# Appendix B 

# 

# List of Assumptions and Limitations of Data Used in the Risk Assessment 

###### 

Below is a summary list of assumptions and a section reference for each
assumption. The assumptions are listed in order of importance to the
modeled risk.

**Limitations in quantifying human health impact** **using attributable
risk:**

**ASSUMPTION** **1:** The level of risk as calculated does not account
for cases originating from chicken and contaminating other foods or the
spread from chicken to other animal hosts and resulting in human
exposure. (refer to Section 3.1)

DISCUSSION 1: The definition of the attributable risk included all cases
of disease which may be attributed to a specific risk factor (122, 83).
One limitation of epidemiologic tools used to determine the attributable
risk or etiologic fraction is that those cases that were exposed to the
risk factor of interest, even though the exposure may not have been the
cause of the disease, would be included in the calculated level of risk,
thereby potentially overestimating the level of actual risk. Conversely,
another limitation of the epidemiologic tools used to determine the risk
from the specific exposure of interest is that spread from the primary
source of the pathogen, in this case chickens, is not included in the
calculation of the level of risk. The magnitude of the bias introduced
by false associations with chicken exposures (false positive
associations) is likely to be much smaller than the lack of inclusion of
the undeterminable cases from spread of the chicken associated resistant
*Campylobacter* to other sources of human exposure. In addition, the
risk assessment does not take into account the spread of the pathogen
from chicken to other food sources. This can occur from cross
contamination of other foods (29) or spread from chicken sources more
proximate to the farm. For example, spread of Campylobacter can occur
via many different pathways: from exposure of birds, insects and run-off
surface water to chicken litter; from the use of chicken litter in
aquaculture to fertilize fish ponds and from the use of litter in cattle
feeds to increase the non-protein nitrogen content. Therefore, the risk
assessment is likely to underestimate the overall risk of acquiring a
resistant *Campylobacter* infection from exposure to chicken due to the
secondary spread of *Campylobacter* from chickens to other sources of
human exposure to the pathogen.

**ASSUMPTION 2:** The current level of risk of contracting
campylobacteriosis from consumption of chicken is contained within the
range of risk ascertained from studies conducted in the 1980's (refer to
Section 3.1)

LIMITATIONS of studies used to determine the proportion of chicken
associated cases:

Limitations of study 1 include: the demographic characteristics of the
population, the frequency of chicken consumption, the proportion of the
population consuming chicken and many other factors may have changed
since this study. For example, the amount of chicken consumed has
increased since 1982, and in 1998 people consumed 54.4% (72.60 lbs/47.02
lbs) more chicken, calculated in ready to cook pounds consumed per
capita (80).

Limitations of study 2 include the lack of representativeness of the
study population and the absence of some exposures, such as travel and
raw milk that are frequently associated with risk in the population at
large. In addition, the study was limited to enteric illnesses because
more invasive infections were not eligible for inclusion in the study,
although these usually comprise less than 1% of cases. These differences
result in difficulty in generalizing the findings to the United States
population but may represent the level of risk in some subgroups of the
population.

DISCUSSION 2: In the two case control studies there was an increased
risk of illness associated with consumption of chicken especially
consumption of undercooked chicken. One study indicated a risk
associated with raw milk consumption although the proportion of
attributable risk was much less than that attributed to chicken. The
proportions of disease attributable to consumption of chicken were 48.5%
and 66.7%. The higher estimate of attributable risk from study 2 of
66.7% in the university student population indicates that in some
subgroups of the population exposures are likely to differ and risk
attributable to consumption of chicken will vary accordingly. These
estimates of the etiologic fraction represent a range of risk that is
likely to reflect the level of risk in the early 1980's. More recent
data do not exist for United States populations. Data analysis of a case
control study, conducted by the CDC and participating State Health
Departments (CA, CT, GA, MD, MN, OR), in 1998 within FoodNet sites is
currently underway and will be published in the near future. The data
from this study will provide updated risk factor information from which
etiologic fractions associated with identified risk factors may be
determined.

## Limitations in data on resistant isolates:

**ASSUMPTION** **3:** The fluoroquinolone resistance observed in persons
ill from campylobacteriosis, (after removal of travelers, those who took
a fluoroquinolone prior to culture and those for whom the time of taking
the fluoroquinolone was unknown) is largely attributed to chickens
(refer to Section 3.2).

