ("How seriously should we take them this time?").

One situation in which paternalistic authority is needed arises when a
single message must be sent to a heterogeneous audience-for example, when
officials must decide whether to declare a particular food "safe." Safety
is a continuous variable, and any cut-off represents a value judgment. For
any given food, different groups may face different risks, derive different
benefits, and want to make different trade-offs. For example, a few people
are strongly allergic to sulfur dioxide as a dried food preservative.
Marketing such foods signals their safety to all. Labels that declare
preservatives in foods allow consumers to customize their risk levels, but
only if they know their own risks (i.e., whether they are strongly
allergic, which they may learn only through a bad reaction whose source
they identify).

The Food and Drug Administration faces a similar challenge in its effort to
standardize risk labels for over-the-counter drugs. For example, other
things being equal, producing bilingual labels will require either reducing
print size or omitting information about some side effects. These
modifications would, in turn, increase the risks for consumers with limited
vision (e.g., some of the elderly) or those particularly sensitive to the
omitted effects. Whatever labeling, warning, or communication strategy is
chosen will leave some residual risk, with an uneven distribution depending
on the heterogeneous sensitivities of the audience. Thus, the strategy
reflects the authorities' notion of the "acceptable level of
misunderstanding" (28).

What that acceptable level should be is a political and ethical question,
which could be resolved by properly constituted public or private groups,
and a scientific question, partially resolvable by research of the sort
described here. Rigorous empirical testing is needed to determine whether
communications fulfill the hopes placed in them (27). Emerging foodborne
pathogens provide a particular challenge to safety communications-and a
particular need for evaluation. Their novelty and ability to produce
outbreaks in diverse places in the world and the food chain encourage
treating them as unique. If a communication strategy is improvised only
when a crisis hits, or as it evolves, the chances for a misstep increase.
Those chances are especially large if the outbreak is the first major risk
problem for the health authorities involved (16).

As a result, communications about these unique situations should be
routine. A standard format for reporting risk information should be
adopted. Funtowicz and Ravetz (29) propose a notation that includes a
best-guess risk estimate (expressed in standard units), a measure of
variability, and a "pedigree" (indicating the quality of the research).
Although new, such notation might become familiar, much as degrees
Fahrenheit, miles per gallon, probability of precipitation, and recommended
daily allowance have become familiar.

Another part of communication planning is to adopt standard scripts for
reporting complex procedural information regarding what citizens should do
and what food specialists are doing. The adoption process should include
empirically testing the comprehensibility of concrete messages with an
audience like the intended audience. Influence diagrams offer one template
for organizing procedural information. Risk analyses provide one way to
identify the crises most likely to occur and may allow not only testing the
most likely messages, but also identifying the persons most likely to do
the communicating and preparing them accordingly. The chemical industry's
Community Awareness and Emergency Response program might provide some
useful lessons in how to organize for unlikely events, although the
challenges of dealing with the relatively identifiable community
surrounding a chemical plant are different from those presented by dealing
with the diffuse national (or even international) audience concerned about
a food. The chemical industry's experience may also provide guidance on how
to achieve voluntary industry compliance with a set of communication
principles. Public goodwill is eroded every time an industry spokesperson
violates the public trust by misrepresenting, or just explaining
inadequately, the state of affairs. Reducing misrepresentation requires
institutional discipline; reducing inadequate communication requires a
scientific approach to communication.

Acknowledgments

Our research was supported in part by the National Institute for Allergy
and Infectious Diseases, the National Institute of Alcohol Abuse and
Alcoholism, and the National Science Foundation.

Address for correspondence: Baruch Fischhoff, Department of Social and
Decision Sciences, Department of Engineering and Public Policy, Carnegie
Mellon University, Pittsburgh, PA 15213, USA; fax: 412-268-6938; e-mail:
baruch@andrew.cmu.edu.

References

  1. Fischhoff B, Bostrom A, Quadrel MJ. Risk perception and communication.
     In: Detels R, McEwen J, Omenn G, editors. Oxford textbook of public
     health. London: Oxford University Press; 1997. p. 987-1002.
  2. Krimsky S, Golding D, editors. Social theories of risk. Westport (CT):
     Praeger; 1992.
  3. Slovic P. Perception of risk. Science 1987;236:280-5.
  4. Lammerding AM. Quantitative microbial risk assessment. Emerg Infect
     Dis. In press 1997.
  5. National Research Council. Improving risk communication. Washington
     (DC): National Academy Press; 1989.
  6. Carter MW, editor. Radionuclides in the food chain. Berlin:
     Springer-Verlag; 1988.
  7. Kahneman D, Slovic P, Tversky A, editors. Judgment under uncertainty:
     heuristics and biases. New York: Cambridge University Press; 1982.
  8. Gilovich T, editor. How we know what isn't so. New York: Free Press;
     1993.
  9. Hasher L, Zacks RT. Automatic and effortful processes in memory. J Exp
     Psychol Gen 1984;108:356-88.
 10. Peterson CR, Beach LR. Man as an intuitive statistician. Psychol Bull
     1967;69:29-46.
 11. Fischhoff B, Slovic P, Lichtenstein S. Fault trees: sensitivity of
     assessed failure probabilities to problem representation. J Exp
     Psychol Hum Percept Perform 1978;4:330-44.
 12. Crouch EAC, Wilson R, editors. Risk/benefit analysis. Cambridge (MA):
     Ballinger; 1981.
 13. Fischhoff B, Watson S, Hope C. Defining risk. Policy Sciences
     1984;17:123-39.
 14. Slovic P, Fischhoff B, Lichtenstein S. Rating the risks. Environment
     1979;21:14-20,36-9.
 15. Fischhoff B, Svenson O. Perceived risk of radionuclides: understanding
     public understanding. In: Harley JH, Schmidt GD, Silini G, editors.
     Radionuclides in the food chain. Berlin: Springer-Verlag; 1988. p.
     453-71.
 16. Fischhoff B. Risk perception and communication unplugged: twenty years
     of process. Risk Analysis 1995;15:137-45.
 17. Schriver KA. Evaluating text quality: the continuum from text-focused
     to reader-focused methods. IEEE Transactions on Professional
     Communication 1989;32:238-55.
 18. Bursztajn HJ, Feinbloom RI, Hamm RA, Brodsky A, editors. Medical
     choices, medical chances: how patients, families, and physicians can
     cope with uncertainty. New York: Routledge; 1990.
 19. Raiffa H, editor. Decision analysis: introductory lectures on choices
     under uncertainty. Reading (MA): Addison-Wesley; 1968.
 20. Merz J, Fischhoff B, Mazur DJ, Fischbeck PS. Decision-analytic
     approach to developing standards of disclosure for medical informed
     consent. Journal of Toxics and Liability 1993;15:191-215.
 21. Burns WJ, Clemen RT. Covariance structure models and influence
     diagrams. Management Science 1993;39:816-34.
 22. Howard RA. Knowledge maps. Management Science 1989;35:903-22.
 23. Fischhoff B, Downs J. Accentuate the relevant. Psychological Science
     1997;8:154-8.
 24. Morgan MG, Henrion M, editors. Uncertainty. New York: Cambridge
     University Press; 1990.
 25. Poulton EC, editor. Bias in quantifying judgment. Hillsdale (NJ):
     Lawrence Erlbaum; 1989.
 26. Bostrom A, Fischhoff B, Morgan MG. Characterizing mental models of
     hazardous processes: a methodology and an application to radon.
     Journal of Social Issues 1992;48:85-100.
 27. Bruhn CM. Consumer education. Emerg Infect Dis. In press 1997.
 28. Fischhoff B, Riley D, Kovacs D, Small M. What information belongs in a
     warning label? Psychology and Marketing. In press 1998.
 29. Funtowicz S, Ravetz J, editors. Uncertainty and quality in science for
     policy. London: Kluwer; 1990.

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Animal Diseases of Public Health Importance

Gregory D. Orriss
Food and Agriculture Organization of the United Nations, Rome, Italy
------------------------------------------------------------------------

      The Food and Agriculture Organization's (FAO) interest in
      emerging diseases caused by foodborne pathogens derives from
      its role as the leading United Nations agency with a mandate
      for food quality and safety matters. The Food Quality and
      Standards Service of FAO's Food and Nutrition Division is
      active in all areas related to food safety and implements the
      FAO/World Health Organization Food Standards Program. Its
      activities include providing assistance to FAO's member nations
      in addressing problems, strengthening infrastructure, 
      promoting standardization as a means of facilitating trade, and
      safeguarding the interests of consumers. This paper considers
      the importance of emerging foodborne diseases from the
      perspectives of the consumer, international trade in food,
      producers and processors, and developing countries and
      addresses prevention and control measures.

In recent years, public concern regarding food safety has increased as a
consequence of the outbreak of bovine spongiform encephalopathy (BSE) in
cattle, the prevalence of Salmonella serotype Enteritidis illnesses (from
poultry, meat, eggs), and the more localized outbreaks of illnesses
associated with Listeria monocytogenes (from dairy products, pt, salads)
and Escherichia coli O157:H7 (from ground or minced beef, unpasteurized
apple juice, vegetables). Emerging pathogens and the appearance of problems
such as BSE have resulted in enactment of specific controls in many
countries, while the general heightening of interest internationally has
prompted health professionals and the food industry in many countries to
scrutinize the control of emerging infectious agents.

Animal Feeding and Food Safety

The Food and Agriculture Organization (FAO) of the United Nations has had a
long-standing interest in the area of food safety and food quality. Because
of problems such as BSE and emerging pathogens, FAO convened an Expert
Consultation on Animal Feeding and Food Safety in Rome in March 1997 to
address these issues and provide the scientific basis for improving
practices in the feeding of animals for the production of food.

The ultimate objective of food industry and safety regulators is to ensure
that food reaching the consumer is safe and wholesome. This objective does
not imply that food can ever be completely free of risk but rather that the
level of risk to the consumer can be acceptable. Foods generally expected
to be safe may become unsafe as a result of hazards introduced during
production, processing, storage, transport, or final preparation by the
consumer. For food derived from animal sources, the hazards may originate
from a number of sources, including the consumption of contaminated feed.

Hazards in food that may relate to animal feed include salmonellosis,
mycotoxicosis, and ingestion of unacceptable levels of veterinary drugs and
agricultural and industrial chemicals. In addition, if the postulated link
between BSE and new variant--Creutzfeldt-Jakob disease is established, this
disease would also be an example of contamination originating from animal
feed.

The FAO consultation limited its considerations to food safety matters that
pertained strictly to animal feeds; it did not consider plant toxins,
radionuclides, or parasites spread by human sewage. The risk to human
health from other infectious agents that may contaminate either feed or
forage appears to be negligible or nonexistent and was, therefore, not
considered by the consultation. Only the standard domestic animals from
which food is derived in large quantities, such as meat and meat products,
milk and milk products, and eggs and egg products, as well as fish products
derived from aquaculture that involves the feeding of fish, were
considered. All aspects of animal feed, other than natural unrestricted
grazing, were considered. The consultation concluded that emerging
pathogens are generally not identified through traditional animal
surveillance and epidemiology.

Hazards Associated with Animal Feed

Mycotoxins are secondary metabolites produced by fungi of various genera
when fungi grow on agricultural products before or after harvest or during
transportation or storage. Some fungi such as Aspergillus spp. and
Penicillium spp. can invade grain after harvest and produce mycotoxins,
while others, such as Fusarium spp., typically infest grains and produce
mycotoxins before harvest. In some circumstances, Aspergilli can grow and
produce mycotoxins before the crop is harvested.

Both intrinsic and extrinsic factors influence fungal growth and mycotoxin
production on a substrate. Intrinsic factors include water activity, pH,
and redox potential; extrinsic factors are relative humidity, temperature,
and availability of oxygen.

Many mycotoxins with different chemical structures and widely differing
biologic activities have been identified. Mycotoxins may be carcinogenic
(e.g., aflatoxins B1, ochratoxin A, fumonisin B1), estrogenic (zearalenone
and I and J zearalenols), nephrotoxic (ochratoxins, citrinin, oosporeine),
dermonecrotic (trichothecenes), or immunosuppressive (aflatoxin B1,
ochratoxin A, and T-2 toxin). Much of the published information on toxicity
comes from studies in experimental animals, and these may not reflect the
effects of mycotoxins on humans and other animals. In addition, their
significance in human foods of animal origin is incompletely understood.
Mycotoxins are regularly found in animal feed ingredients such as maize,
sorghum grain, rice meal, cottonseed meal, groundnuts, legumes, wheat, and
barley. Most are relatively stable compounds, are not destroyed by feed
processing, and may even be concentrated in screenings.

Various animal species metabolize mycotoxins in different ways. In pigs,
ochratoxin A can undergo enterohepatic circulation and is eliminated very
slowly, whereas in poultry species it is rapidly excreted. The polar
mycotoxins such as fumonisins tend to be excreted rapidly. Mycotoxins, or
their metabolites, can be detected in meat, visceral organs, milk, and
eggs. However, their concentration in these food products is usually
considerably lower than in the feed consumed by the animals; at these
levels, mycotoxins are unlikely to cause acute intoxication in humans
consuming these products. Residues in animal products of carcinogenic
mycotoxins, such as aflatoxin B1, M1, and ochratoxin A, pose a threat to
human health, and their levels should be monitored and controlled.

In most instances, the principal source of mycotoxins for humans is
contaminated grains and cereals, rather than animal products. This means
that the hazard is much greater in developing countries in which maize and
other grains form the staple diet and the intake of animal products,
including meat, is low.

Only limited information is available regarding mycotoxin residues in
animal products intended for human consumption. The metabolism of
mycotoxins by animals and the residues of mycotoxins and their metabolites
in animal tissues should be studied further.

Infectious Agents

Agent Causing Transmissible Spongiform Encephalopathies in Ruminants

Transmissible spongiform encephalopathies are nonfebrile neurologic
diseases with a long incubation period and are fatal. These diseases are
associated with incompletely defined agents termed prions, which are
resistant to normal heat treatments of feed and food. Sheep scrapie has
been recognized for over 250 years. BSE was first recognized in the United
Kingdom during 1986. For BSE, the infectious agent enters the feed
primarily through rendered infected tissues (notably the central nervous
system and the reticuloendothelial system) under insufficient heat to
reduce the concentration of the infectious agent to an ineffective dose. In
the case of sheep scrapie, infection is naturally maintained by
transmission between sheep. Humans have likely been exposed to the scrapie
agent by eating brain and other tissues, although there is no evidence that
Creutzfeldt-Jakob disease in humans has been associated with scrapie.

Humans can potentially be exposed to BSE through consumption of infected
tissues. The occurrence of a new variant of the human transmissible
spongiform encephalopathy, Creutzfeldt-Jakob disease, has raised the
possibility of an association with the BSE agent. With the limited number
of cases now, there is no proven link between this new variant and the
possible transmission of the agent from infected bovine tissue to humans.
The FAO consultation recommended risk reduction measures to address the
elimination of BSE from cattle.

Salmonella enterica

The more than 2,000 Salmonella serotypes can be divided into three groups:
species-specific, such as Gallinarum (in poultry); invasive, which may
cause systemic infections in their host, such as Enteritidis (in laying
hens); and noninvasive, which tend to remain within the intestinal tract.
Members of the first group are infrequently feedborne pathogens. Among the
second group, the principal manifestation of human infection is
gastroenteritis, with septicemia occurring in some patients. The third
group may be associated with subclinical infections in farm livestock; it
sometimes causes disease in livestock and is associated with food poisoning
in humans.

Salmonellae are widely distributed, and animal feed is only one of many
sources of infection for farm animals. Animal feed ingredients of both
animal and plant origin are frequently contaminated with salmonellae,
although the most common serotypes associated with human disease,
Enteritidis and Typhimurium, are rarely isolated from animal feed. Feed can
be contaminated from raw ingredients.

Toxoplasma gondii

The protozoon T. gondii is found in cats and, according to serologic
surveys, also in birds and other domesticated species including sheep,
pigs, goats, and horses. The primary source of infection for animals is
feed contaminated with feces of cats and possibly with rodent tissues.

Cats are an important source of infection for humans; however, some human
infections may be due to the handling or consumption of raw meat. Pregnant
women may miscarry or give birth prematurely, and infants often get central
nervous system disorders and ocular disease.

Trichinella spiralis

T. spiralis is a nematode that parasitizes the intestinal tract of mammals,
particularly pigs. The larvae encyst in the tissues, particularly the
muscles, which act as a source of infection for humans who consume raw or
partially cooked meat. The clinical manifestations include fever, muscle
pain, encephalitis, meningitis, myocarditis, and (rarely) death.

The cysts in infected carcasses can be killed by freezing (-18C for 20
days) or traditional rendering temperatures. Adequate cooking of raw meat
and table scraps before feeding to farm animals would eliminate this
hazard.

The FAO consultation also addressed potential hazards associated with
veterinary drugs and agricultural and other chemicals and recommended risk
reduction measures to prevent, eliminate, or reduce the hazards to
acceptable levels. The consultation participants prepared a draft Code of
Practice for Good Animal Feeding to be considered by the Codex Alimentarius
Commission (CAC).

Codex Alimentarius Commission

Since 1962, CAC has been responsible for implementing the Joint FAO/World
Health Organization (WHO) Food Standards Program. "Codex Alimentarius,"
whose name is taken from Latin and translates literally as "food code" or
"food law," was founded in response to the worldwide recognition of the
importance of international trade and the need to facilitate trade while
ensuring the quality and safety of food for the world consumer.

It follows, therefore, that the commission's primary objectives are the
protection of the health of consumers, the assurance of fair practices in
the food trade, and the coordination of all food standards. Food standards,
guidelines, and recommendations are the work of CAC. With the adoption of
the World Trade Organization's Agreement on the Application of Sanitary and
Phytosanitary Measures and the Agreement on Technical Barriers to Trade, a
new emphasis and dimension have been placed on Codex standards.

Codex Committee on Food Hygiene

The Codex Committee on Food Hygiene (CCFH) has overall responsibility for
all provisions of food hygiene prepared by Codex commodity committees and
contained in commodity standards, codes of practice, and guidelines. CCFH
also develops general principles, codes of practice, guidelines for food
hygiene, and microbiologic criteria for food to be applied horizontally
across Codex committees. Food hygiene is defined as "all conditions and
measures necessary to ensure the safety and suitability of food at all
stages of the food chain."

According to the deliberations at the 29th session of CCFH, the
microbiologic safety of foods is principally ensured by control at the
source, product design, process control, and good hygienic practices during
production, processing, handling, distribution, storage, sale, preparation,
and use, preferably in conjunction with the application of the Hazard
Analysis and Critical Control Points (HACCP) system. This preventive system
offers more control than end-product testing because of the limited
effectiveness of microbiologic examination to assess the safety of food.

When they have been established by Codex or national risk managers,
objectives for food safety can be taken up by industry; by applying HACCP
(or an equivalent food safety management system), industry can ensure that
these objectives are met. This is the use of HACCP as a corrective risk
management option: a risk is identified, and a management option is
selected and implemented. HACCP is also used as a preventive risk
management tool. In this case, hazard analysis identifies potential hazards
in raw materials, production line, and line-environments to the consumer.
Hazard analysis is defined as "The process of collecting and evaluating
information on hazards and conditions leading to their presence to decide
which are significant for food safety and therefore should be addressed in
the HACCP plan." Input concerning the potential hazards and their control
could come from risk analysis, but often such information is not available
and industries need to apply their best judgment.

The Revised Principles for the Establishment and Application of
Microbiological Criteria For Foods states, "Microbiological criteria should
be established according to these principles, and be based on scientific
analysis and advice, and where sufficient data are available, on a risk
analysis appropriate to the foodstuff and its uses." These criteria may be
relevant to the examination of foods, including raw materials and
ingredients of unknown or uncertain origin, and may be used when no other
means of verifying the efficacy of HACCP-based systems and good hygienic
practices are available. Microbiologic criteria may also be used to
determine that processes are consistent with the General Principles of Food
Hygiene. Microbiologic criteria are not normally suitable for monitoring
critical limits as defined in the HACCP system.

Establishing microbiologic criteria and food safety objectives in general
is difficult because of the considerable knowledge gap relating to biologic
hazards and their relationship to human illness. This has led to many
evaluations by CCFH, which are based on subjective or qualitative
assessments and serve as the basis for recommendations. Although aware of
these limitations, CCFH is now developing a framework of principles and
guidelines for the application of microbiologic risk assessment. CCFH's
action was in response to the recommendation of the 1995
 J
oint FAO/WHO
Consultation on the Application of Risk Analysis to Food Standards relating
to the application of risk assessment within the Joint FAO/WHO Food
Standards Program. International Commission for Microbiological
Specifications for Foods and CCFH delegations are also in the process of
developing background papers on a number of foodborne pathogens to better
conduct quantitative risk assessments and set subsequent food safety
objectives. Notwithstanding the development of risk analysis approaches by
these groups, the work of CCFH and all Codex committees would benefit from
advice from an expert body on foodborne biologic hazards for purposes of
risk management. The committee could be modeled on the FAO/WHO Joint Expert
Committee on Food Additives and Joint Meeting on Pesticide Residues,
allowing for the unique consideration of epidemiologic and clinical data
related to pathogens causing human illness, and of the dynamics of
microbial populations in food throughout the food chain.

Control of Listeria monocytogenes in foods is an example of the need to
consider a structured risk management approach. Listeria are frequently
consumed in small amounts by the general population without apparent ill
effects. Only higher levels of Listeria are thought to cause serious
disease problems. It is believed that Listeria will always be present in
the environment. Therefore, the critical issue may not be how to prevent
Listeria in foods, but how to control its survival and growth to minimize
the potential risk. In many foods, complete absence of Listeria is
unrealistic and unattainable; trying to achieve this goal can limit trade
without having any appreciable benefit to public health. A relevant risk
management option, therefore, is to focus on foods that have historically
been associated with human disease and support the growth of Listeria to
high levels, rather than focusing on foods that do not support growth.
Thus, establishing tolerably low levels of Listeria in specific foods may
be one food safety objective achieved by risk managers after a rigorous and
transparent risk analysis. Such an approach is now being considered by CCFH
after an initial risk assessment by the International Commission for
Microbiological Specifications for Foods and CCFH delegations.

Although Listeria presents unique challenges in terms of its widespread
occurrence and the particular susceptibility of vulnerable groups,
pathogens such as E. coli O157:H7, Salmonella, and Campylobacter are also
being addressed. These microbial pathogens produce acute foodborne
illnesses and can cause severe chronic sequelae, creating an important
public health problem and food safety concerns.

Codex Codes of Hygienic Practice are based on good manufacturing practices,
HACCP principles, and risk analysis. CCFH is responsible for coordinating
and overseeing the work of specific Commodity Committees in this area. In
the specific area of food hygiene, Codex has revised its main document,
Recommended International Code of Practice: General Principles of Food
Hygiene, to incorporate risk assessment principles and include specific
references to the HACCP system.

FAO Programs on Food Quality and Safety

The Food Quality and Standards Service is a service within the Food and
Nutrition Division of the FAO, located in Rome. The Secretariat of CAC is
also located there. The Regular Program of the Food Quality and Standards
Service provides the technical and scientific basis for FAO for all food
quality matters, including food safety. This includes providing the
Secretariat for the Joint Expert Committee on Food Additives and
participation in both the Joint Meeting of the FAO Panel of Experts on
Pesticide Residues in Food and the Environment and the WHO Expert Group on
Pesticide Residues and in the Joint Expert Committee on Food Irradiation.

The Food Quality and Standards Service develops and publishes guidelines
and manuals (including the FAO Food and Nutrition Paper Series and Manuals
of Food Quality Control), arranges expert consultations and conferences
(e.g., the Joint FAO/WHO Expert Consultation on Biotechnology and Food
Safety, September 30 to October 4, 1996; the Joint FAO/WHO Expert
Consultation on the Application of Risk Management to Food Safety Matters,
January 27-31, 1997; the Joint FAO/WHO Consultation on Food Consumption and
Exposure Assessment to Chemicals, February 10-14, 1997; and the FAO
Consultation on Animal Feeding and Food Safety, March 10-14, 1997), and has
a major and continuing program of providing technical assistance regarding
food standards and food control to member countries, particularly
developing countries and countries in transition from a centrally planned
to a market economy.

The Joint Expert Committee on Food Additives, the Joint Meeting on
Pesticide Residues, and the Joint Expert Committee on Food Irradiation are
expert committees that provide independent scientific advice that forms the
basis for the development of food safety recommendations used in
international trade. These committees are forums in which independent,
invited experts assess the state of scientific knowledge of food additives,
pesticide and veterinary drug residues in food, mycotoxins, other chemical
contaminants in food, and food irradiation treatments and make
recommendations to member governments and to Codex.

FAO's Food Quality and Standards Service also develops and publishes
Manuals of Food Quality Control. These manuals provide recommendations for
the development and operation of food quality and safety systems. While
aimed primarily at providing advice to developing countries, the manuals
document modern approaches, including the development of quality control
programs throughout the food chain that apply to all countries. Such an
approach is instrumental in facilitating international trade in food. Key
titles in the series include Food Inspection, Food for Export, Management
of Food Control Programs, Imported Food Inspection, and Quality Assurance
in the Food Control Laboratory.

The program of technical assistance projects undertaken by the Food Quality
and Standards Service handles assistance in food quality control, including
safety; such projects have established or strengthened the food control
systems in a number of developing countries. Typically, they assist in
establishing the infrastructure for an enhanced food control program,
assessing laboratory service requirements, providing guidance to develop
legislation and procedural manuals, setting up reputable inspection and
certification systems, and providing training and staff development. In
these assistance projects, the standards established by the CAC are basic
guides to international requirements.

Conclusion

Food will always represent some biologic risk; it is the task of the food
industry to maintain the level of risk at the minimum that is practical and
technologically feasible. It should be the role of regulatory bodies to use
risk assessment to determine realistic and achievable risk levels for
foodborne hazards and to base their risk management and food safety
policies on the practical application of the results of these analyses.

Foodborne illnesses are preventable. Adherence to good manufacturing
practices and good hygienic practices and application of the HACCP system
can result in food safety and ensure food quality. Food safety is the
shared responsibly of governments, academia, the food industry, and the
consumer.

Codex standards, guidelines, and recommendations have the objective of
protecting the consumer and facilitating international food trade.
Adherence to Codex provides the basis for food safety and quality and meets
the requirements of international trade.

Address for correspondence: Gregory D. Orriss, Chief, Food Quality and
Standards Service; Food and Nutrition Division; Food and Agriculture
Organization of the United Nations; fax: 39-6-5705-4593; e-mail:
Gregory.Orriss@fao.org.
------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

Foodborne Disease Control: A Transnational Challenge

F. K. Kferstein, Y. Motarjemi, and D. W. Bettcher
World Health Organization, Geneva, Switzerland

    "Disease knows no boundaries and borders are porous to disease."(1)
---------------------------------------------------------------------------

      In the globalized political economy of the late 20th century,
      increasing social, political, and economic interdependence is
      occurring as a result of the rapid movement of people, images,
      values, and financial transactions across national borders.
      Another consequence of the increase in transnational trade,
      travel, and migration is the greater risk of cross-border
      transmission of infectious diseases. As the world becomes more
      interconnected, diseases spread more rapidly and effectively.
      With more than one million people crossing international
      borders every day, and with the globalization of food
      production, manufacturing, and marketing, the risk of
      infectious disease transmission is greater. Economic
      globalization has also increased the need for governmental
      budget austerity, and consequent national preparedness has been
      eroded. The emergence of new infectious diseases, as well as
      the reemergence of old ones, thus represents a crucial
      transnational policy issue. These problems cannot be resolved
      by national governments alone; they require international
      cooperation. This article analyzes the role of foodborne
      disease surveillance programs, nationally and internationally,
      in the control of foodborne diseases.

In the past two to three decades, public health authorities in
industrialized countries have been faced with an increasing number of food
safety problems. In 1983, a Joint Food and Agriculture Organization/World
Health Organization Expert Committee on Food Safety concluded that illness
due to contaminated food was perhaps the most widespread health problem in
the contemporary world and an important cause of reduced economic
productivity (2). More recent data from industrialized countries indicate
that annually up to 10% or more of the population may have a foodborne
disease. The situation is equally serious in developing countries, where
infant diarrhea causes many illnesses and deaths. In addition to known
foodborne diseases, public health communities are being challenged by the
emergence of new or newly recognized types of foodborne illnesses, often
with serious and chronic health consequences. Certain populations (e.g.,
pregnant women, the elderly, infants and children, immunocompromised
persons, and the undernourished) are particularly vulnerable. In economic
terms, foodborne illnesses are very costly for industry, health services,
and society as a whole.

Many factors have contributed to the increase in foodborne disease.
Industrialization, leading to increased wealth and urbanization, has
revolutionized the food supply system, resulting in mass production and an
explosive increase in the number of food service establishments and food
outlets. Mass production, environmental factors, and inadequate knowledge
on the part of food handlers have contributed to increased contamination of
primary foodstuffs.

The increase in international trade has increased the risk for cross-border
transmission of infectious diseases. The globalization of food (and feed)
trade, facilitated by the liberalization of world trade, while offering
many benefits and opportunities, also presents new risks (3). Food, a major
trade commodity, is also an important vehicle for transmission of
infectious diseases. Because food production, manufacturing, and marketing
are now global, infectious agents can be disseminated from the original
point of processing and packaging to locations thousands of miles away.
This multinational approach to food production and distribution and the
progressive opening up of world markets have allowed the international food
trade to flourish. The value of food trade, U.S. $266 billion in 1994, was
more than 300% greater than it was 20 years ago and continues to grow
rapidly (4).

The globalization of foodborne diseases also results from increased travel.
International travel is more accessible today. The World Tourism
Organization estimates world tourist arrivals at 567 million in 1995, and
this figure is expected to rise to 660 million by the year 2000. Over the
past 200 years, the average distance traveled and the speed of travel have
increased 1,000 times while incubation periods for diseases have not
changed. As a result, a person can be exposed to a foodborne illness in one
country and expose others to the infection in a location thousands of miles
from the original source of the infection (5). Depending on their
destination, travelers are estimated to run a 20% to 50% risk of
contracting a foodborne illness.

As international trade and travel increase, foodborne disease outbreaks of
the same origin are more likely to occur in different parts of the globe.
Food safety in the late 20th century represents a transnational challenge
requiring enhanced levels of international cooperation in setting standards
and regulations and in strengthening surveillance systems. Effective food
safety programs, built on a clear understanding of the epidemiology of
foodborne disease, must be developed and implemented. The globalization of
the world's economy has been accompanied by intense economic competition
and increased pressure on governments to downsize. Public sector austerity
has reduced disease surveillance in many countries (6). For example, in
Great Britain, the failure to maintain public health infrastructures has,
in the words of the British Medical Association, resulted in "Britain
returning to the 19th century in terms of public health, with problems such
as dirty water, contaminated food, and old infectious diseases reemerging"
(7). Failing a reversal of this trend, public health authorities and health
services may be overwhelmed in the near future by outbreaks or epidemics of
foodborne diseases. The 1991 epidemic of cholera in Peru and the 1996
outbreak of Escherichia coli O157 in Japan demonstrate how one single
foodborne disease epidemic or outbreak may disrupt the functioning of a
health-care system.

Epidemiologic surveillance of foodborne illness is fundamental to the
planning of food safety programs and the development of a strategy for
prevention and control. There are different methods of surveillance: death
registrations and hospital discharges; disease notification;
laboratory-confirmed cases; sentinel surveillance; surveillance of
investigated outbreaks; population-based surveillance; and case-control
studies of sporadic cases (8). This article examines the role of foodborne
disease surveillance programs, nationally and internationally, in the
control and prevention of foodborne disease.

Foodborne Disease Awareness of Public Health Authorities

Data on the incidence of foodborne illnesses collected through
notifications, laboratory confirmations, and sentinel or population-based
studies can provide a measure of the magnitude of the foodborne disease
problems, their economic consequences, and over the years, an indication of
the trend. Although several weaknesses are associated with the collection
of such data-particularly those collected through notification and
laboratory confirmations (since they represent only the tip of the
iceberg)-they can nevertheless be useful in raising the awareness of public
health authorities about the importance of food safety.

Surveillance data collected in some industrialized countries confirm that
foodborne diseases constitute one of the most widespread health problems
and that they have increased over the last two or three decades (Figures
1-4). Part of the increase may be attributable to recent improvements in
information reporting and collection systems, improved diagnoses, or
greater publicity and concern about food safety in general. However, a real
increase of foodborne disease incidence is not disputed. First, the
increase has been steady and cannot be explained by a one-time improvement
in the surveillance system. Second, increases have been observed in
different countries, including those with no improvement in reporting and
surveillance programs. The general increase, as demonstrated by the results
of surveillance data, has led many public health authorities to take
stringent regulatory and educational measures to improve food safety, with
some successful results (9). For instance, in the United States, active
surveillance of foodborne listeriosis has led to concerted efforts by
industry and government to prevent the disease. As a consequence, the
number of cases and deaths has decreased by 44% and 48%, respectively (10).

[Figure Not Available in ASCII]
Figure 1. Laboratory reports of gastrointestinal infections in England and Wales.

[Figure Not Available in ASCII]
Figure 2. Incidence of foodborne diseases in Venezuela.

[Figure Not Available in ASCII]
Figure 3. Incidence of salmonellosis in the United States, Japan, and Australia.

[Figure Not Available in ASCII]
Figure 4. Incidence of infectious enteritis and typhoid and paratyphoid fevers in Germany.

Public health authorities must be aware of the magnitude and trend of
foodborne illness so that necessary resources can be mobilized to improve
food safety programs. Lack of reliable epidemiologic data in many parts of
the world has impeded the recognition of the public health importance of
food safety and consequently the emphasis on food safety programs.

Early Detection Of Foodborne Disease Outbreaks

Surveillance of foodborne diseases plays an important role in the early
detection of foodborne disease outbreaks and their control. Early
identification of the source of the outbreak is becoming increasingly
important as countries move towards industrialization. Increased mass
production means outbreaks can change from being small and confined to a
family to large, affecting hundreds or even thousands of people (Table).

Table. Examples of large foodborne disease outbreaks
------------------------------------------------
   Country    Year        Disease      No. cases
------------------------------------------------
United
Kingdom      1985    Salmonellosis     1,000
United
States       1985    Salmonellosis     >168,000
United
States       1993    Salmonellosis     224,000
China        1988    Hepatitis A       >310,000
Germany      1993    Salmonellosis     1,000
Australia    1991    Norwalk-like agent>3,050
United               E. coli O157
States       1992-93 infection         >500

Japan        1996    E. coli O157      >6,000
                     infection
------------------------------------------------

Rapid investigation of foodborne disease outbreaks is crucial to prevent
them from taking on massive proportions. In the 1993 French outbreak of
listeriosis due to potted minced pork (affecting 39 persons and causing
eight miscarriages and one death), public health authorities traced its
source within 1 week and thus prevented the outbreak from spreading by
removing the implicated food product from the market and informing the
group at risk about its unsafe nature (11). In an outbreak of botulism in
the United Kingdom traced to hazelnut yogurt, the source was identified
within 3 days, and the product was withdrawn from the market (12).

