United States Office of Science and Technology EPA-822-R-99-001

Environmental Protection Office of Water November 1999

Agency Washington, DC 20460 www.epa.gov

Legionella:

Human Health

Criteria Document
ACKNOWLEDGMENTS

This document was prepared for the U. S. Environmental Protection Agency, Office of Ground Water

and Drinking Water (OGWDW) by the Office of Science and Technology (OST) under contract with ICF 

Consulting Group (Contract No. 68-C6-0029). Overall planning and management for the preparation of this

document was provided by Latisha S. Parker, MS of OST.

EPA acknowledges the valuable contributions of those who reviewed this document. They include Lisa

Almodovar MPH, Robin Oshiro, MS and Stephen Schaub, Ph.D. of the U.S. EPA. EPA also recognizess the

following external peer reviewers for their excellent review and valuable comments on the draft document:

Janette E. Stout Ph.D. and Paul H. Eldelstein M.D. 
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TABLE OF CONTENTS

I. Summary ....................... ....................... ....................... ....... I-1

II. General Information and Properties ....................... ....................... ......... II-1

A. History ....................... ....................... ....................... . . II-1

B. Taxonomy ....................... ....................... ....................... II-2

C. Microbiology, Morphology, and Ecology ....................... ..................... II-6

D. Symbiosis in Microorganisms ....................... ....................... ....... II-7

III. Occurrence ....................... ....................... ....................... ... III-1

A. Worldwide Distribution ....................... ....................... ........... III-1

B. Occurrence in Water ....................... ....................... .............. III-9

1. Natural Surface Water ....................... ....................... ....... III-9

2. Groundwater ....................... ....................... ............. III-10

3. Man-Made Waters ....................... ....................... ........ III-11

C. Occurrence in Soil ....................... ....................... .............. III-19

D. Occurrence in Air ....................... ....................... ............... III-20

E. Specific Disease Outbreaks ....................... ....................... ....... III-21

1. Nosocomial Outbreaks ....................... ....................... ..... III-22

2. Outbreaks Among Travelers ....................... ....................... . III-22

3. Community Outbreaks ....................... ....................... ..... III-32

F. Environmental Factors Affecting Legionella Survival ....................... .......... III-32

1. Symbiotic Microorganisms ....................... ....................... . . III-32

2. Water Temperature ....................... ....................... ........ III-35

3. Other Factors ....................... ....................... ............. III-36

G. Summary ....................... ....................... ..................... III-37

IV. Health Effects in Animals ....................... ....................... ............... IV-1

A. Laboratory Studies ....................... ....................... ............... IV-1

B. Summary ....................... ....................... ...................... IV-4

V. Health Effects in Humans ....................... ....................... ............... V-1

A. Symptoms and Clinical Manifestations ....................... ...................... V-1

B. Clinical Laboratory Findings ....................... ....................... ....... V-5

C. Mechanism of Action ....................... ....................... ............. V-6

D. Immunity ....................... ....................... ...................... V-9

E. Chronic Conditions ....................... ....................... .............. V-11

F. Treatment ....................... ....................... ..................... V-11

G. Summary ....................... ....................... ..................... V-14

VI. Risk Assessment ....................... ....................... ...................... VI-1

A. Hazard Identification ....................... ....................... ............. VI-1

B. Dose-Response Information ....................... ....................... ........ VI-2

C. Potential for Human Exposure to Legionella ....................... .................. VI-3

1. Prevalence of Legionella in the Environment ....................... ............ VI-3

2. Mode of Transmission to Humans ....................... .................... VI-3
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D. Risk Factors ....................... ....................... .................... VI-6

E. Quantification of Potential Health Effects ....................... .................... VI-9

F. Minimizing Risk ....................... ....................... ................ VI-10

G. Summary ....................... ....................... ..................... VI-12

VII. Analysis and Treatment ....................... ....................... ............... VII-1

A. Analysis of Samples ....................... ....................... ............. VII-1

1. Collection of Legionella ....................... ....................... .... VII-1

2. Detection of Legionella in Environmental and Biological Samples ................. VII-3

B. Disinfection as a Water Treatment Practice ....................... .................. VII-8

C. Summary ....................... ....................... .................... VII-13

VIII. Research Requirements ....................... ....................... .............. VIII-1

IX. References ....................... ....................... ....................... .... IX-1
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LIST OF FIGURES

Figure III-1. Summary of Reported Cases of Legionellosis in the 

United States, 1984-1996 ....................... ....................... ....... III-4

LIST OF TABLES

Table II-1. Approved Legionella Species ....................... ....................... .... II-3

Table III-1. Summary of Reported Cases of Legionellosis in the

United States, 1984-1996 ....................... ....................... ....... III-3

Table III-2. Legionnaires’ Disease in 24 European Countries in 1996 ....................... ..... III-7

Table III-3. Occurrence of Legionella Bacteria in Potable Water Supplies and 

Distribution Systems ....................... ....................... ......... III-12

Table III-4. Occurrence of Legionella Bacteria in Cooling Towers ....................... ...... III-16

Table III-5. Occurrence of Legionellosis Outbreaks ....................... .................. III-23

Table V-1. Frequency of Symptoms of Legionnaires’ Disease ....................... .......... V-3

Table V-2. Extrapulmonary Sites of Legionella Infection ....................... .............. V-4

Table V-3. Common Clinical Laboratory Findings in Patients with 

Legionnaires’ Disease ....................... ....................... ......... V-5

Table V-4. Recommendations for Antibiotic Treatment of Legionnaires’ Disease ................. V-13
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I-1

I. Summary

This document was prepared to update information in the Environmental Protection Agency’s (EPA)

Drinking Water Criteria Document on Legionella (EPA 1985) and is intended to serve as an addendum to that

report. Where appropriate, a summary of relevant information from the 1985 document is presented in each

chapter of this addendum. For a more detailed description of information published before 1986, please refer to

the 1985 document. This chapter presents a summary of the information contained in Chapters II through VII. 

Chapter VIII contains a discussion of research recommendations and Chapter IX lists references.

Legionella bacteria are aerobic gram-negative rods associated with respiratory infections. Legionella

pneumophila was first recognized as a disease entity from a pneumonia outbreak at a 1976 Convention of the

American Legion in Philadelphia. Of the 42 known species of Legionella, 18 have been linked to pneumonia

infections in humans. The species L. pneumophila (particularly serogroups 1-6) has been accepted as the

principal cause of human outbreaks of legionellosis, which includes both legionnaires’ disease and Pontiac

fever. 

Legionella are ubiquitous in natural aquatic environments, capable of existing in waters with varied

temperatures, pH levels, and nutrient and oxygen contents. They can be found in groundwater as well as fresh

and marine surface waters. Their widespread survival in nature can be attributed to their relationships with

other microorganisms in the environment. Symbiotic existence with algae and other bacteria, particularly in

biofilms, increases the availability of nutrients. They also are able to infect protozoans and subsequently

reproduce within these organisms. These relationships provide protection against adverse environmental

conditions, including standard water disinfection techniques. Consequently, Legionella are also prevalent in

anthropogenic waters such as potable water, cooling tower reservoirs, and whirlpools.

Aerosol-generating systems such as faucets, showerheads, cooling towers, and nebulizers aid in the

transmission of Legionella from water to air. Human inhalation of contaminated aerosols leads to Legionella

infections and disease outbreaks. Historically, many of the reported outbreaks were nosocomial (i.e., hospitalacquired), resulting from the adulteration of hospital potable water supplies, air conditioning systems, or cooling

towers. Due to increased awareness of the disease, numerous community-acquired and travel-acquired

outbreaks are now reported each year as well. However, most cases of legionnaires’ disease are sporadic (i.e.,

non-outbreak related) and are acquired in the community.
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Collection of Legionella from natural environmental samples, anthropogenic sources such as plumbing

fixtures and potable water systems, and biological specimens is generally done by taking swab samples. These

samples are typically concentrated by filtration, treated with an acid buffer, and isolated on a BCYE agar culture

medium. An array of serological tests then are used to detect the bacteria. The most commonly used tests are

direct and indirect immunofluorescence assays; however, new techniques are consistently being developed and

improved upon as well.

The health effects of Legionella contamination have been studied in animals as well as in humans. 

Experimental studies on guinea pigs and other animals have been conducted to better understand human

infection by Legionella, even though animals are not naturally infected by the bacteria. Infected animals hosting

Legionella in their lungs experience impaired respiratory system performance as a result of the disease process. 

Clinical features include hypoxia, fever, seroconversion, and weight loss. 

Legionella infection in humans occurs when bacteria are inhaled or aspirated into the lower respiratory

tract and subsequently engulfed by enteric pulmonary macrophages. The bacteria rapidly reproduce within the

macrophages and are eventually released when the host cell lyses. Recent research indicates that the ability of

Legionella to infect certain strains of amoeba is a factor in their infection of human lung tissue, as the amoeba

provides a habitat within the pulmonary system in which the bacteria can live and reproduce. Resistance to

Legionella infection is mainly cell-mediated, although humoral immune responses may also play a role. 

Legionellosis in humans has typically been characterized as either an acute self-limiting, non-pneumonic

condition known as Pontiac fever or a potentially fatal pneumonic condition known as legionnaires’ disease. 

Timely treatment of legionnaires’ disease is extremely important for a patient’s recovery. Although

erythromycin has historically been used to treat patients with legionnaires’ disease, newer macrolides and

quinolones are gaining acceptance as the first choice for treatment.

In terms of risk assessment, it is important to realize that the most prevalent source of Legionella

transmission is potable water from large buildings, particularly hospitals. Thus, although Legionella are widely

distributed in both natural and man-made water systems, transmission to humans from a water source results

mainly from inhalation or aspiration of aerosolized contaminated potable water. Potential risks caused by

Legionella in water supplies are not quantifiable by the measures of modern science. However, preventative

and corrective actions have been discovered and implemented to protect the population, especially highly

susceptible individuals (e.g., immunosuppressed people, certain hospital patients). The most effective measures

of treatment have proven to be a combination of systemic sanitization of entire water systems (e.g., thermal
I-3

disinfection, hyperchlorination, copper-silver ionization) and focal disinfection of specific portions of those

systems (e.g., UV light sterilization, instantaneous heating systems, ozonation). These treatment procedures are

very useful in preventing the recolonization of Legionella in most water distribution systems. 

Fresh, innovative methods of detection and treatment of Legionella in water supplies and sources are

consistently being uncovered and tested. In addition, new medications are being developed to treat patients

overcome with legionnaires’ disease. Substantial advancements have been made since the 1985 report, and

modern science presses on with goals of further understanding Legionella and legionnaires’ disease and the

eventual eradication of Legionella colonies in water distribution systems.

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II-1

II. General Information and Properties

A. History

In January 1977, Joseph McDade of the U.S. Centers for Disease Control (CDC) discovered a novel

bacteria while investigating the unexplained pneumonia outbreak at the 1976 American Legion Convention in

Philadelphia (Brenner 1987). Of those attending the convention, 221 became ill with pneumonia, and 34 of

those affected died. The aerobic gram-negative bacteria isolated from infected post-mortem lung tissue and

identified as the causative agent of this pneumonia outbreak was later called Legionella pneumophila, receiving

the name Legionella to honor the stricken American legionnaires and pneumophila from the Greek word

meaning “lung-loving” (Fang et al. 1989). 

The symptoms exhibited in the 1976 outbreak were termed Legionnaires’ disease. Humans can be

affected by Legionella bacteria in two ways: (1) a potentially fatal multi-system disease involving pneumonia

(legionnaires’ disease) and (2) a self-limited influenza-like infection (Pontiac fever) (Hoge and Brieman 1991). 

Pneumonia occurs in approximately 95 percent of Legionella infections (Nguyen et al. 1991).

Subsequent to finding L. pneumophila, additional investigations ensued to determine whether prior

undetected outbreaks had occurred. Research revealed five additional outbreaks of legionellosis (i.e., diseases

caused by Legionella), which were attributed to L. pneumophila. The first occurred in 1965 at St. Elizabeth’s

Hospital in Washington, D.C. Eighty-one patients became ill with pneumonia, and 14 died (Lowry et al. 1993). 