DISCUSSION 3: It is difficult to know what proportion of resistance in
human campylobacteriosis may be attributable to a source when human
exposures are multiple and varied and when the data are limited. A
single source of resistant bacteria may be disseminated from its origins
into the environment or maintained in secondary hosts further spreading
resistant *Campylobacter* to additional sources of human exposure
further complicating the ability to measure the impact.

Fluoroquinolone use has been associated with the development of
fluoroquinolone resistance in *Campylobacter* in clinical trials in
poultry production units (58) in poultry production in the Netherlands
(36) and in the United States (92) after the introduction of veterinary
fluoroquinolones. In countries where fluoroquinolones have been approved
for human and companion animal use but are not allowed in food animals
the level of fluoroquinolone resistance in food animals and human
clinical cases is low (8, 54).

An Extra Label Use Prohibition of fluoroquinolone use in food-producing
animals was published in 1997 (21CFR530.41), limiting food animal drug
use to species listed on the product label. Approvals of fluoroquinolone
drugs for use in animals include feline and canine oral and canine
injectable products (available in 1989), poultry water soluble and
in-ovo injectable products (available in 1995) and feedlot cattle
injectable products (available in October, 1998). There are no
fluoroquinolones currently approved for use in swine.

Campylobacteriosis is primarily an animal derived foodborne disease,
with the predominant source of human infections attributed to poultry
(22, 31, 36 64). There is little surveillance data available to describe
the level of fluoroquinolone resistance in *Campylobacter* isolated from
animal derived food in the United States, either before or after the
approval of these drugs for food animal use. Chicken *Campylobacter*
isolates collected in 1998 indicated an overall level of 13.4%
resistance to Ciprofloxacin (see Section 4.1). Because there was no food
animal fluoroquinolone use other than use in poultry until late 1998,
and only rare, sporadic and isolated resistance was observed prior to
1992 in human cases[^1] it is unlikely that the increase in domestically
acquired fluoroquinolone resistance observed in people since 1996[^2]
can be attributed to a consistently distributed source of resistant
*Campylobacter* exposures. Distribution of resistance from foodborne
sources is more likely to be associated with specific exposures and
limited predominantly to poultry.

DATA GAP 3: Quantification of the proportion of human disease
attributable to various sources and the determination of the level of
resistance carriage within the specific exposures would more precisely
allow the determination of the relative contributions of the various
exposures to fluoroquinolone resistant human disease. This ability to
determine the relative contributions of various sources of infection to
the level of resistance in human cases becomes increasingly important
once fluoroquinolones are available for use in more than one food animal
species. A model intended to determine the human health impact of the
level of resistance in *Campylobacter* attributable to fluoroquinolone
use in food animals will need to distribute the burden of resistant
human disease amongst many different food animal species and attribute
levels of resistance to sources of human infection.

**Limitations in microbiological methods:**

**LIMITATION 4:** The lack of accuracy in the determination of the level
of resistance using a single isolate leading to an underestimation of
the level of resistance in chicken carcasses is currently not
quantifiable. The limitation in the accuracy of reported carcass
prevalence and the lack of reliability in the results needs further
characterization and methods need to be developed to provide more
accurate and reliable data which would improve the ability to measure
the impact on human health. This risk assessment determined the
measurable risk, limiting the model to those parameters for which data
were relevant, valid and available.

**ASSUMPTION** **5:** If a carcass was positive for *Campylobacter*, the
predominant species isolated was *C. jejuni.* (refer to Section 4.1)

DISCUSSION: The prevalence of *Campylobacter* in chickens was estimated
from a 1994-95 survey of **1,297** broiler carcass rinse samples at
88.2% of carcasses, indicating that **1,144** carcasses tested positive
(104). The isolates were speciated using the biochemical hippurate assay
and *C. jejuni* and *coli* were included in the carcass prevalence
estimate.