Because of global food distribution and worldwide travel, an international
exchange of information on foodborne disease incidences and outbreaks and
the foods involved is extremely important to identify international
clusters originating from a common source. For instance, Salm-Net, a
network for the international surveillance of human salmonellosis, has
demonstrated the value of such an interactive international collaboration.
Individual countries with apparently isolated outbreaks can feed their
information into the network and ascertain whether the outbreak is confined
to their country or is of wider international importance. The
identification and investigation of several international outbreaks have
been simplified by the Salm-Net network.

Food, the Transmission of Diseases, and the Identification of Associated
Risk Factors

Information collected through investigation of foodborne disease outbreaks
or case-control studies of sporadic cases provides a better understanding
of the role of food in the transmission of communicable diseases and in the
identification of risk factors leading to disease. Epidemiologic data from
foodborne disease surveillance can provide public health authorities with
important information about the types of food implicated in outbreaks;
populations at risk; practices that lead to contamination, growth, and
survival of foodborne pathogens; and places where foods are often
mishandled. Such data are essential for designing effective intervention
programs. Such programs in industrialized countries, for example, have
demonstrated the relatively greater prevalence and incidence of foodborne
diseases of microbial origin over those of chemical origin and the role of
food handlers in the transmission of diseases; they have identified
campylobacteriosis and salmonellosis (particularly infections caused by
Salmonella Enteritidis) as the leading foodborne diseases. The emergence of
other diseases, such as infections due to E. coli O157 and Listeria
monocytogenes-often with serious sequelae-has been pinpointed as a major
public health problem. These surveillance programs have also alerted public
health authorities to the foods most often implicated and the major risk
factors in food preparation.

Because of the lack of epidemiologic data, the role of food in the
transmission of diseases has been poorly acknowledged, particularly in
developing countries. Diarrheal diseases in infants and children and
diseases such as shigellosis and cholera have been perceived as being
water-borne for many years. For instance, after the cholera epidemic in
Peru (where epidemiologic investigations implicated, among other foods,
seafood, and an embargo was placed on trade in foodstuffs), the role that
food plays in the transmission of the disease began to be fully recognized.

Increased trade in food, international travel and migration, and economic
and technologic development have changed dietary habits. New foods, food
preparations, and dietary habits are introduced into different regions, and
as a consequence, foodborne diseases are emerging or reemerging. Dietary
habits are also changing as a result of nutritional recommendations and
campaigns or may be influenced by food policy, production systems, or
environmental changes that lead to increased access to certain foods. These
changes in dietary habits influence the epidemiology of foodborne illnesses
and contribute to the emergence of foodborne diseases. In the United
States, public information campaigns promote an increased consumption of
fruits and vegetables. To meet the increased demand, these products have to
be imported on a seasonal basis. At certain times of the year, more than
75% of the fresh fruits and vegetables available in grocery stores and
restaurants are imported (13). Epidemiologic data have shown that, partly
as a consequence of the increased consumption of fruits and vegetables, the
proportion of foodborne disease outbreaks has doubled (14).

Data collected through foodborne disease surveillance programs permit the
monitoring of changes in the epidemiology of foodborne diseases and the
identification of new pathogens and new dietary or food preparation habits
that may present a health risk. The data can also determine if existing
programs need to be readjusted to ensure that the food safety program is
adequate and relevant.

A method used in recent years to complement epidemiologic data in
identifying risky practices and behavior is the Hazard Analysis and
Crit
ical Control Points system (HACCP). Application of HACCP to food
preparation permits the identification of practices that may be potentially
hazardous and need to be modified or those that are critical for ensuring
the safety of foods and require specific monitoring. However, the first
principle of HACCP-to conduct a hazard analysis-calls for epidemiologic
data on foodborne diseases, as the process involves an appraisal of the
possibility of hazards and the severity of their effects; the qualitative
and quantitative evaluation of the presence of hazards; the survival and
multiplication of microorganisms of concern; the production or persistence
of toxins, chemicals, or physical agents in foods; and, conditions leading
to the above.

As demonstrated in the decision tree for hazard analysis (Figure 5) (15),
access to information would be difficult without epidemiologic surveillance
of foodborne diseases. Similarly, epidemiologic data are also needed to
develop sampling plans of food, as demonstrated in the decision tree for
Listeria monocytogenes sampling plans of foods (Figure 6) (16).

[Figure Not Available in ASCII]
Figure 5. Hazard identification: identification of potentially hazardous 
microorganisms (15).

[Figure Not Available in ASCII]
Figure 6. Listeria monocytogenes sampling plans of foods that did not receive an
in-pack listericidal treatment (16).

Planning and Evaluating the Effectiveness of Food Safety Programs

The collection of epidemiologic data is important in planning interventions
and setting priorities. Countries with scarce resources, facing an abundant
number of foodborne diseases and food safety problems, need to prioritize
food safety issues. Epidemiologic data provide a basis for identifying
foodborne diseases, groups at risk, or even priority points in the food
chain.

Evaluating the effectiveness and impact of an intervention is an important
element of any plan. Data collected through disease notification or
sentinel studies permit an evaluation of the effectiveness of interventions
and their impact on health, and if necessary, the adjustment of a program
to improve its efficacy and impact. Data on the rising incidence of
foodborne illnesses in many countries demonstrate that present prevention
strategies, mainly based on regulatory measures, are inadequate and
emphasize the need for additional measures (e.g., additional regulatory
initiatives and health education about food safety).

Risk Assessment and International Food Standards

The movement of ever-increasing quantities of food across borders has
resulted in a transnationalization of disease risk (17). Therefore, the
globalization of food trade and the open access to foreign markets need to
be accompanied by effective means of health protection for populations. In
the food sector, international regulatory instruments need to be integrated
with strengthened surveillance and monitoring.

As a result of the Uruguay Round of Multilateral Trade Negotiations and the
increased liberalization of trade facilitated by this agreement, concern
about the safety of imported food has grown. However, provisions in the
Agreement on the Application of Sanitary and Phytosanitary Measures, which
entered into force with the establishment of the World Trade Organization
on January 1, 1995, are designed to address these concerns: according to
the work of the Codex Alimentarius Commission, its standards, guidelines,
and recommendations are recognized as the reference for national food
safety requirements. Countries that are members of the World Trade
Organization may no longer be able to reject foods that meet Codex
standards, guidelines, and recommendations without providing justification.

Moreover, the increased volume of the global food trade underscores the
need for sound epidemiologic information and international risk assessment.
In this regard, Article 5 of the Sanitary and Phytosanitary Measures
agreement explicitly requires World Trade Organization members to conduct
scientific and consistent risk assessments. Furthermore, the World Health
Organization has recommended that the application of the HACCP system at
every stage of the food chain represents an effective approach for
governments to meet the terms outlined in the agreement (18).

Another issue receiving more attention from regulatory agencies and
underlined during the Food and Agriculture Organization/World Health
Organization Conference on Food Standards, Chemicals in Food, and Food
Trade (1991), is the scientific basis of the Codex standards. The
Conference recommended that the Codex, in its norm-setting work on health
and safety, place greater emphasis on risk assessment (19). Epidemiologic
data on foodborne diseases have an important role in risk assessment. One
example is assessing the risk of contracting listeriosis associated with
different levels of Listeria monocytogenes in smoked fish and meat products
(16). However, the need for risk assessment as the basis for setting
standards has shown a great gap in knowledge about foodborne pathogens and
their relation to human illness (20-22). To address the
national/transnational risks caused by foodborne diseases, this gap must be
narrowed.

Risk Assessment Approach

Risk assessment is defined as a scientifically based process that has the
following steps: 1) Hazard identification-The identification of biologic,
chemical, and physical agents present in a particular food or group of
foods that can cause illness. 2) Hazard characterization-The qualitative or
quantitative evaluation of the nature of the illness associated with
biologic, chemical, and physical agents that may be present in food. For
chemical agents, a dose-response assessment should be performed. For
biologic or physical agents, a dose-response assessment should be performed
if the data are obtainable. 3) Exposure assessment-The qualitative or
quantitative evaluation of the likely intake of biologic, chemical, and
physical agents in food as well as exposures from other sources. 4) Risk
characterization-The qualitative or quantitative estimation, including
uncertainties, of the probability of and severity of known or potential
illness in a given population on the basis of hazard identification, hazard
characterization, and exposure assessment.

In many cases, data are not available to support a quantitative risk
assessment of biologic hazards. We discuss next the types of challenges
that make quantitative risk assessment difficult for pathogenic organisms
associated with food and the role of epidemiologic surveillance.

    Hazard Identification

    Because only some foodborne disease outbreaks are adequately
investigated and have the etiologic agents identified, many foodborne
pathogens remain unidentified. Most of the available epidemiologic data are
furnished by industrialized countries, while the situation in developing
countries is largely unknown. The epidemiologic database must be extended
to include information from developing countries. However, investigation
and surveillance systems in developing countries need to be strengthened
before the database can expand.

    Hazard Characterization

    For many foodborne pathogens, dose-response data are limited or
nonexistent. Information on which dose-response estimates can be based is
difficult to obtain and may be inaccurate for various reasons: host
susceptibility to pathogens is highly variable, attack rates from a
specific pathogen may vary widely, virulence of a pathogenic species is
highly variable, pathogenicity is subject to genetic variation resulting
from frequent mutation, antagonism from other bacteria in foods or the
digestive system may influence pathogenicity, and foods may modulate the
ability of bacteria to infect or otherwise affect the host.

    Exposure Assessment

    An exposure assessment will give an estimate of either the number of
pathogenic organisms or the level of toxins consumed in food. Although the
levels of chemical agents in food may change only slightly due to
processing, the population of bacterial agents is dynamic and may increase
or decrease dramatically. Changes in populations of bacteria are affected
by complex interactions of these factors: ecology of the bacterial
pathogen; processing, packaging, and storing of food; preparation steps,
such as cooking, which may inactivate bacterial agents; and cultural
factors relating to consumers.

In addition, for some of the emerging foodborne pathogens, the sources of
exposure are still not fully understood. Information on foodborne disease
outbreaks provides an opportunity to learn about the types of foods that
may harbor the pathogen.

    Risk Characterization

    Characterizing the risk associated with biologic pathogens depends on
information gained in the previous steps. Risk characterization will result
in a qualitative or quantitative estimate of the potential for adverse
effects from a particular pathogen on a specific population. Whether a
quantitative risk assessment approach is possible and appropriate for
characterization of risks associated with foodborne pathogens is not known.
Thus, the qualitative approach to characterizing risk may be the only
alternative.

International Travel

International travel and migration are contributing factors in the spread
of foodborne diseases in some countries. For instance, 80% to 90% of the
incidence of salmonellosis in Scandinavian countries is attributed to
international travel. Surveillance of travel-related foodborne diseases
provides a mechanism for appreciating the relative prevalence of foodborne
diseases in various countries. It also provides a basis for informing
physicians and health services about unfamiliar diseases contracted by
travelers returning from distant places. In this way, advice on
precautionary measures can also be given to travelers. The only foodborne
disease now covered by the International Health Regulations is cholera,
which is reported to the World Health Organization. Since the purpose of
these regulations is to help provide maximum security against the
international spread of diseases with a minimum of interference with world
traffic (i.e., trade and travel) (23), it is timely to consider whether the
regulations should cover additional foodborne diseases.

Conclusion

The globalization of the risks associated with foodborne illness,
specifically increased international travel and trade in food, has resulted
in greater interdependence in terms of food safety. Therefore,
internationally agreed-upon food safety standards and other types of
agreements are becoming increasingly important in addressing the complex
transnational challenge of foodborne disease control. Epidemiologic data
provide a common ground for reaching international consensus on food safety
issues.

As Morris Potter has said, "If one recognizes that ensuring food safety is
inherently uncertain, foodborne illnesses become opportunities to learn
rather than failures to predict. Foodborne disease will occur, and we must
be prepared to react quickly to reduce the risk of new foodborne hazards"
(24).

Address for correspondence: F.K. Kferstein, Food Safety Unit, World Health
Organization, 20 Avenue Appia, CH-1211 Geneva 27, Switzerland; fax:
41-22-791-4807; e-mail: Kfersteinf@who.ch. 

References

  1. Kemel, W. Health dilemma at the borders: a call for global action. In:
     Proceedings of the 34th Session of the WHO Advisory Committee on
     Health Research; 1996 Oct; Geneva, Switzerland. Geneva: World Health
     Organization; 1996.
  2. FAO/WHO. The role of food safety in health and development. Report of
     Joint FAO/WHO Expert Committee on Food Safety. World Health Organ Tech
     Rep Ser; 1984;705.
  3. Fidler D. Globalization, international law and emerging infectious
     diseases. Emerg Infect Dis 1996;2:77-84.
  4. FAO. The world food summit. FAO technical background document no.
     12;1996.
  5. WHO. International response to epidemics and applications of the
     International Health Regulations: report of a WHO consultation.
     Geneva: World Health Organization unpublished
     document;1996;WHO/EMC/IHR/96.1.
  6. Berkelman RL, Bryan RT, Osterholm MT, LeDuc JW, Hughes JM. Infectious
     disease surveillance: a crumbling foundation. Science 1994:264:368-70.
  7. Ferriman A. Doctors warn of a return of past plagues. The Independent
     1997 Mar 7; News section:5.
  8. Borgdorff MW, Motarjemi Y. Foodborne disease surveillance: What are
     the options? World Health Stat Quart 1997;50(1/2):12-23.
  9. Motarjemi Y, Kferstein FK, Moy G, Miyagawa S, Miyagishima K.
     Importance of HACCP for public health and development. The role of the
     World Health Organization. Food Control 1996;7:77-85.
 10. Tappero JW, Schuchat A, Deaver KA, Mascola L, Wenger JD. Reduction in
     the incidence of human listeriosis in the United States. JAMA
     1995;273:1118-22.
 11. Goulet V. Investigation en cas d'pidemie de listriose, Md Mal
     Infect 25 Spcial 1995;184-90.
 12. O'Mahony M, Mitchell E, Gilbert RJ, Hutchinson DN, Begg NT, Rodhouse
     JC, Morris JE. An outbreak of foodborne botulism associated with
     contaminated hazelnut yoghurt. Epidemiol Infect 1990;104:389-95.
 13. Hedberg CW, MacDonald KL, Osterholm MT. Changing epidemiology of
     food-borne diseases: a Minnesota perspective. Clinical Infect Dis
     1994;18:671-80.
 14. Wachsmuth K, Kruse H, Tauxe R, Hedberg C, Potter M. Microbial hazards
     and emerging issues associated with produce. Journal of Food
     Protection. In press 1997.
 15. Notermans S, Mead GC. Incorporation of elements of quantitative risk
     analysis in the HACCP system. Int J Food Microbiol 1996;30:157-73.
 16. Van Schothorst M. Sampling plan for Listeria monocytogenes. Food
     Control 1996;7:203-8.
 17. Nakajima H. Global disease threats and foreign policy. Brown Journal
     of World Affairs. In press 1997.
 18. WTO. Selected World Health Organization activities relevant to the
     application of sanitary and phytosanitary measures. Geneva:
     WTO;1995;G/SPS/W/37.
 19. FAO/WHO. Report of the Joint FAO/WHO Conference on Food Standards,
     Chemicals in Food and Food Trade. Rome: FAO;1991.
 20. FAO/WHO. Report of the Joint FAO/WHO Expert Consultation on
     Application of Risk Analysis to Food Standards Issues, Geneva, 1995
     13-17 Mar. Geneva: World Health Organization, unpublished
     document;1995;WHO/FNU/FOS/95.3.
 21. FAO/WHO. Report of the Joint FAO/WHO Expert Consultation on Risk
     Management and Food Safety Issues, Rome, 1997 27-31 Jan. FAO Food Nutr
     Pap 1997;62.
 22. Bernard DT, Scott VN. Risk assessment and foodborne micro-organisms:
     the difficulties of biological diversity. Food Control 1995;6:329-33.
 23. WHO. International Sanitary Regulations: World Health Regulation No.
     2. Geneva: World Health Organization;1951.
 24. Potter M, Gonzalez-Ayala S, Silarug N. The epidemiology of foodborne
     diseases. In: Doyle MP, Beuchat L, Montville T, editors. Fundamentals
     of Food Microbiology. Washington (DC): American Society for
     Microbiology; 1997.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/kaferste.txt

Consumer Concerns: Motivating to Action

Christine M. Bruhn
University of California, Davis, California, USA
------------------------------------------------------------------------

      Microbiologic safety is consumers' most frequently volunteered
      food safety concern. An increase in the level of concern in
      recent years suggests that consumers are more receptive to
      educational information. However, changing lifestyles have
      lessened the awareness of foodborne illness, especially among
      younger consumers. Failure to fully recognize the symptoms or
      sources of foodborne disease prevents consumers from taking
      corrective action. Consumer education messages should include
      the ubiquity of microorganisms, a comprehensive description of
      foodborne illnesses, and prevention strategies. Product labels
      should contain food-handling information and warnings for
      special populations, and foods processed by newer
      safety-enhancing technologies should be more widely available.
      Knowledge of the consequences of unsafe practices can enhance
      motivation and adherence to safety guidelines. When consumers
      mishandle food during preparation, the health community, food
      industry, regulators, and the media are ultimately responsible.
      Whether inappropriate temperature control, poor hygiene, or
      another factor, the error occurs because consumers have not
      been informed about how to handle food and protect themselves.
      The food safety message has not been delivered effectively.

Consumer Knowledge and Concern

Consumers are receptive to information about microbiologic hazards.
Nationwide surveys by the Food Marketing Institute indicate that more
people volunteer concerns about microbiologic hazards than about any other
potential food safety issue. From 1992 to 1996, volunteered concern about
microbiologic safety increased from 36% to 49% (1). Specifically, concern
about contamination by bacteria or other microorganisms was 77%, more than
concern about pesticide residues (66%), product tampering (66%), antibiotic
residues (42%), or any other food safety risk.

Food-Handling Practices

Although many consumers recognize the potential seriousness of foodborne
bacteria, they lack information on safe handling and storage of food
products (2). Williamson et al. (3) found that consumers under 35 years of
age knew less about food safety terms and concepts than those over 35.
Specific safe food handling was not practiced by 15% to 30% of survey
respondents. For example, consumers did not cool cooked food rapidly, with
29% indicating they would let roasted chicken cool completely before
refrigerating. Only 32% indicated they would use small, shallow containers
to refrigerate leftovers. Consumers did not know that failure to
refrigerate may jeopardize safety, with 18% not concerned or uncertain
about the safety of cooked meat and 14% not concerned about poultry left
unrefrigerated for more than 4 hours. The need for sanitation was not
recognized, with only 54% indicating they would wash a cutting board with
soap and water between cutting raw meat and chopping vegetables.

Food safety experts have identified the most common food-handling mistakes
made by consumers at home. These mistakes include serving contaminated raw
food, cooking or heating food inadequately, obtaining food from unsafe
sources, cooling food inadequately, allowing 12 hours or more between
preparation and eating, and having a colonized person handle implicated
food or practice poor hygiene (4). The same factors were identified in
mishandling associated with specific pathogens (5).

Changing Lifestyles

Many factors have contributed to consumers' lack of familiarity with safe
food handling and increased foodborne illnesses. Increased participation in
the paid labor force has lessened the exposure of young people to
food-handling practices in the home; few schools offer or require food
preparation classes; and partially prepared foods may have different, less
familiar handling requirements (2,6).

People eat out more frequently today, thereby increasing their exposure to
the food service industry, noted for high turnover rates and minimal job
training in personal hygiene (7). Furthermore, the population is shifting,
with an increased percentage of persons at higher risk for foodborne
illness because of age or health status (8,9). Additionally, some food
safety recommendations related to temperature and acidity do not eliminate
risks from some pathogens (2).

Nature and Source of Foodborne Illness

Consumer perceptions and behavior related to foodborne illness changed
little between 1988 and 1993 (10). Consumers misperceived the nature of
foodborne illness and the most likely pathogen source. Consumer belief
about the type of food responsible for foodborne illness--meat, poultry,
seafood, eggs--was consistent with expert opinions; however, consumers
believed that foodborne illness was generally mild, without fever, and
occurred within a day of eating contaminated food. Infections caused by
Salmonella and Campylobacter, the most common foodborne illnesses in the
United States (11), are not consistent with the symptoms consumers
described, because these organisms have longer latency and cause fever.

Most consumers believed that their foodborne illness was caused by food
prepared somewhere other than the home. Williamson (3) found that about
one-third of consumers thought food safety problems most likely occurred at
food manufacturing facilities, and one-third blamed unsafe restaurant
practices. Only 16% thought mishandling was most likely to occur in the
home. Fein et al. (10) found that 65% of consumers attributed foodborne
illness to food prepared at a restaurant, 17% to mishandling at the
supermarket, and 17% to mishandling at home. In contrast, food safety
experts believe sporadic cases and small outbreaks in the home are far more
common than recognized outbreaks (2).

Failure to recognize the home as a likely source of foodborne illness is
not unexpected because illness traced to a food establishment affects many
people and may receive widespread publicity (12). Illness that occurs at
home is rarely reported unless severe (2).

If consumers misperceive the nature and origin of foodborne illness, they
underestimate the frequency of serious consequences and are less motivated
to change. Schafer et al. (13) found that motivation for proper food
handling requires viewing the mishandling of food as a direct threat to
one's health. The failure to associate mishandling of food in the home with
foodborne illness interferes with foodborne disease education efforts (10).

Ubiquity of Organisms

Consumers do not seem to be aware of the ubiquity of microorganisms in the
environment. During foodborne disease outbreaks, press accounts focus on
fecal contamination of food. Government standards classify natural
microorganisms as contaminants, which suggests that microorganisms are only
present as a result of mishandling. In contrast, Hazard Analysis and
Critical Control Points (HACCP) programs recognize and attempt to control
potential dangers related to pathogenic microorganisms.

When consumers in a national sampling were asked on whom they rely for
product safety, the percentage responding "myself as an individual"
decreased from 48% in 1989 to 25% in 1996 (1). As self-reliance decreased,
consumer reliance on food manufacturers and supermarkets increased. This
may be a response to the message that if raw food contains microorganisms,
it is contaminated. It suggests some consumers are shifting the
responsibility for safe food to manufacturers and retailers.

Consumers may not realize they can introduce pathogens during food
handling. In 1990, the Food Marketing Institute asked consumers what steps
they took at home to ensure the safety of food (14). Respondents
volunteered refrigeration (58%), proper storage (35%), checking expiration
dates (26%), washing and cleaning the food (25%), cooking properly (22%),
and wrapping food properly (20%). No one volunteered washing hands or
preventing cross-contamination.

Labeling

Products must contain safety labels instructing consumers how to handle
food. In 1989, the National Advisory Committee on Microbiological Criteria
for Foods recommended a mandatory uniform logo for perishable refrigerated
foods, uniform labeling for frozen food, "Use by" dates, and
time/temperature indicators wherever possible (15).

Although products are currently labeled when they require refrigeration,
the label is ineffective because the warning is difficult to find or read.
As the proportion of older people increases, print must be larger. Labels
should also display symbols to further enhance the effectiveness of the
message.

Safe-handling labels on meat products appear to have made a difference. The
Food Marketing Institute (1) found that 60% of survey respondents had seen
the labels. Of those aware of the labels, 65% said the labels increased
their awareness of safety, and 43% said they changed their behavior as a
result of the information. The most common volunteered change was washing
the counter and utensils after contact with meat (approximately 40%),
followed by washing hands more frequently (approximately 20%) and cooking
to the proper temperature (approximately 20%).

Labels do not consistently contain needed information. When foodborne
illness was related to consumption of unpasteurized apple juice, consumers
were not able to determine from the label which products were pasteurized.
Many major manufacturers do not indicate whether their fresh juice product
is pasteurized.

Consumers are not advised about potential risks for special populations.
Raw milk sold in California must contain a warning statement, but other
states may not have this requirement. Because of inconsistencies in
labeling, unpasteurized juice products may be given to infants. Also,
products that contain honey do not include a warning about potential risk
for infant botulism.

Processing Technology

Consumers do not realize that pathogens can survive minimal processing, as
evidenced by a recent Escherichia coli outbreak associated with fresh apple
juice, which demonstrated that processors also may not recognize potential
risks. A fresh apple juice manufacturer in northern California claimed its
product was safe because the juice was squeezed in small batches and frozen
immediately.

Freezing is not effective against E. coli O157:H7, but other methods are
protective. Several methods have been developed to reduce pathogens and
increase the safety of foods. Once these methods are verified as effective
and safe, the food industry should be free to use them, and consumers
should have the opportunity to select safer foods. In some cases, the
regulatory approval process appears to hinder rather than facilitate the
safer handling of food.

Food irradiation, exposing food to high levels of electromagnetic energy
for specific purposes, has been approved for selected uses. A petition
before the Food and Drug Administration to permit irradiation of meat and
other muscle foods appears to have satisfied safety concerns, but approval
has not yet been granted. The requirement to seek approval for each
application of irradiation prevents rapid response in cases of foodborne
outbreaks. Although this regulatory procedure may have been reasonable when
irradiation was first introduced, it warrants a fresh look in view of the
wealth of data now available on the safety and wholesomeness of food
irradiation (16).

Attitude surveys and marketing experience consistently demonstrate that
consumers will purchase irradiated food (17). National surveys indicate
consumer concern about irradiation was lower than other food-related
concerns. When specifically asked what they considered a serious health
hazard, 29% identified irradiation, 77% identified bacteria, and 66%
identified pesticides (1). The percentage of consumers concerned about
irradiation has decreased significantly over time. In the late 1980s, 42%
to 43% classified irradiation as a serious concern, decreasing to 29% in
1996. A relative ranking of food processing methods surveyed by the Gallup
Organization found that irradiation, food preservatives, and chlorination
generated similar concern (18).

In a nationwide Food Marketing Institute survey, 69% of consumers indicated
they were very or somewhat likely to purchase products irradiated to kill
bacteria or other microorganisms (1). Surveys completed in several areas of
the country indicate 60% to 70% of consumers would prefer irradiated food
(17). In one study, information about irradiation increased interest in
purchasing to 90%, and education plus food samples increased purchase
intent to 99%.

Consumers have purchased irradiated food in select locations across the
United States since 1992 (17). Fruits from the Mainland and Hawaii have
sold well in the Midwest and California (L. Wong, pers. comm.). Irradiated
chicken gained over 60% of market share when priced 10% lower than
nonirradiated chicken and 47% when priced the same (J.A. Fox, pers. comm.).

Irradiated food is not widely available. Special interest groups threaten
companies that exchange information about irradiation processing.
Consumers, however, appear to prefer irradiated foods when the benefits of
these foods are endorsed by health professionals. Food manufacturers and
retailers should offer consumers the choice of safety-enhanced,
energy-pasteurized irradiated food.

Communicating with the Public

To respond to consumers' need for information, a multifaceted program is
needed. The HACCP strategy, which teaches consumers to critically think
through the food safety process to determine how foodborne illness could
occur, has been effective (19,20). The HACCP approach to home food
preservation is logical and highlights key control points (21).

Consumers should be informed that microorganisms are ubiquitous in the
environment, found on raw products of animal or plant origin. Pathogens may
survive minimal processing and preservation treatments. People may
introduce pathogens during any stage of food processing or handling,
including just prior to consumption. Foodborne illness can range from mild
to severe and life-threatening with chronic complications. People have
control and can reduce risks.

Communicating food safety information to the public effectively is another
challenge. Consumers obtain most of their information on food, nutrition,
and science from the media; television is cited most frequently, and
newspapers and magazines follow (22,23). Brochures enforce messages and
serve as useful references, although they are not as widely seen as media
stories.

Developing messages with the press should be a primary activity of a food
safety education program. Consumers judge a message by the credibility of
the person conveying it, its appeal to their common sense, and the
frequency of the message (24). Media presentations can motivate people to
listen and change behavior. Consultants from the USDA hotline say, "We've
seen an explosion in media coverage of food safety, and callers want more
detailed explanations of things they read and hear" (25).

Information on safe food handling must be motivating and memorable. Stories
that capture the public's attention are personal. They relate life
experiences of people with whom the public can identify. Stories of the
consequences of mistakes are memorable. They can be touching, humorous, or
grotesque. It is easy to visualize and remember the infected bakery worker
who made 5,000 people ill when he mixed a vat of buttercream frosting with
his bare hands and arms despite bouts with diarrhea (26).

Stories can be heartrending, as in the experiences of a family who lost a
child to E. coli O157:H7 infection. It is difficult to document the
effectiveness of vivid accounts of doing things right or wrong. How
ever,
when Washington state carried extensive coverage linking the outbreak to
undercooked hamburgers, 13% of the men said they ate undercooked hamburger,
compared with 38% in Colorado (25).

Conclusions

Consumer concerns about foodborne illness can motivate change in regulatory
and industry use of technology, product labeling, and consumer education.

New Technologies

The food industry has both a right and a responsibility to use safe and
effective technology to enhance the safety of the food supply. Regulatory
authorities should expediently evaluate and facilitate new technologies,
such as food irradiation, laser light treatment, and high-pressure
processing, which enhance food safety. Health professionals, the food
industry, and regulators should challenge special interest groups that
distort information and strive to limit consumer choice.

Improved Labeling

Processed and packaged food should bear labels that clearly indicate how
food should be handled. Labels should include warnings about special risks
to select populations. Benefits from special processing that can reduce
microbes should also be encouraged.

Consumer Education

Consumers need to appreciate the seriousness of foodborne diseases. They
must learn to recognize unsafe food-handling practices, the latency period
for some microbes, and the symptoms of foodborne disease. They need to
understand how to protect themselves through kitchen and personal hygiene,
including thoroughness and frequency of hand washing, temperature control,
and safe food choices such as foods processed by heat or energy
pasteurization. Young people should be reached through age-specific school
curricula, such as personal hygiene and special "living skills" units that
address food safety and diet. Food industry and health educators should
work with the media to develop interesting and timely messages to increase
consumer knowledge about safe food handling. Messages must be consistent,
science-based, frequent, and personalized.

Address for correspondence: Christine M. Bruhn, Consumer Behavior,
University of California, Center for Consumer Research, University of
California, Davis, CA 95616-8598, USA; fax: 916-752-3975; e-mail:
cmbruhn@udavis.edu.

References

  1. Abt Associates Inc. Trends in the United States, consumer attitude and
     the supermarket 1996. Washington (DC): Food Marketing Institute; 1996.
  2. Institute of Food Technologists' Expert Panel on Food Safety and
     Nutrition. Scientific status summary, foodborne illness: role of home
     food-handling practices. Food Technology 1995;49:119-31.
  3. Williamson DM, Gravani RB, Lawless HT. Correlating food safety
     knowledge with home food-preparation practices. Food Technology
     1992;46:94-100.
  4. Bryan F. Risks associated with vehicles of foodborne pathogens and
     processes that lead to outbreaks of foodborne diseases. Journal of
     Food Protection 1988;51:663-73.
  5. Bean NH, Griffin PM. Foodborne disease outbreaks in the United States,
     1973-1987; pathogens, vehicles, and trends. Journal of Food Protection
     1990;53:804-17.
  6. Dumagan JC, Hackett JW. Almost half of the food budget is spent eating
     out. Food Review 1995;18:37.
  7. Martin R. Foodborne disease threatens industry. National Restaurant
     News 1990;24:27,30.
  8. Doyle MP. Food safety in the 21st century. Dairy Food Environ Sanit
     1993;13:383.
  9. Food Institute. Population trends 1996-2050. Fair Lawn (NJ): The
     Institute; 1996. p. 31.
 10. Fein SB, Jordan CT, Levy AS. Foodborne illness: perceptions,
     experience, and preventive behaviors in the United States. Journal of
     Food Protection 1995;58:1405-11.
 11. Tauxe RV. Epidemiology of Campylobacter jejuni infections in the
     United States and other industrialized nations. In: Nachamkin J,
     Blaser MJ, Tompkins LS, editors. Campylobacter jejuni: current status
     and future trends. Washington (DC): American Society for Microbiology;
     1992. p. 9-19.
 12. Ollinger-Snyder P, Matthews ME. Food safety issues: press reports
     heighten consumer awareness of microbiological safety. Dairy, Food and
     Environmental Sanitation 1994;14:580-9.
 13. Schafer RB, Schafer E, Bultena GL, Hoiberg EO. Food safety: an
     application of the health belief model. Journal of Nutritional
     Education 1993;25:17-24.
 14. Food Marketing Institute. Opinion research, trends 90, consumer
     attitudes & and the supermarket 1990. Washington (DC): The Institute;
     1990.
 15. Rhodes ME. Educating professionals and consumers about
     extended-shelf-life refrigerated foods. Food Technology 1991;45:162-4.
 16. Diehl JF. Safety of irradiated foods. New York: Marcel Dekker, Inc.;
     1995.
 17. Bruhn CM. Consumer attitudes and market response to irradiated food.
     Journal of Food Protection 1995;58:175-81.
 18. The Gallup Organization. Irradiation: consumers' attitudes. Princeton
     (NJ): Prepared for the American Meat Institute; 1993.
 19. Hoban T. Consumer awareness and acceptance of bovine somatotropin.
     Survey conducted for the Grocery Manufacturers of America; 1994.
 20. Hoban T, Kendall PA. Consumer attitudes about the use of biotechnology
     in agriculture and food production. Raleigh (NC): North Carolina State
     University; 1992.
 21. Bruhn CM, Peterson S, Phillips P, Sakovich N. Consumer response to
     information on integrated pest management. Journal of Food Safety
     1992;12:315-26.
 22. United States Department of Agriculture. The Food Safety Educator
     1996;1:3.
 23. Beware the Norwalk virus. Food Talk (Winter) 1989;4.
 24. Reicks J, Bosch A, Herman M, Krinke UB. Effectiveness of a food safety
     teaching strategy promoting critical thinking. Journal of Nutritional
     Education 1994;26:97-100.
 25. Medeiros LC, George RT, Gruns K, Chandler C, Crusey S, Fittro J, et
     al. The safe food handling for occasional quantity cooks curriculum.
     Journal of Nutritional Education 1996;28:39-42.
 26. United States Department of Agriculture, Food Safety and Inspection
     Service. A margin of safety: the HACCP approach to food safety
     education. Washington (DC): United States Department of Agriculture
     Information and Legislative Affairs; 1990.

------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

Indentifying and Controlling Emerging Foodborne Pathogens: Research Need

Robert L. Buchanan
USDA ARS Eastern Regional Research Center, Wyndmoor, PA, USA
---------------------------------------------------------------------------

      Systems for managing the risks associated with foodborne
      pathogens are based on detailed knowledge of the microorganisms
      and the foods with which they are associated-known hazards. An
      emerging pathogen, however, is an unknown hazard; therefore, to
      control it, key data must be acquired to convert the pathogen
      from an unknown to a known hazard. The types of information
      required are similar despite the identity of the new agent. The
      key to rapid control is rapid mobilization of research
      capabilities targeted at addressing critical knowledge gaps. In
      addition, longer-term research is needed to improve our ability
      to respond quickly to new microbial threats and help us become
      more proactive at anticipating and preventing emergence. The
      type of contingency planning used by the military in
      anticipating new threats serves as a useful framework for
      planning for new emergence.