The second pneumonia outbreak occurred in 1973 in Benidorm, Spain, and the third occurred in 1974 in the

same hotel as the Philadelphia outbreak of 1976. In addition, two outbreaks of Pontiac fever occurred, one in

Pontiac, Michigan, in 1968 and the other in 1973 in James River, Virginia. Aside from outbreaks, sporadic

cases of legionellosis were detected in 1943, 1947, and 1959 (Brenner 1987).

Within two years of identifying L. pneumophila, the second species of Legionella, L. micdadei, was

discovered (Dowling et al. 1992). In the following years, advances in growth and enrichment media, combined

with clinical and environmental studies, allowed for the discovery of numerous species of Legionella (Brenner

1987).

B. Taxonomy
II-2

DNA-DNA hybridization studies, as well as unique cellular fatty acid composition, indicated that the

bacteria causing the pneumonia outbreak of 1976 should be classified as a new species. At the First

International Symposium on Legionnaires’ Disease, held in 1978, the bacteria received the name Legionella

pneumophila and became apart of the new family Legionellaceae (Bangsborg 1997, Brenner 1986). 

The 1985 Legionella Criteria Document discusses the taxonomic approaches and diagnostic techniques

used to classify Legionella species. Molecular techniques used include DNA hybridization, genomic DNA size

comparison using (Guanine+Cytosine) content, oligonucleotide cataloguing of 16s rRNA, and plasmid analysis

(EPA 1985). Comparison of bacterial DNA and the use of antigenic analysis of proteins and peptides are the

best current methods to classify Legionella species, although some phenotypic characteristics (i.e., gram

reactivity, cell membrane fatty acid and ubiquinone content, morphology, and growth on specific media) can be

used to recognize bacteria at the genus level (Bangsborg 1997, Fang et al. 1989, Winn 1988 ). 

Following the initial identification of L. pneumophila in 1977, numerous species have been discovered

within the Legionella genus. The 1985 Legionella Criteria Document listed 22 species within the genus. 

Currently, the genus consists of 42 species, seven of which can be further divided into serogroups (Bangsborg

1997). The bacterial strains within a species that can be divided by serotype are genetically homologous (based

on DNA hybridization experiments), but can be differentiated by specific reactivity to antibodies (EPA 1985).

Eighteen of the 42 species of Legionella have been linked to patients with pneumonia (Bangsborg 1997). A

majority of human infections (70-90%) have been caused by L. pneumophila, especially serogroups 1 and 6 (Lo

Presti et al. 1997). Table II-1 is a compilation of species information.

Table II-1. Approved Legionella Species

Name Implicated in Human Disease?

L. adela idensis No

L. anisa Yes

L. birmin gham ensis Yes

L. bozem anii * (Fluorib acter bo zemana e) SG 1 -2 Yes

L. brun ensis No

L. cherrii Yes

L. cincin natiensis Yes

L. dumoffii* (Fluorib acter dum offii) Yes
Name Implicated in Human Disease?

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L. erythra No

L. fairfielde nsis No

L. feelei* SG 1-2 Yes**

L. geestiana No

L. gorm anii* (Fluorib acter gormanii) Yes

L. gratiana Yes

L. hackeliae* SG 1 -2 Yes

L. israelen sis No

L. jame stownie nsis No

L. jorda nis Yes

L. lansin gensis Yes

L. londiniensis SG 1-2 No

L. longbeachae SG 1-2 Yes

L. lytica* Yes

L. mac eache rnii (Tatloc kia mace achernii) Yes

L. micd adei* (Tatloc kia micd adei) Yes

L. moravica No

L. nautarum No

L. oakridgensis* Yes

L. parisie nsis Yes 

(Lo Presti et al. 1997)

L. pneumophila* SG 1-16 Yes

L. qua teirensis No

L. quinlivanii SG 1-16 Yes

L. rubrilucens No

L. sainthelensi SG 1-2 Yes

L. santicr ucis Yes

L. shakespearei No

L. spiritensis No

L. steigerw altii No

L. tucson ensis Yes

L. wad sworthii Yes
Name Implicated in Human Disease?

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L. waltersii No

L. worsle iensis No

Source: Bangsborg 1997, unless otherwise noted

SG = serogroup

* = species with experimentally documented ability to parasitize amoeba

** = causes Pontiac fever, but rarely pneumonia (Lo Presti et al. 1998)

An ongoing controversy about the taxonomy of the Legionellaceae family involves the single genus

designation. Because several species have a unique phenotypic characteristic (blue white fluorescence in UV

light) and very low DNA-DNA hybridization homology to L. pneumophila, two additional genera, Tatlockia

and Fluoribacter, have been proposed by Garrity et al. and Brown et al. (see Table II-1 for the accepted and

proposed names) (Bangsborg 1997). The new genera have not been accepted by the mainstream scientific

community, but Bangsborg (1997) suggested that the classifications may be justified. 

The method for determining whether two organisms are of the same genus and/or species is based on

DNA-DNA hybridization studies of Enterobacteriaceae (Bangsborg 1997). Members of the same species are

indicated by 70 percent or greater homology under optimal reaction conditions or 60 percent homology under

stringent conditions; 25-60 percent homology indicates genus member status. The Legionella species in the

proposed Tatlockia and Fluoribacter genuses share less than 25 percent DNA sequence homology with L.

pneumophila, suggesting the need for new genera. However, many argue that the use of DNA-DNA

hybridization is not an effective method to distinguish between genera, since the technology is most accurate for

organisms more closely related. Furthermore, the species in dispute exhibit phenotypic characteristics present in

the Legionella species. Finally, infection with these species results in the same human disease and is treatable

with the same antibiotics as all other Legionella species (Bangsborg 1997). 

Bangsborg (1997) examined the multi-genus argument by using crossed immuno-electrophoresis of

proteins from Legionella species. Three rabbit antibody preparations, one against L. pneumophila, a second

against L. micdadei, and a third against L. bozemanii, L. dumoffii, and L. gormanii were used on the

electrophoresed proteins. Findings based on the antigenic profiles suggest that creating the Tacklockia and

Fluoribacter genera is warranted. Further taxonomic investigation is necessary to clarify this debate.
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Finally, identification of species isolates is another highly important taxonomic area of study, since

determining sources of outbreaks is essential to public safety. Molecular methods have been used to identify

individual isolates and will be discussed in Chapter VII, Analysis and Treatment of Legionella. 

C. Microbiology, Morphology, and Ecology

All Legionella species appear as small rods, faintly staining gram-negative. They are unencapsulated,

nonsporeforming, with physical dimensions from 0.3 to 0.9 m in width and from 2 to 20 m in length (Winn

1988). Most exhibit motility through one or more polar or lateral flagella. Legionella cell walls are unique

from other gram-negative bacteria in that they contain significant amounts of both branched-chain cellular fatty

acids and ubiquinones with side chains of more than 10 isoprene units. These bacteria are aerobic,

microaerophillic, and have a respirative metabolism that is non-fermentative and is based on the catabolism of

amino acids for energy and carbon sources (Brenner et al. 1984). 

Ubiquitously found in nature, Legionella species exist primarily in aquatic environments, although some

have been isolated in potting soils and moist soil samples (Fields 1996). Legionella can survive in varied water

conditions, in temperatures of 0-63 oC, a pH range of 5.0-8.5, and a dissolved oxygen concentration in water of

0.2-15 ppm (Nguyen et al. 1991). 

Even though Legionella are ubiquitous in nature, they have specific growth requirements for culturing. 

The 1985 EPA Legionella Criteria document provides a detailed explanation of the process of determining

appropriate growth media to sustain Legionella bacterial growth. A typical media used to grow Legionella is

charcoal yeast extract (BCYE) agar supplemented with -ketoglutarate, L-cysteine, iron salts, and buffered to

pH 6.9 (EPA 1985). Bangsborg (1997) also provides information about Legionella growth mediums. The

BCYE agar can be further supplemented with antibacterial agents to suppress microflora (cefamandole and

vancomycin to inhibit gram-positive bacteria and polymyxin B to inhibit gram-negative bacteria), antifungal

agents (anisomycin for yeast), and inhibitors (glycine) (Nguyen et al. 1991). However, some antibiotics can be

detrimental to Legionella growth. For example, cefamandole can inhibit L. micdadei and several strains of L.

pneumophila (Winn 1993). In addition, pretreatment of respiratory tract specimens with acid before culturing

can be very useful in selecting for Legionella, since these bacteria exhibit acid resistance, unlike most other

bacteria (Nguyen et al. 1991). Optimal temperatures for culturing are 35-37oC (EPA 1985). Bacterial growth
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can be enhanced in a culturing environment with a CO2

 concentration from 2.5- 5 percent, but not in excess of

8-10 percent, which can be inhibitory (EPA 1985, Winn 1993).

D. Symbiosis in Microorganisms

Experiments have demonstrated that Legionella in sterile tap water show long-term survival but do not

multiply, whereas Legionella in non-sterile tap water survive and multiply (Surman et al. 1994). Furthermore,

Legionella viability is maintained when they are combined with algae in culture, whereas Legionella viability

decreases once the algae are removed (Winn 1988). Legionella proliferation is dependent on their relationships

with other microorganisms.

The first evidence that Legionella share a symbiotic relationship with other microorganisms came with

the discovery of L. pneumophila’s co-existence in an algal mat from a thermally polluted lake (EPA 1985). In

contrast, Legionella survive almost entirely as parasites of single-celled protozoa (Fields 1996). This

relationship first became apparent to Rowbotham in 1980, with the demonstration of L. pneumophila’s ability

to infect two types of amoeba, Acanthamoeba and Naegleria (EPA 1985). Currently, Legionella can infect a

total of 13 species of amoebae and two species of ciliated protozoa (Fields 1996). Table II-1 indicates species

of Legionella that have been shown experimentally to infect amoeba.

Legionella also can multiply intra-cellularly within protozoan hosts (Vandenesch et al. 1990). 

Legionella strains that multiply in protozoa have been shown to be more virulent, possibly due to increased

bacterial numbers (Kramer and Ford 1994). The ability to infect and proliferate within hosts provides

Legionella with protection from otherwise harmful environmental conditions. Therefore, they survive in

habitats with a greater temperature range, are more resistant to water treatment with chlorine, biocides and other

disinfectants, and survive in dry conditions if encapsulated in cysts. Enhanced resistance to water treatment has

major implications for disease transmittance and water treatment procedures.

Legionella also grow symbiotically with aquatic bacteria attached to the surface of biofilms (Kramer and

Ford 1994). Biofilms provide the bacteria with protection from adverse environmental conditions (including

during water disinfection) and nutrients for growth. The concentration of Legionella in biofilms depends upon

water temperature; at higher temperatures, they can more effectively out compete other bacteria. Legionella

have been found in biofilms in the absence of amoeba (Kramer and Ford 1994). Because biofilms colonize
II-7

drinking water distribution systems, they provide a habitat suitable for Legionella growth in potable water,

which can lead to human exposure.
III-1

III. Occurrence

Because routine culturing for Legionella in the environment is not a common practice, the occurrence of

these bacteria is often indicated by outbreaks or sporadic cases of legionellosis (i.e., any disease caused by

Legionella). Therefore, this chapter considers the worldwide occurrence or incidence of legionellosis (Section

A) and outbreaks of legionellosis (Section E) as well as the occurrence of Legionella bacteria in water (Sections

B), soil (Section C), and air (Section D). Environmental factors influencing Legionella survival are discussed in

Section F.

A. Worldwide Distribution

Legionellosis has been reported to occur in North and South America, Asia, Australia, New Zealand,

Europe, and Africa (Edelstein 1988). The true incidence of legionellosis is difficult to determine because

identification of cases requires adequate surveillance. Research suggests that legionnaires’ disease is under

reported to national surveillance systems (Marston et al. 1994; Edelstein 1988). Its recognition depends on

physician awareness of the disease and resources available to diagnose it.