Assumptions relating to the use of surrogate data on diarrheal illness
for seeking care, submitting stools and rate of prescription of
antimicrobials for campylobacteriosis:

**ASSUMPTION(s)** **6:** The rate at which people reporting bloody
stools seek care is similar to the rate at which people with
campylobacteriosis reporting bloody stools seek care. The rate at which
people with non-bloody stools seek care for diarrheal illness is similar
to the rate at which people with campylobacteriosis reporting non-bloody
stools seek care. (refer to Section 2.1 for discussion)

DISCUSSION 6: These estimates are for diarrheal illness, and not
campylobacteriosis specifically. Data describing care seeking behavior
for campylobacteriosis was not available. Bacterial foodborne disease is
typically more severe than viral foodborne disease (42) and rates of
seeking care may differ by pathogen.

In the population survey, factors that were most important in
influencing the decision to seek care were fever, vomiting, "how sick
they felt," stomach cramps, reporting blood in stool and duration of
diarrhea (26). Some of these factors were evaluated for diarrheal
illness in the telephone survey and compared with the same
characteristics in individuals who had culture-confirmed *Campylobacter*
infections or diarrheal disease (Table 2.1). Comparing the groups, a
greater proportion of people with culture-confirmed *Campylobacter*
cases were affected by fever and blood in the stool than the people
seeking care for diarrheal illness. Therefore, the actual rate of
seeking care for campylobacteriosis may be underestimated by the 20.5%
for persons with non-bloody and 33.2% for persons with bloody stools.
However, because a greater proportion of people with fever and bloody
stools would be cultured and enrolled in the case control study, such
comparisons are difficult.

DATA GAP 6: Additional studies to define the rate at which people with
campylobacteriosis seek care would be helpful and would provide a more
accurate estimate. These data would require very large community-based
surveys that are likely to require considerable resources to conduct.

**ASSUMPTION** **7:** The probability that a stool specimen was
requested among people with diarrheal illness reporting bloody stools is
similar to the probability that a stool specimen was requested among
people with campylobacteriosis reporting bloody stools. The probability
that a stool specimen was requested among people with diarrheal illness
reporting non-bloody stools is similar to the probability that a stool
specimen was requested among people with campylobacteriosis reporting
non-bloody stools. (refer to Section 2.2)

**ASSUMPTION** **8:** The population survey proportion of cases of all
acute diarrheal illness seeking care, not submitting a stool sample and
receiving an antibiotic (38.1%) is similar to that for persons ill with
campylobacteriosis. (refer to Section 3.5)

DISCUSSION 8: Severity of illness is one of many factors that lead
physicians to prescribe antibiotics to patients with a diarrheal
illness.

## Assumptions relating to bloody vs. non-bloody diarrhea:

**ASSUMPTION** **9:** Patients with campylobacteriosis who have sought
care and been requested to submit stool cultures and have submitted
stool cultures are prescribed antibiotics at a rate that is the same
whether they had bloody or non-bloody diarrhea. Conversely, if patients
have sought care but have not been requested to submit stool cultures,
they are prescribed at another rate that is the same whether they had
bloody or non-bloody diarrhea.(refer to Section 3.5)

## FoodNet to US extrapolation:

**ASSUMPTION** **10:** The incidence rates for culture-confirmed
*Campylobacter* infections in the FoodNet catchment are representative
of incidence rates for culture-confirmed *Campylobacter* infections in
the U. S. (refer to Section 1.2)

DISCUSSION 10: Although the incidence rates varied by site, from
10.2/100,000 in Maryland to 37.7/100,000 in California in 1998 (21), the
overall rate of *Campylobacter* isolation is likely to reflect isolation
rates in the U.S. population. Comparisons of demographic characteristics
between the FoodNet sites and the U.S. population show similar
distributions of sex, age, race and rural/urban distributions (Table
1.1).

In addition to demonstrating similarity in population composition, an
evaluation of potential exposure is important. In a 1994-5 United States
Department of Agriculture, Food Safety Inspection Service, survey, 88%
of chicken carcasses were reported to carry *Campylobacter* at slaughter
(Table 1.2)(104). Another estimate, of Campylobacter carriage on retail
chicken products was demonstrated at a level of 88% in a Minnesota
survey of chicken products in 1997 (92).