The microbiologic safety of food has been advanced substantially by the
introduction and implementation of Hazard Analysis and Critical Control
Points (HACCP). HACCP provides a systematic conceptual framework for
identifying hazards and focusing efforts on the proper functioning of key
food production, processing, and marketing steps. When applied
appropriately, HACCP is a cost-effective means of controlling known hazards
in foods. Its successful implementation depends on knowledge of such issues
as the pathogenic microorganisms' virulence, cultural characteristics, ways
in which they contaminate the food, effects of food processing and
preparation on their survival, and food consumption patterns. Because it
requires substantial knowledge, HACCP cannot be expected to control unknown
hazards, such as emerging foodborne pathogens. Therefore, controlling a new
foodborne microbial threat requires moving the hazard as quickly as
possible from being unknown to being known. The key to this transition is
the timely acquisition of needed research data. This article identifies
classes of research information needed and discusses a conceptual approach
for addressing unknown microbial threats.

Anticipating the Next Emerging Pathogen

Two types of emergence are encountered with pathogenic foodborne
microorganisms. A true emergence, where a microorganism that had not been
identified as a public health threat begins to cause disease, is relatively
rare. More common is reemergence, where a known microorganism causes
disease in a new way, for example, by causing new types of infections,
being associated with new foods, or appearing in new geographic locations.
For both types, the operational requirement is to control an unanticipated
public health threat. The timeliness of response is critical since the
public health and economic costs of an emerging pathogen are directly
related to the time between its emergence and its control.

The events that lead to emergence are often complex, with the cause often
being obscure and only indirectly related to the new agent. Past emergence
of foodborne pathogens has been associated with changes in microbial
genotypes, demographics, food production and processing methods, marketing
and preparation practices, medical diagnostics, globalization of the food
industry, changes in consumer education, and general socioeconomic trends
(1-3). Planning for a microbial threat is a challenge because one does not
know what the agent will be, what food it will be associated with, or where
or when emergence will occur. While there are several potential ways of
anticipating and responding to microbial threats, the contingency planning
used by the military to anticipate threats seems well suited for emerging
pathogens. Military contingency planning can be viewed as having four major
components: intelligence, personnel and facilities, rapid response, and
strategic planning.

Intelligence is the gathering of medical, scientific, and other information
that allows emergence to be identified. In the United States, this role is
filled to a great extent by the Centers for Disease Control and Prevention
(CDC). In addition to providing information on known foodborne pathogens,
CDC works with local public health agencies and the medical and scientific
communities to investigate new disease syndromes and identify unrecognized
foodborne pathogens. This type of intelligence gathering played a pivotal
role in the recent recognition of Cyclospora as a cause of foodborne
gastroenteritis. CDC's new sentinel site program, FoodNet, is expected to
greatly enhance the identification of new foodborne diseases. However,
these surveillance activities are largely limited to the United States,
whereas an effective intelligence system for foodborne disease must be
worldwide. For example, Cyclospora was identified as a likely foodborne or
waterborne pathogen in Asia and South America before an outbreak was
reported in the United States. Intelligence related to foodborne disease
can be acquired from several sources: the World Health Organization's
surveillance program, the U.S. military's international network of
laboratory and medical investigators, medical and scientific reports, and
the Internet. The Internet is increasingly an important source of
intelligence related to emerging pathogens; through news groups and
bulletin boards such as ProMed, scientists and public health practitioners
share their experiences on almost a real-time basis. Such advances in
intelligence gathering are critical to reducing the time between emergence
and control. However, limiting intelligence to medical considerations is
not enough: intelligence gathering must include awareness of changes and
advances in food production methods, agricultural practices and conditions,
veterinary medicine, environmental and water microbiology, food
technologies, consumer trends, and general socioeconomic conditions.

The second component of contingency planning is ensuring sufficient
personnel and facilities to characterize a new biologic agent and develop
control strategies. The inability to predict the agent or the associated
food, coupled with the degree of specialization required of investigators,
requires a broad range of capabilities and resources. However, no one
organization is likely to maintain the capabilities needed to deal with all
contingencies. If we were to follow the military pattern, we would have
reserve groups that could be mobilized as needed. However, even this
approach requires planning and support to ensure the needed expertise and
facilities. For example, the number of researchers and laboratories
studying Clostridium botulinum has dropped to a point where it would be
difficult to rapidly mobilize a research team, despite this pathogen's
history of reemerging in a surprisingly wide range of foods.

Rapid mobilization of resources is the third component. This component is
particularly important for free-living infectious agents because one goal
is to limit their dissemination to prevent them from establishing secondary
reservoirs. It is much easier to fight a small, contained war than a global
one. The mobilization of resources to respond to an emergence must be
appropriate to the severity of the threat. Overreacting hurts the
credibility of the entire system, while underreacting increases both the
public health and economic impact. Rapid response efforts have focused at
identifying new agents and removing suspect food from the marketplace, two
key initial steps. However, research to prevent another occurrence of the
emerging pathogen has been much less organized and timely.

The fourth component of contingency planning, strategic planning, is
actually the first chronologically. This is the phase where members of war
colleges pose "what we would do if" scenarios and plan appropriate
responses. This type of contingency planning has generally received
attention in relation to emerging pathogens only in connection with the use
of biologic warfare agents. This process relies on futurist thinking to
consider how changes in society, economics, technology, agriculture,
medicine, and international trade may affect the microbiologic safety of
the food supply. Such a broad view is needed because more general events or
trends in society cause most disease emergence. This type of strategic
planning is undertaken with the realization that the probability of any
specific "what if" scenario is low, but the probability that one scenario
will materialize is extremely high.

Research Needs

Research, an integral part of responding to a new foodborne microbial
threat, is the key for moving a new or reemerging biologic agent from being
an unknown pathogen to being one for which control measures are available.
Two areas of research can be classified on the basis of time constraints.
Acute research needs are deficiencies in knowledge that must be addressed
to establish control of an emerging pathogen. This research is highly
targeted and specific for the microorganism and food of concern; it must be
accomplished as quickly as possible. Acute needs generally require applied
research, although basic research may have to be conducted if the
deficiencies in knowledge are great. The second class encompasses
longer-term basic and applied research needs not mandatory to immediate
control.

Acute Needs

While the data needed for any single emerging biologic agent are highly
specific, acute research needs fall into general categories that are
virtually the same for all new pathogens. Common research questions include
the following: Are methods available for detecting and categorizing the
agent? What food is the vehicle for the pathogen? How do the implicated
foods become contaminated? What is the pathogen's reservoir in nature? Is
the pathogen's presence in contaminated food the result of an error or
breakdown in normal controls? Does the pathogen grow in foods? Does the
pathogen survive normal food processing, distribution, and preparation? How
infectious/toxigenic is the pathogen? Are there subpopulations of consumers
at increased risk for this pathogen? Is the pathogen's ability to cause
disease restricted to specific strains with identifiable virulence
characteristics? Answering these questions requires specific data that do
not differ substantially from pathogen to pathogen (Table).

Table. Research data needed for most emerging foodborne pathogens.
-------------------------------------------------------------------
Research Area                Knowledge gaps
-------------------------------------------------------------------
                        Sampling and enrichment techniques
                        Cultivating
                        Biochemical/taxonomic char.
                        Antibodies for capture and differentiation
Detection methods       Subtyping
                        Virulence-associated char.
                        Detecting injured or viable-but-nonculturable
                          cells

                        Contaminated foods
                        Reservoirs and routes of transmission
                        Life cycles
Microbial ecology       Geogr. range and seasonality
                        Route of contamination and location of pathogen
                          in food

                        Dis. char. and diagnosis
                        Sequelae
Pathogenicity           Host range
                        Infectious dose
                        Subpopulations at risk
                        Animal models

                        Free-living vs. obligate parasite
Growth                  Growth requirements
characteristics                 Temperature
                                pH
                                Water activity
                                Oxygen
                        Heat resistance
                          D-values
                          Z-values
Survival                Susceptibility to antimicrobial food
characteristics           additives
                        Acid resistance
                        Sensitivity to disinfectants or dessication
                        Sensitivity to radiation
                           UV
                           Ionizing

                        Effectiveness of food preservation
Control                 Inspection systems to segregate contamination
                          materials

-------------------------------------------------

The criteria for classifying needs as acute are reasonably straightforward:
Is the research needed to prevent a recurrence of the disease or to modify
current HACCP plans? However, these questions have different priorities,
which depend on when the information is needed. To deal with emerging
pathogens, we should learn from modern business practices, especially the
concept of "just-in-time" research. Little consideration has been given to
how to assess and set research priorities for emerging foodborne pathogens.
One attempt was provided as an appendix of the U.S. Pathogen Reduction Task
Force. A relatively simple decision tree used a series of questions to
identify what research was the limiting step in responding to the foodborne
pathogen (4).

The timeliness of addressing research needs must be an integral part of the
planning process, but has been generally overlooked. Past research
mobilization efforts for new foodborne microbial threats can be best
described as haphazard, likely because they reflect the way research is
funded. The traditional means of ensuring strong research programs,
competitive funding of projects after proposals have undergone extensive
peer review, is time-consuming and often not appropriate for the acute
phase of responding to an emerging foodborne pathogen. Further, the
peer-review process tends not to select the often mundane research needed
during the acute phase of an emergence. Two alternative approaches may be
more effective. The first is to have a series of designated laboratories
that have as part of their mission and funding the task of being able to
modify their research programs to address acute research needs. Such
laboratories would need to have a critical mass of facilities and expertise
in various aspects of food safety microbiology. The second approach is to
have a group of reserve scientists with unique expertise or access to
facilities not available at the designated laboratories or needed to
supplement those capabilities. Funds could be earmarked to noncompetitively
fund such reserve scientists on an as-needed basis, with the understanding
that research needs designated as acute would take precedence over other
research needs.

Longer-Term Needs

The three areas of longer-term research associated with emerging pathogens
are amenable to more traditional funding. The first area, specific to the
new pathogen, consists of research for improvements or alternatives to the
detection and control methods initially devised. With initial disease
control established, basic and applied research can seek to understand the
microorganism and develop more optimal approaches for its prevention,
control, or elimination. The second area concerns activities to help reduce
the time between the emergence of a pathogen and its initial control (e.g.,
improved surveillance through the development of new diagnostic methods and
further identification and characterization of virulence determinants and
modes of pathogenicity to accelerate detection of new agents). Just as
important as acquiring research data is rapid data dissemination. The
continuing development of computer-based information networks is a
component of this second research area.

The third area focuses on identifying research factors that will allow new
micr
obial threats to be anticipated. Of necessity, the current response to
emerging pathogens is almost entirely reactive. The public health community
detects a new syndrome, and only then is research-mobilized, often during a
crisis. While reactive response will always be part of dealing with
emerging microbial threats, a more proactive approach is needed if
prevention is to be even partially realized. In military terms, war is the
last resort and represents the failure of diplomats to predict and prevent
a crisis. Microbial threats, like wars, do not spontaneously emerge but are
the result of a series of events or conditions. There is a need to
reexamine how food is produced, processed, marketed, and prepared to
identify conditions that contribute to emergence. For example, organic
acids are used extensively throughout the food industry to control spoilage
and pathogenic microorganisms. Archer (5) hypothesized that over time,
exposure to pH conditions that stress but do not kill may lead to the
emergence of hardier and possibly more virulent foodborne pathogens. It is
already well established that the induction of acid tolerance can enhance
both the survival and virulence of foodborne pathogens (6). Further, one of
the basic tenets of microbial genetics is that conditions that kill most,
but not all, of a bacterial population foster the development of
resistance. This is supported by recent studies that suggest that bacterial
stress responses may select for hypermutability (7,8). While these findings
do not mean that organic acids should not be used as a tool for controlling
foodborne pathogens, they suggest that proactive research should be
conducted to find ways of using these agents that minimize the potential
for resistance. Proactive research, including research that might appear
unrelated to the emergence of foodborne pathogens, can draw on the already
substantial body of basic research related to the conditions and
requirements for gene transfer among biologic agents. For example, Baur et
al. (9) reported on the conditions that led to the competence of
Escherichia coli for genetic transformation in freshwater environments.
Maximal competence occurred when the bacterium was exposed to 2 mM Ca2+ as
temperatures increased from 10C to 20C. With such information,
researchers could examine food processing operations to determine the
presence and importance of such conditions. For example, fruits and
vegetables are often treated with calcium under fluctuating temperatures to
enhance the texture during later processing.

A key to being more proactive in addressing the threat of microbial
foodborne pathogens-consideration of root causes-will likely require food
microbiologists to become involved in nontraditional research areas. If new
biologic agents arise as the result of changes in technology, society, or
global economics, predicting and preventing emergence will ultimately
require better understanding of how such factors influence pathogen
introduction and dissemination.

Conclusions

One of the critical lessons of the past 10 years is that we cannot become
complacent about infectious diseases (1). Only a few diseases (e.g.,
smallpox) have actually been eliminated. The rest, including virtually all
foodborne diseases, we hold in check, winning battles but not the war.
Eventually, our weapons (e.g., antibiotics) become obsolete; pathogens
(e.g., E. coli) become more dangerous; or we become complacent. Contingency
planning must be developed and undertaken with a long-term commitment.
Without that commitment and without understanding that planning is
successful when problems are avoided or minimized, programs of this type
lapse quickly. In the long term, the costs of planning, both in terms of
economics and human suffering, are a fraction of those incurred as the
result of the emergence of a major microbial threat.

Address for correspondence: Robert L. Buchanan, USDA ARS Eastern Regional
Research Center, 600 East Mermaid Lane, Wyndmoor, PA, 19038, USA; fax:
215-233-6445; e-mail: rbuchanan@arserrc.gov.

References

  1. Lederberg J, Shope RE, Oaks S. Emerging infections, microbial threats
     to health in the United States. Washington (DC): U.S. National Academy
     of Sciences Press; 1992.
  2. Nathanson N. The emergence of infectious diseases: societal causes and
     consequences. American Society of Microbiology News 1996;62:83-8.
  3. Potter ME. Factors for the emergence of foodborne disease. In: Amgar
     A, editor. Food Safety `96: Proceedings of the 4th ASEPT International
     Conference. Laval, France: ASEPT; 1996:185-95.
  4. United States Department of Agriculture. Subcommittee report for the
     pathogen reduction taskforce: identifying research and education
     needs. Washington (DC): USDA Food Safety and Inspection Service; 1994.
  5. Archer DL. Preservation microbiology and safety: evidence that stress
     enhances virulence and triggers adaptive mutations. Trends in Food
     Science Technology 1996;7:91-5.
  6. Foster JW. Low pH adaptation and the acid tolerance response of
     Salmonella typhimurium. Crit Rev Microbiol 1995;21:215-37.
  7. Moxon ER, Rainey PB, Nowak MA, Lenski RE. Adaptive evolution of highly
     mutable loci in pathogenic bacteria. Curr Biol 1994;4:24-33.
  8. LeClerc JE, Li B, Payne WL, Cebula TA. High mutation frequencies among
     Escherichia coli and Salmonella pathogens. Science 1996;274:1208-11.
  9. Baur B, Hanselmann K, Schlimme W, Jenni B. Genetic transformation in
     freshwater: Escherichia coli is able to develop natural competence.
     Appl Environ Microbiol 1996;62:3673-8.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/buchanan.txt

Maximizing the Usefulness of Food Microbiology Research

Terry A. Roberts
Food Safety Consultant; Chairman, International Commission on
Microbiological Specifications for Foods
---------------------------------------------------------------------------

      Funding for food microbiology research often follows disease
      outbreaks: botulism from vacuum-packed white-fish chubs,
      listeriosis from soft cheeses, or illness due to Salmonella
      Enteritidis or Escherichia coli. As a consequence of research,
      detection, identification, and subtyping methods improve, and
      more is learned about pathogenicity and virulence. Research
      also explores the organisms' capacity to multiply or survive in
      food and to be killed by established or novel processes.
      However, rarely is there a critical overview of progress or
      trustworthy statements of generally agreed-on facts. That
      information is not maintained in a form that can readily be
      used by regulatory departments and the food industry to ensure
      a safe food supply. A centralized system is urgently needed
      that is accessible electronically and carries information in a
      standardized format on the essential properties of the
      organisms, including pathogenicity, methods of detection,
      enumeration and identification, alternative prevention and
      control methods, and growth and survival characteristics.

Most countries aim to regulate food and the food industry to ensure a safe
and wholesome supply. Although the technology of food processing is based
on tradition (drying, freezing, fermentation), research has resulted in a
wide range of processes to meet the ever-changing requirements of
consumers. Canning of low-acid foods set an enviable example of
microbiologic safety. Today, consumers demand less heavily processed foods
that contain a minimum of chemical preservatives, have a long shelf life,
and are "fresh" yet microbiologically safe. Competitive food retailing
requires such products to be no more expensive than existing products. In
the United Kingdom, there has been a swing toward convenience foods and
foods with ethnic flavors, sometimes with a premium price. Commercial
success is evident, but relatively few of those foods have been evaluated
experimentally for microbiologic safety. Emphasis is on risk assessment, on
Hazard Analysis and Critical Control Points (HACCP) and process control,
all of which rely heavily on existing knowledge. That knowledge, and the
raw data from which it is derived, could be better organized and easier to
use.

The International Commission on Microbiological Specifications for Foods
(ICMSF)

The history of microbiologic aspects of food control is reflected in the
publications of ICMSF. ICMSF was established in 1962 as foodborne disease
greatly increased microbiologic testing of foods. Increased testing, in
turn, created widespread practical and regulatory problems in the
international food trade. At that time, analytical methods were not
standardized, sampling plans were of doubtful statistical validity, and the
interpretation of results applied different concepts of biologic
significance and acceptance criteria that confused and frustrated industry
and regulatory agencies alike.

ICMSF was founded to assemble, correlate, and evaluate evidence about the
microbiologic safety and quality of foods; to consider whether
microbiologic criteria would improve and ensure the microbiologic safety of
particular foods; to propose, where appropriate, such criteria; and to
recommend methods of sampling and examination.

The primary purpose of the commission remains to give guidance on
appraising and controlling the microbiologic safety of foods, with due
attention to microbiologic quality. Meeting the commission's objectives
would facilitate international trade; the work of national control
agencies, the food industry, and international agencies concerned with the
humanitarian aspects of food distribution; and the health of consumers.

An advisory body, the ICMSF provides basic information through extensive
study and experience and makes recommendations, without prejudice, based on
that information. Results of the studies are published as books or papers.

At its meetings, the ICMSF functions as a working group. Its membership
consists of 18 food microbiologists from 11 countries whose interests
include research, public health, official food control, education, product
and process development, and quality control. Members are from government
laboratories in public health, agriculture, and food technology;
universities; and the food industry. ICMSF also seeks assistance from
consultants specializing in particular areas of microbiology. Four
subcommissions (Balkan and Danubian, Latin American, Middle East and North
African, and Southeast Asian) promote activities similar to those of ICMSF
among food microbiologists on a regional scale and facilitate worldwide
communication.

ICMSF raises its own funds and pays for its activities and meetings. The
commission obtained support from government agencies, the World Health
Organization, International Union of Microbiological Sciences and
International Union of of Biological Sciences, and the food industry (more
than 80 food companies and agencies in 13 countries).

During its first 25 years, ICMSF devoted the major portion of its effort to
methodology research, improved standardization of methods, and 17 refereed
publications (1). One important finding was that in analyzing salmonellae,
samples could be composited, making it practical to collect and analyze the
large number of samples recommended in some stringent sampling plans. With
the evolution of alternative methods, rapid test kits, and the
everexpanding list of biologic agents involved in foodborne illness, the
commission reluctantly discontinued its program of comparison and
evaluation of methods.

The long-term objective of enhancing the microbiologic safety of foods in
international commerce was addressed initially in books recommending
uniform analytical methods (2), sound sampling plans and criteria (3), and
the microbial ecology of foods (4,5), which familiarized the analyst and
food technologist with processes used in the food industry and foods
submitted to the laboratory. Knowledge of the microbiology of major foods
and the factors affecting their microbial content helps interpret
analytical results.

At an early stage, the commission concluded that no food sampling plan
could ensure the absence of a pathogen. Testing foods at ports of entry, or
end-product testing elsewhere in the food chain, cannot guarantee food
safety. This led the commission to investigate the potential value of HACCP
for enhancing food safety. A joint ICMSF/World Health Organization meeting
in 1980 led to a report on the use of HACCP for controlling microbiologic
hazards in food, particularly in developing countries (6-8).

The commission published the principles of HACCP and procedures for
developing HACCP plans (9); the publication discussed the relative
importance of controlling the production and harvesting, preparing, and
handling of foods. Recommendations are given for the application of HACCP
at each step in the food chain-from production and harvest to consumption.

The commission recognized that a major weakness in the development of HACCP
plans is hazard analysis; knowing about the many biologic agents
responsible for foodborne illness has become difficult. The commission's
latest book (1) proposes to facilitate the development and assessment of
HACCP plans, improve the safety of foods, and facilitate risk assessment. A
thorough concise review of reports on growth and death responses of
foodborne pathogens, it is intended as a relatively quick reference manual
to assist in making decisions. The commission is revising Microbial Ecology
of Foods: (Vol. 2) Food Commodities with publication planned as
Microorganisms in Foods 6 in late 1997.

ICMSF submitted its recommendations for sampling foods and acceptance
criteria for Listeria monocytogenes to the Secretariat of Codex
Alimentarius in 1993. To stimulate discussion, the recommendations were
initially provided to the Codex; they were subsequently published (10) and
revised after further discussion (11). At the request of the Secretariat of
Codex, the commission developed recommendations for the revision of
Principles for the Establishment and Application of Microbiological
Criteria for Foods, which appears in the Procedural Manual of Codex.

Members of ICMSF believe that the original objectives of the commission
still apply. The formation of the European Union, the many other worldwide
political changes, the growth of developing countries seeking export
markets, and the increased worldwide interest in international trade, as
evidenced by the passage of the General Agreement on Tariffs and Trade and
the North American Free Trade Agreement, all point to the continuing need
for unprejudiced guidance and advice. Import and export policies must be as
uniform as possible and based on scientific facts. The commission will
strive to meet this goal through a combination of educational materials,
promotion of HACCP, and recommendations, where appropriate, of sampling
plans and microbiologic criteria. In addition, commission members will
continue to transfer information on food safety through symposia, meetings,
committee activities, and in their daily work. The future success of ICMSF
will continue to depend on extensive support from consultants and those who
provide the financial support essential to the commission's activities.

Research and Its Current Uses

Research Procedures and Funding

In most countries, funding for research has become more competitive, and
has even decreased in recent years. The following observations are based on
experience in the United Kingdom and the European Union.

The researcher first identifies the sources of funds and writes a proposal.
If funds are granted, the research proceeds. The output may include
confidential reports to the sponsor, refereed scientific publications, and,
increasingly, a patent or a licensed process or technique. Because funding
has so many different sources, writing proposals takes a substantial
proportion of the researchers' time. In many cases, the best researchers
also write the most persuasive proposals, but is this an efficient use of
the researchers' time?

In the United Kingdom, research generally falls into two categories:
research funded by the parent body and research supported by a contract.
The Institute of Food Research falls under the Biotechnology & Biological
Sciences Research Council (BBSRC), formerly the Agriculture and Food
Research Council. A proportion of the institute's funds comes directly from
the BBSRC, although these funds are declining and facing increased
competition among BBSRC institutes.

In addition, funds may be obtained from the central government: the
Ministry of Agriculture, Fisheries, and Food (MAFF) and the Department of
Health, both of which have research programs focusing on food microbiology.
In addition, Scotland and Northern Ireland have separate research budgets,
which traditionally have been spent in those countries. Funds from these
sources sometimes support projects for 3 years, but more recently for
shorter periods.

The European Union supports research across very wide areas, of which only
a small part is food technology and food microbiology, e.g., Food-Linked
Agro-Industrial Research, now ended, and European Cooperation in Scientific
and Technical Research. European Union programs run for 4 or 5 years, but
funding is usually less than 100% (often 50% and dependent on financial
support from the institute or on parallel funding from one of the above
sources).

In the United Kingdom, research funds from the food industry are usually
restricted either to very short trouble-shooting projects or to
confidential investigations. MAFF encourages industrial support of research
in LINK programs, with the government contributing up to 50% of the total
funding, provided that the research contains elements of novelty and a
consortium of companies, sometimes only three or four.

Not only is the researcher faced with attracting funds from a variety of
sources, but the requirements of those funding sources differ greatly.
Research on genetic or physiologic reasons for phenomena, such as the
unusual acid tolerance of Vero cytotoxin-producing Escherichia coli
strains, might meet BBSRC's stringent requirements of scientific quality.
MAFF publishes a Requirements Document each year, identifying areas and
topics that fit into its policy to maintain a safe and wholesome food
supply. Some of the research is basic science, e.g., developing novel
methods of detecting or identifying microbes; some is more mundane, e.g.,
improving the effectiveness of sanitizing surfaces in food processes. MAFF
also supports a limited program of surveillance for particular foodborne
pathogens. These funds are available for research institutes, universities,
and other laboratories, and proposals are judged by both MAFF staff and
independent assessors from outside. Research is organized into programs on
hygienic food processing, separation and detection of pathogens and their
toxins, physicochemical principles underlying microbial growth, growth
conditions for pathogens, ending in 1997-98, and new programs on assessing
microbiologic hazards and risks and managing microbiologic hazards and
risks of salmonellae and campylobacters in poultry. The Department of
Health also supports food safety research-partly as surveillance and partly
as programs targeted at a specific organism, e.g., E. coli O157:H7. The
Department of Health also responds to topics identified by the Advisory
Committee on Microbiological Safety of Foods.

University departments in England are subjected to a different form of
scrutiny to determine the amount of research funding they receive from the
central government, judged by the Higher Education Funding Council for
England. The quality of publications is judged against the number of
researchers, and the funding is directed toward research excellence. Newly
established departments find it difficult to compete.

Research Output

Output from research has been regarded as the report to the contractor,
sometimes confidential and sometimes public. More recently, funding
agencies have encouraged exploitation of research through patents or
licensing agreements. To the researcher, expertise must be recognized
through refereed publications. Although in addition to detailed papers,
overviews and sometimes summaries for the industry are provided, there is
generally less consideration for the research user. In recent years, many
reviews have become mere compilations of completed research. Moreover, most
computerized databases terminate in about 1970, thus ignoring a wealth of
earlier information. HACCP and risk assessment would be greatly facilitated
if data on microbial responses (growth, survival, death) were collated in a
more standardized form. Details should include the food commodity (e.g.,
meat, fish, vegetables, or fruit), the subgroup (e.g., for meat, beef,
pork, or lamb), and the process applied (e.g., cooked, cured, smoked, or
fermented). Details of the packaging should include the pack atmosphere
(%CO2, %O2, %ballast gas). In the longer term, it might become important to
know the exact identity of the organism including serotype, phage type,
strain, and, in time, virulence factors and even sequences for 16S rRNA and
for toxin genes. Other items that should be tabulated include inoculum
concentration; prior history of the inoculum, especially if that history
might alter the response; incubation temperature(s); measure of water (%
brine, aw); pH; other factors (e.g., nitrite, sorbate); microbiologic
method(s); response (growth rate, D value, time to toxin); and estimate of

the reliability of the measures of the responses.

Many readers assume that these details are included in most, if not all,
publications. However, our experience in compiling published data for
comparisons with models of growth, thermal death, or survival (nonthermal
death) illustrates that much desirable information is lacking (12-15).
Attempts to define the boundary between growth and no growth are not new
(16-19). Despite different methods and strains of microbes, compilations of
reported data often show a trend (Figure).
 
[Figure Not Available in ASCII]

 Figure. Specific growth rate of Listeria monocytogenes predicted by
 different models and independent observers (provided by J. Baranyi,
 Institute of Food Research, Reading Laboratory). Estimates of specific
 growth rate from published papers are shown vertically. The response
 surfaces were generated from predictive models and are not a surface
 fitted to the values.

Research Users

Despite an overall neglect of the research user's needs, some useful
listings are available (20) as are helpful overviews and discussions
(21-26). There remains, however, an opportunity for the researcher to make
a product more attractive to the research user, e.g., the food industry
(both national and international), regulatory bodies, and informed
consumers.

Additional Research Needs

Required research includes developing a database of reliable information on
microbial responses to food processing conditions. The database needs to be
easy to access and use. In addition, because understanding of growth
responses and thermal death is greater than our understanding of survival,
i.e., little or no loss of viability over many months, we need to have a
better understanding of the factors determining that survival and of the
underlying microbial physiology. In recent years, we have been faced with
such new microbiologic challenges as Salmonella Enteritidis Phage Type 4,
multidrug-resistant S. Typhimurium Definitive Type 104, verocytotoxigenic
E. coli, Shiga like toxin-producing E. coli, and increasingly
vancomycin-resistant Enterococcus faecium, and we lack the capacity to
anticipate emergence of new pathogens. Finally, new technologies make it
possible to study genetic stability and variation in populations, perhaps
identifying why particular strains are, or become, more virulent and more
pathogenic.

Control of Food Safety

Food safety has traditionally been controlled by inspection and compliance
with ordinances or codes of practice. Much faith has been placed in
sampling plans and associated microbiologic criteria. However, this
approach is retrospective, depending on the microbiologic examination of
product that has often been dispatched or consumed. The number of samples
that can be examined statistically is too small to detect low levels of
"defectives." HACCP was a substantial step forward, with the concept that
safety could be designed into a food process. However, HACCP demands
substantial knowledge about the characteristics of the food, the process,
and the microbes of concern. The knowledge required to implement HACCP
effectively is not organized into systems that are easy to use.

Address for correspondence: Terry A. Roberts, Ph.D., 59 Edenham Crescent,
Reading RG1 6HU, United Kingdom; fax: 44-118-956-6998.

References

  1. International Commission on Microbiological Specifications for Food.
     Microorganisms in foods. 5. Characteristics of microbial pathogens.
     London: Blackie Academic & Professional;1996. p. 511-3.
  2. International Commission on Microbiological Specifications for Food.
     Microorganisms in foods. 1. Their significance and methods of
     enumeration. 2nd ed. Toronto: Univ. of Toronto Press;1978.
  3. International Commission on Microbiological Specifications for Food.
     Microorganisms in foods. 2. Sampling for microbiological analysis:
     principles and specific applications. 2nd ed. Toronto: Univ. of
     Toronto Press;1986.
  4. International Commission on Microbiological Specifications for Food.
     Microbial ecology of foods. Vol. 1. Factors affecting life and death
     of microorganisms. New York: Academic Press;1980.
  5. International Commission on Microbiological Specifications for Food.
     Microbial ecology of foods. Vol. 2. Food commodities. New York:
     Academic Press;1980.
  6. International Commission on Microbiological Specifications for Food.
     Report of the WHO/ICMSF meeting in hazard analysis: critical control
     point system in food hygiene. Geneva: World Health Organization;1982.
     VHP/82.37.
  7. International Commission on Microbiological Specifications for Food.
     Prevention and control of foodborne salmonellosis through application
     of the hazard analysis critical control point system. Report of an
     International Commission on Microbiological Specifications for Foods
     (ICMSF) ad hoc committee. Geneva: World Health Organization;1986.
     WHO/CDS/VPH/86.65.
  8. Simonsen B, Bryan FL, Christian JHB, Roberts TA, Tompkin RB, Silliker
     JH. Report from the International Commission on Microbiological
     Specifications of Foods (ICMSF). Prevention and control of food-borne
     salmonellosis through application of hazard analysis critical control
     point (HACCP). Int J Food Microbiol 1987;4:227-47.
  9. International Commission on Microbiological Specifications for Food.
     Microorganisms in foods. 4. Application of the Hazard Analysis
     Critical Control Point (HACCP) System to ensure microbiological safety
     and quality. London: Blackwell Scientific Publications, Ltd.;1988.
 10. International Commission on Microbiological Specifications for Food.
     Choice of sampling plan and criteria for Listeria monocytogenes. Int J
     Food Microbiol 1994;22:89-96.
 11. van Schothorst M. Sampling plans for Listeria monocytogenes. Food
     Control 1996;7:203-8.
 12. Graham AF, Mason DR, Peck MW. Predictive model of the effects of
     temperature, pH and sodium chloride on growth from spores of
     non-proteolytic Clostridium botulinum. Int J Food Microbiol
     1996;31:69-85.
 13. Sutherland JP, Bayliss AJ. Predictive modelling of growth of Yersinia
     enterocolitica-the effects of temperature, pH and sodium chloride. Int
     J Food Microbiol 1994;21:197-215.
 14. Sutherland JP, Bayliss AJ, Roberts TA. Predictive modelling of growth
     of Staphylococcus aureus: the the effects of temperature, pH and
     sodium chloride. Int J Food Microbiol 1994;21:217-36.
 15. Sutherland JP, Bayliss AJ, Braxton DS. Predictive modelling of growth
     of Escherichia coli O157:H7: the effects of temperature, pH, and
     sodium chloride. Int J Food Microbiol 1995:25:29-49.
 16. Riemann H, Lee WH, Genigeorgis C. Control of Clostridium botulinum and
     Staphylococcus aureus in semi-preserved meat products. Journal of Milk
     and Food Technology 1972;35:514-23.
 17. Bean PG. Developments in heat treatment processes for shelf-stable
     products. Food microbiology: advances and prospects. In: Roberts TA,
     Skinner FA, editors. London: Academic Press; 1983. p. 97-112.
 18. Brown KL. Use of a computer data file for storage of heat resistance
     data on bacterial spores. J Appl Bacteriol 1988;65:49-54.
 19. Brown KL. (1992) Heat resistance of bacterial spores [dissertation].
     Nottingham, England: Univ of Nottingham;1992.
 20. Bryan FL. Diseases transmitted by foods: a classification and summary.
     2nd ed. Atlanta (GA): Centers for Disease Control and Prevention;1982.
 21. Council for Agricultural Science and Technology. Foodborne pathogens:
     risks and consequences. Ames (IA): the Council; 1994; Task Force Rep.
     No. 122.
 22. Advisory Committee on the Microbiological Safety of Food. Report on
     vacuum packaging and associated processes. London: HMSO; 1992.
 23. Advisory Committee on the Microbiological Safety of Food. Interim
     report on Campylobacter. London: HMSO;1993.
 24. Advisory Committee on the Microbiological Safety of Food. Report on
     Salmonella in eggs. London: HMSO;1993.
 25. Advisory Committee on the Microbiological Safety of Food. Report on
     verotoxin-producing Escherichia coli. London: HMSO;1995.
 26. Advisory Committee on the Microbiological Safety of Food. Report on
     poultry meat. London: HMSO;1996.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/roberts.txt

Epidemiology and Detection as Options for Control of Viral and Parasitic
Foodborne Disease

Lee-Ann Jaykus
North Carolina State University, Raleigh, North Carolina, USA
---------------------------------------------------------------------------
      Human enteric viruses and protozoal parasites are important
      causes of emerging food and waterborne disease. Epidemiologic
      investigation and detection of the agents in clinical, food,
      and water specimens, which are traditionally used to establish
      the cause of disease outbreaks, are either cumbersome,
      expensive, and frequently unavailable or unattempted for the
      important food and waterborne enteric viruses and protozoa.
      However, the recent introduction of regulatory testing
      mandates, alternative testing strategies, and increased
      epidemiologic surveillance for food and waterborne disease
      should significantly improve the ability to detect and control
      these agents. We discuss new methods of investigating foodborne
      viral and parasitic disease and the future of these methods in
      recognizing, identifying, and controlling disease agents.

Emerging infectious diseases can be defined as infections that have newly
appeared in a population or have existed but are rapidly increasing in
incidence or geographic range (1). Agents for which a particular route of
transmission is newly recognized and agents (previously unidentifiable)
that are now known because of advances in detection methods should also be
included in this definition. Advances in epidemiologic and detection
methods during the last 10 to 20 years have placed food and waterborne
human enteric viruses and protozoal parasites within this category.