Although legionellosis is widely distributed geographically throughout the world, most cases have been

reported from the industrialized countries. The ecological niches that support Legionella (complex recirculating

water systems and hot water at 35-55°C) are not as common in developing countries, so the incidence of

legionellosis may be comparatively low in these countries (Bhopal 1993). However, most geographical

variation in the incidence of legionellosis is probably artifact due to differences in definitions, diagnostic

methods, surveillance systems, and data presentation (Bhopal 1993).

The 1985 Legionella Criteria Document focused mainly on the distribution of legionellosis in the United

States because, at that time, national surveillance data for the United States were available from the Centers for

Disease Control (CDC), whereas surveillance programs in many other countries had not yet been developed. 

Surveillance in England and Wales began in 1979, but these data were not included in the 1985 report. Since

1985, many European countries as well as Australia and New Zealand have implemented surveillance programs

to monitor the occurrence of legionellosis. Recent findings of national surveillance programs are summarized

below. 
III-2

United States

The CDC first began collecting data on the occurrence of legionellosis in 1976. The 1985 Legionella

Criteria Document provides a detailed summary of the occurrence and distribution of legionellosis in the United

States through 1983. Data regarding the occurrence of legionellosis in the United States reported to CDC from

1984-1996 are summarized in Table III-1 and in Figure III-1. In the United States, the number of cases per

million population rose from 3.5 in 1984 to a peak of 6.3 in 1994 and then began to decline to 4.7 in 1996.

An analysis of data reported to the CDC during the period 1980-1989 examined 3,524 confirmed cases

of legionnaires’ disease in the United States (Marston et al. 1994). Disease rates did not vary by year, but rates

were higher in northern states and during the summer. L. pneumophila, serogroup 1, constituted 71.5 percent of

the fully identified isolates of Legionella. Risk factors for morbidity and/or mortality included older age, male

gender, African-American ethnicity, smoking, nosocomial acquisition of the disease, immunosuppression, endstage renal disease, and cancer (see Chapter VII, Section D for further discussion of risk factors).

Marston et al. (1994) also concluded that legionnaires’ disease is under reported to the CDC. They cite

two studies in which diagnostic tests for legionellosis were routinely performed; Legionella infections

accounted for 3.4 and 4.6 percent of community-acquired pneumonia cases requiring hospitalization. By

projecting this proportion to the estimated total number of community-acquired pneumonia cases in the United

States annually (500,000 cases), they estimate that there would be 17,000-23,000 cases of legionnaires’ disease

leading to hospitalization annually. However, fewer than 500 cases of legionnaires’ disease are reported to the

CDC annually; therefore, the surveillance system detects fewer than 5 percent of Legionella pneumonia cases in

the United States.

Table III-1. Summary of Reported Cases of Legionellosis in the United States, 1984-1996

Year Number

of Cases

Cases per Million

Population

1984 750 3.5

1985 830 3.7

1986 980 4.3

1987 1,038 4.3
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1988 1,085 4.4

1989 1,190 4.8

1990 1,370 5.5

1991 1,317 5.3

1992 1,339 5.3

1993 1,280 5.0

1994 1,615 6.3

1995 1,241 4.8

1996 1,198 4.7

Sources: CDC 1994, CDC 1996, CDC 1997b

Figure III-1. Summary of Reported Cases of Legionellosis in the United States, 1984-1996

1984 3.5

1985 3.7

1986 4.3

1987 4.3

1988 4.4

1989 4.8

1990 5.5

1991 5.3

1992 5.3

1993 5

1994 6.3

1995 4.8

1996 4.7

Sources: CDC 1994, CDC 1996, CDC 1997b

United Kingdom
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Legionnaires’ disease is not a statutorily notifiable disease in England and Wales; therefore, cases are

reported on a voluntary basis. The National Surveillance Scheme for Legionnaires’ Disease for residents of

England and Wales was set up in 1979 by the Public Health Laboratory Service (PHLS) Communicable Disease

Surveillance Centre (CDSC), and data have been collected each year since. In addition, the PHLS CDSC

obtains information about cases of legionnaires’ disease in residents of England and Wales that are the result of

travel, either abroad or in the United Kingdom, from the European Surveillance Scheme for Travel-Associated

Legionnaires’ Disease, which was established in 1987. Data on the occurrence of legionnaires’ disease in

residents of England and Wales in 1996 were reported in the Communicable Disease Report (Joseph et al. 1997)

and are summarized below. 

In 1996, 201 cases of legionnaires’ disease were reported to the PHLS CDSC (Joseph et al. 1997). The

number of cases associated with various sources of infection were: 101 (50%) cases resulting from travel, either

abroad or in the United Kingdom; two (1%) hospital-acquired cases; and 98 (49%) community-acquired cases. 

The number of cases linked to outbreaks or clusters was 55 (27%), and the remaining 146 cases (73%) were

reported as single cases. Six outbreaks were associated with industrial sites, and nine outbreaks or clusters were

associated with travel.

A total of 3,005 cases of legionnaires’ disease in residents of England and Wales were reported during

the period 1980-1996 (Joseph et al. 1997). Overall, travel and community cases each accounted for 46 percent,

and hospital-acquired infections accounted for the remaining 8 percent. The annual totals of reported cases

fell between 1989 and 1991, following a peak of 279 cases reported in 1988. Since 1993, the annual totals have

been increasing; there was a sharp increase in the number of cases of legionnaires’ disease reported in 1996 (201

cases) compared to 160 in 1995. The 201 cases reported in 1996 was the highest total recorded since 1989. 

Cases associated with travel abroad accounted for the second highest number of travel cases reported since

1980, and community-acquired cases the largest since 1989. In contrast, the number of hospital-acquired cases

was lower in 1996 than in any of the previous years. 

Legionnaires’ disease has been a notifiable disease in Scotland since 1988 (Joseph et al. 1997); data are

reported to the Scottish Centre for Infection and Environmental Health (SCIEH). The most recent data available

are for 1996, which were summarized in the Communicable Disease Report (Christie 1997).

In 1996, 24 cases of legionnaires’ disease in residents of Scotland were reported to SCIEH (Christie

1997). A total of 15 cases resulted from travel, 13 from travel outside the UK and two from travel within the
III-5

UK. The travel cases were associated with three clusters and two linked groups (cases linked to the same

accommodation but who became ill more than six months apart). There were no cases of hospital-acquired

infection in 1996. Two cases may have been associated with workplace exposure (article does not specify

occupation). The remaining seven cases were presumed by the author to have been acquired in the community. 

The total number of cases reported in Scotland in 1996 (24) is 14 fewer than in 1995.

Europe

Since 1993, 24 collaborating European countries have been submitting information on cases of

legionnaires’ disease in Europe through completion of the annual reporting forms prepared by the PHLS CDSC

in London. The annual results for 1996 were reported in the Weekly Epidemiological Record (Anonymous

1997b) and are summarized below.

In 1996, 1,566 cases of legionnaires’ disease were reported in 24 European countries including England,

Wales, and Scotland (Anonymous 1997b). The number of cases as well as the rate of infection for each of the

24 countries is shown in Table III-2. Four countries reported more than 100 cases each: Spain, 430; France,

294; England and Wales, 200; and Germany (North and South-East), 181. The highest rates of infection (per

million) occurred in Germany (30.17), Croatia (16.00), Denmark (14.40), Spain (11.03), Greece (7.00), and

France (5.25). In all other countries, the rate of infection was less than 5.00 per million population.

In 1996, there were nearly 300 more cases than in 1995 and nearly 400 more cases than in 1994

(Anonymous 1997b). The increase was attributed mainly to a large community outbreak in Spain in 1996. In

addition, the average European rate of 4.45 cases per million population in 1996 reflected an increase of almost

1 case per million population from 1995.

Table III-2. Legionnaires’ Disease in 24 European Countries in 1996

Country Cases Population

(millions)

Rate per Million

Population

Austria 20 8 2.50

Belgium 16 10 1.60

Croatia 24 1.5 16.00

Czech R epublic 12 10.5 1.14
Country Cases Population

(millions)

Rate per Million

Population

III-6

Denmark 72 5 14.40

England and Wales 200 52 3.85

Finland 18 5 3.60

France 294 56 5.25

Germany (No rth and S outh-east) 181 6 30.17

Greece 7 1 7.00

Ireland 0 3.5 0.00

Italy 84 57 1.47

Malta 0 0.4 0.00

Netherlands 40 15.5 2.58

Northern Ireland 0 1.6 0.00

Norway 1 4.3 0.23

Portugal 16 10 1.60

Russian Federation (Moscow) 45 10 4.50

Scotland 24 5 4.80

Slovakia 3 5 0.60

Spain 430 39 11.03

Sweden 40 9 4.44

Switzerland 26 7 3.71

Turkey 13 30 0.43

Total 1,566 352.3 4.45

Source: Anonymous 1997b

The distribution of cases between various sources of infection were: 16 percent of cases resulting from

travel; 6 percent hospital-acquired cases; 40 percent community-acquired cases; and 38 percent of unknown

origin (Anonymous 1997b). The proportion of community-acquired cases rose from 16 percent in 1994 and 21

percent in 1995 to 40 percent in 1996 largely due to a decline in the proportion of cases from unknown origin,

which represented 55 percent in 1994, 50 percent in 1995, and 38 percent in 1996. 

In 1996, individual European countries detected 22 outbreaks: two linked to hospitals, eight to the

community, and 12 to travel (Anonymous 1997b). This distribution represents a decline in nosocomial

outbreaks and a rise in community outbreaks in comparison to 1995 data. The number of outbreaks and the
III-7

number of cases linked to outbreaks may be largely under reported because many countries are still unable to

provide any epidemiological data associated with the cases of legionnaires’ disease they diagnose. For example,

the European Surveillance Scheme for Travel-Associated Legionnaires’ Disease detected around 20 travelrelated outbreaks, whereas individual countries detected only 12 travel-related outbreaks. It also is likely that

many industrial-related community outbreaks remain undetected in countries without enhanced surveillance.

The majority of European cases (75%) reported in 1996 were caused by L. pneumophila, serogroup 1

(Anonymous 1997b). L pneumophila of other or undetermined serogroups accounted for 18 percent, and the

remaining 7 percent were attributed to other or unknown Legionella species.

Australia

The Communicable Diseases Network Australia New Zealand collects data on cases of legionellosis in

Australia and New Zealand as part of the National Notifiable Diseases Surveillance System. There have been

1,041 notifications of legionellosis in Australia since 1991, with similar numbers of cases reported each year

(Anonymous 1997a). Since 1995, 255 notifications provided species identification. The majority of cases

were caused by L. pneumophila (41%); however, at least 22 percent of legionellosis cases were attributed to L.

longbeachae (2 percent of cases attributed to other species, and 35 percent of cases not speciated). The report

suggests that these data indicate a microbiological difference in the incidence of legionellosis in Australia

because L. pneumophila has been reported as responsible for at least 90 percent of legionellosis infections in

other countries (Anonymous 1997a).

B. Occurrence in Water

The 1985 Legionella Criteria Document states that Legionella are widely distributed in the aqueous

environment in the United States and, apparently, wherever they are sought (EPA 1985). Since 1985, research

has revealed that Legionella thrive in biofilms, and interaction with other organisms in biofilms is essential for

their survival and proliferation in aquatic environments (Kramer and Ford 1994, Yu 1997, Lin et al. 1998a). 

Legionella survival is enhanced by symbiotic relationships with other microorganisms; sediment within biofilms

stimulates the growth of these commensal microflora, which stimulate the growth of Legionella (see Section F

in this chapter for further discussion of symbiotic microorganisms). This section considers the specific

occurrence of Legionella in natural water bodies (surface water and groundwater) as well as man-made waters

(e.g., potable water, cooling towers, whirlpools, etc.).
III-8

1. Natural Surface Water

Legionella are considered to be ubiquitous in the aqueous environment, although few studies examine

natural nonepidemic surface waters for their presence. The 1985 Legionella Criteria Document cited several

studies that clearly demonstrate the widespread occurrence of Legionella from natural surface freshwater

sources (e.g, lakes and streams) in the United States. At the time of the 1985 report, there was little evidence

that the marine environment is a normal habitat for Legionella although they had been isolated from estuarine

waters in Puerto Rico (EPA 1985). More recent studies indicate that Legionella are fairly common in marine

waters (Ortiz-Roque and Hazen 1987, Palmer et al. 1993).