Another factor affecting incidence rates may be the sensitivity of stool
culture methods as the methods for culturing stools are extremely
diverse. Specimen handling is another factor that can greatly decrease
the sensitivity of stool culture methods. In a review of non-typhoidal
salmonellosis, an assumed estimate of the sensitivity of culture was 70%
and was used to estimate the burden of salmonellosis in the United
States (2). This estimate was adopted for determining the burden of
campylobacteriosis in a recent review of foodborne disease (9).

DATA GAP 10: Incomplete knowledge of the sensitivity and specificity of
culturing specimens for *Campylobacter* exists*.*

## Invasive disease assumptions:

**ASSUMPTION** **11:** All invasive campylobacteriosis cases seek care,
have a specimen collected that yields *Campylobacter*, and is
ascertained by FoodNet. (refer to Section 1.3)

DISCUSSION 11: It is not known precisely what proportion of persons with
invasive *Campylobacter* infections seek care, but because persons with
invasive *Campylobacter* infections will be moderately to severely ill,
it is likely that most of these patients will seek care.

There is little knowledge of the completeness of ascertainment of
invasive campylobacteriosis; the frequency with which laboratories are
requested to test blood, CSF or other sterile specimens for
*Campylobacter* and the sensitivity and specificity of the diagnostic
tests used for isolation from blood and other sterile sites. Blood
cultures usually represent more than 99% of all invasive isolations and
most currently used blood culture systems are good for isolating
*Campylobacter,* when it is present. The lack of information on the
frequency of diagnostic requests and sensitivity may result in an
underestimate of actual invasive disease rates. However, because the
currently ascertained proportion of invasive cases is very small,
approximately 1.0% of all confirmed cases, and most cases are likely to
seek care, an increase in isolation of specimens classified as invasive
is unlikely to have much impact on the overall number of cases of
campylobacteriosis in the U.S.

DATA GAP 11: Data describing rates or cases of invasive disease seeking
care, requests for diagnostic tests and the sensitivity of diagnostic
procedures, such as blood culture, are not available.

**ASSUMPTION** **12:** The proportion of fluoroquinolone prescriptions
of total antibiotic prescriptions is the same for patients with invasive
campylobacteriosis treated by their health care providers as it is for
patients with enteric campylobacteriosis treated by their health care
providers. (refer to Section 3.6)

**ASSUMPTION** **13:** Because of the severity of illness upon
presentation, all cases with invasive disease are presumed to seek care
and are presumed to take antibiotics for their illness. (refer to
Section 3.5)

[^1]: In two surveys encompassing 474 human isolates from 1982 to 1992
    in the United States, only a single Ciprofloxacin resistant isolate
    was identified and subsequently speciated as *C. lari* (70).

[^2]: After removal of persons who had traveled within 7 days of illness
    onset and removal of those taking fluoroquinolones prior to culture,
    quinolone resistance in Minnesota was observed in 0.8% of isolates
    in 1996 and had increased to 3.0% in 1998 (chi square for linear
    trend, 9.8; p[\<]{.underline}0.002) (71). In Minnesota quinolone
    resistance, screened by nalidixic acid disc diffusion was highly
    correlated with resistance to ciprofloxacin using the E-Test,
    (sensitivity 99.6%, specificity 98.4%) (71). A survey of
    *Campylobacter* isolated from 88% of 91 chicken products resulted in
    *C. jejuni* from 67(74%) and *C. coli* from 19 (21%) of samples and
    six samples were the source of both pathogens. Products carrying
    resistant isolates were purchased from 11 stores representing 8
    franchises and originated in seven processing plants in five states
    (70, 71) indicating widespread resistance in chicken campylobacter
    isolates. Molecular subtyping was performed using PCR restriction
    endonuclease length polymorphism typing of the flagellin gene in the
    C. jejuni human and chicken product isolates. 12 subtypes were
    identified from 13 *C. jejuni* positive chicken products. Six of
    seven resistant subtypes in the chicken products were also
    identified in the quinolone resistant human isolates. For people
    acquiring infections during 1997, excluding cases that had taken
    fluoroquinolones prior to culture, persons with non-traveler
    resistant infections were more likely to have C. jejuni subtype also
    found in the quinolone resistant C. jejuni from chicken products
    (odds ratio 15.0, 5^th^ and 95^th^ percentile 1.9 to 321.8) (70).