Human enteric viruses and protozoa are parasitic agents that replicate in
the intestines of infected hosts and are excreted in the feces. In general,
the viruses are limited to human hosts, while the parasitic agents (in the
form of cysts or oocytes) have a variety of human and nonhuman animal
hosts. Both are transmitted primarily by the fecal-oral route, and as a
result, the major source of contamination for foods and water is through
contact with human and animal fecal pollution. This contamination may occur
directly, through contaminated meat carcasses or poor personal hygiene
practices of infected food handlers, or indirectly, through contact with
fecally contaminated water or other cross-contamination routes. Viruses and
parasites differ from foodborne bacterial pathogens in important ways.
Because they are environmentally inert, they do not replicate in food,
water, or environmental samples. Additionally, unlike bacterial pathogens,
human enteric viruses and protozoal parasites are environmentally stable
(2), are resistant to many of the traditional methods used to control
bacterial pathogens (2), and have notably low infectious doses (3,4). This
allows virtually any food to serve as a vehicle for transmission and
enables these agents to withstand a wide variety of commonly practiced food
storage and processing conditions (2,5).

Epidemiology

Human Enteric Viruses

Human enteric viruses are increasingly recognized as important causes of
foodborne illness. A recent report issued by the Council for Agricultural
Science and Technology ranked human enteric viruses as fifth and sixth
among identified causes of foodborne disease in the United States (6). A
review of U.S. national surveillance data for 1979 showed that 14 (44%) of
32 foodborne disease outbreaks in institutional settings were
epidemiologically typical of viral gastroenteritis (7). Viral
gastroenteritis was reported as the most common foodborne illness in
Minnesota from 1984 to 1991, predominantly associated with poor personal
hygiene of infected food handlers (8). Furthermore, recent data indicate
that 10% of the 4,617 outbreaks of foodborne disease of unconfirmed
etiology reported from 1973 to 1987 met at least two of the clinical
criteria for outbreaks of acute viral gastroenteritis (8,9). The apparent
failure to confirm a viral etiology in such outbreaks has been due largely
to the lack of available tests and the reluctance of public health
officials to use epidemiologic criteria in the classification of foodborne
viral disease (9-11). The unavailability of food specimens and the failure
to report outbreaks of mild gastrointestinal disease have also contributed
to reporting difficulties. All of these factors have resulted in a drastic
underestimate of the true scope and importance of foodborne viral infection
(8).

The most common types of foodborne viral disease are infectious hepatitis
due to hepatitis A virus and acute viral gastroenteritis associated with
the Norwalk agent and other related small, round-structured
gastrointestinal viruses (12). The human enteroviruses may also be
transmitted by foodborne routes (13) and are the most commonly isolated
agents in surveys of naturally occurring viral contamination in foods (14).
Foodborne outbreaks due to small round viruses, parvoviruses, and
astroviruses are occasionally reported (15). Rotaviruses, some
adenoviruses, and hepatitis E virus are important causes of waterborne
disease outbreaks, particularly in developing countries (12). Foodborne
outbreaks associated with human enteric viruses are almost always due to
the consumption of fecally contaminated raw or undercooked shellfish and
ready-to-eat products contaminated by infected food handlers (12).
Postrecovery and secondary transmission are a concern (16,17). Recent
outbreaks are summarized in Table 1 (18-24).

Table 1. Recent outbreaks of foodborne viral disease
---------------------------------------------------------------------------
Agent   Location   Date      No. Cases   Food        Confirmation(a)   Ref.
---------------------------------------------------------------------------
HAV     Shanghai,  Jan. 1988 300,000     Raw clams   Yes               18
          China              (4% total               (IEM,
                             population)             Hybridization,
                                                     Cell culture,
                                                     Experimental
                                                     infection)
HAV     U.S. (AL,  July-Aug. 61          Raw oysters Yes               19
        GA,        1988                              (Antibody
        FL, TN,                                      capture-
        HI)                                          RT-PCR)
SRSV    U.S. (LA)  Nov. 1993 40          Raw oysters No                21
        U.S. (LA,
SRSV    MD,        Nov. 1993 180         Raw/steamed No                22
        MS, FL, NC                         oysters
                                         Steamed/
SRSV    U.S. (GA)  Dec. 1994 34 clusters   roasted   No                22
                                         oysters

SRSV    U.S. (FL,  Jan. 1995 3           Oysters     No                20
        TX)

Norwalk U.S. (DE)  Sept.1987 191         Commercial  No                23
                                           ice
                                         Celery/
Norwalk U.S. (CO)  July,     1440          chicken   No                24
                   1988                     salad
                                         
---------------------------------------------------------------------------

(a)Confirmation of the virus, viral antigen, or viral acid in food specimens
HAV = hepatitis A virus; IEM = immune electron microscopy; RT-PCR = reverse
transcriptase polymerase chain reaction; SRSV = small round-structured
gastrointestinal virus

----------------------------------------------------------------

Parasitic Protozoa

Since 1981, enteric protozoa have become the leading cause of waterborne
disease outbreaks for which an etiologic agent could be determined (5). A
recent study reported that 21% of drinking water-associated outbreaks
between 1991 and 1992 were attributable to parasitic agents (25).
Furthermore, these agents are frequent contaminants of potable water
supplies (26,27). The potential for transmission of these agents by
foodborne routes is increasingly recognized (28). For instance, from 1988
to 1992, seven food-associated outbreaks of giardiasis, comprising 184
cases, were reported in the United States (29). However, since foodborne
transmission is only recently documented and more than half of all reported
foodborne disease outbreaks have undetermined etiology, the true importance
of foodborne transmission of parasitic protozoa is unknown.

The most common human enteric parasitic infections in the United States are
caused by Cryptosporidium parvum and Giardia lamblia. Cyclospora is also an
emerging enteric protozoon that has recently been associated with the
consumption of contaminated fruit (30). Large communitywide waterborne
outbreaks of parasitic protozoa are usually associated with surface water
supplies that are either unfiltered or subjected to inadequate flocculation
and filtration (5). Two large waterborne outbreaks have occurred in the
United States within the last 10 years (31,32); one of these was the
largest recorded waterborne disease outbreak in U.S. history (32).

Limitations

Most of the information about viral and parasitic food and waterborne
disease comes from outbreak investigations by state and local health
departments and surveillance programs directed by the Centers for Disease
Control and Prevention (CDC). However, since many of these diseases are not
reportable and surveillance is based on voluntary reporting by state health
departments, the magnitude of this disease problem is underestimated. This
is exacerbated by a reluctance to use epidemiologic criteria in the
classification of foodborne viral disease and the failure to report mild
outbreaks of gastrointestinal disease. Furthermore, since investigation
generally follows an outbreak, important information and samples may have
been destroyed, consumed, or lost to inaccurate recall. Since many of these
outbreaks are small and confined, epidemiologic investigation may be
limited by the resources available to state and local health departments.
Difficulties in investigating and reporting are further complicated by the
fact that the enteric protozoa cause common opportunistic infections in the
immunocompromised, and the role of foods in these diseases has not been
studied. Likewise, the role of the foodborne transmission route in sporadic
disease and the importance of carrier states and secondary illness after a
primary foodborne disease outbreak are poorly characterized.

Detection

Clinical Samples

Illness caused by human enteric viruses can be suspected epidemiologically
by considering incubation period and illness duration analysis, classic
viral gastroenteritis symptoms, and the absence of bacterial or parasitic
pathogens in stool samples (10,11). Laboratory confirmation of human
enteric viral infection has been based on a rise in specific antibody to
the virus, or alternatively, the demonstration of virus particles, antigen,
or nucleic acid in stools. The detection methods most often applied to
clinical samples have included immune electron microscopy,
radioimmunoassay, and enzyme immunoassay (33). The usefulness of these
assays has been reduced by low detection limits (>104-105 particles/ml) and
the inability to cultivate Norwalk-like viruses in vitro, which has limited
the supply of viral antigen available for developing reagents (8). In
addition, the Norwalk agent is only one of several small round-structured
gastrointestinal viruses that cause outbreaks with similar clinical and
epidemiologic features (8).

Before 1981, parasitic disease in humans was diagnosed histologically by
identifying the life cycle stages of parasitic agents in the intestinal
mucosa (34). More recently, clinical diagnosis has involved methods to
concentrate parasitic agents from stool specimens followed by a variety of
fluorescent or immunofluorescent staining techniques and subsequent
microscopic examination (34). Serodiagnostic methods have been developed
(35,36), but additional evaluations are needed to confirm the diagnostic
utility of these methods.

Environmental, Food, and Water Samples

Failure to confirm a viral and parasitic etiology in foodborne outbreaks
has also been due to the lack of adequate methods to detect the causative
agents in environmental samples. Like bacterial pathogens in food and
water, viruses and parasites are frequently present in small numbers.
However, unlike traditional food microbiologic techniques, which have
relied on cultural enrichment and selective plating to increase cell
numbers and differentiate pathogens in background microflora, techniques to
detect human enteric viruses and parasitic protozoa require live mammalian
cells for growth. For this reason, standard methods to detect enteric
bacteria in foods cannot be used; instead, detection requires an initial
concentration step, often from large volumes of food or water, followed by
mammalian cell culture infectivity assay or immunofluorescent staining.
Concentration methods are usually cumbersome, and yields are less than
optimal. Both cell culture infectivity and immunofluorescent staining are
expensive and slow and require highly trained personnel. Furthermore,
mammalian cell culture lines are largely unavailable for the
epidemiologically important foodborne viruses. Alternative detection
methods based on immunologic and molecular methods have been reported;
recent methodologic developments have focused on overcoming barriers to
detection, such as improving recovery efficiencies and detection limits and
preventing inhibitions due to food-related compounds.

Human Enteric Viruses in Foods

    Concentration

    Two general schemes for the concentration of human enteric viruses from
foods have been reported: extraction-concentration methods and
adsorption-elution-concentration methods (Figure). The general purpose of
concentration is to provide a high recovery of infectious virus in a
low-volume aqueous solution free of cytotoxic materials. Both schemes
employ conditions favoring the separation of viruses from shellfish tissues
(most of the developmental work has used shellfish as model food
commodities), primarily through the use of filtration, precipitation,
polyelectrolyte flocculation, and solvent extraction. While either method
can be used, adsorption-elution-precipitation methods have been favored in
recent years (2,37). Virus yields after concentrations are 10% to 90% (38).

[Figure Not Available in ASCII]

Figure. General steps in the isolation of human enteric viruses and
parasitic protozoa from foods.

    Detection

    Traditional methods to directly detect viruses in foods after
concentration have been based on the ability of enteric viruses to infect
live mammalian cells in culture. Quantal and enumerative methods using a
variety of mammalian cell culture lines, generally from primate kidneys,
have been reported. Such approaches have been limited because levels of
contaminating virus generally are low (1-200 infectious units per 100 grams
of shellfish) (39), residual food components interfere with assays (38),
and the epidemiologically important viruses do not replicate (small
round-structured gastrointestinal viruses) or replicate poorly (hepatitis A
virus) in mammalian cell culture (2). Alternative methods such as
enzyme-linked immunosorbent assay (ELISA) and DNA/RNA probes have been
reported but are limited by high detection limits (>103 infectious units),
unavailability of reagents, and poor sample quality (2). These difficulties
are illustrated by the confirmation of viral contamination in a food in
only two reported instances (18,19).

The in vitro enzymatic amplification method of polymerase chain reaction
(PCR) offers an opportunity to enrich a single specific nucleic acid
sequence up to a millionfold and hence provides a sensit
ive and specific
method with a theoretical detection limit of one virus unit. This method is
readily adaptable to the detection of RNA viruses by preceding the PCR with
a brief reverse transcription (RT) step, hence the designation RT-PCR. The
recent cloning of the Norwalk agent and related small round-structured
gastrointestinal viruses has provided an opportunity to develop effective
molecular detection methods for these previously nondetectable agents
(40,41).

The application of PCR methods to the detection of human enteric viruses in
foods is an area of active research. However, the development of such
methods is complicated by low levels of contamination, high sample volumes,
and the presence of food components, which may interfere with enzymatic
amplification reactions. To address these issues, three alternative
approaches have been used to simultaneously reduce sample volumes and the
level of interfering compounds. The most frequently applied approach
involves isolating and purifying nucleic acids (RNA) from the food sample
before RT-PCR (42-47). A second approach combines capture of the virus with
specific antibody followed by nucleic acid amplification by using RT-PCR
(19,48). In the third approach, the intact virus particle is concentrated
and purified from the complex food matrix resulting in sample volume
reduction and removal of inhibitors, followed by subsequent heat release of
viral nucleic acid from the virion capsid and RT-PCR (49-51). All three
methods have been applied to various shellfish species, and in some cases,
to other food commodities and naturally contaminated field shellfish
specimens. The methods are summarized in Table 2, along with optimized
virus detection levels (19,42-51). A combined approach was reported by
Chung et al. (51), who successfully detected human enteroviruses and
hepatitis A virus in naturally contaminated oyster samples after viral
amplification in mammalian cell cultures. Despite enormous strides in the
ability to detect human enteric viruses with PCR, the technique is still
limited by the absence of effective concentration methods, the presence of
enzymatic inhibitors, and the inability to distinguish between infectious
and noninfectious virions.

Table 2. Emerging detection methods for human enteric viruses in foods
---------------------------------------------------------------------------
                                       Detection            Field
Pathogen         Sample                limit                app.  Ref.
---------------------------------------------------------------------------
Nucleic Acid     Extraction/RT-PCR

HAV              Clams                 2000 particles/g     No    44
                                       (10 PFU/g)

Poliovirus       Oysters               38 PFU/20 g          No    42
                                       (2 PFU/g)

HAV              Clams/Oysters         100 PFU/1.5 g        No    43
                                       (67 PFU/g)

Norwalk          Clams/Oysters         5-10 PCRU/1.5 g      No    43
                                       (3-7 PCRU/g)

Poliovirus       Oysters/Mussels       10 PFU/5 g           No    45
                                       (2 PFU/g)
SRSV             Oysters/Mussels       Not specified        Yes   46
Polivirus,                             100 PFU/50 g
HAV              Clams                 (2 PFU/g)            No    50

Norwalk          Clams                 1000 PCRU/50 g       No    50
                                       (20 PCRU/g)

Norwalk          Various               20-200 PCRU/10 g     No    47
                                       (2-20 PCRU/g)

Antibody-Capture/RT-PCR
HAV              Clams/Oysters         Not specified        No    48
HAV              Oysters               Not specified        Yes   19

Virion Concentration
Poliovirus,                            10 PFU/50 g
HAV              Oysters               (0.02 PFU/g)         Yes   49,

Norwalk          Oysters               4500 PCRU/50 g       No    49, 51
                                       (90 PCRU/g)
Poliovirus,                            1000 PFU/50 g
  HAV            Clams                 (20 PFU/g)           No    50

Norwalk          Clams                 100 PCRU/50 g        No    50
                                       (2 PCRU/g)
---------------------------------------------------------------------------

HAV = hepatitis A virus; RT-PCR = reverse transcriptase polymerase chain
reaction; PCRU = PCR-amplifiable units; SRSV = small round-structured
gastrointestinal virus.

Enteric Parasitic Agents

    Concentration

    Like viruses, parasitic protozoa are usually present in low
concentrations in contaminated water and hence must be concentrated from
large volumes of water before detection (Figure). Giardia cysts and
Cryptosporidium oocysts are concentrated from 100 to 1,000 liters of water
by filtration through yarn-wound filters. Retained particulates are eluted
from the filters and reconcentrated by centrifugation. The pelleted cysts
and oocysts are then separated from particulate debris by flotation on
Percoll-sucrose gradients, followed by subsequent detection. With this
method, a concentrate of less than 5 ml can be provided for final detection
(52). While recovery efficiencies as high as 100% have been reported (53),
recovery is generally poor and greatly affected by water quality and
particulate matter (53,54). Alternative methods using calcium carbonate
precipitation can concentrate Cryptosporidium oocysts from water with
concentration efficiencies as high as 63% (55).

    Detection

    Traditional methods to detect parasitic agents from water sample
concentrates have been based on immunofluorescent staining of filtered
sample concentrates (52). The sample concentrate is filtered through
cellulose acetate filters and commercial kits that use fluorescein
isothiocyante-labeled monoclonal antibodies applied for immunofluorescent
staining. The stained filters are examined under an ultraviolet microscope,
and cysts and oocysts are classified according to immunofluorescence, size,
shape, and internal morphologic characteristics. The results are reported
as presumptive and confirmed cysts and oocysts per 100 liters of water
(52). Confirmation is based on the ability to visualize organelles under
light microscopy. This method is extremely limited because it is
time-consuming and expensive, requires highly skilled personnel, and does
not indicate viability of the cysts or oocysts; in addition,
cross-reactions of monoclonal antibodies with algal cells and debris
interfere with the interpretation of results (56).

Because of the limitations of immunofluorescence assays, alternative
strategies for the detection of protozoal parasites in environmental and
water samples are being sought. Most of the approaches use the traditional
methods of concentration, in conjunction with alternative detection
methods. In many cases, the inclusion/exclusion of fluorogenic dyes is used
to enhance morphologic examination; in particular, 4,6-diamidino-2-phenyl
indole and propidium iodide have been used to facilitate detection and also
assess viability (57). Two recent methodologic developments include cell
sorting/particle counting approaches and molecular approaches. In many
cases, a combination of approaches is used. Particle-counting approaches
include the Fluorescence-Activated Cell Sorting system, a laser-based
particle counter that is able to simultaneously sort particles, sense
fluorescence, and determine size (58). This system is being used to detect
Giardia and Cryptosporidium in water samples in England and Australia (55).
Differentially stained cysts and oocysts may be visualized microscopically
with cooled charge couple devices (58,59). Molecular approaches that apply
DNA hybridization to the detection of Giardia species have been reported
(60). A method that combines fluorescence and in situ hybridization with
confocal microscopy has been applied to both detect and speciate Giardia
cysts (61). PCR methods have been developed to detect Giardia and
Cryptosporidium (62-64). While PCR methods have the potential to detect one
single infectious unit and may be applied to discriminate pathogenic from
nonpathogenic species, they remain limited because of enzymatic inhibition,
the inability to discriminate between viable and nonviable organisms, and
the current absence of quantitative assay.

Several methods in development are combinations of multiple detection
approaches. A combined method, designated the electrorotation assay,
couples filtration and subsequent elution with affinity immunocapture by
using paramagnetic beads. By inducing an electric field on a microscope
with a special stage attachment, the organism-bead complexes rotate in a
characteristic pattern that enables detection of parasitic protozoa (52).
Several investigators are also working on methods that couple cell culture
infectivity with immunostaining, thereby providing detection with
simultaneous indication of viability (M. Sobsey, pers. comm.). Clinical
ELISA kits have been evaluated for use in environmental water samples with
reported detection limits of fewer than 10 cysts or oocyts (65); however,
cross-reaction with algae continues to be a problem. Emerging detection
approaches are summarized in Table 3 (58-64). Methods to concentrate and
detect parasitic protozoa specifically from foods are under development.

Table 3. Emerging detection methods for parasitic protozoa in water
---------------------------------------------------------------------------

Pathogen            Detection   Viability         Differentiation      Ref.
                    limit
---------------------------------------------------------------------------
Flow Cytometry with Fluorescent Imaging and CCD
                                Yes, using
Cryptosporidium     NR          differential      NR                   58,
                                fluorogenic vital                      59
                                dyes

DNA Hybridization
Giardia
  (16s-like rDNA)   1 cyst      NR                No                   60
                                                  Yes-in situ
Giardia                                           hybridization
  (16s-like rDNA)   NR          NR                with differential    61
                                                  fluorescence
                                                  G. lamblia; G. muris

Polymerase Chain Reaction
                                Partial, using
                                mRNA
Giardia                         as target;
  (Giardin gene)    1 cyst      Depends on        NR                   62
                                inactivation
                                method
Giardia                                           Yes-primer sequence
(Giardin gene)      1 cyst      NR                G. duodenalis        63
Cryptosporidium
(Target not         20 oocysts  No                Yes-by hybridization 64
specified)                                        C. parvum
---------------------------------------------------------------------------

CCD = cooled charge couple device; NR = not reported

Considerations

Although prototype alternative and rapid methods to detect human enteric
viruses and parasitic protozoa in foods and water have been reported,
multiple barriers must be overcome before these methods are applicable to
routine monitoring. To obtain sample representation and detection
sensitivity adequate for the low levels of contamination found with these
enteric pathogens in naturally contaminated environmental specimens, large
sample volumes of food and water need to be processed. While some of the
detection methods begin with large samples, many do not consider sample
size, which limits the sensitivity of the assay procedure from the
beginning. The approaches then applied to concentrate and purify the
pathogens from the samples do not only need to be reasonably efficient but
also need to produce a concentrate low in volume and free of inhibitory
compounds. To complicate matters further, different food commodities have
differing types and levels of inhibitors, many of which have remained
recalcitrant to almost all removal processes.

The relationship between detection by molecular and immunologic approaches
and the subsequent viability or infectivity of these enteric pathogens
remains a concern. While investigators have addressed these issues using in
vitro excystation, inclusion and exclusion of vital dyes, animal
infectivity (parasitic protozoa) (57,66,67), and assays combining mammalian
cell culture infectivity with alternate detection strategies (viruses) (51;
M. Sobsey, pers. comm.), these methods are not useful for the nonculturable
human enteric viruses; they are expensive, time-consuming, and not readily
amenable to routine diagnostic work. Establishing quantitative detection
methods also needs further research.

The development of detection methods is also limited by the current state
of knowledge. For instance, while recent sequencing evidence indicates that
the Norwalk-like small round-structured gastrointestinal virus group
consists of multiple members having the same physical and genomic
characteristics as other viruses in the family Caliciviridae (40,41),
considerable sequence and antigenic diversity remain among the members of
this group (68-70). While PCR primers for these genetically diverse agents
have been reported (46,71), development of universal detection methods is
clearly limited until more complete information about this group of human
enteric viruses is available.

Research is needed to develop and refine the prototype protocols into
collaboratively tested methods that could be routinely and expeditiously
used to evaluate the microbiologic safety of food products. In general,
future research needs for the routine application of alternative methods to
detect enteric viral and parasitic protozoal contamination in foods
requires development of the following: 1) simple, rapid, and cost-effective
extraction and concentration procedures; 2) simple and reliable methods for
the removal of inhibitors; 3) methods that are not restricted by food
product; and 4) quantitative approaches for assessing the relative levels
of contamination.

Conclusions

Improved epidemiologic surveillance, predominantly through the creation of
population-based centers that will focus on the epidemiology and prevention
of food and waterborne infectious disease (72), should improve knowledge
regarding viral etiology in foodborne disease outbreaks and the clinical
and economic importance of viral and parasitic disease agents. Data
obtained from these studies may elucidate the role of foods in sporadic
disease as well as in secondary spread.

Current research programs under way at CDC, U.S. Department of Agriculture,
and Food and Drug Administration, as well as extramural funding through the
National Institutes of Health and U.S. Department of Agriculture programs,
should promote the development, testing, and dissemination of rapid and
accurate detection methods for viral and parasitic foodborne disease
agents. Such research will continue to be important as testing regulations,
such as the landmark Environmental Protection Agency Information
Collections Requirement Rule, and the new U.S. Department of Agriculture
Pathogen Reduction: Hazard Analysis and Critical Control Points (HACCP)
Systems Rule, emerge in the future. From a clinical standpoint, the
development of inexpensive and widely available reagents has been improved
through recent developments in molecular biology (33). This will increase
the number of facilities able to perform diagnostic testing, which will
ultimately facilitate epidemiologic investigation. From a food safety
standpoint, improved detection methods should eventually provide a
regulatory option for monitoring food safety, particularly in the
economically important shellfish species and in detecting viral
contamination due to human h
andling, or parasitic protozoal contamination
from animal wastes. The development of rapid detection methods will also
aid in the evaluation of control strategies for viral and protozoal
contamination of foods. For instance, depuration, thermal processing, and
irradiation for the control of foodborne viral contamination in shellfish
need to be further evaluated. In the case of contamination by infected food
handlers and animal wastes, the availability of rapid methods will increase
our understanding of this transmission route and help determine critical
control points for HACCP approaches to control the transmission of these
foodborne disease agents. This will allow the integration of a true
farm-to-table food safety approach for the control of viral and parasitic
food and waterborne disease.

In the United States, there is currently a clear regulatory mandate to
evaluate food safety risks within a systematic conceptual framework in the
form of quantitative microbial risk assessment. The application of
quantitative risk assessment to food safety issues has been hampered by the
lack of available data regarding prevalence, transmission, infectious dose,
and behavior of microorganisms in foods; this has been particularly true
for emerging pathogens. Improved epidemiologic and detection methods should
dramatically affect the ability to evaluate food safety risks through risk
assessment strategies. For instance, by using emerging epidemiologic data
in conjunction with epidemiologic modeling, statistical overview analysis
(meta-analysis), and geographic information systems, scientists should be
able to improve hazard analysis and exposure assessment and provide a
clearer picture of disease transmission patterns. More rapid, accurate, and
readily available detection methods should allow determination of
prevalence of contamination and disease, and when quantitative, should
provide a means of assessing dose-response relationships. Together these
will improve subsequent risk characterization, allowing regulators to
quantitate the magnitude of the problem, evaluate risk reduction
strategies, and prioritize competing risks.

The combination of increased surveillance, improved detection methods, and
testing requirements should result in a marked improvement in the ability
to detect, investigate, and control food and waterborne enteric viral and
parasitic protozoal agents. Taken together, these approaches promise to
provide increased information necessary to assess risks, control disease,
and ultimately improve public health in the next century.

Address for correspondence: Lee-Ann Jaykus, Department of Food Science,
North Carolina State University, Box 7624, Raleigh, NC 27695 USA; fax:
919-515-7124; e-mail: leeann_jaykus@ncsu.edu.

References

  1. Morse SS. Factors in the emergence of infectious disease. Emerg Infect
     Dis 1995;1:5-10.
  2. Jaykus LA, Hemard MT, Sobsey MD. Human enteric pathogenic viruses. In:
     Hackney CR, Peirson MD, editors. Environmental indicators and
     shellfish safety. New York: Chapman and Hall; 1994.
  3. DuPont HL, Chappell CL, Sterling CR, Okhuysen PC, Rose JB, Jakubowski
     W. The infectivity of Cryptosporidium parvum in healthy volunteers. N
     Engl J Med 1995;332:855-9.
  4. Haas CN. Estimation of risk due to low doses of microorganisms: a
     comparison of alternative methodologies. Am J Epidemiol
     1983;118:573-82.
  5. Moe CL. Waterborne transmission of infectious agents. In: Hurst CJ,
     Knudsen GR, McInerney MJ, Stenzenbach LD, Walter MV, editors. Manual
     of environmental microbiology. Washington (DC): American Society for
     Microbiology; 1996.
  6. Council for Agricultural Science and Technology. Foodborne pathogens:
     risks and consequences. Washington (DC): Library of Congress No. 122;
     1994.
  7. Jenkins S, Horman JT, Isreal E, Cukor G, Blacklow NR. An outbreak of
     Norwalk-related gastroenteritis in a boys' camp. American Journal of
     Diseases of Children 1985;139:787-9.
  8. Hedberg CW, Osterholm MT. Outbreaks of foodborne and waterborne viral
     gastroenteritis. Clin Microbiol Rev 1993;6:199-210.
  9. Bean NH, Griffin PM. Foodborne disease outbreaks in the United States,
     1973-1987: pathogens, vehicles, and trends. Journal of Food Protection
     1990;53:804-17.
 10. Kaplan JE, Feldman R, Campbell DS, Lookabaugh C, Gary W. The frequency
     of a Norwalk-like pattern of illness in outbreaks of acute
     gastroenteritis. Am J Public Health 1982;72:1329-32.
 11. Kaplan JE, Gary GW, Baron RC, Singh N, Schonberger LB, Feldman R,
     Greenberg HB. Epidemiology of Norwalk gastroenteritis and the role of
     Norwalk virus in outbreaks of acute nonbacterial gastroenteritis. Ann
     Intern Med 1982;96:756-76.
 12. Cliver DO, Ellender RD, Fout GS, Shields PA, Sobsey MD. Foodborne
     viruses. In: Vanderzant C, Splittstoesser DF, editors. Compendium of
     methods for the microbiological examination of foods. Washington (DC):
     American Public Health Association; 1992.
 13. Centers for Disease Control. Foodborne and waterborne disease
     outbreaks annual summary 1976. Washington (DC): U.S. Department of
     Health, Education, and Welfare Publication No. CDC 78-8185; 1978.
 14. Gerba CP, Goyal SM. Detection and occurrence of enteric viruses in
     shellfish: a review. Journal of Food Protection 1978;41:743-54.
 15. Appleton H. Small round viruses: classification and role in food-borne
     infections. In: Ciba Foundation Symposium. Novel Diarrhoea Viruses.
     Chinchester, England: Wiley Publishers; 1987.
 16. Heun EM, Vogt RL, Hudson PJ, Parren S, Gary GW. Risk factors for
     secondary transmission in households after a common-source outbreak of
     Norwalk gastroenteritis. Am J Epidemiol 1987;126:1181-6.
 17. White KE, Osterholm MT, Mariotti JA, Korlath JA, Lawrence DH, Ristinen
     TL, Greenberg HB. A foodborne outbreak of Norwalk virus
     gastroenteritis: evidence for post-recovery transmission. Am J
     Epidemiol 1986;124:120-6.
 18. Xu ZY, Li ZH, Wang JX, Xiao ZP, Dong DH. Ecology and prevention of a
     shellfish-associated hepatitis A epidemic in Shanghai, China. Vaccine
     1992;(Suppl 1):S67-8.
 19. Desenclos JCA, Klontz KC, Wilder MH, Nainan OV, Margolis HS, Gunn RA.
     A multistate outbreak of hepatitis A caused by the consumption of raw
     oysters. Am J Pub Health 1991;81:1268-72.
 20. Centers for Disease Control and Prevention. Multistate outbreak of
     viral gastroenteritis associated with consumption of
     oysters-Apalachicola Bay, Florida, December 1995-January 1995. MMWR
     Morb Mort Wkly Rpt 1995;44:37-9.
 21. Centers for Disease Control and Prevention. Viral gastroenteritis
     associated with consumption of raw oysters-Florida, 1993. MMWR Morb
     Mort Wkly Rpt 1994;43:446-9.
 22. Centers for Disease Control and Prevention. State outbreak of viral
     gastroenteritis related to consumption of oysters-Louisiana, Maryland,
     Mississippi, and North Carolina, 1993. MMWR Morb Mort Wkly Rpt
     1993;42:945-8.
 23. Cannon RO, Poliner RJ, Hirshhorn RB, Rodeheaver DC, Silverman PR,
     Brown EA, et al. A multistate outbreak of Norwalk virus
     gastroenteritis associated with consumption of commercial ice. J
     Infect Dis 1991;164:860-4.
 24. Warner RD, Carr RW, McClesky FK, Johnson PC, Goldy Elmer LM, Davison
     VE. A large nontypical outbreak of Norwalk virus: gastroenteritis
     associated with exposing celery to nonpotable water and with
     Citrobacter freundii. Arch Intern Med 1991;151:2419-24.
 25. Centers for Disease Control and Prevention. Surveillance for
     waterborne disease outbreaks-United States, 1991-1992. MMWR Morb Mort
     Wkly Rpt 1993;42:1-22.
 26. LeChevallier MW, Norton WD, Lee RG. Giardia and Cryptosporidium spp.
     in filtered drinking water supples. Appl Environ Microbiol
     1991;57:2617-21.
 27. Rose JB, Gerba CP, Jakubowski W. Survey of potable water supplies for
     Cryptosporidium and Giardia. Environ Sci Technol 1991;25:1393-400.
 28. Bean NH, Griffin PM, Goulding JS, Ivey CB. Foodborne disease
     outbreaks, 5-year summary, 1983-1987. MMWR Morb Mort Wkly Rpt
     1990;39(SS-1):15-57.
 29. Bean NH, Goulding JS, Lao C, Angulo FJ. Surveillance for foodborne
     disease outbreaks-United States, 1988-1992. MMWR Morb Mort Wkly Rpt
     1996;45(SS-5):1-66.
 30. Ortega YR, Sterling CR, Gilman RH, Cama VA, Diaz F. Cyclospora
     speciesA new protozoan pathogen of humans. N Engl J Med
     1993;328:1308-12.
 31. Hayes EB, Matte TD, O'Brien TR, McKinley TW, Logsdon GS, Rose JB, et
     al. Large community outbreak of cryptosporidiosis due to contamination
     of a filtered public water supply. N Engl J Med 1989;320:1372-6.
 32. MacKenzie WR, Hoxie NJ, Proctor ME, Gradus MS, Blair KA, Peterson DE,
     et al. A massive outbreak in Milwaukee of Cryptosporidium infection
     transmitted through the public water supply. N Engl J Med
     1994;331:161-7.
 33. Kapikian AJ, Estes MK, Chanock RM. Norwalk group of viruses. In:
     Fields BN, Knipe DM, Howley PM, editors. Fields Virology.
     Philadelphia: Lippencott-Raven Publishers, Inc.; 1996.
 34. Current WL, Garcia LS. Cryptosporidiosis. Clin Microbiol Rev
     1991;4:324-58.
 35. Campbell PN, Current WL. Demonstration of serum antibodies to
     Cryptosporidium sp. in normal and immunodeficient humans with
     confirmed infections. J Clin Microbiol 1983;18:165-9.
 36. Ungar BLP, Soave R, Fayer R, Nash TE. Enzyme immunoassay detection of
     immunoglobulin M and G antibodies to Cryptosporidium in
     immunocompetent and immunocompromised persons. J Infect Dis
     1986;153:570-8.
 37. DeLeon R, Jaykus L. Detection of bacteria and viruses in shellfish.
     In: Hurst CJ, Knudsen GR, McInerney MJ, Stenzenbach LD, Walter MV,
     editors. Manual of environmental microbiology. Washington (DC):
     American Society for Microbiology; 1996.
 38. Sobsey MD, Carrick RJ, Jensen HR. Improved methods for detecting
     enteric viruses in oysters. Appl Environ Microbiol 1978;36:121-8.
 39. Cole MT, Kilgen MB, Reily LR, Hackney CR. Detection of enteroviruses,
     bacterial indicators, and pathogens in Louisiana oysters and their
     overlying waters. Journal of Food Protection 1986;49:596-601.
 40. Jiang X, Graham DY, Wang K, Estes MK. Norwalk virus genome cloning and
     characterization. Science 1990;250:1580-3.
 41. Lambden PR, Caul EO, Ashley CR, Clarke IN. Sequence and genome
     organization of a human small round-structured (Norwalk-like) virus.
     Science 1993;259:516-9.
 42. Atmar RL, Metcalf TG, Neill HF, Estes MK. Detection of enteric viruses
     in oysters by using the polymerase chain reaction. Appl Environ
     Microbiol 1993;59:631-5.
 43. Atmar RL, Neill HF, Romalde JL, LeGuyader F, Woodley CM, Metcalf TG,
     Estes MK. Detection of Norwalk virus and hepatitis A virus in
     shellfish tissues with the PCR. Appl Environ Microbiol 1995;61:3014-8.
 44. Goswami BB, Koch WH, Cebula TA. Detection of hepatitis A in Mercenaria
     mercenaria by coupled reverse transcription and polymerase chain
     reaction. Appl Environ Microbiol 1993;59:2765-70.
 45. Lees DN, Henshilwood K, Dore WJ. Development of a method for detection
     of enteroviruses in shellfish by PCR with poliovirus as a model. Appl
     Environ Microbiol 1994;60:2999-3005.
 46. Lees DN, Henshilwood K, Green J, Gallimore CI, Brown DWG. Detection of
     small round structured viruses in shellfish by reverse
     transcription-PCR. Appl Environ Microbiol 1995;61:4418-24.
 47. Gouvea V, Santos N, Carmo Timenetsky M, Estes MK. Identification of
     Norwalk virus in artificially seeded shellfish and selected foods. J
     Virol Methods 1994;48:177-87.
 48. Deng MY, Day SP, Cliver DO. Detection of hepatitis A virus in
     environmental samples by antigen-capture PCR. Appl Environ Microbiol
     1994;60:1927-33.
 49. Jaykus L, DeLeon R, Sobsey MD. A virion concentration method for
     detection of human enteric viruses in oysters by PCR and oligoprobe
     hybridization. Appl Environ Microbiol 1996;62:2074-80.
 50. Dix A. Development of methods to extract human enteric viruses from
     hard-shelled clams for detection by reverse transcriptase-polymerase
     chain reaction (RT-PCR) and oligoprobe hybridization (OP) [thesis].
     Raleigh (NC): North Carolina State University; 1997.
 51. Chung H, Jaykus L, Sobsey MD. Improved detection of human enteric
     viruses in field oyster specimens by in vivo and in vitro
     amplification of nucleic acids. Appl Environ Microbiol 1996;62:3772-8.
 52. Schaefer III FW. Detection of protozoan parasites in source and
     finished drinking waters. In: Hurst CJ, Knudsen GR, McInerney MJ,
     Stenzenbach LD, Walter MV, editors. Manual of Environmental
     Microbiology. Washington (DC): American Society for Microbiology;
     1996.
 53. LeChevallier MW, Norton WD, Siegel JE, Abbaszadegan M. Evaluation of
     the immunofluorescence procedure for detection of Giardia cysts and
     Cryptosporidium oocyts in water. Appl Environ Microbiol 1995;61:690-7.
 54. Nieminski EC, Schaefer III FW, Ongerth JE. Comparison of two methods
     for the detection of Giardia cysts and Cryptosporidium oocytes in
     water. Appl Environ Microbiol 1995;61:1714-9.
 55. Versey G, Slade JS, Byrne M, Shepard K, Fricker CR. A new method for
     the concentration of Cryptosporidium oocysts from water. J Appl
     Bacteriol 1993;75:82-6.
 56. Rodgers MR, Flanigan DJ, Jakubowski W. Identification of algae which
     interfere with the detection of Giardia cysts and Cryptosporidium
     oocysts and a method for alleviating this interference. Appl Environ
     Microbiol 1995;61:3759-63.
 57. Campbell AT, Robertson LJ, Smith HV. Viability of Cryptosporidium
     parvum oocytes: correlation of in vitro excystation with
     inclusion/exclusion of fluorogenic vital dyes. Appl Environ Microbiol
     1992;58:3488-93.
 58. Campbell AT, Robertson LJ, Smith HV. Novel methodology in the
     detection of Cryptosporidium parvum: a comparison of cooled charge
     coupled device (CCD) and flow cytometry. Water Science Technology
     1993;27:89-92.
 59. Campbell AT, Haggart R, Robertson LJ, Smith HV. Fluorescent imaging of
     Cryptosporidium using a cooled charge couple device (CCD). J Microbiol
     Methods 1992;16:169-74.
 60. Abbaszadegan M, Gerba CP, Rose JB. Detection of Giardia cysts with a
     cDNA probe and applications to water samples. App Environ Microbiol
     1991;57:927-31.
 61. Erlandsen SL, VanKeulen H, Gurien A, Jakubowski W, Schafer III FW,
     Wallis P, et al. Molecular approach to speciation and detection of
     Giardia: fluorochrome rDNA probes for identification of Giardia
     lamblia, Giardia muris, and Giardia ardeae in laboratory and
     environmental samples by in situ hybridization. In: Thompson RCA,
     Reynoldson JA, Lymbery AJ, editors. Giardia: from molecules to
     disease. Oxford: CAB International; 1994.
 62. Mahbubani MH, Bej AK, Perlin M, Schaefer III FW, Jakubowski W, Atlas
     RM. Detection of Giardia cysts by using polymerase chain reaction and
     distinguishing live from dead cysts. Appl Environ Microbiol
     1991;57:3456-61.
 63. Mahbubani MH, Bej AK, Perlin M, Schaefer III FW, Jakubowski W, Atlas
     RM. Differentiation of Giardia duodenalis and other Giardia spp. by
     using polymerase chain reaction and gene probes. J Clin Microbiol
     1992;30:74-8.
 64. Webster KA, Pow JDE, Giles M, Catchpole J, Woodward MJ. Detection of
     Cryptosporidium parvum using a specific polymerase chain reaction. Vet
     Parasitol 1993;50:35-44.
 65. de la Cruz AA, Sivaganesan M. Detection of Giardia and Cryptosporidium
     spp. in source water samples by commercial enzyme-immunosorbant assay.
     In: Proceedings 1994 Water Technology Conference; 1994 Nov 6-10; San
     Francisco, California. Denver (CO): American Water Works Association,
     1995.
 66. Sauch JF, Flanigan D, Galvin ML, Berman D, Jakubowski W. Propidium
     iodide as an indicator of Giardia cyst viability. Appl Environ
     Microbio
l 1991;57:3243-7.
 67. Schupp DG, Erlandsen SL. A new method to determine Giardia cyst
     viability: correlation of florescein diacetate and propidium iodide
     staining with animal infectivity. Appl Environ Microbiol
     1987;53:704-7.
 68. Moe CL, Gentsch J, Ando T, Grohmann G, Monroe SS, Jiang X, et al.
     Application of PCR to detect Norwalk virus in fecal specimens from
     outbreaks of gastroenteritis. J Clin Microbiol 1994;32:642-8.
 69. Green J, Norcott JP, Lewis D, Arnold C, Brown DWG. Norwalk-like
     viruses: demonstration of genomic diversity by polymerase chain
     reaction. J Clin Microbiol 1993;31:3007-12.
 70. Wang J, Jiang X, Madore HP, Gray J, Desselberge U, Ando T, et al.
     Sequence diversity of small, round-structured viruses in the Norwalk
     virus group. J Virol 1994;68:5982-90.
 71. Ando T, Monroe SS, Gentsch JR, Jin Q, Lewis DC, Glass RL. Detection
     and differentiation of antigenically distinct small round-structured
     viruses (Norwalk-like viruses) by reverse transcription-PCR and
     southern hybridization. J Clin Microbiol 1995;33:64-71.
 72. Centers for Disease Control and Prevention. Addressing emerging
     infectious disease threats: a prevention strategy for the United
     States. Atlanta (GA): U.S. Department of Health and Human Services,
     U.S. Public Health Service; 1996.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/jaykus.txt