Ortiz-Roque and Hazen (1987) investigated the occurrence of Legionella at twenty-six sampling sites in

Puerto Rico (16 marine, 8 freshwater, and 2 estuarine). L. pneumophila was the most abundant species at all

sites, with highest densities reported for sewage-contaminated coastal waters. L. pneumophila was found in

densities several orders of magnitude higher than those in corresponding natural aquatic habitats in the United

States, which the researchers attributed to the presence of higher concentrations of organic matter in the water. 

Several other species were widely distributed at all sites, including L. bozemanii, L. dumoffii, L. gormanii, L.

longbeachae, and L. micdadei. The study notes the occurrence of Legionella in water samples taken from

epiphytic rain forest plants, which further demonstrates the ubiquitous nature of these organisms in natural

surface water.

Palmer et al. (1993) studied the occurrence of Legionella in ocean water as part of an investigation of

their presence in raw and treated sewage and nearby receiving waters in California. Ocean-receiving water

located five miles offshore from where treated sewage was discharged contained Legionella; however, ocean

water between the discharge site and coastal bathing beaches was negative. The presence of Legionella at a

nearby beach swimming area was attributed to surface runoff from a flood control channel and river, which

tested positive for Legionella.

2. Groundwater

The 1985 Legionella Criteria Document reported that no studies had documented the occurrence of

Legionella in groundwater (EPA 1985). Recognizing the need for data on the occurrence of Legionella in

groundwater, the U.S. EPA and the American Water Works Association Research Foundation (AWWARF)

sponsored a study in which untreated groundwater samples from 29 public water supply system wells were
III-9

analyzed for the presence of L. pneumophila (Lieberman et al. 1994). A variety of hydrogeologic settings were

represented by the wells selected. Samples positive for L. pneumophila were collected from six (21%) of the

sampling sites. In contrast, Campo and Apraiz (1988) sampled water coming from wells in Spain that were not

subject to disinfection; of the 29 samples from eight wells, none were positive for Legionella.

3. Man-Made Waters

As noted previously, Legionella thrives in biofilms. Because bacteria in biofilms are relatively resistant

to standard water disinfection procedures, Legionella are able to enter and colonize potable water supplies

(Kramer and Ford 1994, Lin et al. 1998a). Artificial aquatic habitats (e.g., components of water distribution

systems and cooling towers) are believed to function as amplifiers or disseminators of Legionella present in

potable water (EPA 1985). The 1985 Legionella Criteria Document clearly establishes that these bacteria occur

in a variety of man-made water sources, including components of internal plumbing systems (e.g., faucets and

showerheads), cooling towers, respiratory-therapy equipment, humidifiers, and whirlpools. 

Potable Water Supplies and Distribution Systems

In 1980, British investigators first demonstrated that plumbing fixtures in potable water systems

contained Legionella (EPA 1985). The 1985 Legionella Criteria Document provides extensive evidence of

Legionella occurrence in a variety of plumbing equipment, including faucets, shower heads, hot water tanks,

and water storage tanks. Since that time, numerous studies have continued to document the occurrence of

Legionella in components of potable water distribution systems; these studies are summarized in Table III-3.

As awareness of the ecology and epidemiology of Legionella has increased, attention has shifted from

heat-exchange units, such as cooling towers, to potable water distribution systems as sources of human exposure

and infection. The 1985 Legionella Criteria Document notes the 
III-10

Table III-3. Occurrence of Legionella Bacteria in Potable Water Supplies and Distribution Systems

Setting Year Location Species (Serogrou p) References

community, hospitals,

hotels, residential

1987 Alicante, S pain L. pneu moph ila (serogroups 1,8,6) Campo and Apraiz 1988

community 1986-1987 North West England L. pneu moph ila Jones and Ashcroft 1988

community 1985-1987 England L. pneu moph ila (serogroup 1) Colbourne et al. 1988,

Colbourne and Dennis 1989

community NS NS NS Hsu 1986

community NS Columbus, Oh io L. pneu moph ila (serogroup 1) Voss et al. 1985

community NS Columbus, Oh io L. pneu moph ila (serogroup 1) Voss et al. 1986

community NS Adelaid e, Australia propo sed nam e: L. waltersii Benson et al. 1996

community NS Pittsburg h, Penns ylvania L. pneu moph ila (serogroups 1,3,4-6,12) Stout et al. 1992a

community NS U.S. Virgin Islands L. pneu moph ila (serogroups 1-6)

L. micdadei

L. gorm anii

Broadhead et al. 1988

hospitals 1994-1995 Alleghen y County, P ennsylvan ia L. pneu moph ila (serogroups 1,3,5) Goetz et al. 1998

hospital 1993-1994 Taiwan L. pneu moph ila (serogroup 1) Pan et al. 1996

hospital 1990-1992 England and Scotland L. pneu moph ila (serogroups 1,4 6) Liu et al. 1993

hospital 1990 Halifax, Nova Scotia, Canada L. pneu moph ila (serogroups 1,5) Bezanson et al. 1992

hospital 1986-1990 Halifax, Nova Scotia, Canada L. pneu moph ila (serogroup 1) Marrie et al. 1992

hospital 1989 Stanford University Medical

Center, C alifornia

L. dum offii Lowry et al. 1991

hospital and hotel 1985-1987 Lower Saxony, Germany L. pneu moph ila (serogroups 1-6,9,10)

L. dum offii

L. anisa

Habicht and Müller 1988

hospital 1984-1986 Brussels, Belgium L. pneu moph ila (serogroups 6,10) Ezzeddine et al. 1989

hospital 1985 London, England L. pneumophila (serogroups 1,4) Oppenheim et al. 1987

hospital 1984-1985 Dublin, Ireland L. pneu moph ila (serogroups 3,5,6) Haugh et al. 1990
Setting Year Location Species (Serogrou p) References

III-11

hospital 1984-1985 Torino , Italy L. pneu moph ila (serogroup 1) Moiraghi Ruggenini et al. 1989

hospitals 1983 Canada L. pneu moph ila (serogroups 1,3)

L. dum offii

Tobin et al. 1986

hospital 1982-1983 NS L. pneu moph ila Stout et al. 1985b

hospital 1982 France L. anisa Bornstein et al. 1985

hospital 1981 Pittsburg h, Penns ylvania L. pneu moph ila (serogroup 1) Stout et al. 1982

hospitals 1980-1981 Chicago , Illinois

Los An geles, Ca lifornia

L. anisa Gorman et al. 1985

hospital NS Germany L. pneu moph ila Botzenhart et al. 1986

hospital NS England L. pneu moph ila (serogroup 1) Ribeiro et al. 1987

hospital NS Duesseldorf, Germany L pneu moph ila (serogroups 1,6) Hell 1989

hospital NS Quebec, Canada L. pneu moph ila (serogroups 1-6,8)

L. longbeachae (serogroups 1,2)

L. micdadei

Alary and Joly 1992

hospitals NS Sao P aulo, Br azil L. pneu moph ila (serogroups 1,6) Pellizari and Martins 1995

hotel, residential, and

industrial

NS Bangladesh L. pneu moph ila Hossain and Hoque 1994

hotel 1986 Greece L. pneu moph ila (serogroups 1,8) Alexiou et al. 1989

laboratory 1989 Detroit, Michigan NS Paszko-Kolva et al. 1991

residential 1989-1991 Finland L. pneu moph ila Zacheus and Martikainen 1994

residential 1982-1983 Chicago , Illinois L. pneu moph ila (serogroups 1-6) Arnow et al. 1985

residential NS Germany

The Netherlands 

Austria

L. pneu moph ila Tiefenbrunner et al. 1993

residential NS South-eastern Germany L. pneu moph ila (serogroups 1,3,6,10) Lück et al. 1993

residential and

institutional

NS South Africa NS Augoustinos et al. 1995
Setting Year Location Species (Serogrou p) References

III-12

residential NS Pittsburg h, Penns ylvania L. pneu moph ila Stout et al. 1992b 

residential NS Pittsburg h, Penns ylvania L. pneu moph ila Lee et al. 1988

residential NS Vermont L. pneu moph ila Witherell et al. 1988

NS 1987-1988 New S outh W ales, Austra lia NS Hedges and Roser 1991

NS = not specified
III-13

presence of Legionella in water distribution systems of hospitals, hotels, clubs, public buildings, homes,

and factories; recent studies confirm that these systems continue to be a major source of Legionella

exposure (see Table III-3 for examples).

The 1985 Legionella Criteria Document stated that no isolations of Legionella had been reported

from the extramural components of community water distribution systems. Based on indirect evidence,

water in distribution systems was believed to be contaminated with Legionella infrequently and with low

numbers of organisms (EPA 1985). At that time, Legionella were thought to be introduced into the

distribution systems through cross connections with equipment such as cooling towers, evaporative

condensers, lawn sprinkling equipment, and hoses (EPA 1985). Since 1985, studies have shown that

Legionella are present in all segments of community water supplies, including treatment facilities

(Campo and Apraiz 1988, Colbourne and Dennis 1989, Colbourne et al. 1988, Voss et al. 1986).

Cooling Towers

The first outbreak of Pontiac fever in 1968 was later linked to the presence of Legionella in a

defective evaporative condenser in a county health department building (EPA 1985). The 1985

Legionella Criteria Document notes numerous outbreaks of legionellosis that have been linked to

cooling towers and evaporative condensers in hospitals, hotels, and public buildings, clearly establishing

these water sources as habitats for Legionella. Table III-4 summarizes more recent studies that

document the continued presence of Legionella in cooling towers and evaporative condensers.

Whirlpools and Spas

Whirlpools and spas serve as an ideal habitat for Legionella because they are maintained at

temperatures ideal for their growth (Hedges and Roser 1991). In addition, organic nutrients suitable for

bacterial growth often accumulate in these waters. Whirlpools and spas can produce 
III-14

Table III-4. Occurrence of Legionella Bacteria in Cooling Towers

Setting Year Location Species (Serogrou p) References

commercial NS NS L. pneu moph ila (serogroups 1,3,5,6,10) Kusnetsov et al. 1993

commercial NS São P aulo, Br azil L. pneu moph ila (serogroups 1,6)

L. bozem anii

Pellizari and Martins 1995

commercial NS NS NS Cappabianca et al. 1994

commercial NS San Juan, Puerto Rico L. pneu moph ila (serogroups 1-6)

L. bozem anii

L. micdadei

L. gorm anii

L. dum offii

Negron-Alvira et al. 1988

hospital 1993-1994 Taiwan L. pneu moph ila (serogroup 1) Pan et al. 1996

hospital 1985 Singapore L. pneu moph ila (serogroups 1,4) Nadarajah and Goh 1986

hotels, universities,

hospitals

1983 Canada L. pneu moph ila (serogroups 1,4,6) Tobin et al. 1986

industrial 1980-1981 Jamestown, New Y ork L. anisa Gorman et al. 1985

industrial NS Bangladesh L. pneu moph ila Hossain and Hoque 1994

sewage treatment plant NS Adelaid e, Australia informal n ame: L. genomospecies 1 Benson et al. 1996

NS 1993 Fall River, Massa chusetts L. pneu moph ila (serogroup 1) Keller et al. 1996

NS 1988-1991 Adelaid e, Australia L. pneu moph ila (serogroups 1-14)

L. anisa

L. rubrilucens

Bentham 1993

NS 1988-1991 United States NS Shelton et al. 1994

NS 1987-1988 New S outh W ales, Austra lia NS Hedges and Roser 1991

NS 1987 Singapore L. pneu moph ila (serogroups 1,5,7,8)

L. dum offii

Meers et al. 1989

NS 1983-1987 Israel NS Shuval et al. 1988

NS NS South Africa NS Grabow et al. 1991
Setting Year Location Species (Serogrou p) References

III-15

NS NS Japan L. pneu moph ila (serogroups 1,3,6) Ikedo and Yabuuchi 1986

NS = not specified
III-16

water droplets of respirable size that have the potential to transmit Legionella to humans (Jernigan

1996). The 1985 Legionella Criteria Document notes two outbreaks resulting from the presence of

Legionella in whirlpools, one involving a therapeutic whirlpool and another involving a recreational

whirlpool (EPA 1985). Recent studies document the continued presence of Legionella in whirlpools. 

Hsu et al. (1986) detected Legionella in 5 of 140 (13%) spa whirlpool samples. Hedges and Roser

(1991) tested spas in New South Wales, Australia and found that 11 of 43 (26%) contained Legionella. 

In addition, several spa filters were found to have higher Legionella counts than the water contained in

the pool, suggesting that spa filters can act as protective reservoirs or niches for Legionella. Fallon and

Rowbotham (1990) also isolated Legionella from whirlpool water and filters while investigating a large

outbreak of legionellosis at a leisure complex in Scotland. Jernigan et al. (1996) isolated Legionella

from the sand filter in a cruise ship whirlpool spa following an outbreak of legionnaires’ disease among

cruise ship passengers.

Other related sources of Legionella include spring water spas and saunas. Spring water therapy

is medicinally accepted in many European countries and often involves aerosol exposure or bathing in

certain spring waters (thermal or non-thermal). During an epidemiologic survey of spa waters in France,

15 different Legionella species were isolated, including a species that had never before been identified,

L. gratiana (Bornstein et al. 1989a, 1989b). Den Boer et al. (1998) reported a case of legionnaires’

disease linked to an air-perfused footbath at a sauna in The Netherlands that was found to be

contaminated with L. pneumophila.

Wastewater

The 1985 Legionella Criteria Document reports only a few instances of Legionella isolation from

wastewater (EPA 1985). The 1985 document notes the difficulty of isolating Legionella from

wastewater because it contains so many other microorganisms. Palmer et al. (1993) conducted an

extensive study to determine whether Legionella were present in the influent of a major metropolitan

sewage treatment plant and to determine how well the bacteria could survive the different stages of

sewage treatment. They found that Legionella were always present in all phases of the sewage treatment

process, including the secondary effluent that was discharged through an ocean outfall. They also noted

that population numbers did not significantly decline in different stages of the treatment process. 
III-17

In a later study, Palmer et al. (1995) examined tertiary treated (including chlorination) sewage

effluents that are used as reclaimed water and aerosols obtained from above a secondary sewage

treatment basin for the presence of Legionella. The bacteria were detected in samples of reclaimed

water at all four sites tested using two detection methods: polymerase chain reaction and direct

fluorescent antibody (see Chapter 7, Section A for explanation of detection methods). The researchers

noted that they were not able to culture Legionella obtained from any of the reclaimed water samples,

suggesting that chlorine may injure Legionella and cause them to enter a viable but nonculturable state. 

Legionella were detected in the air obtained from above secondary treatment (activated sludge) aeration

tanks at one site using polymerase chain reaction, direct fluorescent antibody, and plate culture.

C. Occurrence in Soil

The 1985 Legionella Criteria Document reported that Legionella had been isolated from mud and

sandy, moist soil on the edge of streams containing the bacteria. The 1985 document noted a lack of

data indicating soil is involved in the transmission of Legionella to humans although excavations and

other soil disturbances had been associated with some Legionella epidemics. At that time, Legionella

had only been from mud or moist soil (EPA 1985). More recently, one species, L. longbeachae, was

shown to inhabit and thrive in soil (Steele et al. 1990). Following an outbreak of legionellosis due to L.

longbeachae in South Australia in 1988 and 1989, Steele et al. (1990) analyzed a number of water and

soil samples to find the source of the organism. L. longbeachae was not isolated from any of the water

samples or natural soil samples; however, the bacteria was isolated from three samples of potting soil

mixes and from soil surrounding two potted plants. L. longbeachae was able to persist for seven months

in two potting mixes stored at room temperature. The researchers concluded that the isolation and

prolonged survival of L. longbeachae in potting mixes suggest that soil rather than water is the natural

habitat of this species and may be a source of human exposure.

D. Occurrence in Air

As discussed in Sections B and C of this chapter, the natural habitat for Legionella appears to be

aquatic bodies and perhaps, for L. longbeachae, soil. However, Legionella can be found in air as part of

aerosols. The 1985 Legionella Criteria Document establishes aerosolization as an important component

of Legionella transmission from the aquatic environment to the human respiratory system (see Chapter

VI, Section C.2 for further discussion of transmission to humans). At the time of the 1985 report,
III-18

aerosol-generating systems that had been linked to disease transmission included cooling towers,

evaporative condensers, plumbing equipment (e.g., faucets, showerheads, hot water tanks), humidifiers,

respiratory-therapy equipment (e.g., nebulizers), and whirlpool baths (EPA 1985). Studies published

after the 1985 report have confirmed the presence of Legionella in aerosols from several of these

systems (Bollin et al. 1985, Seidel et al. 1987).

In most cases, disease outbreaks resulting from Legionella aerosolization have involved indoor

exposure and outdoor exposure to within 200 meters. However, Addiss et al. (1989) describe an

outbreak that occurred in Wisconsin in which aerosolized L. pneumophila from an industrial cooling

tower was disseminated at least one mile (1.6 km) and perhaps up to two miles (3.2 km). 

Meteorological conditions that suppress vertical mixing and favor horizontal transport of aerosols (e.g.,

fog, high humidity, and cloud cover) occurred before and intermittently during the outbreak and

presumably contributed to the lengthy transport.

E. Specific Disease Outbreaks

Legionellosis can occur as sporadic cases or as outbreaks. The majority of cases of legionnaires’

disease are sporadic rather than outbreak related (Stout et al. 1992a). The study of outbreaks caused by

Legionella has yielded essential information about these bacteria and the illnesses they cause. Early

outbreaks illustrated the clinical course of legionnaires’ disease and Pontiac fever. Subsequently,

epidemics provided information regarding the sources of human exposure, risk factors for the

development of disease, and the efficacy of treatment options.

Legionellosis outbreaks have been attributed most frequently to exposure to contaminated

cooling towers, potable water, or components of water distribution systems. Outbreaks of legionellosis

caused by contaminated cooling towers can be explosive with numerous cases over a short period of

time (e.g., Addiss et al. 1989, Fiore et al. 1998, Gecewicz et al. 1994, O’Mahoney et al. 1990). 

Legionellosis outbreaks due to contaminated water or water distribution systems tend to be more

insidious and may only be revealed after active surveillance is introduced (e.g., Brady 1989, Colville et

al. 1993, Goetz et al. 1998, Guiget et al. 1989, Hanrahan et al. 1987, Helms et al. 1988, Le Saux et al.

1989, Meenhorst et al. 1985, Schlech et al. 1985, Struelens et al. 1992).
III-19

Establishing the source of Legionella bacteria causing a legionellosis outbreak can be

problematic due to their ubiquitous nature in the environment. Epidemiologic investigations of

outbreaks often rely on multiple molecular subtyping techniques to match clinical isolates of Legionella

with isolates from environmental samples (Johnston et al. 1987, Mamolen et al. 1993, Struelens et al.

1992, Whitney et al. 1997). Detection of Legionella in environmental and biological samples is

discussed further in Chapter VII.

Outbreaks of legionellosis typically are categorized as nosocomial (i.e., hospital-acquired),

travel-acquired, or community-acquired. Table III-5 summarizes outbreaks of legionellosis that have

been reported since 1985, including the type of outbreak, the setting in which the outbreak occurred, the

source of the outbreak, the number of individuals affected, the species implicated, and the location and

time of the outbreak. In addition, specific characteristics and features of the various types of outbreaks

are described below.

1. Nosocomial Outbreaks

Studies have linked nosocomial legionellosis to air conditioning systems and cooling towers;

however, numerous studies demonstrate the importance of hospital potable water supplies as a source of

nosocomial infections (see Table III-5 for examples). L. pneumophila has most commonly been

implicated as the causative agent in hospital-acquired legionellosis outbreaks (see Table III-5 for

examples).

2. Outbreaks Among Travelers

Travelers are usually exposed to Legionella via contaminated hotel potable water or

contaminated whirlpool spas at hotels, resorts and cruise ships (see Table III-5 for examples). Two

reported outbreaks resulted from exposure to Legionella-contaminated water in decorative fountains

(Fensterheib et al. 1988, Hlady et al. 1993). As with nosocomial legionellosis outbreaks, the most

commonly implicated species is L. pneumophila (see table III-5 for examples).

Among U.S. residents, travel-associated legionellosis outbreaks are extremely difficult to detect,

and extensive case investigations often are required. The European Surveillance Scheme for Travel

Associated Legionnaires’ Disease has greatly enhanced detection of travel-associated outbreaks in
III-20

European cities because individual cases are entered into a centralized database, which is then searched

for other cases linked to the same place of accommodation (Joseph et al. 1997).
III-21

Table III-5. Occurrence of Legionellosis Outbreaks

Setting Dates Location Source # Affected

Species

(Serogroup) References

Nosocomial

hospital 1996 Arizona hot water

distribution

system

8 L. pneu moph ila

(serogroups 6,10)

Kioski et al. 1997

hospital January-June, 1996 Ohio hot water

distribution

system

2 L. pneu moph ila

(serogroup 1)

Kioski et al. 1997

hospital July 2-12, 1995 Franklin Coun ty,

Pennsylv ania

cooling towers

and roo ftop air

samples

22 L. pneu moph ila

(serogroup 1)

Fiore et al. 1998

hospital January, 1985-

April, 1993

Innsbruck

University

Hosp ital, Austria

hot water system 14 L. pneu moph ila

(serogroup 1)

Prodinger et al. 1994

hospital March, 1992 Albany Medical

Center, New York

potable water

system used in

nasogastric tubes

2 L. pneu moph ila

(serogroup 6)

Venezia et al. 1994

hospital February-March,

1992

Providence, Rhode

Island

potable water 2 L. pneu moph ila Mermel et al. 1995

hospital March, 1983-

September, 1991

Ontario, Canada tap water, shock

absorb ers within

water pipes

13 L. pneu moph ila

(serogroup 1)

Memish et al. 1992

hospital December, 1990-

February, 1991

Varnamo, Sweden hot water s upply 31 L. pneu moph ila

(serogroup 1)

Darelid et al. 1994

hospital June-October,

1990

Glasgow Royal

Infirmary, Scotland

fire hydrants 

connec ted to

main water

supply

3 L. pneu moph ila

(serogroup 1)

Patterson et al. 1994

hospital and

community

1988-1990 São P aulo, Br azil NS 5 L. pneu moph ila

(serogroups 1)

Levin et al. 1993
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-22

hospital June-August, 1989 NS cooling towers 3 L. pneu moph ila

(serogroup 1)

Shelton et al. 1994

hospital May, 1989 Stanford

University 

Medical Center

bath water 3 L. dum offii Lowry et al. 1991

hospital July, 198 8-April,

1989

Nottingham,

England

domestic hot

water system 

12 L. pneu moph ila

(serogroup 1)

Colville et al. 1993

hospital 1984-1988 Atlanta, G eorgia nebulizer and

water system

13 L. pneu moph ila

(serogroup 3)

Mastro et al. 1991

hospital 1977-1988 Charlotte sville

Virginia

study sugge sts

potable water

16 L. micdadei Doebbeling et al. 1989

hospital October, 1985-

September, 1987

Brussels, Belgium water system 32 L. pneu moph ila

(serogroup 1)

Struelens et al. 1992

hospital September, 1985-

February, 1986

Paris, France shower su pply

and water tank

4 L. pneu moph ila

(serogroup 1)

Meletis et al. 1987

hospital OctoberDecember, 1985

Glasgow Royal

Infirmary, Scotland

cooling tower 16 L. pneu moph ila

(serogroup 1)