Quantitative Microbiology: A Basis for Food Safety

T. A. McMeekin, J. Brown, K. Krist, D. Miles, K. Neumeyer, D.S. Nichols, J.
Olley, K. Presser, D. A. Ratkowsky, T. Ross, M. Salter, and S. Soontranon
University of Tasmania, Hobart, Australia
---------------------------------------------------------------------------

      Because microorganisms are easily dispersed, display
      physiologic diversity, and tolerate extreme conditions, they
      are ubiquitous and may contaminate and grow in many food
      products. The behavior of microbial populations in foods
      (growth, survival, or death) is determined by the properties of
      the food (e.g., water activity and pH) and the storage
      conditions (e.g., temperature, relative humidity, and
      atmosphere). The effect of these properties can be predicted by
      mathematical models derived from quantitative studies on
      microbial populations. Temperature abuse is a major factor
      contributing to foodborne disease; monitoring temperature
      history during food processing, distribution, and storage is a
      simple, effective means to reduce the incidence of food
      poisoning. Interpretation of temperature profiles by computer
      programs based on predictive models allows informed decisions
      on the shelf life and safety of foods. In- or on-package
      temperature indicators require further development to
      accurately predict microbial behavior. We suggest a basis for a
      "universal" temperature indicator. This article emphasizes the
      need to combine kinetic and probability approaches to modeling
      and suggests a method to define the bacterial growth/no growth
      interface. Advances in controlling foodborne pathogens depend
      on understanding the pathogens' physiologic responses to growth
      constraints, including constraints conferring increased
      survival capacity.

Ensuring the microbial safety and shelf life of foods depends on minimizing
the initial level of microbial contamination, preventing or limiting the
rate of microbial growth, or destroying microbial populations. With many
foods, these strategies have been practiced successfully for thousands of
years. However, in the last decade, the incidence of foodborne disease has
increased in the industrialized world (1), despite the introduction of the
Hazard Analysis and Critical Control Points (HACCP) concept and the
promulgation of regulations in food safety. The increased incidence of
foodborne disease is caused by changes in agricultural and food processing
practices, increasing international trade in foods, and social changes
(which include changed eating habits and increased population mobility)
(2).

This article develops the propositions that available quantitative
information, properly applied, is a basis for improved food safety; the
information available, largely an empiric description of microbial behavior
in foods, highlights a lack of understanding of the physiology of foodborne
pathogens; and knowledge of the physiology may lead to more precise control
of foodborne bacteria and novel protocols to ensure the microbiologic
safety of foods.

Characteristics of Bacteria

Bacteria have inhabited the earth for approximately three and a half
billion years and have colonized almost every conceivable habitat (3). In
fact, the development of microbial populations is probably precluded only
when liquid water is absent or conditions are so extreme that rapid
denaturation of proteins outpaces their rate of replacement. The major
characteristics that underpin the success of prokaryotes are small size and
ease of dispersal, physiologic diversity, and tolerance of extreme
conditions (4). The temperature range over which microbial populations
develop is -12C (the temperature at which intracellular water freezes) to
+112C (the temperature at which liquid water is maintained only under
elevated pressure). The pH range is pH 1 to pH 12, and the salinity range
is zero to saturated (4).

Langeveld et al. (5), who studied microbial development in biofilms in a
tubular heat exchanger used to pasteurize milk, report the exploitation of
different ecologic niches by bacteria. Through the ascending temperature
range of the tube (~20C to ~90C), the dominant microbiota changed from
gram-negative bacteria such as Acinetobacter, to coliforms to Streptococcus
thermophilus to thermophilic bacilli. At the highest temperature, the wall
of the exchanger was colonized by Thermus thermophilus. Thus, it appears
that contaminants deposited along the length of the tube were selected by
the in situ temperature, with the fastest-growing organism dominating.

Factors Affecting Microbial Behavior in Foods

Most studies in food microbiology are concerned with the rapid growth of
populations, but in many ecosystems, the survival characteristics of the
population also need to be considered. The longevity of bacterial spores
and their resistance to harsh conditions are well documented. However, the
ability of vegetative cells to resist stressful conditions is increasingly
recognized as an important ecologic trait (6). Attention also needs to be
given to relatively slow-growing populations in various situations, e.g.,
when the shelf life of a product is extended by control of rapidly growing
spoilage organisms. The behavior of foodborne microorganisms, be it the
growth or death of microbial populations, is based on the time of exposure
to environmental factors affecting population development; for example,
equivalent kills of bacteria in milk are achieved by low temperature-long
time pasteurization (60C/30 min) and high temperature-short time
pasteurization (72C/15 sec). When populations are in the biokinetic range,
the rate at which they develop is determined by factors such as
temperature, water availability, and pH applied in food preservation
procedures. The extent of microbial growth is a function of the time the
population is exposed to combinations of intrinsic food properties (e.g.,
salt concentration and acidity) and extrinsic storage conditions (e.g.,
temperature, relative humidity, and gaseous atmosphere).

Different factors assume dominance in different foods and preservation
strategies. In many foods, the full preservation potential of a single
property is restricted because of considerations related to the esthetic,
organoleptic, and nutritional properties of the product. However, several
properties or conditions may be combined to provide a desired level of
stability (7). In situations where the preservation strategy is designed to
slow the rate of population growth, the effect will always be increased by
storage temperature. Temperature control in processing, distribution, and
storage (the cold chain) is crucial to ensure the adequate shelf life and
safety of many common foods, including meat, fish, poultry, and milk. Newer
technologies, including modified atmosphere packaging and sophisticated
products such as sous-vide meals, do not obviate the need for strict
temperature control. Indeed, the requirement for vigilance increases with
increased shelf life and the possibility of growth of psychrotrophic
pathogens over an extended period.

Predictive Microbiology

Predictive microbiology involves knowledge of microbial growth responses to
environmental factors summarized as equations or mathematical models. The
raw data and models may be stored in a database from which the information
can be retrieved and used to interpret the effect of processing and
distribution practices on microbial proliferation. Coupled with information
on environmental history during processing and storage, predictive
microbiology provides precision in making decisions on the microbiologic
safety and quality of foods. The term "quantitative microbial ecology" has
been suggested as an alternative to "predictive microbiology" (8).

The development, validation, and application of predictive microbiology has
been extensively reviewed in the last decade (9,10). Modeling studies have
concentrated on descriptions of the effect of constraints on microbial
growth (rather than survival or death), often using a kinetic model
approach (rather than probability modeling) and most often describing the
effect of temperature as the sole or one of a number of controlling
factors. For example, the temperature dependence model for growth of
Clostridium botulinum demonstrated a good fit to data, but the authors
noted that "care must be taken at extremes of growth, as no growth may be
registered in a situation where growth is indeed possible but has a low
probability" (11).

The emphasis in modeling efforts on temperature (often in combination with
other factors) may be justified, given its crucial role in the safe
distribution and storage of foods. Surveys carried out over several decades
in the United Kingdom, United States, Canada, and Australia point to the
predominant role of temperature abuse in outbreaks of foodborne disease
(12-14).

Problems with Predictive Microbiology and Research Needs

Several commonly perceived problems with predictive modeling (8) are
reviewed below. While considerable progress has been made in defining
philosophic approaches and experimental protocols for growth model
development and many models have been developed and published, more
validation studies are required, particularly involving independent and
industry-based trials. More emphasis should be placed on modeling the death
kinetics of foodborne pathogens with low infective doses.

Measurement of environmental factors (e.g., temperature) can be achieved
with precision, but in some situations, (e.g., in chilling of meat
carcasses), it is more difficult. Location of the sensor can be an
important consideration (15,16). In meat chilling, where control of
microbial development is a function of the combined effects of falling
temperature and water activity, development of a technique to measure water
activity in situ at the carcass surface would provide valuable information.
Furthermore, development of techniques to measure constraints such as water
activity, pH, or redox potential on a microscale might provide useful
information for a complex food such as salami. This would allow definition
of the role of the microenvironment in determining microbial behavior. The
concept of a microenvironment is well developed in soil microbiology (17)
but has been neglected in food microbiology.

The inherent variability of response times (generation time and lag phase
duration) as an issue in predictive microbiology was first raised by
Ratkowsky et al. (18), who related the variance of responses on a
transformed rate scale such as V(k) or V(lnk) to the variances of
responses on a time (0) scale. The variance was shown to be proportional to
the square or cube of the response time. It was later confirmed (19-21)
that nonnormal gamma and inverse Gaussian distributions described the
distribution of response times in predictive microbiology, which indicate
that variance is proportional to the square or cube of the response time,
respectively.

The practical implication of these findings for the application of kinetic
models is that inherent biologic variability increases markedly with
increasing response times, and thus the confidence limits associated with
predictions also increase markedly. However, if the probability
distribution of the response time is known, one can determine the
probability that an organism will grow more quickly than a predicted
response time (21). Thus, kinetic models are appropriate to describe
consistent microbial growth responses, but under extreme conditions a
probability approach may be required.

Models are normally developed under static conditions (growth rates and lag
times are measured at a series of set temperatures, water activity values,
and pH levels), and the results are combined to describe the effects of
each factor or a combination of factors on population development.
Subsequently, models must be validated in foods under conditions that mimic
situations encountered in normal practice, e.g., decreasing temperature and
water activity during active chilling of meat carcasses or fluctuating
temperatures during the distribution and storage of many food commodities.

Shaw (22) and later several other authors (23-26) reported on the effect of
fluctuating temperatures. Depending on the magnitude of the temperature
deviation, the organism may change its rate of growth to a rate
characteristic of the new temperature, or it may stop growing if a lag
phase is introduced. In both situations, Salmonella Typhimurium has
responded nearly as predicted by the model (24,25). Baranyi et al. (26)
presented similar results for the spoilage bacterium Brocothrix
thermosphacta. When cycled between 25C and 5C, the model predicted
behavior well in both the growth rate and lag phase duration. However, a
temperature shift from 25C to 3C caused deviations from model predictions
due to decline in viable cell numbers or extended lag phases. During the
final extended phase of growth at 2.8C, the rate resumed at that
predicted. Baranyi et al. (26) also examined the perceived problem of
modeling lag phase duration. The difficulty in predicting lag phase
duration in foods is not due to the lack of a suitable model: the
difficulty comes from the lack of knowledge of the physiologic status of
the organisms contaminating the food. The organisms may include cells that
are actively growing, exhibiting a physiologic lag phase, damaged and under
repair, exhibiting physiologic (endospores) or exogenous dormancy (VNC
cells), damaged but unable to reproduce because of ineffective repair
mechanisms, and dead. At least part of the confusion surrounding the
measurement of lag phase duration arises from experiments in which inocula
of different physiologic statuses were used (27,28).

Methods to define the physiologic status of foodborne contaminants under
various conditions need to be developed. This will require observations on
individual cells or small populations of cells either directly by
microscopy or an indicator of single-cell metabolic activity (26).
Luminescent Salmonella strains have been used as real-time reporters of
growth and recovery from sublethal injury (29). Alternatively, a parameter
to describe the suitability of cells to grow in a new environment may be
incorporated in the model (26).

Current Status of Predictive Microbiology

Some problems with predictive microbiology have been discussed, and, for
each, needed research has been suggested. Opinions vary on the efficacy of
models to predict outcomes under real life conditions. At one end of the
scale, accuracy such as that described for the growth of Pseudomonas in
minced beef (Figure 1) can be obtained in trials conducted independently of
the laboratory in which the model was developed. At the other end, models
developed in laboratory broth systems have been reported to be
inappropriate to describe growth on food (30). Dalgaard (31) reported
similar discrepancies and suggested an iterative approach to model
development using food, rather than laboratory media, as the growth
substrate for model development. Such reports emphasize the need for
rigorous validation of models under practical conditions. Deviations from
predictions do not necessarily imply that the model is defective but more
likely that knowledge of some food ecosystems is incomplete and factors
other than those used in model development have an effect on microbial
behavior.

[Figure Not Available in ASCII]

 Figure 1. Validation of Pseudomonas predictor in minced beef . Printed
 with permission of G. Thomson, Defence Force Food Science Centre,
 Scottsdale, Tasmania, Australia.

The common theme of the problems in predictive microbiology discussed above
is that of uncertainty-uncertainty in terms of the starting conditions
(e.g., initial microbial numbers and types) and the microbial response in a
give

n or changing environment. Uncertainty translates to variability if the
distribution of response times is understood and the variance can be
described. As we have indicated above, the variability associated with very
long response times limits the utility of kinetic models and requires a
probability approach. Thus, while in the last decade predictive modelers
were justified in their selection of temperature as a primary factor to
model in kinetic approaches, the next decade may see a return to
probability modeling as pioneered by Genigeorgis (32) and Roberts (33).
This shift will derive impetus from the emergence of dangerous pathogens
with very low infective doses, and continued kinetic modeling will
concentrate on survival and death rather than growth of populations.

The first kinetic death model to find widespread use in the food industry
was for thermal destruction (34). One can consider a model describing a
12-log cycle reduction in C. botulinum spores in a short time with
considerable certainty. However, as we move toward less severe processes
with longer response times and the added complications of "shoulders" and
"tails" to define the growth/no growth interface, biologic variability will
again dictate a probability approach to describe the survival and slow
decline of microbial populations.

Defining the Growth/No Growth Interface

Because growth of pathogenic bacteria in foods always increases the risk
for foodborne disease, defining the conditions at which no growth is
possible is of considerable practical significance for food manufacturers
and regulators. Bacterial growth/no growth interface models quantify the
combined effect of various hurdles on the probability of growth and define
combinations at which the growth rate is zero. Increasing the level of one
or more hurdles at the interface by only a small amount will significantly
increase the probability of "fail safe" events and decrease the probability
that a few cells in the population will resolve the lag phase and begin to
grow (a "fail dangerous" event) (7). The growth/no growth interface also
has great physiologic significance because at that point biosynthetic
processes are insufficient to support population growth, and survival
mechanisms are in place.

A procedure to derive the interface was proposed by Ratkowsky and Ross
(35); it employs a logistic regression model to define the probability of
growth as a function of one or more controling environmental factors. From
this model, the boundary between growth and no growth, at some chosen level
of probability, can be determined. The form of the expression containing
the growth limiting factors is suggested by a kinetic model, while the
response at a given combination of factors is either presence or absence
(i.e., growth/no growth) or probabilistic (i.e., the fraction of positive
responses in n trials). This approach represents an integration of
probability and kinetic approaches to predictive modeling.

Microbial Responses to Stress and Microbial Physiology

Bacteria have physiologic mechanisms enabling them to survive in
environments that preclude their growth. While some tolerance to
environmental insults is adaptive, a wide range of protective mechanisms is
induced when cells enter a stationary phase or become starved. These
phenomena are under the control of the rpoS gene, which codes for a
stationary-phase-specific sigma factor, expression of which triggers the
development of a semidormant state in which bacteria can better resist
multiple physical challenges (36). This factor and the gene products whose
expression it controls are of vital significance to food microbiology; they
form the basis for a global stress response in which one stress can confer
protection to a wide range of other stresses. Under the influence of this
factor, bacterial cells respond very quickly to unfavorable changes in
their environment. Often these responses are phenotypic and remain in place
only during stress (37).

Low pH Tolerance

Brown et al. (37) demonstrated "acid habituation" (38), a phenotypic
response to an environmental insult, for five strains of Escherichia coli.
These strains showed a wide range of intrinsic acid tolerance, which for
each strain was enhanced by exposure to nonlethal acidity (pH 5) before
exposure to a lethal acid challenge (pH 3). Neutralization of the growth
medium partially reversed tolerance to acid stress, underlining that acid
habituation is a phenotypic response. Furthermore, acid tolerance was
correlated with changes in the fatty acid composition of the cell membrane.
During acid habituation, monounsaturated fatty acids (16:1w7c and 18:1w7c)
in the phospholipids of E. coli were either converted to their cyclopropane
derivatives (cy17:0 and cy19:0) or replaced by saturated fatty acids. The
degree of acid tolerance of the five strains of E. coli was highly
correlated with the membrane cyclopropane fatty acid content, which may
enhance the survival of cells exposed to low pH.

Low Water Activity Tolerance

Bacterial cells, when confronted by lowered water activity, regulate the
internal environment by rapidly accumulating compatible solutes such as
glycine betaine or carnitine (39). The solutes, which may be scavenged by
the cell, exert their influence at very low concentrations; the effect is
demonstrated both in limits and rate growth. These compounds appear also to
provide cryotolerance as well as osmotolerance (40; Figure 2).

 [Figure Not Available in ASCII]
 Figure 2. Effect of betaine on the growth of Escherichia coli
 in glucose-minimal medium. Without added NaCl the growth
 rate yield and minimum growth temperature are the same with
 and without betaine. With 4% NaCl the growth rate and yield
 are lower without betaine and the actual minimum temperature
 for growth is approximately 9C lower than with betaine
 (K. Krist, unpub. data).

Energy Diversion

Microbial responses to stressful conditions may constitute a drain on the
energy resources of the cell, e.g., in relation to the accumulation of
compatible solutes (41). Knochel and Gould (42) argued "that restriction of
the availability of energy will interfere with a cell's reaction to osmotic
stress." The energy diversion hypothesis was supported by McMeekin et al.
(9) on the basis of observations on the growth of Staphylococcus xylosus at
nine different levels of water activity. Though Tmin, the theoretic minimum
temperature for growth, remained constant, the actual minimum temperature
at which growth was observed increased with decreasing water activity,
suggesting energy expenditure to cope with aw stress.

Krist and Ross (unpub. data), however, challenged this explanation because
of findings from growth rate and yield experiments on E. coli growing in a
glucose-limited minimal minerals medium. With both decreasing temperature
and water activity, the growth rate declined gradually, but the yield was
not greatly affected until close to the point where growth ceased. As the
substrate was converted to the same amount of biomass, this suggests that
the stresses imposed by suboptimal temperature or water activity are not a
major drain on the cell's energy reserves. Compatible solutes likely
ameliorate the effect of both factors by maintaining enzymes in an active
configuration (39). With pH, the growth rate of many organisms is
unaffected across a wide range of pH values. To maintain intracellular pH,
the cell uses considerable energy to export protons (43), and thus it is
anticipated that yield will be affected.

Increasing knowledge of the physiologic response of bacterial cells to
individual constraints and combinations of constraints will provide greater
precision in defining growth-limiting conditions and possibly allow
development of novel protocols to ensure the microbial safety of foods. As
an example, the remarkable effect of compatible solutes on the growth rate
and growth range conditions is an obvious advantage for bacteria growing
under stressful conditions (Figure 2). Compatible solutes, such as betaine
and carnitine, are widely distributed in foods of plant and animal origin
and are easily available to bacteria and rapidly taken up by specific
transport mechanisms (39,40). It is unlikely that growth might be
controlled by "creating a hostile environment devoid of osmolytes," as
suggested by Smith (44). However, it might be possible to use the specific
uptake mechanism to deliver a compatible solute analogue with lethal
effects on the cell. Alternatively, if the cell is moved from an
environment in which growth is possible to one where growth ceases,
compatible solutes may also allow enzymic reactions to continue within the
cell, depleting energy reserves and inducing a greatly extended lag phase
or death. This hypothesis is supported by the observations of many authors
that survival is better at low rather than ambient temperatures. For
example, Clavero and Beuchat (45) state, "Regardless of the pH and aw,
survival of E. coli O157:H7 was better at 5C than at 20C or 30C."
Furthermore, preliminary experiments in this laboratory suggest that E.
coli declines more rapidly in the presence of betaine than in its absence
(Krist, unpub. data).

Application of Predictive Models

The incorporation of predictive models into devices such as temperature
loggers has been described for E. coli (46) and Pseudomonas (47), as has
the development of expert systems from predictive modeling databases
(48,49).

The current food poisoning crisis indicates that existing quantitative
information on microbial growth, survival, and death, if properly applied,
would have an immediate impact on the incidence of foodborne disease in the
industrialized world. Even without the synthesis of data into mathematical
models, simply logging the temperature history of food processing,
distribution, and storage operations would provide much useful information.
Loggers provide a hard copy of a temperature profile in real time and thus
evidence of temperature abuse and the source of the abuse.

For loggers with appropriate software (46,47), the temperature profile may
be interpreted in terms of microbial growth. However, the interpretation
must be based on an informed analysis of the temperature history by a
trained operator. The operator may, for example, be required to enter
default values for initial bacterial numbers or provide an estimate of lag
phase duration under specified conditions. Estimates of both imply general
knowledge of food microbiology and specific knowledge of the process and
products under consideration. The equivalence of an estimate of microbial
growth derived from temperature profile to that obtained from conventional
microbiologic criteria may also be necessary (15).

An alternative to the use of temperature loggers is the development of in-
or on-package temperature tags as recommended in the U.S. Food Safety
Initiative draft document Food Safety from Farm to Table (50). With
temperature tags, informed interpretation is not required because abuse is
indicated directly by the tag response. Therefore, the tag must indicate
the significance of the environmental history for microbial behavior. The
time/temperature tags available are based on physical or chemical changes
that follow Arrhenius kinetics (9). While these may give a reasonable
approximation of microbial growth in the normal range, the deviation of
microbial responses becomes increasingly large as conditions move from
normal to stressful. The intriguing possibility of a universal
time/temperature indicator was flagged (51) on the basis of observations
made of temperature effects on foodborne pathogens in this laboratory and
by Snyder (52). The universal indicator is based on a relationship that
describes the maximum specific growth rate of a continuum of organisms from
psychrophiles to thermophiles in terms of Arrhenius kinetics with an
apparent activation energy of ~80 kJ/mole. This value can be related to the
activation energy/growth rate at any other temperature by a relative rate
function derived from Belehradek (square root) kinetics.

Conclusions

We have argued that a thorough understanding of microbial ecology and
physiology offers the best opportunity to control microbial populations in
food and reverse the upward trend in the incidence of foodborne disease.
Many food preservation strategies have their origin in empirical
observations practiced for thousands of years. The systematic collection
and collation of data on microbial behavior in foods spawned the discipline
of food microbiology, within which predictive microbiology (quantitative
microbial ecology) has accelerated our understanding of the microbial
ecology of foodborne bacteria. Studies in microbial physiology will further
enhance our knowledge and offer new possibilities for food preservation.

The most disturbing aspect of the current crisis is that simple application
of existing knowledge would lead to a marked reduction in the incidence of
foodborne disease. Education of food handlers and consumers in basic
hygiene and the consequences of temperature abuse is urgently needed as is
a greater depth of understanding for those in technical positions in the
food industry or those with regulatory responsibilities. Furthermore, an
appreciation of the need for shared responsibility for food safety within
all sectors of the continuum from farm to table, including the consumer,
has to be developed. The U.S. Food Safety Initiative draft document
emphasizes this point, as does the structure of the Australian Meat
Research Corporation's Microbial Food Safety Key Program (53).

Address for correspondence: Professor T.A. McMeekin, Department of
Agricultural Science, University of Tasmania, GPO Box 252-54, Hobart, TAS
7001, Australia; fax: 61-3-62-26-2642; e-mail: Tom.McMeekin@utas.edu.au.

Acknowledgments

The work described in this manuscript was funded largely by the Australian
Meat Research Corporation (MRC) through a Core Funding Project and the
Microbial Food Safety Key Program. The authors are indebted to Dr. Stefan
Fabiansson for his support and encouragement of research into the ecology
and physiology of foodborne pathogens.