Winter et al. 1987

hospital December, 1984-

December, 1985

Manitoba, Canada water system,

renal transplant

unit sink

6 L. pneu moph ila

(serogroup 1)

Le Saux et al. 1989

hospital August, 1982-

December, 1985

Columbus, Oh io potable water,

showers

7 L. pneu moph ila Brady 1989

hospital January, 1983-

December, 1985

Berlin, Germany water sup ply

system

35 L. pneu moph ila Ruf et al. 1988

hospital April 16-May 16,

1985

District General

Hospital, Stafford,

England

air conditioning

unit

68 confirmed

35 suspected

L. pneu moph ila Anonymous 1985

Dennis 1991

O’Mahony et al. 1990

hospital May-September,

1984

University of Utah

School of

Medicine, Utah

cooling tower 4 L. pneu moph ila

(serogroup 1)

Johnston et al. 1987
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-23

hospital June-July, 1984 Halifax, Nova

Scotia

shower heads,

faucets, ac filter

8 L. pneu moph ila

(serogroup 1)

Martin et al. 1988

hospital 1983 NS hot water NS L. pneu moph ila Palmer 1986

hospital August, 1978-

November, 1983

Leiden U niversity

Hospital, The

Netherlands

hot pota ble

water system

21 L. pneu moph ila

(serogroups 1,10)

Meenhorst et al. 1985

hospital June 27 -August 

25, 1983

 Rhode Island water in cooling

tower 

15 L. pneu moph ila

(serogroup 1) 

Garbe 1985

hospital November, 1982-

March, 1983

Paris, France water sup ply 47 L. pneu moph ila

(serogroup 1)

Guiguet et al. 1987

hospital FebruarySeptember, 1982Upstate New Yo rk potable water,

showers, and

water system

7 confirmed

4 suspected

L. pneu moph ila

(serogroup 1)

Hanrahan et al. 1987

hospital 1981 Iowa City, Iowa hot and c old

water systems

16 L. pneu moph ila

(serogroup 1)

Helms et al. 1988

hospital 1981 Paris, France hot water system 6 L. pneu moph ila

(serogroup 1)

Neill et al. 1985

hospital NS Queb ec City distilled water 5 L. dum offii Joly et al. 1986

hospital NS NS hot water s upply

system

19 L. pneu moph ila

(serogroup 1)

L. anisa

Bornstein et al. 1986

rehabilitation

center

NS Germany potable water 11 L. pneu moph ila

(serogroup 1 )

Nechwatal et al. 1993

renal transplant

unit

June, 1989-March,

1990

São P aul, Braz il potable water

system

8 L. pneu moph ila

(serogroup 1)

Levin et al. 1991

Travel-Acquired

cruise ship July-August, 1994 Cruise ship to

Bermuda

whirlpool 14 L. pneu moph ila

(serogroup 1)

Guerrero et al. 1994

Guerrero and Filippone

1996
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-24

cruise ships April, 1994 New Y ork City to

Bermuda

whirlpool spas

and aero sols

16 confirmed

34 pro bable

L. pneu moph ila

(serogroup 1)

Jernigan et al. 1996

hotel SeptemberOctober, 1996

Mamo ra Bay,

Antiqua

solar powered

hot water system

3 L. pneu moph ila

(serogroup 1)

Joseph et al. 1997

hotel May 1996 Mino rca, Spa in hot water system 4-5 L. pneu moph ila

(serogroup 1)

Joseph et al. 1996, 1997

hotel May-August, 1995 Kusadasi, Turkey water sup ply 7 L. pneu moph ila

(serogroup 1)

Anonymous 1995a, 1995b

hotel January 6-February

2, 1992

Orlando, Florida decorative

fountain

5 L. pneu moph ila

(serogroup 1)

Hlady et al. 1993

hotel 1986-1990 Ischia Island,

Naple s, Italy

hot-water s upply 6 NS Castellani P astoris et al.

1992

hotel 1988 Santa Clara

County, C alifornia

fountain in lobby 34 L. anisa Fenstersheib et al. 1990

hotel August-September,

1987

Yugo slavia NS 15 NS Anonymous 1988

hotels 1973-1987 Northe rn Italy potable water 117 L. pneu moph ila

(serogroups 1,3,4)

Passi et al. 1990

hotel 1979-1982 U.S. Virgin Islands potable water

system

27 L. pneu moph ila

(serogroup 1)

Schlech et al. 1985

leisure complex 1988 Lochgoilhead,

Scotland

whirlpool and

filter

NS L. micdadei Fallon and Rowbotham

1990

leisure complex January-March,

1995

Northwest England whirlpoo l 8 confirmed

32 possible 

L. micdadei Newton et al. 1996

ski lodges October, 1987 Vermont water sources,

whirlpool spa

17 L. pneu moph ila

(serogroup 1)

Mamolen et al. 1993

ski resort January, 1991 Vermont hot tub 6 L. pneu moph ila Thomas et al. 1993
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-25

Community-Acquired

artesian we ll

construc tion site

October, 1990 Apulia, Ita ly groundwater 2 L. pneu moph ila Miragliotta et al. 1992

business district April 11-20, 1992 Fairfield, Sydney,

Australia

not determined 26 L. pneu moph ila

(serogroup 1)

Levy et al. 1994

coal mine 1979-1982 South Wales, UK open pit pond 3 L. pneu moph ila Davies et al. 1985

commercial

building (BBC)

April, 1988 London, England cooling systems NS L. pneu moph ila

(serogroup 1)

Dennis 1991

community September 11-

October 18, 1996

Alcala de Henares,

Spain

cooling towers,

water storage

tanks

49 confirmed

197 p ossible

L. pneu moph ila

(serogroup 1)

Anonymous 1996

community SeptemberNovember, 1991Chorley, United

Kingdom

cooling tower 11 L. pneu moph ila Peiris et al. 1992

community 1988-1990 São P aulo, Br azil NS 3 L. pneu moph ila

(serogroups 1,5)

Levin et al. 1993

community May, 1987-June,

1989

South A ustralia potting soils,

mixes

30 L. longbeachae Steele et al. 1990

community

(Piccadilly Circus)

January-F ebruary, 

1989

London, England cooling towers 33 confirmed

10 suspected

L. pneu moph ila

(serogroup 1)

Watson et al. 1994

community May 30, 1986 and

August 27-

October 27, 1986

Gloucester,

England

wet cooling

towers

15 L. pneu moph ila

(serogroup 1)

Hunt et al. 1991

community August 10-29,

1986

Sheboygan,

Wisco nsin

industrial

cooling tower

29 L. pneu moph ila

(serogroup 1)

Addiss et al. 1989

grocery store October 10-

November 13,

1989

Bogalusa,

Louisiana

mist machine,

aeroso ls

33 L. pneu moph ila

(serogroup 1)

Mahoney et al. 1992
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-26

home and butcher

shop

September 1986 Italy shower and

condensation

water

3 L. pneu moph ila

(serogroup 1)

Castellani P astoris et al.

1988

hospital July 2-12, 1995 Franklin Coun ty,

Pennsylv ania

cooling towers

and roo ftop air

samples

22 L. pneu moph ila

(serogroup 1)

Fiore et al. 1998

hot spring 1986 France spring water

system

5 L. pneu moph ila

(serogroups 1,3)

Bornstein et al. 1989a

hotel April 22-27, 1993 Sydney, A ustralia cooling towers 4 L. pneu moph ila

(serogroup 1)

Bell et al. 1996

industrial October, 1988 Lostock, England water cooling

system

57 NS Anonymous 1989

industrial estate 1996-1997 Northamptonshire

Enlgand

cooling towers 20 L. pneu moph ila

(serogroup 1)

Joseph et al. 1997

industrial foundries OctoberNovember, 1996West Midland s,

England

cooling tower 7 L. pneu moph ila

(serogroup 1)

Joseph et al. 1997

industrial plant June-August, 1994 Birmingham,

England

cooling towers 8 L. pneu moph ila

(serogroup 1)

Joseph et al. 1995

industrial p lants July, 1987 NS potable water 3 L. pneu moph ila

(serogroup 1)

Muraca et al. 1988

locker room May 15-17, 1982 Michigan whirlpool aerator 14 L. pneu moph ila

(serogroup 6)

Mangione et al. 1985

nursing home 1994 Ontario, Canada water system 10 L. sainthelensi

(serogroup 1)

Tang et al. 1995

nursing home 1994 Ontario, Canada water system 9 L. sainthelensi

(serogroup 1)

Tang et al. 1995

nursing home December, 1990 Nagasaki, Japan not determined 2 L. pneu moph ila

(serogroup 1)

Maesaki et al. 1992
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-27

office building January-Febru ary,

1990

Christchurch, New

Zealand

cooling tower 4 confirmed

3 suspected

L. pneu moph ila

(serogroup 1)

Mitchell et al. 1991

office building April, 1984 New Y ork City,

New York

cooling tower 86 L. pneu moph ila

(serogroup 1)

Friedman et al. 1987

plastics factory OctoberNovember, 1996

Trent, England unregistered

cooling tower

2 NS Joseph et al. 1997

plastics factory 1996 Wales cooling towers 4 L. pneu moph ila

(serogroup 1)

Joseph et al. 1997

plastics factory August, 1996 Yorkshire, England water from an

uncovered

outdoor tank

2 L. pneu moph ila

(serogroup 1)

Joseph er al. 1997

police HQ building October, 1985 United Kingdom air conditioning

system 

6 L. pneu moph ila

(serogroup 1)

O’Mahony et al. 1989

power station SeptemberOctober, 1981 United Kingdom small cap acity

cooling towers

3 confirmed

2 suspected

L. pneu moph ila

(serogroup 1)

Morton et al. 1986

prison August-September,

1993

Michigan cooling towers 17 L. pneu moph ila

(serogroup 1)

Gecewicz et al. 1994

recycling plant June, 1994 South England cooling tower 5 L. pneu moph ila

(serogroup 1)

Joseph et al. 1995

retail store September 29-

October 22, 1996

Southwestern

Virginia

whirlpool spa

display

23 L. pneu moph ila Hershey et al. 1997

retail store May-June, 1986 Maryland not determined 27 L. pneu moph ila

(serogroup 1)

Redd 1990

retirement home June 10-July 22,

1988

Los Angeles,

California

evaporative

condenser and

potable water 

6 L. pneu moph ila

(serogroup 1)

Breiman et al. 1990
Setting Dates Location Source # Affected

Species

(Serogroup) References

III-28

sauna December, 1992-

January, 1996

The Netherlands hot water system 6 L. pneu moph ila Den Boer et al. 1998

town building July August, 1993 Fall River,

Massa chusetts

cooling towers 11 L. pneu moph ila

(serogroup 1)

Gecewicz et al. 1994

Keller et al. 1996

town building August -October,

1993

Rhode Island cooling towers 17 L. pneu moph ila

(serogroup 1)

Gecewicz et al. 1994

Whitney et al. 1997

Unknown

NS March-April, 1993 Georgia and

Florida

NS 1 confirmed

24 suspected

NS Anonymous 1993

NS 1986-1996 Singapore cooing towers,

fountains, spa

pools

22 confirmed

236 presumed

L. pneu moph ila Heng et al. 1997

NS = not specified
III-29

3. Community Outbreaks

Cooling towers and potable water are the most common causes of community outbreaks of

legionellosis (see Table III-5 for examples). Other less common sources reported include a whirlpool

spa display at a retail store (Hershey et al. 1997) and a grocery store mist machine (Mahoney et al. 1992). 

Some outbreaks involve residential exposure (e.g., Breiman et al. 1990, Tang et al. 1995), whereas

others involve exposure at the workplace (e.g., Anonymous 1989, Dennis 1991, Joseph et al. 1997,

Joseph et al. 1995, Muraca et al. 1988). Community-acquired outbreaks have often been associated with

urban rather than rural areas (Joseph et al. 1997), which is not surprising given the increased availability

of artificial water bodies in urban areas. As with nosocomial and travel outbreaks, L. pneumophila is the

species most commonly implicated in community-acquired outbreaks. 

As noted previously, the vast majority of cases of legionnaires’ disease are community-acquired

sporadic (i.e., non-outbreak related) (Stout et al. 1992a). Straus et al. (1996) studied 146 adults

diagnosed with having nonepidemic, community-acquired legionnaires disease and the possible link to

residential potable water. Legionella was isolated from water in six percent of case patients homes (1-8

sites per home) compared to three percent of control patients homes. The researchers suggest that

transmission of Legionella from domestic water may have occurred in more instances than the study

results indicate, since sampling occurred as much as six weeks after a patient’s illness.

F. Environmental Factors Affecting Legionella Survival

1. Symbiotic Microorganisms

Legionella can only exist on artificial cultured media in very specific conditions and under

particular temperature, pH, and nutritional requirements. Nevertheless, they survive in an extremely

wide range of conditions in natural and man-made aquatic habitats. Their survival is enhanced by

symbiotic relationships with other microorganisms such as protozoa, algae, and other bacteria, which

provide them with advantages in the natural environment as well as in anthropogenic potable water

distribution systems. Legionella have the unique ability to multiply within protozoan cells, which helps
III-30

them survive over a wide temperature range and resist the effects of chlorine, biocides, and other

disinfectants.

Amoebae and Other Protozoa

Many species of Legionella can infect amoebae and other protozoa and subsequently reproduce

within these protozoans. Legionella have been found to infect and incorporate themselves into at least

13 species of amoebae including Acanthamoeba, Hartmanella, Valkampfia, and Naegleria, and two

strains of ciliates, Tetrahymena and Cyclidium (Lee and West 1991, Paszko-Kolva et al. 1993, States et

al. 1989, Kramer and Ford 1994, Henke and Seidel 1986, Fields 1996, Vandenesch et al. 1990). Further,

a study by Vandenesch et al. (1990) illustrated that L. pneumophila can infect and reproduce within the

amoeba Acanthamoeba, even when the ratio of Legionella cells to amoebae is low. Various species of

Legionella have been detected recently that are able to grow intracellularly in protozoan cells even

though they have never been capable of growth on standard Legionella media. These organisms have

been called LLAP (Legionella-like amoebal pathogens) organisms, and they have the ability to infect

and propagate in many mammalian and protozoan cells (Fields 1996). After the bacteria are

phagocytosed by amoebae, they multiply within their vesicles and remain encapsulated in the cysts until

the vesicles and/or amoeba rupture (States et al. 1989). Because Legionella replicate rapidly

intracellularly within protozoan hosts for prolonged periods of time, amoebic vesicles can contain

hundreds of Legionella cells at once (Berk et al. 1998, Lee and West 1991). In addition, replication

within protozoa can contribute to enhanced virulence of Legionella (Kramer and Ford 1994).

The fact that Legionella have the ability to infect and grow in protozoa is extremely critical to

their maintenance and survival. Not only can they multiply quickly within protozoan cells, but they also

obtain protection from disinfectants and other adverse environmental conditions. For example,

Legionella caught in encysted protozoa have demonstrated better resistance to chlorine than E. coli, a

common indicator of water quality (Paszko-Kolva et al. 1993, States et al. 1989, Kramer and Ford 1994). 

Legionella trapped in the amoeba A. polyphaga have been shielded from the effects of exposure to 50

mg/L of free chlorine (Paszko-Kolva et al. 1993, Fields 1996). Intracellularly grown Legionella are also

more resistant to biocides, chemical disinfection, and other physical stresses than Legionella grown on

cultured media. Because protozoa ingest virulent strains of L. pneumophila, they also augment growth
III-31

of the bacteria in cooling towers and other epidemic sources (Barbaree et al. 1986). In addition,

encapsulation in cysts allows Legionella to survive in the dry conditions of an aerosol for extended time

periods, thus allowing the bacteria to persist, disperse, and infect human hosts (Fields 1996). 

Algae and Other Bacteria

Certain algae such as the cyanobacterium Fischerella and the green algae Scenedesmus,

Chlorella, and Gleocystis have fostered the growth of Legionella, but only in the presence of light (Lee

and West 1991, Kramer and Ford 1994, States et al. 1987, Henke and Seidel 1986, Paszko-Kolva et al.

1993). States et al. (1987) found that the highest incidence of Legionella multiplication came from

samples gathered from zones affected by the accumulation of algal materials and leaf litter. Legionella

growth is further supplemented by their utilization of the nutrients supplied by the decomposition and

excretion of algae, as well as decaying organic matter from leaf litter (States et al. 1987).

Legionella also have formed colonies in media deficient in cysteine or iron salts, which they

require for growth. The colonies have been found around strains of common aquatic bacteria such as

Flavobacterium, Pseudomonas, Alcaligenes, and Acinetobacter, which are presumed to provide these

nutrients (Lee and West 1991, Paszko-Kolva et al. 1993, Kramer and Ford 1994, Stout et al. 1985a). 

Legionella have also been found attached to the surface of biofilms in water systems (Kramer and Ford

1994). Biofilms are encased microcolonies made up of bacterial cells and attached to a conglomerate of

polysaccharides. They trap nutrients for growth and provide a protective layer for many microbes. 

Legionella survive in these biofilms via nutritional symbiosis with other inhabiting organisms (Kramer

and Ford 1994).

2. Water Temperature

Legionella exhibit the ability to survive in an incredibly wide range of temperatures. As a lower

limit, Bentham (1993) observed growth at a water temperature of 16.5°C. The highest water

temperature of a sample cultivated by Botzenhart et al. (1986) was 64°C, while Henke and Seidel (1986)

claimed Legionella to be a “thermoresistant” organism, exhibiting survival in natural warm waters of up
III-32

to 60°C and artificially heated waters of 66.3°C. Optimum temperatures for Legionella reproduction

range from 32 to 45°C (Vickers 1987, Kramer and Ford 1994).

Nevertheless, temperature has a formidable effect on the persistence and dissemination of

Legionella in aquatic habitats. While Legionella populations seem to be controlled by extremely low

temperatures, they are enhanced by heat and elevated temperatures found in areas like whirlpools, hot

springs, and blast zones (Henke and Seidel 1986, Lee and West 1991, Verissimo et al. 1991). Colbourne

and Dennis (1989) contend that although Legionella are not thermophilic, they exhibit thermo-tolerance

at temperatures between 40 and 60°C, which gives them a survival advantage over other organisms

competing in man-made warm water systems. Although temperatures between 45 and 55°C are not

optimal for Legionella, these temperatures enable them to reach higher concentrations than other

bacteria commonly found in drinking water, thus providing Legionella with a selective advantage over

other microbes (Kramer and Ford 1994). Legionella were found in natural surface waters of Puerto Rico

in densities several orders of magnitude higher than those in corresponding natural habitats in the United

States (Ortiz-Roque and Hazen 1987) although these differences may be due to factors other than

temperature (e.g., increased nutrient availability). In contrast, the distribution and abundance of

Legionella in south-eastern Australia is comparable to the United States and Europe (Hedges and Roser

1991).

3. Other Factors

Although interaction with other microorganisms and water temperature are the most significant

and evident factors affecting Legionella growth and survival, there are a few other factors, such as

sediment and metals content, that are notable influences as well. These factors are usually amplified by

ideal water temperature or coexisting environmental microflora.

Stout et al. (1985a) tested different external influences of Legionella growth and sustenance. The

results indicated that growth of L. pneumophila declined in the absence of environmental microflora

such as algae and amoebae. The results also showed that as the amount of sediment increased, so did the

population of L. pneumophila. This was largely attributed to the fact that the scale, or mineral deposits,

and detritus, or decaying plant matter, that make up sediment, are used by Legionella organisms as a
III-33

major source of nutrients. In this study, the greatest effect on Legionella growth and survival was caused

by the presence of both sediment and other microbes. The researchers theorized that the sediment

stimulates the growth of environmental microorganisms, which prompts the growth of Legionella that

rely on their environmental by-products and availability as hosts (Stout et al. 1985a).

States et al. (1987) noted that Legionella growth was more evident at the corners and bottoms of

tanks, sedimentation basins, and reservoirs than anywhere else due to the excess sediment and scale in

those areas. It follows that total organic carbon and turbidity are also factors that motivate Legionella

growth since these influences are found in water zones rich in sediment. Vickers et al. (1987) studied

the design of water distribution systems and concluded that vertical tanks were more prone to Legionella

growth due to thicker accumulation of sediment at the bottom of the tank. Also, greater amounts of

scale and sediment in older tanks may contribute to increased growth of Legionella. Sediment is

important to Legionella growth because it provides essential nutrients, aids in the growth of other

coexisting microflora, and shelters the organism as well (Vickers et al. 1987).

Changes in water pressure and flow rates of water distribution systems may cause disruption of

the biofilm, resulting in increased concentrations of Legionella in water supplies (Kramer and Ford

1994). Mermel et al. (1995) remarked that repressurization of potable water upon completion of a

construction project may lead to increased concentration of Legionella in the water. They noted that this

phenomenon could occur in the absence of construction (i.e., any situation in which the water pressure is

changed). Straus et al. (1996) reported that recent residential plumbing repair is an independent risk

factor for community-acquired legionnaires’ disease.

Water hardness is determined primarily by the amount of calcium and magnesium in scale

deposits. Legionella have been found to flourish in areas where these metallic cations are present

(Vickers et al. 1987). Low levels of iron, zinc, and vanadium also may stimulate the growth of

Legionella (Kusnetsov 1993, States et al. 1987, Stout et al. 1992b), while higher concentrations of

metals like copper, iron, manganese, and zinc may actually be toxic (Kusnetsov 1993).

G. Summary
III-34

Cases of legionellosis have been documented throughout the world; however, the true incidence

of the disease is unknown due to inadequate surveillance. Geographical variation in the incidence of

legionellosis has been attributed to differences in definitions, diagnostic methods, surveillance systems,

and data presentation. National surveillance programs currently are conducted in the United States, 24

European countries (including England), and Australia and New Zealand. In the United States, the

number of cases per million population rose from 3.5 in 1984 to a peak of 6.3 in 1994 and then began to

decline to 4.7 in 1996. In England and Wales, annual totals declined briefly after a peak in 1988 but

have been increasing since 1993.

Legionella are widely distributed in the aqueous environment, including both natural water

bodies (surface water and groundwater) and man-made waters (e.g., potable water, cooling towers,

whirlpools, etc.). The presence of Legionella has been documented in fresh surface water sources (e.g,

lakes and streams), estuarine and marine surface water sources, and groundwater. Legionella thrive in

biofilms, and interaction with other organisms in biofilms is essential for their survival and proliferation

in aquatic environments. Bacteria in biofilms are relatively resistant to standard water disinfection

procedures, and therefore, Legionella are able to enter potable water supplies. Legionella find niches

suitable for survival and growth in artificial aquatic habitats (e.g., internal plumbing systems, cooling

towers, respiratory-therapy equipment, humidifiers, and whirlpools), which function as amplifiers or

disseminators of these bacteria.

Although water has been the most documented source of Legionella in the environment, these

bacteria have been isolated from mud, moist soil, and potting soil. Legionella can be transmitted from

water to air by aerosol-generating systems such as cooling towers, evaporative condensers, plumbing

equipment (e.g., faucets, showerheads, hot water tanks), humidifiers, respiratory-therapy equipment

(e.g., nebulizers), and whirlpool baths. Inhalation of Legionella-contaminated aerosols is an important

source of human exposure and infection.

Human exposure to Legionella-contaminated sources can result in outbreaks of legionellosis. 

Outbreaks can be categorized as nosocomial (i.e., hospital-acquired), travel-acquired, or communityacquired. Nosocomial outbreaks have been linked to hospital potable water supplies as well as air

conditioning systems and cooling towers. Travelers are usually exposed to Legionella in contaminated
III-35

hotel potable water or contaminated whirlpool spas. Community outbreaks are caused by exposure to

the widest variety of sources, but potable water and cooling towers are the most common. L.

pneumophila has most frequently been implicated as the causative agent for all three types of outbreaks. 