References

  1. Maurice J. The rise and rise of food poisoning. New Scientist
     1994;144:28-33.
  2. McMeekin TA, Olley J. Predictive microbiology and the rise and fall of
     food poisoning. ATS Focus 1995;88:14-20.
  3. Pace NR. Perspective on the natural microbial world: molecular
     microbial ecology. ASM News 1996;63:463-70.
  4. Schelgel HG, Jannasch HW. Prokaryotes and their habitats. In: Balows
     A, Truper HG, Dworken M, Harder W, Scheifer K-H, editors. The
     prokaryotes. New York: Springer Verlag; 1992. p. 75-125.
  5. Langeveld PLM, van der Klijn W, CB van der Waals. Bacterie groei in
     indampers voor milk en wei. Overdruk van Voedingsmiddelentechnologie
     1990;23:13-7.
  6. Archer DL. Preservation microbiology and safety: evidence that stress
     enhances virulence and triggers adaptive mutations. Trends in Food
     Science Technology 1996;7;91-5.
  7. Leistner L. Food preservation by combined methods. Food Research
     International 1992;25:151-8.
  8. Ross T, McMeekin TA. Predictive microbiology and HACCP. In: Pearson
     AM, Dutson TR, editors. HACCP in meat, poultry and fish processing.
     London: Blackie Academic and Professional; 1995. p. 330-53.
  9. McMeekin TA, Olley J, Ross T, Ratkowsky DA. Predictive microbiology:
     theory and application. Taunton, UK: Research Studies Press; 1993.
 10. Whiting RC, Buchanan RLB. Predictive modeling. In: Doyle MP, Beuchat
     LR, Montville TJ, editors. Food microbiology fundamentals and
     frontiers. Washington (DC): American Society for Microbiology Press
     1997. p. 728-39.
 11. Graham A, Lund BM. The effect of temperature on the growth of non
     proteolytic type B Clostridium botulinum. Letters in Applied
     Microbiology 1993;16:158-60.
 12. Davey GR. Food poisoning in New South Wales: 1977-84. Food Technology
     in Australia 1985;37:453-7.
 13. Enneking U. Hazard analysis of critical control points (HACCP) as part
     of the Lufthansa in-flight service quality assurance. International
     Food Safety News 1993;2:52-3.
 14. Adams M, Moss MO. Food microbiology. Cambridge: Royal Society of
     Chemistry; 1995.
 15. Gill CO, Harrison JCL, Phillips DM. Use of a temperature function
     integration technique to assess the hygienic efficiency of a beef
     carcass cooling process. Food Microbiology 1991;8:83-94.
 16. Gill CO, Jones SDM, Tong AKW. Application of a temperature function
     integration technique to assess the hygienic efficiency for spray
     chilling beef carcasses. Journal of Food Protection 1991;54:731-6.
 17. Madigan MT, Martinko JM, Parker J. Microbial ecology. In: Brock,
     editor. Biology of microorganisms. 8th ed. New Jersey: Prentice-Hall;
     1996.
 18. Ratkowsky D, Ross T, McMeekin TA, Olley J. Comparison of
     Arrhenius-type and Belehradek-type models for the prediction of
     bacterial growth in foods. J Appl Bacteriol 1996;71:452-9.
 19. Ratkowsky DA. Predicting response times in predictive microbiology.
     Occasional paper. Tasmania, Australia: Research and Development Unit,
     Biometrics Section, Dept of Primary Industry, Fisheries and Energy;
     1991.
 20. Alber SA, Schaffner DW. Evaluation of data transformations used with
     the square root and Schoolfield models for predicting bacterial growth
     rate. Appl Environ Microbiol 1992;58:3337-42.
 21. Ratkowsky DA, Ross T, Macario N, Dommett TW, Kamperman L. Choosing
     probability distributions for modelling generation time variability. J
     Appl Bacteriol 1996;80:131-7.
 22. Shaw MK. Effect of abrupt temperature shift on the growth of
     mesophilic and psychrophilic yeasts. J Bacteriol 1967;93:1332-6.
 23. Labuza TP, Fu B. Growth kinetics for shelf-life prediction: theory and
     practice. J of Industrial Microbiology 1993;12:309-23.
 24. Mitchell GA, Brocklehurst TF, Parker R, Smith AC. The effect of
     transient temperatures on the growth of Salmonella typhimurium LT2. I:
     Cycling within the growth region. J Appl Bacteriol 1994;77:113-9.
 25. Mitchell GA, Brocklehurst TF, Parker R, Smith AC. The effect of
     transient temperature on the growth of Salmonella typhimurium LT2. II:
     Excursions outside the growth region. J Appl Bacteriol 1995;79:128-34.
 26. Baranyi J, Robinson TP, Kaloti A, Mackey BM. Predicting growth of
     Brocothrix thermosphacta at changing temperature. Int J Food Microbiol
     1995;27:61-75.
 27. Walker SJ, Archer P, Banks JG. Growth of Listeria monocytogenes at
     refrigeration temperatures. J Appl Bacteriol 1990;68:157-62.
 28. Dufrenne J, Delfgou E, Ritmeester W, Notermans S. The effect of
     previous growth conditions on the lag phase of some foodborne
     pathogenic microorganisms. Int J Food Microbiol 1997;34:89-94.
 29. Chen J, Griffiths MW. Luminescent Salmonella strains as real time
     reporters of growth and recovery from sublethal injury in food. Int J
     Food Microbiol 1996;31:27-44.
 30. Gill CO, Greer GG, Dilts BD. The aerobic growth of Aeromonas
     hydrophila and Listeria monocytogenes in broths and on pork. Int J
     Food Microbiol 1997;35:67-74.
 31. Dalgaard P. Predictive microbiological modelling and seafood quality.
     In: Seafood from producer to consumer, integrated approach to quality.
     Proceedings of the International Seafood Conference, 1995
     Noordwijkerhout. The Netherlands: Elsevier, Amsterdam. In press 1997.
 32. Genigeorgis CA. Factors affecting the probability of growth of
     pathogenic microorganisms in foods. J Am Vet Med Assoc
     1981;179:1410-7.
 33. Roberts TA, Gibson AM, Robinson A. Prediction of toxin production by
     Clostridium botulinum in pasteurised pork slurry. Journal of Food
     Technology 1981;16:337-55.
 34. Stumbo CR, Prokit KS, Ramarkrishnan TV, Evans DA, Franaas FJ. CRC
     handbook of lethality guides for low-acid canned foods. 1. Boca Raton
     (FL): CRC Press; 1983.
 35. Ratkowsky DA, Ross T. Modelling the bacterial growth/no growth
     interface. Letters in Applied Microbiology 1995;20:29-33.
 36. Lee IS, Lin J, Hall HK, Bearson B, Foster JW. The stationary-phase
     sigma factor (RpoS) is required for a sustained acid tolerance
     response in virulent Salmonella typhimurium. Mol Microbiol
     1995:17:155-67.
 37. Brown J, Ross T, Nichols PD, McMeekin TA. Acid habituation of
     Escherichia coli and the potential role of cyclopropane and fatty
     acids in low pH tolerance. Int J Food Microbiol. In press 1997.
 38. Goodson M, Rowbury RJ. Resistance of acid-habituated Escherichia coli
     to organic acid and its medical and applied significance. Letters in
     Applied Microbiology 1989;8:211-4.
 39. Galinski EA. Compatible solutes of halophilic eubacteria: molecular
     principles, water solute interaction, stress protection. Experientia
     1993;49:487-96.
 40. Ko R, Smith LT, Smith GC. Glycine betaine confers enhanced
     osmotolerance and cryotolerance on Listeria monocytogenes. J Bacteriol
     1994;176:426-31.
 41. Csonka LN. Physiological and genetic responses of bacteria to osmotic
     stress. Microbiol Rev 1989;53:121-47.
 42. Knochel S, Gould G. Preservation microbiology and safety: quo vadis?
     Trends in Food Science and Technology 1995;6:127-31.
 43. Davidson PM. Chemical preservatives and natural antimicrobial
     compounds. In: Doyle MP, Beuchat LR, Montville TJ, editors. Food
     microbiology fundamentals and frontiers. Washington (DC): American
     Society for Microbiology Press, 1997.
 44. Smith LT. Role of osmolytes in adaptation of osmotically stressed and
     chill stressed Listeria monocytogenes grown in liquid media and on
     processed meat surfaces. Appl Environ Microbiol 1996;62:3088-93.
 45. Clavero MRS, Beuchat LR. Survival of Escherichia coli O157:H7 in broth
     and processed salami as influenced by pH, water activity and
     temperature and suitability of media for its recovery. Appl Environ
     Microbiol 1996;61:2735-40.
 46. Gill CO, Phillips DM. Hygienically appropriate time/temperature
     parameters for raw meat processing. Proceedings of the 36th
     International Congress of Meat Science and Technology; 27 Aug_2 Sep
     1990; Havana, Cuba; 1990. p. 458-70.
 47. McMeekin TA, Ross T. Modeling applications. Journal of Food Protection
     1996 (Suppl): 1-88.
 48. Adair C, Briggs PA. The concept and application of expert systems in
     the field of microbiological safety. Journal of Industrial
     Microbiology 1993;12:263-7.
 49. Jones JE. A real-time database/models base/expert system in predictive
     microbiology. Journal of Industrial Microbiology 1993;12:268-72.
 50. Glickman D, Salala DE, Browner CM. Food safety from farm to table: a
     new strategy for the 21st Century. Washington (DC): U.S. Department of
     Agriculture;1997.
 51. McMeekin TA, Ross T. Shelf life prediction: status and future
     possibilities. Int J Food Microbiol 1996;33:65-83.
 52. Snyder OP. Use of time and temperature specifications for holding and
     storing food in retail operations. Dairy Food Environ Sanitat
     1996;116:374-88.
 53. Fabiansson S, Wrigley J, Sumner J, McMeekin TA, Orr C. Meat Research
     Corporation Business Plan, Microbial Food Safety Key Program, 1996.
     Sydney, Australia: Meat Research Corporation; 1996.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/mcmeekin.txt

Strategies for Rapid Response to Emerging Foodborne Microbial Hazards (superscript 1)

Jesse Majkowski
U.S. Department of Agriculture, Washington, D.C., USA
---------------------------------------------------------------------------

      The foodborne outbreak paradigm has shifted. In the past, an
      outbreak affected a small local population, had a high attack
      rate, and involved locally prepared food products with limited
      distribution. Now outbreaks involve larger populations and may
      be multistate and even international; in many the pathogenic
      organism has a low infective dose and sometimes is never
      isolated from the food product. Delay in identifying the
      causative agent can allow the outbreak to spread, increasing
      the number of cases. Emergency intervention should be aimed at
      controlling the outbreak, stopping exposure, and perhaps more
      importantly, preventing future outbreaks. Using epidemiologic
      data and investigative techniques may be the answer. Even with
      clear statistical associations to a contaminated food, one must
      ensure that the implicated organism could logically and
      biologically have been responsible for the outbreak.

Mobilizing Resources

In the traditional paradigm of a foodborne disease outbreak, the cases were
from a small local group, and the attack rate was high. Local health
officials generally detected and investigated the outbreak. In the emerging
paradigm, a diffuse outbreak may be spread over a very wide area, perhaps
several counties or states, even with a low infective dose and a low attack
rate. The outbreak may be registered only as an increase in sporadic cases
and detected only because of specific laboratory-based subtyping
surveillance. Whether Salmonella serotyping or another molecular method
identifies a cluster of related cases, the investigations are more complex,
and often no obvious food-handling error is found. Industry contamination
may be involved, and implications may be industrywide or nationwide.

Detecting a widespread outbreak requires increased reliance on laboratory
subtyping by state public health laboratories at a time when some states
are considering eliminating or privatizing their laboratories. Surveillance
data must be rapidly compared over increasingly broad regions, not just at
the county level but at the state, regional, and national levels. Increased
awareness is needed throughout the system that a local outbreak may herald
a broader problem. Moreover, investigations must be conducted quickly to
prevent future cases. Because of the low levels of microbiologic
contamination, the right specimens and samples of food must be used.
Available epidemiologic data should guide this selection so that the most
likely vehicles are sampled. Traceback must extend beyond the immediate
preparation of the implicated food to the whole chain of preparation, i.e.,
sources of ingredients, processing, storage, and transportation;
cooperation at all levels of industry is required. The goals are to control
the ongoing outbreak, remove the contaminated product from the market, and
learn how to prevent similar outbreaks. Intervention in outbreaks must
change. Emergency intervention must be based on solid epidemiologic data
(appropriate study design and sample size and clear statistical association
with logical and biologic plausibility) and cannot always wait for
laboratory confirmation of a contaminated product. Illnesses will not wait
for laboratory examination to yield the pathogen; the pathogen may not be
detectable with current technologies, the food may not be available, and
the delay can be critical.

The Human Side of Foodborne Disease

Public health officials in Washington state screened children with bloody
diarrhea and hemolytic uremic syndrome (HUS) as the result of a large
outbreak of Escherichia coli O157:H7 infection in 1993. One isolate of the
organism caused perhaps 75% of the cases, not 100%. As a result, the source
of the infection was identified, and regulatory action was taken to halt
future cases. Hospitals reported fewer new cases after this action.
However, 500 Washington state residents, two-thirds of them children under
15 years of age, became infected before the incriminating food could be
removed from the market. The HUS attack rate was approximately 12% for
children under the age of 16 years. The organism was recovered from the
food product (hamburger), and DNA fingerprinting was initiated by several
techniques. The number of colony-forming units of E. coli O157:H7 in the
hamburger was relatively low.

This organism can cause severe life-threatening infection, even with an
inoculum rate too low to be detected by testing. In the United States, the
incidence of HUS is approximately 1.7 cases per 100,000 children under the
age of 15 years. This figure is based on data from King County, Washington,
in the early 1980s and indicates that there are 1,000 cases of HUS in the
United States per year or an average of 2.8 cases per day in a population
of 250 million. The Centers for Disease Control and Prevention has recently
reported that only half of the country's microbiologists screen for this
organism.

Clostridium botulinum, A Reemerging Pathogen

The outstanding property of C. botulinum is its ability to synthesize a
neurotoxin of extraordinary potency (lethal dose is approximately one
microgram). C. botulinum is an unusual foodborne pathogen in that it causes
neuroparalytic rather than diarrheal disease. During illness, the first
nerves affected are the cranial nerves in the head and eye; the paralysis
can descend and affect every peripheral nerve in the body. The toxin can
enter into foods, but in recent years, C. botulinum has also been found to
colonize the intestinal tract of infants under 1 year of age and of adults
that have undergone intestinal surgery or have been treated with
antibiotics. The number of cases of adults with unusual clostridia that
produce botulinum toxin is increasing.

Botulism occurs worldwide. The highest incidence is found in Poland and in
Asia and is related to food handling (in Poland, home canned meats).

New food processes and packaging have been associated with the reemergence
of botulism. A clam chowder outbreak in California involved a boxed food
that was not properly stored. Because boxed foods generally do not require
refrigeration, the food was kept at ambient temperature; however, it was
not shelf stable and should have been refrigerated.

A large outbreak (30 cases) of botulism occurred in a restaurant in El Paso
when potatoes were cooked in foil, left wrapped, and then used in potato
salad. Baking had eliminated vegetative organisms, but the spores of
botulinum were not killed. In this case, a low-acid food was held under
anaerobic conditions.

Botulism outbreaks are probably the most reported type of foodborne
illness. Changes in processing and ingredients in foods and formulations
can inadvertently lead to the growth of C. botulinum. Failure to thoroughly
heat a food product may allow the botulinum toxin to survive.

DNA Fingerprinting

Responding to the threat of emerging foodborne diseases requires public
health surveillance that is based on epidemiologic methods and close
collaboration between epidemiologists and public health laboratories.
Surveillance for foodborne diseases should include pathogen-specific
surveillance to identify clusters of cases caused by the same organism and
epidemiologic investigations to identify the source.

The Minnesota State Department of Health is developing a new approach to
foodborne disease surveillance. A Salmonella Enteritidis (SE) outbreak was
first recognized by the public health laboratory when the number of SE
isolates suddenly increased. Because many of the isolates came from
clinical laboratories in southeastern Minnesota, the outbreak initially
seemed regional. However, within 48 hours of initiating a case-control
study, a nationally distributed food product was identified as the vehicle,
and the nationwide scope of the outbreak was documented. This was an
example of consequential epidemiology. When the association between food
consumption and SE was announced, the evidence implicating a particular
brand was limited to a single case-control study of 15 matched pairs.
Laboratory isolation of SE from official samples was not reported until 10
days later. This prompt action, based on epidemiologic data, prevented at
least 10 days of potential exposure for thousands of consumers.
Consequential epidemiology produces results that translate into disease
prevention. We need to continue to develop models for how to rapidly
evaluate and act on epidemiologic data and how to better coordinate
activities regionally and nationally.

A critical element of the success of this type of foodborne disease
surveillance is the specificity with which we can match isolates that may
be epidemiologically linked. Molecular subtyping schemes such as
pulsed-field gel electrophoresis can greatly improve identification of
clusters of the same organisms such as particular Salmonella serotypes.

Another example involved analysis of a typical epidemiologic curve for a
seemingly single outbreak. When isolates were analyzed, however, a cluster
of small outbreaks was found. One was caused by infected food handlers at a
restaurant. This outbreak would have continued, and the infected food
handlers would have provided an ongoing source of infection to patrons had
the cluster not been identified through subtype-specific surveillance. This
incident serves as a model for how foodborne disease surveillance systems
must be developed and used. Molecular subtyping has revolutionized our
ability to conduct meaningful surveillance. We consider it an integral part
of disease prevention and control and continue to explore its usefulness.

Parasites

Cyclospora cayetanensis is a protozoan coccidian parasite. A one-celled
organism, it is related to other organisms such as Toxoplasma and
Cryptosporidium. It is a prototypical emerging pathogen. C. cayetanensis is
unusual in that it is not immediately infectious when excreted. Under
optimal conditions, it matures in days to weeks, so direct person-to-person
spread is very unlikely. An outbreak following a meal is probably not
caused by the food handler. The organism appears to be seasonal, and in
most places where it has been studied, it occurs in the spring or summer
and causes little or no disease during the fall or winter. Infection has
been reported throughout the world, and the key studies have been conducted
in Peru and Nepal. Disease caused by C. cayetanensis is characterized by
watery stools, nausea, weight loss, low-grade fever, fatigue, or any
combination of these symptoms. The disease (which is easily treatable) can
be quite protracted, and without treatment, relapse can occur. The mean
incubation period of 1 week complicates the epidemiology; cases may not be
recognized until 2 weeks after people have been exposed.

In 1996, more than 1,450 cases of Cyclospora were reported in the United
States (87%) and Canada (13%). Approximately half of them were in clusters;
the other half were sporadic (not epidemiologically linked to other cases).
More than 65% of the 1,450 cases were laboratory confirmed; 22 infected
patients were hospitalized. Fifty-five clusters were reported, 47 in the
United States and 8 in Canada. An average of 28 attendees per event and a
very high attack rate were reported. The attack rate was 56% for attendees,
not for people who ate the implicated food. At least one type of fresh
berry was served at every event and, despite other types of exposures, no
other food was implicated in any cluster investigation. The berry did not
always achieve statistical significance, largely because of the small
number of attendees at a specific event. The berry most likely linked to
the cases was later determined. That type of berry was served at 50% to 91%
of the 55 events; it was the only kind served at 10 or 11 of the events.

We had overwhelming epidemiologic evidence, but we never identified
Cyclospora on any raspberries. Two factors, at least, contributed to this.
The test for C. cayetanensis did not exist when this outbreak occurred;
therefore, no implicated raspberries were tested. The question in this
investigation as in others is how much epidemiologic evidence is needed to
implicate a food as the vehicle of disease? A review team may be needed to
look at the epidemiologic data and determine if they are adequate to
warrant informing the public about a hazardous food. We are seeing new
pathogens, new species. As outbreaks cross into other states, the need for
coordination between health officials in the states and in the federal
government becomes more urgent.

Conclusions

Early identification of the outbreak and the organism can prevent future
cases. Recent investigations have found that the presence of an organism
even at low levels can cause serious consequences. Fingerprinting organisms
for identification during outbreaks is extremely important. In some
instances, fingerprinting has helped identify several small outbreaks that
initially appeared to be one large outbreak. We can no longer afford to
wait for all the evidence and laboratory results to be collected and
reported; we must use epidemiologic data. Once the outbreak is identified,
DNA fingerprinting is needed to identify whether other outbreaks are
occurring simultaneously.

A rapid and coordinated response is needed among state officials and
federal agencies. Interventions should stop outbreaks and identify products
causing illness so they can be removed from the market. Then health
officials need to take the next step-investigate what happened and
determine the cause so that similar outbreaks can be prevented.

Address for correspondence: Jesse Majkowski, U.S. Department of
Agriculture, Room 3715 Franklin Court, 14th Street & Independence Avenue,
S.W. Washington, DC 20250, USA. fax: 202-501-7515; e-mail:
jesse.majkowski@usda.gov.

(superscript 1) The author has summarized the transcripts of panel discussions by Dennis
Lang, Craig Hedberg, Eric Johnson, Suzanne Binder, and Philip Tarr.
---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/majkows.txt

Foodborne Illness: Implications for the Future

Richard L. Hall
---------------------------------------------------------------------------

      Many outbreaks of foodborne illness, even those involving newly
      recognized pathogens, could have been avoided if certain
      precautions had been taken. This article will draw on existing
      information to suggest realistic measures that, if implemented,
      are most likely to avert or diminish the impact of new
      foodborne disease outbreaks.

I do not see microbiologic infections as totally different from other food
hazards, or microorganisms as totally different from other life forms. We
are all hosts and prey, parasites and predators. Even though we differ in
size, complexity, and weaponry, we employ many similar strategies-the
result of a shared planet, and a substantially shared genetic base. That is
also why we have much to learn from animal disease surveillance.

Writing on emerging diseases, many argue forcefully for a broad and
well-integrated view (1,2); however, aspects absolutely essential to
understanding the problem are often omitted. This short article will
doubtless make the same error. So, as Shakespeare had Prologue plead at the
opening of Henry the Fifth, "Piece out our imperfections in your minds."

Factors Contributing to Emergence

Outbreaks occur whenever pathogenic agents in sufficient number or quantity
encounter a susceptible population without effective interceptive measures.
Then, if we did not expect it, we say "it emerged."

Genetic Variability

The large genetic variability of microorganisms is the principal reason why
so often some survive after any unfavorable environmental change. Some
strains are hypermutable, which reinforces the potential for survival, and
have very short generation times, with bacterial minutes comparable to
human years. As Dr. Lederberg notes, microorganisms are opponents with whom
we cannot race-on their terms.

Environment

Environmental factors also contribute to emergence. Hot, humid climates
favor the growth of fungi and the production of mycotoxins. To borrow an
example outside foodborne pathogens, an unusually wet season produced a
sharp increase in the deer mice population and the consequent outbreak of
hantavirus in the Four Corners area of the United States.

Behavior

Human actions and behavior directly affect food safety. People are vectors
for disease, traveling far more often, farther, and more rapidly than ever
before (3), and moving far more swiftly than rats, lice, and mosquitoes.
Political boundaries frequently and perversely act as leaky sieves, letting
diseases through unimpeded, while blocking measures for disease prevention,
control, and treatment (1).

Urbanization

Urbanization is a major factor in emergence. Crowding increases human
contact and opportunities for transmission. Particularly in developing
countries, public health services lag far behind the rush from farm to
city. Cities, especially in industrialized nations, are economic and
governmental centers and harbor institutions of culture and learning.
However, cities are also massive projects in the intensive monoculture of
humans. With agriculture, it is entirely possible to carry out monoculture
effectively and productively-esthetic considerations aside-if one provides
for and tightly controls all essential inputs and conditions, monitors the
process closely, and is prepared for prompt and effective intervention if
something goes wrong. That hardly describes cities anywhere.

Raw Food Production

The effect of changes in raw food production and harvest practices on
opportunities for foodborne outbreaks have been discussed. Centralized
processing and wide distribution are the principal characteristics of the
"new scenario," as compared to the "old scenario" of local production, home
processing, and intrafamily consumption. A classic example is the recent
Japanese outbreak of hemorrhagic Escherichia coli.

Denial

A behavior that encourages outbreaks is denying the existence of an
epidemic-a practice more common in developing countries concerned about the
effect of outbreak publicity on tourist trade and exports (4). One of our
own attitudes is indifference to outbreaks perceived as commonplace and
distant (5).

Economics

War and economic collapse provide unparalleled opportunities for disease
outbreaks (e.g., cholera in central Africa). The infrastructure that
provides clean water, community medicine, disease surveillance, and food
control, even where it exists, is a fragile fabric, easily torn by
economic, social, and physical disruption.

Technology

In spite of their benefits, technologies often bring new or enlarged risks.
This is not an argument for returning to a state of nature. The invention
of sausage doubtless increased the incidence of botulism; indeed, "botulus"
is the Latin word for sausage. Without proper processing, any modern
packaging that excludes oxygen can have the same effect.

Risk Factors

Factors such as age, illness, and medical treatment increase the risk for
foodborne illness. Such increases also result from behavior that promotes
the incidence of other diseases (e.g., AIDS).

Failure to Prevent and Control

The most common human action that adversely affects food safety is the
avoidable lack of or failure to use effective prevention and control
measures. That failure is why 85% of all outbreaks are traceable, about
equally, to mishandling in homes or in food service establishments.

Interacting Factors

In much of the developing world, an interrelated and mutually reinforcing
set of problems keeps foodborne disease at a high level (Figure).
Approximately five million children under the age of 5 years living in the
tropics die each year of malnutrition and diarrheal disease (6). Palliation
is temporary; only economic and technical development can break through
this net.

 [Figure Not Available in ASCII]
 Figure. Problems causing foodborne disease in developing countries.

The contributing factors already mentioned would cause us problems, even
acting singly. However, they interact, often synergistically. The
combination of bacterial genetic variability and the ease and frequency of
mutation of strains present a threat because the process enhances selection
for new and more dangerous pathogens. Acid rain and recycling through
ruminants may have encouraged the increased environmental durability of
acid-tolerant E. coli O157:H7. The increasing popularity of marinades in
the preparation of foods may have had the same effect.

The globalization of the food trade pulls together several of these
contributing factors. One country's contaminated water leads to another
country's contaminated raspberries. Refrigeration and controlled atmosphere
can preserve pathogens, as well as foods, and spread them all over the
world. The Salmonella Enteritidis outbreak from contaminated eggs in ice
cream was a typically broad problem.

Technologic change combines many of these contributing factors. The
intensive monoculture of plants and animals presents concentrated
opportunities. For example, an outbreak of southern corn blight was
attributed to the narrow genetic base of a popular hybrid corn.
Human-guided plant genetics was defeated by the much more rapid genetic
adaptability of microorganisms.

Indiscriminate use of antibiotics and the ability of microorganisms to
exchange genetic information has led to increasing resistance. Minimal
processing of food, warmly received by natural food lovers, is an open
invitation to slightly more durable pathogens to take over the food supply.
Centralized processing and mass distribution push us even further in the
race Dr. Lederberg reminded us we cannot win. Immunosuppression, due to
disease or medication, combined with the failure to take simple sanitary or
health precautions make the emergence of pathogens inevitable.

The Role of Public Health Measures

Achieving optimum public health requires many measures, each necessary but
not sufficient in itself. The most basic measure is clean water, as recent
incidents with strawberries and other fresh produce washed with
contaminated water illustrate.

Effective food control structures (e.g., statutory and regulatory
frameworks and inspection and enforcement agencies) are essential. A
surveillance system has always been required and should be directed to
populations at high risk. Newer methods of typing pathogens, faster
response times, and enhanced analytical sensitivity are making those
systems far more effective than they have been.

Family health programs and education are equally necessary. Food safety
still depends heavily on how each of us chooses and uses (or abuses) our
food, and each of us is not expert.

The Roles of Food Processing

We process food for the following main reasons: nutrition, safety,
preservation, distribution, and esthetics. Cooking typically increases
protein digestibility and destroys antinutrients. Fortification and
enrichment add, restore, or increase essential nutrients. Cooking and
canning destroy pathogenic organisms; cooking and steeping remove natural
toxicants, such as cyanide from cassava. Canning, dehydration, salting,
smoking, and preservatives, combined with proper packaging and storage,
decrease spoilage. Canning, freezing, refrigeration, and controlled
atmosphere storage provide variety and year-round availability. Flavors,
colors, pre-sale preparation, and stabilizers that prevent separation or
crystallization increase consumer appeal and convenience.

Food Processing Technologies

Food processing technologies reduce our exposure to dietary pathogens in
three ways, summarized by Professor E. M. Foster as "the three Ks": keep
them out, kill all you can, keep the rest from growing.

One group of technologies removes or destroys microorganisms and naturally
occurring toxicants by careful sanitation; heat treatment (e.g., cooking,
retorting, pasteurizing, high temperature/short time treatment, ultrahigh
temperature treatment, ohmic heating, and other newer techniques);
radiation (widely useful and recently applied to all of our current
methods); physical separation, (e.g., air separation, gravity tables,
visual scanners, and other methods for reducing "natural and unavoidable
defects" including gross contamination); dehydration; and new technologies
(e.g., hydrostatic pressure, pulsed light).

A second group of technologies keeps contaminants well below dangerous
levels. These include raw material quality control; careful sanitation;
chemical preservation (i.e., fermentation, bacteriostats, pH control, water
activity control, and controlled atmosphere storage); dehydration; and
freezing and refrigeration.

The third group of technologies prevents recontamination through sanitation
of the general environment and at the point of service, protective
packaging, and proper handling to ensure packaging integrity.

Beyond these three groups of technologies are general principles governing
the use of all such techniques. All foods require incoming quality control
and careful sanitation. For nearly all foods except fresh produce and dried
grains, one technology from each group listed above should be used. All
technologies used must be applied effectively, or none is effective. These
technologies must be used in combination, in a systems approach. Failure to
apply properly a preceding technology can cause later technologies to
increase microbiologic hazards. If oxygen is later excluded from improperly
processed food by packaging (canning, foil, an oil layer), anaerobic
pathogens, such as Botulinum, are free to multiply.

Future Options

Food safety objectives must be established. There is a clear need for
priorities and for employing the "principle of commensurate effort" (i.e.,
applying effort to risks in the order of their probable impact).

There is also the need for a broad view (1,2). The interaction of several
factors, not all of which can be seen in advance, makes a team approach
essential.

More effort at understanding the evolution of virulence (7) could provide
us with insights on the genetic characteristics and the environmental
conditions needed to minimize risk. The 1918-19 influenza epidemic can be
used as an example; the present search for its genetic makeup suggests the
value of prior knowledge. Knowing or estimating the probability of present
or future virulence could prepare us for the more effective use of other
measures. These include newer, faster, more sensitive methods of detection.

Improved detection would lead to more effective prevention and control
measures. The need to establish priorities suggests here, as in cancer
research, the value of biomarkers. Biomarkers are genetic or biochemical
indicators of impending risk in the potential pathogen, preceding clinical
or epidemiologic indications. Such biomarkers might indicate potential
virulence, increased environmental durability, or increased resistance to
heat, cold, unfavorable pH, preservatives, or antibiotics.

We need to apply available knowledge more effectively. Most outbreaks of
foodborne disease are due to mishandling food in ways we already know how
to avoid. The points that follow are not new, but as our food supply
increases in complexity and geographic reach, these weapons must grow
apace.

Clean water, public sanitation, disease surveillance, and food control are
more important than ever. The value of effective disease surveillance has
been demonstrated. But what we now have and do is not enough. A national
response team must be created. Data should be shared more broadly
(electronic linkage of laboratory and surveillance results) if we are to
cope effectively with a global food supply, multistage processing by
different firms at different locations, and national and international
distribution. The effective application of these resources requires a
Hazard Analysis and Critical Control Points (HACCP) approach (8).

Private health measures must grow in effectiveness and reach. We must make
continuing and increasingly effective use of food processing technologies
that reduce microbiologic risks. HACCP cannot be generic or static; it must
be adapted to each specific product and processing facility and must be
updated with continuing feedback on the hazards to be avoided. HACCP,
fortified by the public health measures just described and by the results
of the research discussed at this conference, will enable the food
processing industry to meet the challenges of emerging pathogens.

Even with the steps just described, approximately 85% of all outbreaks
occur as a result of food mishandling in food service establishments or
homes. We need to extend HACCP principles to the food service sector as
well. HACCP probably cannot reach into homes. If we are to communicate
better, we need first to find out what consumers already know, what they
want to know, and what they need to know. In short, if we are to educate
effectively, we must have direct evidence of how well the information
process is working.

Finally, we will never succeed in achieving desirable consumer
risk-management practices until consumers understand the inevitability of
some risks and minimize them. Food risks have been categorized into six
groups in decreasing order of size: microbiologic, nutritional, natural
toxicant, environmental contaminant, pesticide residues, and food additives
(9). The take-home message on food safety for all of us, as consumers, can
be summed up in three words: sanitation, variety, moderation. Sanitation
deals effectively with microbiologic risks; variety and moderation deal
with nutritional risks, minimizing the impact of the four remaining risks
to the extent they are even potentially insignificant. Sanitation, variety,
and moderation are within our own control as individual consumers. That is
doubtless why we have done only a mediocre job of implementing them. Unless
all of us c
an and do pursue these three objectives effectively, we are all
at some unnecessarily increased risk.

Address for correspondence: Richard L. Hall, 7004 Wellington Court,
Baltimore, MD 21212; fax: 410-377-2909.

References

  1. Farmer P. Social inequalities and emerging infectious diseases. Emerg
     Infect Dis 1996;2:259-69.
  2. Vidaver AK. Emerging and reemerging infectious diseases. American
     Society for Microbiology News 1996;62:583.
  3. Wilson M. Travel and the emergence of infectious diseases. Emerg
     Infect Dis 1995;1:39-46.
  4. Vidaver AK. Emerging and reemerging infectious diseases. American
     Society for Microbiology News 1996;62:585.
  5. Lederberg J, Shope RE, Oaks SC. Emerging infections: microbial threats
     to health in the United States. Washington (DC): National Academy
     Press; 1992.
  6. Snyder JD, Merson MH. The magnitude of the global problem of acute
     diarrhoeal disease: a review of active surveillance data. Bull World
     Health Organ 1996;60(4):605-13.
  7. Ewald P. Guarding against the most dangerous emerging pathogens:
     insights from evolutionary biology. Emerg Infect Dis 1996;2:245-56.
  8. International Committee on Microbiological Safety in Foods. HACCP in
     microbiological safety and quality. In: Microorganisms in Food. Book
     4. Oxford: Blackwell Scientific Publications Ltd.; 1988. p. 28-43.
  9. Wodicka VO. Wodicka rates food additive hazard as low. Food Chemical
     News 1971;12(49):11.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/hall.txt

Vero Cytotoxin-Producing Escherichia coli O157 Outbreaks in England and
Wales, 1995: Phenotypic Methods and Genotypic Subtyping

Geraldine A. Willshaw,* Henry R. Smith,* Thomas Cheasty,* Patrick G. Wall,
and Bernard Rowe*
*Central Public Health Laboratory, London, United Kingdom; Communicable
Disease Surveillance Centre, London, United Kingdom

---------------------------------------------------------------------------
      Vero cytotoxin-producing Escherichia coli O157 belonging to
      four phage types (PTs) caused 11 outbreaks of infection in
      England and Wales in 1995. Outbreak strains of different PTs
      were distinguishable by DNA-based methods. Pulsed-field gel
      electrophoresis best discriminated among strains belonging to
      the same PT, distinguishing six of the seven PT2 outbreak
      strains and both PT49 outbreak strains.

Vero cytotoxin-producing Escherichia coli O157 (O157 VTEC), first
associated with outbreaks of human disease in North America in 1982, has
since emerged as an important human pathogen; it causes mild nonbloody
diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome, as well as
less common manifestations such as thrombotic thrombocytopenic purpura with
neurologic symptoms (1). Laboratory-confirmed infections with O157 VTEC in
England and Wales increased from fewer than 10 in 1983 to 250 in 1990, 792
in 1995, and 660 in 1996 (2;3; Laboratory of Enteric Pathogens, unpub.
data). The incidence of O157 VTEC infection in Scotland relative to its
population is up to six times that in England and Wales, but wide
geographic variations exist in England, Wales, and Scotland (4,5). The main
reservoir for O157 VTEC appears to be healthy cattle, although recently
organisms have been found in sheep (4,6,7). O157 VTEC infections are
usually foodborne, associated with consumption of undercooked minced beef
(most commonly as beefburgers), unpasteurized milk, and a variety of other
vehicles such as salami, cheese, yogurt, water, salad vegetables, and fruit
juice (7,8). Other routes of infection include animal contact and
person-to-person spread, both in families and institutional settings (4).

O157 VTEC is differentiated by phage typing into more than 80 phage types
(PTs) (9, R. Khakhria, pers. comm.). Polynucleotide DNA probes identify
Vero cytotoxin (VT) genotypes and divide O157 VTEC into strains with VT1,
VT2, or VT1+2 genes. However, certain phage and VT types predominate: thus
in England and Wales approximately 50% of O157 VTEC are PT2 and produce VT2
(2,3). A range of DNA-based methods is available for further strain
discrimination (4), and we have applied some of these to compare strains
associated with outbreaks of O157 VTEC infection in England and Wales in
1995.

Eleven general outbreaks during 1995 affected members of more than one
household or residents of an institution (Table), whereas 18 general
outbreaks were reported during 1992 to 1994 (10). Most of the outbreaks in
1995 occurred in late summer and in the community (Table, outbreaks 5, 9,
and 10); institutions (Table, outbreaks 1, 4, and 11); catering
establishments (Table, outbreaks 3, 6, and 7); or a mixture of these
(Table, outbreaks 2 and 8). Outbreaks occurred throughout England and
Wales; of the 11, four were in the northern region. The incidence of
hemolytic uremic syndrome was 0% to 36% in individual outbreaks and 8%
overall. The case-fatality rate was 6%, mainly among the elderly. In six
outbreaks, there was epidemiologic evidence for foodborne infection, but
O157 VTEC was never isolated from food. Person-to-person spread was
probably important in three outbreaks. The environmental and epidemiologic
investigations of two of these incidents have been reported (11, 12).

Table. Outbreak of infection with O157 VTEC in England and Wales 1995,
properties of outbreak strains

---------------------------------------------------------------------------
 Out- Month Region/setting (ref) Cases Phage VT     VT2      RFLP    PFGE   Likely
 break      (a)                  (HUS/ type  probe  subtype  (phi)   XbaI   transmission
 No.                             Fatal)      (b)    (c)      32511   (e)    of infection
                                                             (d)
 ----------------------------------------------------------------------------------------
 1    Jan   Northern/nursing     7     2     2      2+2c     PT2-A   PT2-1  Person-
            home                 (0/2)                                      to-person
 2    May   Wessex/Community;    26    2     2      2        PT2-C   PT2-2  Foodborne
            hospital             (2/0)
 3    Jul   N.W. Thames/Hotel    5     1     1+2    2        PT1-A   PT1-1  Foodborne
                                 (0/0)
 4    Jul   N.                   3     2     2      2+2c     PT2-A   PT2-1  Person-
            Western/Residential  (1/3)                                      to-person
            home; hospital
 5    Jul   Northern/Community   12    2     2      2+2c     PT2-A   PT2-4  Foodborne
            (11)                 (0/1)
 6    Jul   Northern/Restaurant  5     2     2      2+2c     PT2-A   PT2-1a Foodborne
                                 (1/0)
 7    Jul   East Anglia/Holiday  4     49    2      2+2C     PT49-A  PT49-1 Foodborne
            Camp                 (0/1)
 8    Aug   Wales/Day nursery;   49    2     2      2+2c     PT2-B   PT2-3  Foodborne,
            community            (2/0)                                      person-
                                                                            to-person
 9    Oct   W. Midlands/         11    2     2      2+2c     PT2-AvarPT2-1b Foodborne
            Community (12)       (4/0)
 10   Oct   Various/Community    3     RDNC  1+2    2+2c     RDNC-A  RDN    Unknown
                                 (0/0) (f)                           C-1
 11   Dec   Northern/Day nursery 2     49    2      2+2c     PT49-B  PT49-2 Unknown
                                 (0/0)
 ----------------------------------------------------------------------------------------

HUS=hemolytic uremic syndrome; VT= Vero cytotoxin; RFLP=restriction
fragment length polymorphisms; PFGE=pulsed field gel electrophoresis
(a) Investigation of the epidemiology of two outbreaks has been reported
previously (11,12)
(b) Determined by hybridization with digoxigenin-labeled polynucleotide
probes for VT1 and VT2 genes (2,3).
(c) Based on polymerase chain reaction amplification with a sense primer
specific for either the VT2 or VT2c sequence and a degenerate antisense
primer that would anneal to known VT2 sequences (14).
(d) Hybridization with a probe comprising digoxigenin-labelled fragments of
the VT2-encoding bacteriophage from strain E32511(15). Patterns were
designated according to the phage type of the strain and a letter denoting
a unique pattern type. The PT2-Avar pattern differed from PT2-A by the
possession of a single extra hybridizing fragment.
(e) Profiles of XbaI digested genomic DNA. Patterns were designated
according to the phage type of the strain and differentiated by number.
Thus patterns PT2-1, PT2-2, PT2-3, and PT2-4 differed from each other by at
least three fragment positions. Where there were single unique band
differences from PT2-1 these were designated PT2-1a, etc.
(f) The designation RDNC indicates that the strain reacts with the typing
phages but does not conform to a currently defined pattern.

For most outbreaks, epidemiologic investigation, phage typing of isolates,
and further tests with VT1 and VT2 probes (Table) initially discriminated
probable outbreak-associated cases from sporadic infections at the same
time in the same area. Seven outbreaks were due to strains of PT2, VT2; two
to PT49, VT2; and one to PT1, VT1+VT2. The other outbreak was associated
with a VT1+VT2 strain that reacted with the typing phages but did not
conform to a recognized type (reacts but does not conform [RDNC]). In
outbreak 8, the O157 VTEC was resistant to sulphonamides and tetracyclines,
whereas the other outbreak strains were sensitive to antimicrobial agents.
In relation to community outbreaks 2, 5, and 8, several cases were infected
with O157 VTEC that were similar to the outbreak strains, but the patients
had no known epidemiologic link with the outbreak. Such strains were
therefore included in the tests described below to provide evidence for the
possible involvement of these cases. O157 VTEC that hybridize with the VT2
polynucleotide probe may carry different VT2 sequences (13). Strains from
humans commonly possess VT2 or VT2c or both sequences. Polymerase chain
reaction (14) showed that strains from all outbreaks except 2 and 3
possessed both types of VT2 sequences (Table). O157 VTEC belonging to PT1,
such as those from outbreak 3, usually carry VT1 and only the VT2 sequence
(13). The presence of the VT2, but not the VT2c, sequence differentiated
the PT2 strain from outbreak 2 from all the other PT2-associated outbreaks.
This property was exploited during the course of the outbreak to exclude
patients infected with PT2 strains that had the VT2+VT2c genotype and not
the outbreak VT2 genotype.

The O157 VTEC strains were further analyzed by Southern blot hybridization
of Eco RI restriction enzyme-digested genomic DNA with a probe comprising
digoxigenin-labeled fragments of the VT2-encoding bacteriophage
[Image]32511 (15). Strains of O157 VTEC give an array of up to 20
hybridizing fragments; these restriction fragment length polymorphisms
(RFLPs) are different for strains of different PTs and can distinguish
between strains of the same PT. The RFLP patterns of the PT1 and RDNC
strains from outbreaks 3 and 10 distinguished them from each other and from
all the remaining strains. The technique indicated that the PT49 strains
from outbreaks 7 and 11 had distinct RFLP patterns (Table), but it did not
differentiate the PT2 strains from outbreaks 1, 4, 5, and 6. These strains
gave a single pattern (identical to that of a PT2 strain reported
previously [15]) that we have found most commonly in strains of this PT
(unpub. data). The PT2 strains from outbreaks 2 and 8 gave RFLP patterns
that were different from this common pattern and from each other (Table).
In both instances, the test was used to exclude those cases outside the
outbreak.

Pulsed-field gel electrophoresis (PFGE) has been applied widely as a highly
discriminatory method for fingerprinting bacterial pathogens. The method of
Barrett et al. (16), modified as indicated in the Figure, identified
between 15 and 20 XbaI-generated fragments of the O157 VTEC strains
(Figure). PFGE patterns of PT1, PT49, and RDNC were reproducible and
clearly distinct from each other; they differed from the most common PT2
pattern by at least six fragment positions. The PT49 strains associated
with outbreaks 7 and 11 were distinguished from each other by their PFGE
profile. Results of PFGE of O157 VTEC of PT2 from the outbreaks are shown
in the Figure. The strains that caused outbreaks 1 and 4 were
indistinguishable by all methods including PFGE (lanes 2 and 4). Two other
strains, from outbreaks 6 and 9, were very similar to these two, differing
at only one fragment position (lanes 6 and 8), but the patterns were
distinct and reproducible. The PT2 strains from outbreaks 5 and 8 (lanes 5
and 7) differed from those associated with outbreaks 1 and 4 by at least 3
fragment positions and were distinguishable from each other. The strain
from outbreak 2, which possessed only the VT2 gene, was clearly distinct
from all the other PT2 strains by PFGE (lane 3). Criteria for the
interpretation of patterns produced by PFGE have been published by Tenover
et al. (17). Results of the analysis of O157 VTEC by PFGE suggest these
criteria need modification for closely related organisms such as O157 VTEC
(18).

[Figure Not Available in ASCII]

Figure. Pulsed-field gel electrophoresis of XbaI-digested genomic DNA of
O157 VTEC PT2 isolated from outbreaks in 1995. Digests were separated on
1% agarose for 42h at a voltage gradient of 5.6 volts per cm with a pulse
ramp time of 5 to 50 sec. Lane 1 contained a phage lambda DNA 48.5 kb
ladder (Sigma). Lanes 2 to 8 contained digests of PT2 strains from
outbreaks as follows: lane2, outbreak 1; lane3, outbreak 2; lane4,
outbreak 4; lane5, outbreak 5; lane6, outbreak 6; lane7, outbreak 8;
lane8, outbreak 9.

Although phage typing and polynucleotide probes for VT1 and VT2 genes
rapidly characterized strains from the outbreaks in 1995, DNA-based methods
were valuable in distinguishing outbreak cases from outlying ones. VT2 gene
subtyping was rapid but not very discriminatory, whereas RFLP and PFGE
techniques differentiated strains but were time-consuming. The highest
level of discrimination (by PFGE) distinguished certain O157 VTEC that
appeared identical by all other methods. Some of the differences detected
by PFGE were minor, but for each outbreak, they were reproducible with all
the strains examined in this study. The epidemiologic or evolutionary
significance of these minor variations is difficult to evaluate.

Investigation of the epidemiology of outbreaks of O157 VTEC infection
requires a combined use of typing and fingerprinting methods in a
hierarchic manner consistent with practical and economic constraints. In
outbreaks well defined by epidemiologic studies, phage typing and the
identification of VT1 and VT2 genes, including VT2 subtyping, are likely to
be sufficient. In outbreaks less clearly defined epidemiologically,
DNA-based methods may assist in identifying those strains not associated
with the outbreak; this is particularly helpful when the outbreak is due to
a common phage type. DNA-based methods have been useful in linking human
infections with associated foods (15) and animals (19).

Although PFGE gives a high level of discrimination between closely related
O157 VTEC, it has certain disadvantages. It is time-consuming and may not
be suitable for rapid identification of large numbers of strains. We
recommend the use of a combination of phage typing, VT typing, and PFGE to
provide good discrimination of O157 VTEC strains in epidemiologic
investigations.

Acknowledgments

We thank laboratories in England and Wales for sending clinical samples and
bacterial isolates and colleagues in the Laboratory of Enteric Pathogens
for technical assistance.

Address for correspondence: Geraldine A. Willshaw, Ph.D., Laboratory of
Enteric Pathogens, Central Public Health Laboratory, 61, Colindale Avenue,
London, NW9 5HT, United Kingdom; fax: 44-181-905-9929.

References

  1. Karmali MA. Infection by Verocytotoxin-producing Escherichia coli.
     Clin Microbiol Rev 1989;2:15-38.
  2. Thomas A, Chart H, Cheasty T, Smith HR, Frost JA, Rowe B. Vero
     cytotoxin-producing Escherichia coli, particularly serogroup O157,
     associated with human infections in the United Kingdom: 1989 to 1991.
     Epidemiol Infect 1993;110:591-600.
  3. Thomas A, Cheasty T, Frost JA, Chart H, Smith HR, Rowe B. Vero
     cytotoxin-producing Escherichia coli, particularly serogroup O157,
     associated with human infections in England and Wales: 1992-4.
     Epidemiol Infect 1996;117:1-10.
  4. Advisory Committee on the Microbiological Safety of Food. Report on
     Verocytotoxin-producing Escherichia coli. London: HMSO; 1995.
  5. Sharp JCM, Reilly WJ, Coia JE, Curnow J, Synge BA. Escherichia coli
     O157 infection in Scotland: an epidemiological overview, 1984-94. PHLS
     Microbiology Digest 1995;12:134-40.
  6. Chapman PA, Siddons CA, Harkin MA. Sheep as a potential source of
     verocytotoxin-producing Escherichia coli O157. Vet Rec 1996;138:23-4.
  7. Griffin PM, Tauxe RV. The epidemiology of infections caused by
     Escherichia coli O157:H7, other enterohemorrhagic E. coli, and the
     associated hemolytic uremic syndrome. Epidemiol Rev 1991;13:60-98.
  8. Neill MA, Tarr PI, Taylor DN, Trofa AF. Escherichia coli. In: Hui YH,
     Gorham JR, Murrell KD, Oliver DO, editors. Foodborne disease handbook
     1994. New York: Marcel Dekker; 1994. p. 169-213.
  9. Khakhria R, Duck D, Lior H. Extended phage-typing scheme for
     Escherichia coli O157:H7. Epidemiol Infect 1990;105:511-20.
 10. Wall PG, McDonnell RJ, Adak GK, Cheasty T, Smith HR, Rowe B. General
     outbreaks of Vero cytotoxin-producing Escherichia coli O157 in England
     and Wales from 1992-1994. Commun Dis Rep CDR Rev 1996;6:R26-33.
 11. Stevenson J, Hanson S. Outbreak of Escherichia coli O157 phage type 2
     infection associated with eating precooked meats. Commun Dis Rep CDR
     Rev 1996;6:R116-8.
 12. Gammie AJ, Mortimer PR, Hatch L, Brierley AFM, Chada N, Walters JB.
     Outbreak of Vero cytotoxin-producing Escherichia coli O157 associated
     with cooked ham from a single source. PHLS Microbiology Digest
     1996;13:142-5.
 13. Thomas A, Smith HR, Rowe B. Use of digoxigenin-labelled
     oligonucleotide DNA probes for VT2 and VT2 human variant genes to
     differentiate Vero cytotoxin-producing Escherichia coli strains of
     serogroup O157. J Clin Microbiol 1993;31:1700-3.
 14. Thomas A, Cheasty T, Chart H, Rowe B. Isolation of Vero
     cytotoxin-producing Escherichia coli serotypes O9ab:H- and O101:H-
     carrying VT2 variant gene sequences from a patient with haemolytic
     uraemic syndrome. Eur J Microbiol Infect Dis 1994;13:1074-6.
 15. Willshaw GA, Thirlwell J, Jones AP, Parry S, Salmon RL, Hickey M. Vero
     cytotoxin-producing Escherichia coli O157 in beefburgers linked to an
     outbreak of diarrhoea, haemorrhagic colitis and haemolytic uraemic
     syndrome in Britain. Letters in Applied Microbiology 1994;19:304-7.
 16. Barrett TJ, Lior H, Green JH, Khakhria R, Wells JG, Bell BP, et al.
     Laboratory investigation of a multistate food-borne outbreak of
     Escherichia coli O157:H7 by using pulsed-field gel electrophoresis and
     phage typing. J Clin Microbiol 1994;32:3013-7. Escherichia coli O157
     infections in Minnesota by molecular subtyping. N Engl J Med
     1997;337:388-94.
 17. Trevena WB, Willshaw GA, Cheasty T, Wray C, Gallagher J. Vero
     cytotoxin-producing E. coli O157 infection associated with farms.
     Lancet 1996;347:60-1.

---------------------------------------------------------------------------

Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/willshaw.txt

Genetic Polymorphism Among Cryptosporidium parvum Isolates: Evidence of Two
Distinct Human Transmission Cycles

---------------------------------------------------------------------------

      We report the results of molecular analysis of 39 isolates of
      Cryptosporidium parvum from human and bovine sources in nine
      human outbreaks and from bovine sources from a wide geographic
      distribution. All 39 isolates could be divided into either of
      two genotypes, on the basis of genetic polymorphism observed at
      the thrombospondin-related adhesion protein (TRAP-C2) locus.
      Genotype 1 was observed only in isolates from humans. Genotype
      2, however, was seen in calf isolates and in isolates from a
      subset of human patients who reported direct exposure to
      infected cattle or consumed items thought to be contaminated
      with cattle feces. Furthermore, experimental infection studies
      showed that genotype 2 isolates were infective to mice or
      calves under routine laboratory conditions, whereas genotype 1
      isolates were not. These results support the occurrence of two
      distinct transmission cycles of C. parvum in humans.

Long considered a veterinary disease, cryptosporidiosis has emerged as an
important infectious disease in humans (1). In immunocompetent persons, the
disease is usually self-limiting; however, in the immunocompromised, it is
frequently chronic, more severe, and sometimes fatal. Cryptosporidiosis is
one of the major secondary diagnoses in people with AIDS and is associated
with a twofold greater hazard of death than other AIDS-defining diagnoses
(2).

A number of major waterborne outbreaks of cryptosporidiosis have occurred
in urban settings (3); however, the disease also occurs sporadically. Since
most Cryptosporidium parvum infections are self-limiting and
symptomatically similar to other diarrheal diseases, the disease may often
be undiagnosed or misdiagnosed in the absence of a recognized outbreak.
Consequently, the actual incidence of cryptosporidiosis and the relative
importance of each of its many modes of transmission are largely unknown.
For these reasons, laboratory tools are needed for quantitative and
qualitative environmental sampling and for strain analysis of
Cryptosporidium isolates. These tools would be extremely valuable for
source identification and outbreak investigations, for correlation with
clinically important phenotypes, and for determining risk factors in
nonepidemic settings.

A number of new nucleic acid-based approaches have been developed for
detection, diagnosis, and typing of C. parvum, among them polymerase chain
reaction (PCR)-based tests that focus either on random amplification of DNA
polymorphisms or on specific polymorphic genetic loci (4-7). These
PCR-based tests suggest the existence of strain variation and the
possibility of two distinct transmission cycles among C. parvum isolates
that infect humans. In this study, we examined genetic polymorphism among
C. parvum isolates from human and nonhuman sources to identify
strain-specific markers that could be correlated with epidemiologically
important phenotypes.

Analytic Approach

Parasite Isolates

Thirty-nine isolates were examined from stool samples positive for C.
parvum: 17 were obtained from humans or calves during outbreaks in the
United States and Canada (Table 1); one was a calf isolate (Iowa calf)
routinely passaged in neonatal Holstein calves in our laboratories; and 21
were obtained from cattle from Georgia, Alabama, Ohio, Oklahoma, Kansas,
Iowa, Idaho, Utah, and Washington. All samples were collected and placed
directly into a 2.5% potassium dichromate solution and were stored at 4C.
Oocysts were purified by using discontinuous sucrose and Percoll or cesium
chloride gradients (8,9).

Table 1. Cryptosporidium parvum isolates examined
------------------------------------------------------------
Isolate                   Implicated source Host        Ref.
------------------------------------------------------------
Maine,1993                apple cider       human       16
Wisconsin, 1993           drinking water    human       3
Wisconsin, 1996           drinking water    human       *
Georgia (day care),1995   person-to-person  human       *
Georgia (water-park),
1995                      recreational waterhuman       17
Florida, 1995             drinking water    human       18
British Columbia, 1996    drinking water    human       15
Texas, 1996               unknown           human       *

Pennsylvania, 1997        bovine contact    human,      *
                                            calf
------------------------------------------------------------

*Reference is this article.

Isolation of Genomic DNA

Parasite DNA was isolated as described by Kim et al. (10). Briefly, oocysts
were ruptured by using five freeze-thaw cycles (dry ice ethanol bath and
65C) in a lysis buffer (120 mM NaCl, 10 mM EDTA, 25 mM Tris pH 7.5, 1%
Sarkosyl) containing proteinase K. The samples were incubated for 1 hour at
55C to inactivate nucleases. Then DNA was extracted with
phenol/chloroform/isoamyl alcohol (25:24:1) and chloroform/isoamyl alcohol
(24:1), precipitated with absolute ethanol, washed with 70% ethanol, and
resuspended in TE buffer (10 mM Tris pH 8.0, 1 mM EDTA).

PCR Amplification and Sequencing and Analysis

The gene fragment of interest, a 369-bp region of the
thrombospondin-related adhesive protein (TRAP-C2) of C. parvum, was
amplified with the following primers: 5'-CAT ATT CCC TGT CCC TTG AGT TGT-3'
and 5'-TGG ACA ACC CAA ATG CAG AC-3', which correspond, respectively, to
positions 812 to 835 on the coding strand and positions 1,161 to 1,180 on
the negative strand, of GenBank sequence X77586. The reactions were
performed with Perkin-Elmer (Perkin-Elmer Corporation, Foster City, CA) PCR
reagents, including 1X PCR buffer, 2.5 mM MgCl2, 0.2 mM each dNTP, 0.4 mM
of each specific primer, and 2.5 U of Taq DNA polymerase. After a 1-minute
hot start at 94C, the reactions went through 35 to 40 cycles of denaturing
at 94C for 30 seconds, annealing at 45C for 30 seconds, and extension at
72C for 1 minute, followed by a 72C incubation for strand completion.

An aliquot of each PCR product was examined by agarose gel electrophoresis;
the remaining PCR product was purified by using the Wizard PCR Prep Kit
(Promega Corporation, Madison, WI). Purified PCR fragments were sequenced
directly on an ABI 377 automated sequencer by fluorescent cycle sequencing
using dye-terminator chemistry with AmpliTaq FS (Perkin-Elmer-Applied
Biosystems) according to the manufacturer's recommended procedure. The same
primer sets used initially for PCR were used again for sequencing, diluted
to a concentration of 10 pMoles in the final sequencing reaction.
Downstream analysis of sequence data was accomplished by using the Sequence
Navigator (Perkin-Elmer-Applied Biosystems). Multiple sequence alignments
were performed by using the Pileup program (11). Animal isolates were
manually sequenced by the dideoxy chain-termination method (12), using the
Sequenase Version 2.0 kit (J.T. Baker, Phillipsburg, NJ) with the sequences
of a few isolates confirmed by automated sequencing.

Experimental Infection Studies

Purified oocysts ranged in age from 1 to 6 months at the time of
inoculation of cell cultures or animals. This age range is well within the
storage time that maintains oocyst viability and infectivity (e.g.,
laboratory-passaged isolates are 40% to 50% viable after storage for 6
months). Approximately (superscript 10)6 oocysts were administered orally to 2-day-old
calves or to 4- to 6-day-old BALB/c or SCID mice by using established
procedures (8,13). Beginning at day 5, stools were collected and examined
daily by light microscopy or by immunofluorescent flow cytometry for C.
parvum oocysts (14).

Findings

Sequence Determination and Analysis

A single specific band of 369 bp, corresponding to bases 812 to 1180 of the
1.1 kb C. parvum TRAP-C2 gene (GenBank accession number X77586) was
amplified from 39 different isolates (Figure 1). Although the sequence
similarity was very high among all gene fragments, multiple alignments
showed two primary genotypes. These genotypes could be established on the
basis of nucleotide substitutions at five independent positions, three
being silent changes and the other two resulting in amino acid changes
(Figure 2). Of the five changes, four were transitions, and one was a
transversion.

[Figure Not Available in ASCII]

Figure 1. Alignment of TRAP-C2 nucleotide positions that show polymorphism
among Cryptosporidium parvum isolates from human and nonhuman sources.
Published calf sequence refers to Genbank accession number X77586. Other
bovine (n=21) refers to 21 samples (from Georgia, Alabama, Ohio, Oklahoma,
Kansas, Iowa, Idaho, Utah, and Washington) that had the same genotype.

[Figure Not Available in ASCII]

Figure 2. DNA and putative amino acid sequences of Cryptosporidium parvum
TRAP-C2 genotypes 1 and 2.

Genotype 1 included human isolates from Wisconsin, Georgia, Florida, and
Texas. Genotype 2 contained human isolates from Maine and British Columbia,
human and calf isolates from Pennsylvania, the Iowa calf laboratory strain,
and 21 bovine isolates from various locations around the country. Two human
isolates (one from Florida and the Texas isolate) appeared to represent a
variant of genotype 1. In both cases, they shared the first four positions
with the other genotype 1 isolates. The fifth position, however, was the
same as that of genotype 2 isolates.

Experimental Infection Studies

The results of experimental infection studies are shown in Table 2. The
genotype 2 isolates from human outbreaks in Maine and Pennsylvania and from
a calf in Iowa all readily infect both mice and calves. The genotype 2
isolate from British Columbia was also reported to be infective to
immunosuppressed C57BL/6 mice (15). None of the genotype 1 isolates from
humans-from Wisconsin, Florida, a Georgia day-care facility, and a Georgia
water park-could be established in either mouse or calf. A single sample,
the Georgia day-care isolate, was examined for its ability to infect a
neonatal pig. This isolate caused a brief moderate infection in a neonatal
pig host (data not shown) but not in calves or mice. One of the Wisconsin
isolates and the Georgia day-care isolate were tested for their infectivity
to MDCK cell cultures; both successfully infected this cell line (data not
shown).

Table 2. Experimental infection studies with Cryptosporidium parvum
isolates from various sources
------------------------------------------------------------
                                Experimental
Isolate                         host            Infection
------------------------------------------------------------
Maine, 1993                     mouse           +
                                calf            +
Wisconsin, 1993                 mouse           -
                                calf            -
Georgia (day care), 1995        mouse           -
                                calf            ND
Georgia (water park), 1995      mouse           -
                                calf            -
Florida, 1995                   mouse           -
                                calf            ND
Iowa (bovine), 1984             mouse           +
                                calf            +
------------------------------------------------------------

ND = not done

Conclusions

All isolates examined in this study could be grouped easily into two
distinct genotypes defined by nucleotide substitutions at five positions
within the TRAP-C2 locus, with genotype 1 containing a variant at the fifth
position that was represented by two isolates. All isolates in genotype 1
were from human stool. The isolates in genotype 2, however, were from both
human and bovine sources. In the limited number of isolates that were
tested in experimental infection studies, all genotype 2 isolates could be
established readily in mice and calves. None of the genotype 1 isolates,
however, could be shown to be infective to either of these hosts. The
genotype and experimental infection data suggest the possibility of two
distinct populations of C. parvum cycling in humans. One population appears
to involve zoonotic transmission from calf-to-human with subsequent
human-to-human and human-to-calf transmission. The other population appears
to involve an anthroponotic transmission cycle, exclusively in humans. This
hypothesis is consistent with the data from the epidemiologic
investigations from which the isolates were obtained.

Genotype 2 characteristics were identified in human isolates from the Maine
1993, British Columbia 1996, and Pennsylvania 1997 outbreaks, and in all
isolates from bovine sources. Both the Maine and Pennsylvania outbreaks
could be directly linked to a calf source of C. parvum. The Maine outbreak
was associated with contaminated apple cider (16). Interestingly, C. parvum
oocysts were isolated directly from apple cider, the press used for
preparing the cider, and a calf stool specimen from the farm that supplied
the apples.

The Pennsylvania focus involved three families that together purchased
three young calves that subsequently developed scours. Nine members of
three families had diarrhea, and two were hospitalized. C. parvum oocysts
were isolated from two calves and five humans; all isolates examined
demonstrated the genotype 2 pattern.

The British Columbia isolate came from a human patient infected in an
outbreak (approximately 2,000 cases) that occurred in the small rural
community of Cranbrook (15). During the outbreak investigation,
Cryptosporidium oocysts were identified in human fecal specimens, in cattle
manure specimens found near the watershed, and in water samples from the
reservoir intake.

Of the genotype 1 isolates examined, epidemiologic investigations were
conducted for the Georgia water park 1995, Florida 1995, and Wisconsin 1993
outbreaks. In the Georgia water park outbreak, approximately 2,900 persons
met the case definition for clinical cryptosporidiosis. In a sample of
these patients, the following risk factors were evaluated in telephone
interviews: swimming in lakes or pools, exposure to day care or to persons
with diarrhea, contact with young animals, drinking water from various
sources, chronic illness, and water park attendance. The only factor
independently associated with diarrheal disease was water park attendance
(17).

The Florida 1995 outbreak occurred at a day camp in central Florida and had
approximately 70 cases (18). Risk factors examined included participating in
camp activities, eating lunches provided at the camp, and drinking water
from various specified sources. C. parvum oocysts were observed in the
stools of 16 persons and in water from an outside tap. Fecal contamination
(of unknown origin) of the tap was the suspected source of the outbreak.
Five specimens were examined from this outbreak, all of which belonged to
the genotype 1 grouping; one displayed additional polymorphism at
nucleotide position 244 (Figure 1). The Texas 1995 isolate showed this same
polymorphism, which we think is most accurately described as a subset of
genotype 1.

The Wisconsin 1993 outbreak, which affected more than 403,000 people, is
the largest waterborne disease outbreak ever recorded in the United States.
Four isolates were examined, three isolated during the original outbreak
and a fourth isolated in 1996 from an AIDS patient with a chronic infection
who had initially been infected in the 1993 outbreak.

During the Wisconsin outbreak, possible sources of contamination of Lake
Michigan with Cryptosporidium oocysts included cattle along two rivers that
fed Milwaukee Harbor, slaughterhouses, and human feces (3,19). The
genotypic and experimental infection data from the four isolates we
examined suggest a human rather than 
bovine source. However, these results
come from the analysis of only four samples from a massive outbreak, and
the degree to which these samples are representative of the entire outbreak
remains uncertain.

All genotype 2 isolates examined in this study came from persons that had
direct links or potential exposure to C. parvum from an infected animal.
All samples tested in experimental infection studies were also infective to
both mice and calves. In the genotype 1 isolates, however, while the
initial source of the cases was never directly determined experimentally,
no confirmed links to bovine sources were found, but exposure to water
contaminated with human feces could have occurred. Furthermore, of the
isolates tested in experimental infection studies, none could successfully
infect laboratory animals. These results lead us to suggest the possibility
of a second transmission cycle that is anthroponotic and maintained through
person-to-person contact or through human sewage contamination of the water
supply.

The observations reported here with respect to genotypic variation among C.
parvum isolates from humans and animals are very similar to those reported
by other groups. These studies generally reported one allozyme pattern or
genotype associated with human isolates and a second genotype or allozyme
in bovine samples and a subset of human samples. The specific genes or
regions examined differed in each study but included electromorphs of
phosphoglucomutase and hexokinase (20), random amplified polymorphic DNA
(RAPD) analysis of an unspecified region (4), a repetitive DNA sequence
(6), the 18S rRNA gene and adjacent internal transcribed spacer (ITS)
region I (5), the Cryptosporidium oocyst wall protein (COWP) locus (7), and
the dihydrofolate reductase-thymidylate synthase (DHFR-TS) gene (21). At
least two additional studies suggest the possibilities of two transmission
cycles, on the basis of epidemiologic or experimental observations (22,23).

The TRAP-C2 protein is a member of a class of proteins present in all
apicomplexans examined to date (24-26). This protein is associated with the
cell surface and micronemal complex of these parasites and is thought to be
involved in surface attachment; consequently, changes in this protein could
affect attachment specificity and the resultant host range. Were this the
case, such a mechanism could explain why the host range of one genotype
might be different from that of a second genotype, resulting in distinct
transmission cycles. In the two isolates that make up the variant of
genotype 1, the T-to-C transition results in a change in the amino acid
sequence from tyrosine to histidine. If variations in this protein affect
host preference, the histidine-to-tyrosine change would have to be
inconsequential with respect to protein function and host specificity.
Observations in Plasmodium suggest that the WCSP motif in the TRAP gene is
the functional domain involved in surface attachment; however, the
polymorphism we observed in C. parvum did not involve this region.
Additional studies are needed to clarify the relationship, if any, of
polymorphism in this gene to host range.

The conclusion that two transmission cycles exist for C. parvum is now
supported by the results of independent groups, using markers at six
different genetic loci. This conclusion, if valid, may have important
implications for the prevention and control of cryptosporidiosis in urban
settings. Cattle have been the most commonly implicated source of water
contamination in outbreaks outside the United States but not conclusively
within the United States. Measures for preventing water contamination have
in some cases included the removal of cattle from watershed areas in or
around municipalities. If, however, sewer overflows and inadequate sewage
treatment are the primary source of water contamination in urban settings
where anthroponotic cycles are being maintained, focusing solely on cattle
could fail to eliminate a very important source of infection.

The results of this study suggest the need to 1) combine the typing
approaches of various groups into a multilocus approach for genetic typing
of C. parvum that would result in a reliable and robust method for strain
typing, 2) apply multilocus typing to a large number of C. parvum isolates
both from epidemic and isolated cases and from a large geographic
distribution to determine the prevalence of these two genotypes and their
quantitative importance as indicators of specific risk factors, and 3)
identify additional genetic loci that will allow more precise determination
of strain variation and linkage of genotypic variation to specific clinical
and epidemiologically important outcomes.

Acknowledgments

We thank Thomas R. Navin, David G. Addiss, and Dennis D. Juranek, Centers
for Disease Control and Prevention (CDC), for critical reading of the
manuscript; Gisela Withers, Alfred L. Loban, Susan Yeager, Barbara Fisher,
and Marshall P. Deasy, Pennsylvania Department of Health, Charles Brummit,
St. Lukes Medical Center, Milwaukee, WI, and Michael J. Beach, CDC, for
providing samples; Brian Holloway and staff, CDC, for providing DNA
oligonucleotide primers; Robert W. Ryder, Yale University School of
Medicine, and Mark L. Wilson, The University of Michigan, for help in
coordinating the study; and Kimberley B. Donaldson, Donghyun Hahn, Kimberly
Y. Won, Chunfu Yang, and Lillian Escalante, CDC, for technical support in
completing the work.

Michael M. Peng,* Lihua Xiao, Amanda R. Freeman, Michael J. Arrowood,
Ananias A. Escalante, Andr C. Weltman, Corinne S.L. Ong, William R. Mac
Kenzie, Altaf A. Lal, and Charles B. Beard
*The University of Michigan, Ann Arbor, Michigan, USA; Centers for Disease
Control and Prevention, Atlanta, Georgia, USA; Pennsylvania Department of
Health, Harrisburg, Pennsylvania, USA; and University of British Columbia,
British Columbia, Canada

References

  1. Guerrant RL. Cryptosporidiosis: an emerging, highly infectious threat.
     Emerg Infect Dis 1997;3:51-7.
  2. Colford JM, Tager IB, Hirozawa AM, Lemp GF, Aragon T, Petersen C.
     Cryptosporidiosis among patients infected with human immunodeficiency
     virus. Am J Epidemiol 1996;144:807-16.
  3. MacKenzie WR, Hoxie NJ, Proctor ME, Gradus MS, Blair KA, Peterson DE,
     et al. A massive outbreak in Milwaukee of Cryptosporidium infection
     transmitted through the public water supply. N Engl J Med
     1994;331:161-7.
  4. Morgan UM, Constantine CC, O'Donoghue P, Meloni BP, O'Brien PA,
     Thompson RCA. Molecular characterization of Cryptosporidium isolates
     from humans and other animals using random amplified polymorphic DNA
     analysis. Am J Trop Med Hyg 1995;52:559-64.
  5. Carraway M, Tzipori S, Widmer G. Identification of genetic
     heterogeneity in the Cryptosporidium parvum ribosomal repeat. Appl
     Environ Microbiol 1996;62:712-6.
  6. Bonnin A, Fourmaux MN, Dubremetz JF, Nelson RG, Gobet P, Harly G, et
     al. Genotyping human and bovine isolates of Cryptosporidium parvum by
     polymerase chain reaction-restriction fragment length polymorphism
     analysis of a repetitive DNA sequence. FEMS Microbiol Lett
     1996;137:207-11.
  7. Spano F, Putignani L, McLauchlin J, Casemore DP, Crisanti A. PCR-RFLP
     analysis of the Cryptosporidium oocyst wall protein (COWP) gene
     discriminates between C. wrairi and C. parvum, and between C. parvum
     isolates of human and animal origin. FEMS Microbiol Lett
     1997;150:209-17.
  8. Arrowood MJ, Sterling CR. Isolation of Cryptosporidium oocysts and
     sporozoites using discontinuous sucrose and isopycnic Percoll
     gradients. J Parasitol 1987;73:314-9.
  9. Arrowood MJ, Donaldson K. Improved purification methods for
     calf-derived Cryptosporidium parvum oocysts using discontinuous
     sucrose and cesium chloride gradients. J Eukaryot Microbiol
     1996;43:S89.
 10. Kim K, Gooze L, Petersen C, Gut J, Nelson RG. Isolation, sequence and
     molecular karyotype analysis of the actin gene of Cryptosporidium
     parvum. Mol Biochem Parasitol 1992;50:105-14.
 11. Wisconsin Package [Computer program]. Version 9.0. Madison (WI):
     Genetics Computer Group; 1996.
 12. Sanger F, Miklen S, Coulson AR. DNA sequencing with chain-terminating
     inhibitors. Proc Natl Acad Sci U S A 1977;74:5463-7.
 13. Mead JR, Arrowood MJ, Sidwell RW, Healey MC. Chronic Cryptosporidium
     parvum infections in congenitally immunodeficient SCID and nude mice.
     J Infect Dis 1991;163:1297-304.
 14. Arrowood MJ, Hurd MR, Mead JR. A new method for evaluating
     experimental cryptosporidial parasite loads using immunofluorescent
     flow cytometry. J Parasitol 1995;81:404-9.
 15. Ong CSL, Pearce M, Eisler D, Goh SH, King AS, Bowie WR, and
     Isaac-Renton JL. An outbreak of cryptosporidiosis in southeastern
     British Columbia, Canada. In: Proceedings of the Symposium on
     Waterborne Cryptosporidium American Water Works Association. In press
     1997.
 16. Millard PS, Gensheimer KF, Addiss DG, Sosin DM, Beckett GA,
     Houck-Jankoski A, Hudson A. An outbreak of crytposporidiosis from
     fresh-pressed apple cider. JAMA 1994;272:1592-6.
 17. Beach MJ, McNeil M, Arrowood M, Kreckman L, Craig A, Donaldson K, et
     al. Cryptosporidium in a water park: largest U.S. recreational
     waterborne outbreak [abstract]. 45th Annual Epidemic Intelligence
     Service Conference, Atlanta. 1996.
 18. Centers for Disease Control and Prevention. Outbreak of
     cryptosporidiosis at a day camp-Florida, July-August 1995. MMWR Morb
     Mortal Wkly Rep 1996;45:442-4.
 19. Addiss DG, Mac Kenzie WR, Hoxie NJ, Gradus MS, Blair KA, Proctor JE,
     et al. Epidemiologic features and implications of the Milwaukee
     cryptosporidiosis outbreak. In: Betts WB, Casemore D, Fricker C, Smith
     H, Watkins J, editors. Protozoan parasites and water. Cambridge,
     United Kingdom: Royal Society of Chemistry; 1995. p. 19-25.
 20. Awad-El-Kariem FM, Robinson HA, Dyson DA, Evans D, Wright S, Fox MT,
     McDonald V. Differentiation between human and animal strains of
     Cryptosporidium parvum using isoenzyme typing. Parasitology
     1995;110:129-32.
 21. Vasquez JR, Gooze L, Kim K, Gut J, Petersen C, Nelson RG. Potential
     antifolate resistance determinants and genotypic variation in the
     bifunctional dihydrofolate reductase-thymidylate synthase gene from
     human and bovine isolates of Cryptosporidium parvum. Mol Biochem
     Parasitol 1996;79:153-65.
 22. Pozio E, Gomez Morales MA, Barbieri FM, La Rosa G. Cryptosporidium:
     different behaviour in calves of isolates of human origin. Trans R Soc
     Trop Med Hyg 1992;86:636-8.
 23. Hojlyng N, Molbak K, Jepsen S. Cryptosporidiosis in human beings is
     not primarily a zoonosis. J Infect 1985;11:270-2.
 24. Robson KJH, Hall JRS, Davies LC, Crisanti A, Hill AVS, Wellems TE.
     Polymorphism of the TRAP gene of Plasmodium falciparum. Proc R Soc
     Lond B Biol Sci 1990;242:205-16.
 25. Muller HM, Scarselli E, Crisanti A. Thrombospondin related anonymous
     protein (TRAP) of Plasmodium falciparum in parasite-host cell
     interactions. Parassitologia 1993;35 (suppl):69-72.
 26. Templeton TJ, Kaslow DC. Cloning and cross-species comparison of the
     thrombospondin-related anonymous protein (TRAP) gene from Plasmodium
     knowlesi, Plasmodium vivax and Plasmodium gallinaceum. Mol Biochem
     Parasitol 1997;84:13-24.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/beard.txt

Irradiation Pasteurization of Solid Foods: Taking Food Safety to the Next
Level

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In the 19th century, milk from diseased cattle produced in unsanitary
surroundings and distributed under filthy conditions to an increasingly
urbanized population sickened and killed consumers by the thousands (1,2).
Wide acceptance of the germ theory and the sanitary awakening that followed
led to vast improvements in animal health and hygiene, and the safety of
dairy products improved substantially. Dairy farmers in northern Europe
discovered that heating milk fed to their calves further reduced the risk
for tuberculosis in their herds, and after overcoming concerns that the new
thermal pasteurization technology would corrupt the dairy industry, destroy
the nutritional value of milk, and lead to serious public health problems,
the same level of protection was offered to human consumers a few decades
later (3,4). More recently, thermal pasteurization has been suggested for
eliminating low level contamination of juice by foodborne pathogens (5,6).
However, for the safety of solid foods that enter kitchens as raw
agricultural commodities, including meat, poultry, and seafood, we continue
to rely solely on animal health programs and sanitation. Therefore, as we
approach the 21st century, preventable illness and death caused by
vegetative bacterial and parasitic foodborne pathogens remain substantial
public health problems (7,8).

Irradiation pasteurization of solid foods with low doses of gamma rays,
X-rays, and electrons will effectively control vegetative bacterial and
parasitic foodborne pathogens (9-11). Public concerns, similar to those
raised against thermal pasteurization of milk, have been advanced in
opposition to irradiation pasteurization, and it has been claimed that if
we but paid more attention to sanitation and proper cooking, these products
could be safely consumed without introducing new technologies.

Perhaps. However, the residual risk for infection that remains after
state-of-practice sanitation during production, harvest, processing,
distribution, and preparation yields an unacceptable level of illness and
death. In addition, the admonition to properly cook works only if
culturally acceptable food preferences do not include undercooked and raw
foods. Increased interest (encouraged by the public health and nutrition
community) in fresh produce as part of a high fiber, low fat diet, further
reduces the effectiveness of proper cooking as a disease control strategy
(12).

Recent outbreaks of foodborne illness associated with undercooked meat and
uncooked fresh produce, and the emergence of the previously unrecognized
foodborne hazards that spawned the conference whose proceedings are
reported in this issue of Emerging Infectious Diseases, have stimulated
interest in methods of pasteurizing solid food without altering its raw
appearance and characteristics. Research is under way on a variety of
promising approaches, including pulsed energy, bright light, high pressure,
and other nonthermal technologies, but few are ready for immediate
application (13,14; Fed Reg 61:42381-83, 1997). Irradiation pasteurization,
on the other hand, is a well-established process with clearly documented
safety and efficacy that can be put into widespread use as quickly as
facilities can be sited and built (15).

Good practice guidelines and Hazard Analysis and Critical Control Points
programs (HACCP) can result in raw meat, poultry, seafood, and produce with
sufficiently low levels of pathogen contamination that irradiation doses as
low as 1 to 3 kGy yield adequate margins of safety for common foodborne
pathogens such as Campylobacter, Cryptosporidium, Escherichia coli,
Listeria, Salmonella, and Toxoplasma (9). No other control for
Campylobacter contamination of poultry meat is apparent, and other
approaches to ground beef safety have proven inadequate to prevent
intermittent low level contamination with E. coli O157:H7. Likewise, no
other solutions are immediately available to control pathogen contamination
of produce intended for raw consumption, and irradiation doses used appear
adequate for the bacterial and parasitic pathogens involved in recent
outbreaks (Donald Thayer, pers. comm.)

The food industry appears reluctant to fully embrace irradiation
pasteurization despite the obvious and painful failure of alternative
approaches to prevent foodborne infections. Much of this reluctance stems
from the perception that consumers reject the process and will refuse to
buy irradiated food. Indeed, surveys have shown considerable consumer
confusion and ignorance about food irradiation (16), and reports on public
antipathy toward things radioactive abound. However, consumer surveys also
demonstrate profound and growing public concern about microbial food
safety, and decreasing concerns about the safety of irradiated food (17).
Knowing as little about it as they appear to, approximately half of the
consumers surveyed have expressed willingness to try irradiated food if it
will decrease their risk for illness (16). In addition, when educated about
food irradiation, 90% of survey participants expressed interest in
purchasing irradiated foods; sampling such food increased interest to 99%
(18).

Irradiation pasteurization is not the cure for all food safety ills.
Pasteurization of any sort is no match for bad sanitation and substandard
practices, and irradiation pasteurization can be overwhelmed by large
numbers of pathogens. Just as obviously, foods produced and processed under
appropriate conditions that are then properly packaged and irradiated are
subject to postpasteurization contamination. In addition, the doses of
irradiation used to pasteurize fresh meat and poultry are not sufficient to
kill bacterial spores. Thus, if anaerobic packaging is the method used to
protect irradiated foods from postpasteurization contamination, Clostridium
botulinum could pose a risk if the cold chain is disrupted.

Vibrio infections associated with consumption of raw molluscan shellfish
can be prevented with irradiation pasteurization, but the Norwalk-like
viruses also frequently associated with raw shellfish appear to be more
radioresistant than vegetative bacterial pathogens. Levels of irradiation
an order of magnitude greater than pasteurizing doses for meat and poultry
also are necessary to inactivate hepatitis A virus. To reduce the risk for
foodborne hepatitis A and Norwalk virus infections, it will be necessary to
reduce the level of exposure of food to human feces. This is true
regardless of whether or not foods are to be pasteurized. Although
irradiation pasteurization will not eliminate all seafood-borne pathogens,
it will reduce the potential of seafood to cause illness. Seafood HACCP and
advances in viral diagnostics and environmental virology will help ensure
that prepasteurization conditions are sufficient to yield seafood
appropriate for irradiation pasteurization (19,20). Just as thermal
pasteurization works well for liquid foods like milk and juice, but not for
solid foods for which raw characteristics are desired, irradiation
pasteurization works well for meat, poultry, seafood, and soft fruit, but
wilts leafy vegetables and sprouts. That irradiation pasteurization does
not work for every food and every pathogen is poor justification for not
applying it for those food/pathogen combinations for which it has been
shown to work so well.

Consumer surveys have demonstrated public concerns over worker and
environmental safety that have also contributed to the reluctance of some
to build and use food irradiation facilities. These concerns are
appropriate and addressable. Because food irradiation and irradiation
sterilization of nonfood items like medical supplies are so well
established, proper facilities design and operating characteristics are
well known. The relatively short half-life of Cobalt 60 and its
insolubility in water reduce environmental concerns, which can be
eliminated altogether by using electricity-generated X-rays and electron
beams instead of a radioactive source. Proper education and training has
protected employees of irradiation sterilization facilities; employees of
food irradiation facilities should not be qualitatively different from
other employees in similar industries.

Thus, a broadly applicable solution to many of our food safety problems
exists and has existed for a number of decades. It is disappointing that
the public health community has been so silent for so long on this issue.
Faced with the liability of marketing hazardous foods, it is puzzling why
the food industry has not stepped into the vacuum created by this lack of
leadership from public health. Presentations at the Conference on Emerging
Foodborne Pathogens make it clear that new foodborne hazards are being
stacked on top of old, unresolved food safety problems-broadly applicable
solutions are desperately needed. Just as thermal pasteurization of milk
protected us from E. coli O157:H7 before we knew it was in raw milk,
irradiation pasteurization can protect us from tomorrow's emerging
foodborne pathogen.

Michael T. Osterholm* and Morris E. Potter
*Minnesota Department of Health, Minneapolis, Minnesota, USA; and Centers
for Disease Control and Prevention, Atlanta, Georgia, USA

References

  1. Magruder GL. Further observations on the milk supply of Washington,
     D.C. JAMA 1910;55:581-9.
  2. Kelley ER, Osborn SH. Further evidence as to the relative importance
     of milk infection in the transmission of certain communicable diseases
     of man. Am J Pub Health 1920;10:66-73.
  3. Steele JH, Engel RE. Radiation processing of food. J Am Vet Med Assoc
     1992;201:1522-9.
  4. Potter ME, Kaufmann AF, Blake PA, Feldman RA. Unpasteurized milk:
     hazards of a health fetish. JAMA 1984;252:2048-52.
  5. Besser RE, Lett SM, Weber JT, Doyle MP, Barrett TJ, Wells JG, Griffin
     PM. An outbreak of diarrhea and hemolytic uremic syndrome from
     Escherichia coli O157:H7 in fresh-pressed apple cider. JAMA
     1993;269:2217-20.
  6. Millard PS, Gensheimer KF, Addiss DG, Sosin DM, Beckett GA,
     Houk-Jankowski A, Hudson A. An outbreak of cryptosporidiosis from
     fresh-pressed apple cider. JAMA 1994;272:1592-6.
  7. Foegeding PM, Roberts T. Foodborne pathogens: risks and consequences.
     Ames (IA): Council for Agricultural Science and Technology; 1994; Task
     Force Report No. 122.
  8. Tauxe RV. Emerging foodborne disease: an evolving public health
     challenge. Emerg Infect Dis 1997;3:425-34.
  9. Monk JD, Beuchat LR, Doyle MP. Irradiation inactivation of food-borne
     microorganisms. Journal of Food Protection 1995;58:197-208.
 10. Radomyski T, Murano EA, Olson DG, Murano PS. Elimination of pathogens
     of significance in food by low-dose irradiation: a review. Journal of
     Food Protection 1994;57:73-86.
 11. Lagunas-Solar MC. Radiation processing of foods: an overview of
     scientific principles and current status. Journal of Food Protection
     1995;58:186-92.
 12. Foerster SB, Kizer KW, DiSogra LK, Bal DG, Krieg BF, Bunch KL.
     California's 5 a dayfor better health campaign: an innovative
     population-based effort to effect large-scale dietary change. A J
     Prevent Med 1995;11:124-31.
 13. Paul P, Chawla SP, Thomas P, Kesavan PC. Effect of high hydrostatic
     pressure, gamma-irradiation and combination treatments on the
     microbiological quality of lamb meat during chilled storage. Journal
     of Food Safety 1997;16:263-71.
 14. Shigeshia T, Ohmori T, Saito A, Taji S, Hayashi R. Effect of high
     hydrostatic pressure on characteristics of pork slurries and
     inactivation of microorganisms associated with meat and meat products.
     Int J Food Microbiol 1991;12:207-16.
 15. Thayer DW, Josephson ES, Brynjolfsson A, Giddings GG. Radiation
     pasteurization of food. Ames (IA): Council for Agricultural Science
     and Technology; 1996; Issue Paper No. 7.
 16. Resurreccion AVA, Galvez FCF, Fletcher SM, Misra SK. Consumer
     attitudes toward irradiated food: results of a new study. Journal of
     Food Protection 1995;58:193-6.
 17. Bruhn CM. Consumer attitudes and market response to irradiated food.
     Journal of Food Protection 1995;58:175-81.
 18. Bruhn CM. Consumer concerns: motivating to action. Emerg Infect Dis
     1997;3:511-15.
 19. Jaykus LA. Epidemiology and detection as options for control of viral
     and parasitic foodborne disease. Emerg Infect Dis 1997;3:529-39.
 20. Garrett ES, Lima dos Santos C, Jahncke ML. Public, animal, and
     environmental health implications of aquaculture. Emerg Infect Dis
     1997;3:453-57.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/osterhol.txt

Letters

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Non-O157 Shiga Toxin-Producing Escherichia coli Infections in Europe

To the Editor: Shiga toxin-producing Escherichia coli (STEC) infections are
an important cause of severe human disease. Although most infections are
caused by strains of serogroup O157, STEC pathogenic to humans may belong
to other serogroups usually referred to as non-O157 STEC.

Recently, Tarr et al. (1) and Acheson et al. (2) described infections
attributable to STEC O103 and expressed concern that non-O157 STEC may pose
an underestimated threat to public health in the United States. In fact,
non-O157 STEC is often overlooked in clinical microbiology laboratories
because the toxigenic phenotype is not exploited to identify such
pathogens. Rather, most laboratories use sorbitol MacConkey agar and
serotyping (which cannot detect most non-O157 STEC) to identify E. coli
O157:H7.

Since the end of the 1980s, non-O157 STEC infections have caused as many as
10% to 30% of sporadic cases of hemolytic uremic syndrome (HUS) in Germany
(3), Italy (4), and the United Kingdom (5). Moreover, HUS outbreaks have
been associated with STEC O111:H- in Italy (6) and France (7).

During 1996, we observed a sudden increase in infections attributable to
STEC O103 and O26 in Germany and Italy. In our laboratory in Germany, E.
coli O103:H2 was not identified among 345 non-O157 STEC isolated between
1985 and 1995 but represented 12 (18.2%) of 66 of the non-O157 STEC
isolated during 1996. HUS developed in two infected patients.

Among cases reported to Italy's nationwide HUS surveillance system from
1988 to 1995, evidence of infection with STEC O103 or O26 was found in two
(1.5%) and nine (6.6%) of 135 cases, respectively. Since 1996, infection
with STEC O103 and O26 has been diagnosed in three (11%) and nine (33%) of
27 HUS cases, respectively.

These observations indicate that identification of non-O157 STEC should be
considered by clinical laboratories. Immunoenzymatic tests (based on either
toxin antibodies or receptors) that detect Shiga toxins produced by fecal
bacterial isolates or present in stool specimens are now available (8,9).
Use of these tests should be considered in analyzing the stools of patients
with HUS, bloody diarrhea, or painful nonbloody diarrhea, if classic
microbiologic analysis fails to yield E. coli O157:H7 or another standard
enteric pathogen, such as Campylobacter, Salmonella, or Shigella.

The sudden appearance or increase of rare non-O157 STEC in our populations
is worrisome. Most non-O157 STEC, as well as the sorbitol fermenting
O157:H- strains (10) associated with HUS in several European countries,
would be missed by laboratories using standard microbiologic detection
methods, such as sorbitol MacConkey agar screening. Because of the
considerable clinical and epidemiologic urgency, clinical microbiologists
and physicians should seek out these such pathogens in appropriate clinical
situations.

Alfredo Caprioli,* Alberto E. Tozzi,* Gianfranco Rizzoni, and Helge Karch

*Istituto Superiore di Sanit, Rome, Italy; Ospedale Pediatrico Bambino
Ges, Rome, Italy; Universitat Wuerzburg, Germany

References

  1. Tarr PI, Fouser LS, Stapleton AE, Wilson RA, Kim HH, Vary JC, Clausen
     CR. Hemolytic-uremic syndrome in a six-year-old girl after a urinary
     tract infection with Shiga-toxin-producing Escherichia coli O103:H2. N
     Engl J Med 1996;335:635-8.
  2. Acheson DWK, Wolf LE, Park CH. Escherichia coli and the
     hemolytic-uremic syndrome. N Engl J Med 1997;336:515.
  3. Bitzan M, Moebius E, Ludwig K, Muller-Wiefel DE, Heesemann J, Karch H.
     High incidence of serum antibodies to Escherichia coli O157
     lipopolysaccharide in children with the hemolytic uremic syndrome. J
     Pediatr 1991;119:380-5.
  4. Caprioli A, Luzzi I, Rosmini F, Pasquini P, Cirrincione R, Gianviti A,
     et al. Hemolytic-uremic syndrome and vero-cytotoxin-producing
     Escherichia coli infection in Italy. J Infect Dis 1992;166:154-8.
  5. Kleanthous H, Smith HR, Scotland SM, Gross RJ, Rowe B, Taylor CM,
     Milford DV. Haemolytic-uraemic syndromes in the British Isles, 1985-8:
     association with verocytotoxin producing Escherichia coli. Part 2:
     microbiological aspects. Arch Dis Child 1990;65:722-7.
  6. Caprioli A, Luzzi I, Rosmini F, Resti C, Edefonti A, Perfumo F, et al.
     Communitywide outbreak of hemolytic-uremic syndrome associated with
     non-O157 verocytotoxin-producing Escherichia coli. J Infect Dis
     1994;169:208-11.
  7. Boudaillez B, Berquin P, Mariani-Kurkdjian P, Bef D, Cuvelier B, Capek
     J, et al. Possible person-to-person transmission of Escherichia coli
     O111-associated hemolytic uremic syndrome. Pediatr Nephrol
     1997;11:36-9.
  8. Kehl KS, Havens P, Behnke CE, Acheson DWK. Evaluation of the premier
     EHEC assay for detection of Shiga-toxin-producing Escherichia coli. J
     Clin Microbiol 1997;35:2051-4.
  9. Basta M, Karmali M, Lingwood C. Sensitive receptor-specified
     enzyme-linked immunosorbent assay for Escherichia coli verocytotoxin.
     J Clin Microbiol 1989;27:1617-22.
 10. Gunzer F, Bohm H, Russmann H, Bitzan M, Aleksic S, Karch H. Molecular
     detection of sorbitol-fermenting Escherichia coli O157 in patients
     with hemolytic-uremic syndrome. J Clin Microbiol 1992;30:1807-10.

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The Taxonomy of Cyclospora

To the Editor: In the article by N.J. Pieniazek and B.L. Herwaldt (1) on
the rRNA gene of Cyclospora cayetanensis, the authors suggest that
Cyclospora should be placed in the genus Eimeria because the rRNA genes of
the two genera have similar sequences. The article refers to Norman D.
Levine's chapter on the Apicomplexa in the Illustrated Guide to the
Protozoa (2). Regrettably, the authors failed to read the whole chapter and
to recognize that the initial characteristics for placing the oocyst of a
coccidium in its proper genus are the number of sporocysts and then the
number of sporozoites in each sporocyst. The genus Eimeria has four
sporocysts and two sporozoites in each sporocyst. The genus Cyclospora has
two sporocysts, each of which has two sporozoites.

The original taxonomists (3) of C. cayetanensis recognized that it should
be placed in the taxonomic family Eimeriidae, close to Eimeria, but they
adhered to the traditional designation for genera of coccidia. Pieniazek
and Herwaldt should be cognizant of the rules of zoologic nomenclature as
well as the fact that certain morphologic characteristics of protists have
served us well for many decades and continue to be useful. There are
serious consequences to changing the classification of an organism, and it
should not be thought that one can make such a change casually. I encourage
the editors of Emerging Infectious Diseases to seek the advice of those who
understand what should be done with respect to the classification and
nomenclature of organisms.

William C. Marquardt
Colorado State University, Fort Collins, Colorado, USA

References

  1. Pieniazek NJ, Herwaldt BL. Reevaluating the molecular taxonomy: Is
     human-associated Cyclospora a mammalian Eimeria species? Emerg Infect
     Dis 1997;3:381-3.
  2. Levine ND. Apicomplexa. In: Lee JJ, Hutner SH, Boyce EC, editors. An
     illustrated guide to the protozoa. Lawrence (KS): Society of
     Protozoologists; 1985. p.322-74.
  3. Ortega YR, Gilman RH, Steling CR. A new coccidian parasite
     (Apicomplexa:Eimeriidae) from humans. J Parasitol 1994;80:625-9.

Reply to W.C. Marquardt: Dr. Marquardt's advocacy for reliance on
morphologic characteristics even if phylogenetic data become available that
lead to a different conclusion runs counter to that expressed in an article
he coauthored, which supports the importance of molecular data (1). The
introduction of that paper states the following:

"Early systematists relied largely on light microscopic structures and life
cycle patterns to separate protozoa taxonomically.... Apicomplexans display
enormous variations in life cycle patterns, physiology, cytology, and
biochemistry. There is no consensus on which characteristics should be
relied upon to infer phylogenetic relationships. Developmental and
ultrastructural features have been used to infer evolutionary relationships
among representative genera in the class Sporozoea. However, comparisons of
phenotypic characters are qualitative and lack objective quantitative
assessment to infer genetic relationships. Sequence similarities between
proteins or genes which share a common evolutionary history can be used to
infer quantitative phylogenetic relationships. The small subunit (16S-like)
rRNAs and their coding regions are especially useful for estimating the
extent of genetic relatedness over broad evolutionary ranges."

That paper concludes with the statement that "ribosomal RNA sequence
analyses of other apicomplexans are required in order to test the validity
of relationships inferred from structures and life cycle patterns."
Similarly, we concluded our paper as follows: "Reports based on morphologic
features alone may suffer from poor resolution of features needed for
classification of closely related organisms. To improve our understanding
of the taxonomy of human-associated Cyclospora, molecular evaluation of
isolates of additional Cyclospora and Eimeria species, especially other
mammalian species, is needed."

Norman J. Pieniazek and Barbara L. Herwaldt
Centers for Disease Control and Prevention, Atlanta, Georgia, USA

References

  1. Gajadhar AA, Marquardt WC, Hall R, Gunderson J, Ariztia-Carmona EV,
     Sogin ML. Ribosomal RNA sequences of Sarcocystis muris, Theileria
     annulata and Crypthecodinium cohnii reveal evolutionary relationships
     among apicomplexans, dinoflagellates, and ciliates. Mol Biochem
     Parasitol 1991;45:147-54.

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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: http://www.cdc.gov/ncidod/EID/vol3no4/letters.htm

News and Notes

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Foodborne Diseases Active Surveillance Network (FoodNet)

The Foodborne Diseases Active Surveillance Network (FoodNet) is the
foodborne disease component of the Emerging Infections Program (EIP) of the
Centers for Disease Control and Prevention (CDC). A collaborative project
of CDC, the seven EIP sites, the U.S. Department of Agriculture (USDA), and
the U.S. Food and Drug Administration (FDA), FoodNet consists of active
surveillance for foodborne diseases and related epidemiologic studies
designed to help public health officials better understand the epidemiology
of foodborne diseases in the United States. FoodNet was established in 1995
in five locations: Minnesota, Oregon, and selected counties in Georgia,
California, and Connecticut. The total population of these sites, or
catchment areas, is 14.7 million, or 6% of the population of the United
States. FoodNet was expanded to selected counties in Maryland and New York
in 1997. The goals of FoodNet are to describe the epidemiology of new and
reemerging bacterial, parasitic, and viral foodborne pathogens; estimate
the frequency and severity of foodborne diseases that occur in the United
States per year; and determine how much foodborne illness results from
eating specific foods, such as meat, poultry, and eggs.

Foodborne diseases are common; an estimated 6 to 33 million cases occur
each year in the United States. Although most of these infections cause
mild illness, severe infections and serious complications do occur. The
public health challenges of foodborne diseases are changing rapidly; in
recent years, new and reemerging foodborne pathogens have been described,
and changes in food production have led to new food safety concerns.
Foodborne diseases have been associated with many different foods,
including some previously thought to be safe, such as eggs and fruit juice,
both of which have transmitted Salmonella during recent outbreaks. Public
health officials in the seven EIP sites are monitoring foodborne diseases,
conducting epidemiologic and laboratory studies of these diseases, and
responding to new challenges from these diseases. Information gained
through this network will lead to new interventions and prevention
strategies for addressing the public health problem of foodborne diseases.

Current "passive" surveillance systems rely upon reporting of foodborne
diseases by clinical microbiology laboratories to state health departments,
which in turn report to CDC. Although foodborne diseases are extremely
common, only a fraction of them are routinely reported to CDC through these
surveillance systems. Inadequate reporting results from a complex chain of
events that must occur before a case is reported, and a break at any
linkage along the chain results in a case not being reported (Figure).
FoodNet is an "active" surveillance system, meaning public health officials
frequently contact microbiology laboratory directors to find new cases of
foodborne diseases and report these cases electronically to CDC. In
addition, FoodNet is designed to monitor each of the events that occurs
along the foodborne diseases pyramid and thereby allow more accurate and
precise estimates and interpretation of the prevalence of foodborne
diseases over time. Because most foodborne infections cause diarrheal
illness, FoodNet focuses these efforts on persons who have a diarrheal
illness.

[Figure Not Available in ASCII]

 Figure. The prevalence of illness pyramid. Passive surveillance data
 represent only the tip of the iceberg. For a bacterial infection to be
 included in the passive surveillance system, it must pass through the
 following steps: a person becomes ill with a diarrheal disease, the
 patient must go to a doctor, the doctor must order a bacterial stool
 culture, the assigned microbiology laboratory must culture for this
 organism and report the infection to the state health department, and the
 state health department in turn must report the infection to CDC. This
 passive surveillance system is the means by which the number of cases of
 foodborne illness is currently determined at CDC; if any step does not
 occur, foodborne illness is not reported. FoodNet is designed to collect
 information along each step of this pyramid.

FoodNet Components

Active Laboratory-Based Surveillance

The core of FoodNet is population-based active surveillance at over 300
clinical microbiology laboratories that test stool samples in the seven
participating sites. In active surveillance, the laboratories in the
catchment areas are contacted regularly by collaborative FoodNet
investigators to collect information on all laboratory-confirmed cases of
diarrheal illness. Since January 1996, information has been collected on
every laboratory-diagnosed case of Salmonella, Shigella, Campylobacter,
Escherichia coli O157, Listeria, Yersinia, and Vibrio infection among
residents of the catchment areas of the five original sites; this
information is transmitted electronically to CDC. The result is a
comprehensive and timely database of foodborne illness in a well-defined
population.

Survey of Clinical Laboratories

In October 1995, collaborative FoodNet investigators conducted a baseline
laboratory survey of all microbiology laboratories in the five original
catchment areas to determine which pathogens are included in routine
bacterial stool cultures, which tests must be specifically requested by the
physician, and what specific techniques are used to isolate the pathogens.
A baseline survey will be conducted in the two new sites, and a follow-up
survey to assess any recent changes in laboratory practices was conducted
in the original sites in 1997. Practices in clinical laboratories have been
found to vary; some laboratories look for a wider variety of bacteria than
others. The methods used to collect and examine specimens are being
investigated because these can influence whether the laboratory finds
disease-causing bacteria.

Survey of Physicians

To obtain information on physician stool-culturing practices, collaborative
FoodNet investigators mailed a survey questionnaire to 5,000 physicians
during 1996. Analysis of these data is ongoing. Because laboratories test
stool specimens from a patient only upon the request of a physician or
other health-care provider, it is important to measure how often and under
what circumstances physicians order these tests. As changes occur in the
way health care is provided in the United States, stool-culturing practices
may also change. The practices of physicians who send stool samples to
laboratories within the catchment areas will be monitored by surveys and
validation studies.

Survey of the Population

Collaborative FoodNet investigators contact randomly selected residents of
a catchment area and ask whether the person has had a recent diarrheal
illness, whether the person sought treatment for the illness, and whether
the person had consumed certain foods known to have caused outbreaks of
foodborne illness. During 1996, 750 residents of the catchment areas were
interviewed by telephone each month (9,000/year). Because many who become
ill with diarrhea do not see a physician, little is known about the number
of cases of diarrhea in the general population and how often persons with
diarrhea seek medical care. The population survey is an essential part of
active surveillance for foodborne illness because it allows for an estimate
of the population who seeks medical care when affected by diarrheal
illness.

Case-Control Studies

In 1996, the FoodNet began case-control studies of E. coli O157 and
Salmonella serogroup B and D infections. More than 60% of Salmonella
infections in the United States are caused by serogroup B and D Salmonella.
These large case-control studies will provide new and more precise
information about which food items or other exposures may cause these
diseases. To allow the most precise classification of the isolates from the
patients in these studies, the Salmonella and E. coli O157:H7 laboratory
specimens from these patients are sent from FoodNet sites to CDC for
further study, including antibiotic resistance testing, phage typing, and
molecular subtyping.

Accomplishments

Since becoming operational on January 1, 1996, FoodNet has tracked the
rates of foodborne diseases. Even in the first year of data collection,
numerous interesting patterns and outbreaks were detected. Surprisingly
high isolation rates for Y. enterocolitica in Georgia and Campylobacter in
California were detected. An outbreak of Salmonella infections caused by
contaminated alfalfa sprouts was detected in Oregon. Two outbreaks of E.
coli O157:H7 infections were detected in Connecticut, one due to lettuce
and one to apple cider. FoodNet has also provided the infrastructure for
conducting active surveillance for new and reemerging diseases. When an
association between bovine spongiform encephalopathy in cattle and
variant_Creutzfeldt-Jakob disease in humans was suspected in the United
Kingdom, EIP personnel conducted surveillance for this rare human disease.
EIP personnel also collaborated in the investigation of a multistate
outbreak of Cyclospora infections associated with consumption of
raspberries from Guatemala.

Future FoodNet Projects

Several projects in 1997 will focus on Campylobacter, including a
case-control study to determine the risk factors for infection and
determination of antibiotic resistance patterns among Campylobacter
strains.

Collaborative FoodNet investigators will establish active surveillance for
hemolytic uremic syndrome, a serious complication of E. coli O157:H7
infection.

FoodNet Working Group*

*Frederick Angulo, David Swerdlow, Robert Tauxe, Patricia Griffin, Drew
Voetsch, Thomas Boyce, Sudha Reddy, Mary Evans, Sam Yang (CDC); Duc Vugia,
Ben Werner, Kevin Reilly (California Department of Health Services); Sue
Shallow, Gretchen Rothrock, Pam Daily, Felicia Chi (California EIP); Paul
Blake, Jane Koehler (Georgia Department of Human Resources); Monica Farley,
Wendy Baughman, Molly Bardsley, Suzannne Segler, Shama Desai (Georgia EIP);
James Hadler, Pat Mshar (Connecticut Department of Public Health); Ruthanne
Marcus, Terry Fiorentino (Connecticut EIP); Michael Osterholm, Craig
Hedberg, Jeff Bender, Julie Hogan, Valerie Deneen, Heidi Kassenborg
(Minnesota Department of Health); Paul Cieslak, John Townes, Beletshachew
Shiferaw, Maureen Cassidy, Theresa McGivern, Regina Stanton (Oregon Health
Division); Diane Dwyer, Peggy Pass (Maryland Department of Health and
Mental Hygiene); and Dale Morse, Julia Kiehlbauch, Hwa-Gan Chang, Cathy
Stone (New York Department of Health); I.Kaye Wachsmuth, Jill
Hollingsworth, Peggy Nunnery, Art Baker, Phyllis Sparling (USDA-FSIS); Ken
Fauchi, Bing Garthright, Sean Altekruse (FDA-CFSAN).
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[Figure Not Available in ASCII]

          International Conference on Emerging Infectious Diseases

                             March 8-11, 1988
                             Atlanta, Georgia

                    Abstract Deadline: October 31, 1997

The Centers for Disease Control and Prevention, the Council of State and
Territorial Epidemiologists, the American Society for Microbiology, and the
National Foundation for CDC, along with more than 50 agencies and
organizations are cosponsoring the 1998 International Conference on
Emerging Infectious Diseases. This conference will offer new opportunities
for education, collaboration, and partnership with colleagues worldwide to
explore the most current research, surveillance, and prevention and control
programs addressing all aspects of emerging infectious diseases. Attendance
is limited to 2,500 participants.

The meeting will consist of general and plenary sessions, symposia, and
roundtables with invited speakers, presentations on emerging infections
activities, oral and poster presentations based on submission of an
accepted abstract, and exhibits. Conference topics will include work on
surveillance, epidemiology, research, communication, training, and
prevention and control of emerging infectious diseases, as well as
emergency preparedness and response.

Abstracts should address new, reemerging, and drug-resistant infectious
diseases that affect human health. Deadline for abstract submission is
October 31, 1997. Information on later submission of abstracts for
consideration as late breakers is included in the program materials. For
additional information, access announcements at
www.cdc.gov/ncidod/EID/eid.htm or the National Center for Infectious
Diseases (www.cdc.gov/ncidod/whatsnew.htm), send an e-mail to
meetinginfo@asmusa.org, or call 202-942-9248. Proceedings of the conference
will be published in the Emerging Infectious Diseases journal.
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Errata

    Vol. 3, No. 1

In "Fluoroquinolone Resistance in Neisseria gonorrhoeae" by J.S. Knapp et
al., in the Table, p. 35, under the column R of Equivalent MIC (mg/ml), and
on the row Ofloxacin, 400 mg subrow Ofx, 5, which cited reference 6, the
number should be 2.0, not 1.0. The reference for this criterion
(reference 6 on p. 3) should read "Detection of quinolone-resistant
Neisseria gonorrhoeae."

    Vol. 3, No. 3

In "Host Genes and HIV: The Role of the Chemokine Receptor Gene CCR5 and
Its Allele (DCCR5) by J.M. McNicholl et al., the following figure should
have been printed as Figure 2, p. 264. The figure legend is correct.

[Figure Not Available in ASCII]

In "Emerging Foodborne Diseases" by S.F. Altekruse et al., in Table 1, p.
285, the number of Escherichia coli O157:H7 cases per year in the United
States should be 25 x 103, not 725 x 103. The legend to Figure 2, p. 288
should read "Percentage of U.S. population over 65 years of age, 1900-2040
(projected)."

In "Molecular Epidemiologic Investigations of Mycoplasma gallisepticum
Conjunctivitis in Songbirds by Random Amplified Polymorphic DNA Analyses"
by D.H. Ley et al., in Figure 1, p. 376, the photographs of the house finch
and American goldfinch were inadvertently reversed.

We apologize to our readers for these errors.
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Emerging Infectious Diseases
National Center for Infectious Diseases
Centers for Disease Control and Prevention
Atlanta, GA

URL: ftp://ftp.cdc.gov/pub/EID/vol3no4/ascii/newsnote.txt