The majority of cases of legionnaires’ disease, however, are community-acquired sporadic (i.e., nonoutbreak related).

The growth and survival of Legionella in the environment is enhanced by their ability to form

symbiotic relationships with other microorganisms. Legionella are able to infect and multiply

intracellularly within at least 13 species of amoebae, allowing them to survive over a wider range of

environmental conditions and resist the effects of chlorine, biocides, and other disinfectants. Because

Legionella replicate rapidly intracellularly within these protozoan hosts, often for prolonged periods of

time, a single amoebic vesicle can contain hundreds of Legionella. Relationships with certain algae and

bacteria in biofilms also foster the growth of Legionella, presumably due to the increased availability of

nutrients and resistence to disinfection. Other factors influencing the survival of Legionella in the

environment include water temperature, presence of sediment, and metal content.
III-36

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IV-1

IV. Health Effects in Animals

A. Laboratory Studies

Although Legionella are widely distributed in the environment, there are no reports of their

isolation from naturally infected animals, and they are considered to be strictly human pathogens. As

discussed in detail in the 1985 Legionella Criteria Document, there is considerable serological evidence

that exists to support exposure or possible subclinical infection in animals, such as horses, cattle, sheep,

swine, nonhuman primates, goats, dogs, and protozoa. It should be noted, however, that controversy

exists in the applicability of the utilization of titer criteria in animals, an evaluation method which is

established for measurement of antibody levels in humans.

Animals have been primarily used as hosts for the isolation of the Legionella, models for the

study of the disease process in human legionellosis, models for the study of the virulence of various

Legionella species, as well as for the testing of new diagnostic techniques, immunological responses,

and possible therapeutic approaches. Guinea pigs have been studied extensively due to similarities

between the natural legionnaires’ disease in humans, and the experimental disease in guinea pigs. Other

species including rats, gerbils, mice, hamsters, rabbits, nonhuman primates and embryonated hens’ eggs

have also been utilized for study of infection by Legionella.

Experimental routes of exposures have been primarily respiratory, including small particle

aerosols, intranasal instillation, nose-only inhalation and intratracheal injections. Infections have also

been induced by ingestion (drinking water and gastric intubation) and intraperitoneal injection routes.

Clinical Features and Symptomatology in Guinea Pigs

The disease process, following an inhalation exposure of L. pneumophila in guinea pigs, has

been characterized by investigators as fever for several days, bacteremia, and fibrinopurulent pneumonia

with congestion and eventually consolidation (Baskerville 1984, Davis et al. 1983c). The most striking

clinical symptoms are fever and weight loss (Twisk-Meijssen et al. 1987). In fact, weight loss, fever and

seroconversion are considered to be the only dependable clinical criteria of aerosol infection (Berendt et
IV-2

al. 1980). Clinical symptoms and mortality are dose-dependent in nature, and are discussed at length in

the 1985 Legionella Criteria Document. The defense mechanism initially involves resident alveolar

macrophages and polymorphonuclear cells (PMNs), followed by the presence of immunospecific

antibodies (Davis et al. 1983b). The multiplication of the Legionella in recruited macrophages results in

destruction of the macrophages and the release of toxic products in the lungs of susceptible animals. 

The alveolar membrane integrity is destroyed, and serum proteins, together with PMNs, macrophages,

bacteria, and debris, fill the air sacs producing hypoxia and impairing respiratory function.

In contrast, intraperitoneal exposure in guinea pigs to L. pneumophila causes a diffuse

fibrinopurulent peritonitis involving the liver and spleen (Chandler et al. 1979c, Hambleton et al. 1982). 

In addition, foci of inflammation and necrosis may also be found in the lungs, lymph nodes, pancreas,

and heart. The histological features of the pneumonitis induced by intraperitoneal inoculation are quite

different from those observed in animals infected by the aerosol route; lesions are more focal, the

interstitium is more strongly involved, and necrosis and fibrin in the alveolar exudate is minimal. It was

also noted by Hambleton et al. (1982) that extrapulmonary symptoms, such as diarrhea, kidney or liver

failure, or neurologic disturbances, that are observed with intraperitoneal infections are seldom observed

in guinea pigs infected by the aerosol route. Biochemical changes observed in guinea pigs infected by

the intraperitoneal route include hyponatremia, striking changes in serum trace metals, amino acids and

proteins, changes in liver enzymes indicating hepatic necrosis, and evidence of leukocytosis followed by

leukopenia (Hambleton et al. 1982).

Guinea pigs infected with L. pneumophila by an oral route of exposure demonstrate a febrile

disease with mild pneumonitis and splenitis (Katz and Matus 1984). In one study, subacute exposure to

L. pneumophila in drinking water over a period of 17 days did not cause clinical illness, and none of the

guinea pigs seroconverted (Conner and Gilbert 1979).

Other Animal Models

Rats have also been used as models for L. pneumophila infection. Winn et al. (1982) found that

acute pneumonia occurred in both rats and guinea pigs; however, the rats appeared to be more resistant

to lethal infection and extrapulmonary inflammatory lesions. Davis et al. (1983a) also demonstrated a
IV-3

milder illness in rats similarly exposed. Exposure of marmosets to small-particle aerosols of L.

pneumophila produces acute fibrinopurulent pneumonia like that observed in guinea pigs (Baskerville et

al. 1983b). Rhesus monkeys are less susceptible than the marmosets or guinea pigs, and the pulmonary

lesions are less severe.

Mice have also been used as models for Legionella infection. Fitzgeorge et al. (1983) found that

Porton mice were highly resistant to aerosol infection with L. pneumophila; mice remained healthy and

did not develop antibodies. ICR mice infected by intraperitoneal injection had a moderate to low

susceptibility to infection by Legionella, and Mongolian gerbils were found to be highly susceptible

(Patton et al. 1979). A tabular summary of the dose responses of various animals to experimental L.

pneumophila infection provided in the 1985 Legionella Criteria Document aids in emphasizing that there

is great species variation in susceptibility to Legionella infection.

LD50 Data

LD50 data and median 50% infection doses (ID50) have been documented in guinea pigs exposed

to L. pneumophila by the aerosol route:

• ID50 of <129 bacteria with an LD50 of 1.4x105

 organisms (Berendt et al. 1980)

• LD50 in the range of 500-5000 retained CFU and a fever production ID50 dose of 20 CFU

(Huebner et al. 1984)

• retained LD50 of 104 (Baskerville 1984)

Long-Term Effects

The long-term effects of Legionella-induced pneumonia are pulmonary fibrosis and functional

impairment of the lung. Studies in surviving guinea pigs, Rhesus monkeys and marmosets exposed to

aerosol infections of L. pneumophila have shown that alveolar fibrosis, cellular infiltration of alveolar

walls, and blockage of some terminal airways are common features 10 days after exposure, and were still

present in guinea pigs after one month (Baskerville et al. 1983a). The infecting organism did not persist

in the lungs, and pulmonary abscesses did not develop. In Syrian hamsters intratracheally infected with
IV-4

L. pneumophila, the alveolar response to the infection was still prominent after 90 and 180 days in some

lungs, and the severity of the inflammation was correlated with a persistent restrictive defect in lung

elasticity (Parenti et al. 1989).

B. Summary

 Although animals are not naturally infected by Legionella, their use as models for the study of

human legionellosis is beneficial in understanding the etiology of its clinical manifestations. 

Experimental studies of legionellosis in animals, particularly guinea pigs exposed by the respiratory

route of infection, provide useful information on human legionellosis due to the close similarities of 

these diseases. These similarities are discussed in detail in the 1985 Legionella Criteria Document.

The disease process in the lungs of susceptible animals is characterized by multiplication of the

Legionella in recruited macrophages; destruction of the macrophages eventually results in hypoxia and

impaired respiratory function. Clinical features include weight loss, fever and seroconversion. The LD50

for guinea pigs exposed to L. pneumophila by the aerosol route is somewhat less than 105

 cells. The

long-term effects of Legionella-induced pneumonia are pulmonary fibrosis and functional impairment of

the lung.

There are varying degrees of susceptibility to Legionella infection among animal species. In

comparison to guinea pigs, which have been studied extensively, rats, monkeys, marmosets and mice are

more resistant to infection by Legionella aerosols. Gerbils are highly susceptible to infection by the

intraperitoneal route.
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V-1

V. Health Effects in Humans

Legionellosis in humans has typically been characterized as either a non-pneumonic condition

known as Pontiac fever or a pneumonic condition known as legionnaires’ disease. This chapter

summarizes new information available since publication of the 1985 Legionella Criteria Document on

legionellosis in humans, specifically symptoms and clinical manifestations, clinical laboratory findings,

mechanism of action, immunity, chronic conditions, and treatment.

A. Symptoms and Clinical Manifestations

Pontiac fever is described as an acute, self-limiting illness with "flu-like" symptoms. The illness

is characterized by an attack rate of greater than 90 percent of exposed persons and an incubation period

ranging from 24 to 48 hours (Nguyen and Yu 1991, Roig et al. 1994). The symptoms include fever,

chills, headache, myalgia, and malaise (Muder et al. 1989, Nguyen and Yu 1991). The illness typically

resolves without complications within two to five days (Muder et al. 1989). Upper or lower respiratory

tract symptoms have not been associated with this illness. No additional information on Pontiac fever

was located.

Since publication of the 1985 Legionella Criteria Document, the course of legionnaires’ disease

has been more precisely defined (Davis and Winn 1987, Ampel and Wing 1990; Nguyen et al. 1991,

Stout and Yu 1997). The incubation period for legionnaires’ disease is two to ten days, although

incubation periods exceeding ten days have been reported (WHO 1990). Malaise, myalgia, anorexia,

and headache typically occur within 48 hours. These symptoms are usually accompanied by a rapidly

rising fever that frequently reaches 39°C or 40°C. Chills may also occur with the fever. A dry cough is

typically present in the early stages of the illness. Although the cough may become productive with

minimally or moderately purulent sputum within several days, hemoptysis is rarely observed. Other

common early features of the illness include neurologic abnormalities (e.g., confusion, disorientation,

lethargy) and gastrointestinal symptoms (e.g., nausea, vomiting, watery diarrhea). As the illness

progresses, chest pain (often pleuritic), dyspnea, and respiratory distress may be observed.
V-2

Frequencies of these common symptoms vary. Table V-1 summarizes frequencies for these

symptoms based on estimates provided in two review articles. An important point to note is that the

clinical features described for legionnaires’ disease do not distinguish it from other bacterial pneumonias

(Roig et al. 1994). Recent studies have shown that symptoms initially thought to occur with greater

frequencies in patients with legionnaires’ disease are actually not distinctive. For example, Edelstein

(1993) reported that diarrhea, which has historically been considered a distinctive feature of

legionnaires’ disease, occurred with similar frequency in patients with legionnaires’ disease (0-25%)

compared to patients with pneumonias caused by other agents (3-36%). Similarly, bradycardia and

neurologic abnormalities have been "discredited" as distinctive features (Roig et al. 1994, Stout and Yu

1997). Edelstein (1993) concluded that prospective comparative studies have demonstrated that no one

clinical feature can be used to distinguish legionnaires’ disease from pneumonia caused by other agents.

Extrapulmonary diseases resulting from legionnaires’ disease are rare, but have been reported

with increasing frequency since publication of the 1985 Legionella Criteria Document. Infections in

which Legionella species have been implicated include sinusitis (Lowry and Tompkins 1993), cellulitis

(Waldor et al. 1993, Kilborn et al. 1992), pancreatitis (Kesavan et al. 1993, Eitrem et al. 1987),

peritonitis (Lowry and Tompkins 1993), brain abscess (Andersen and Sogaard 1987), perirectal abscess

(Lowry and Tompkins 1993), acalculous cholecystitis (Earle and Hoffbrand 1990), transient aplastic

anemia (Martinez et al. 1991), myositis (Warner et al. 1991), and various wound infections (Lowry and

Tompkins 1993). Stout and Yu (1997) stated that the heart is the most common extrapulmonary site. 

This assertion is supported by numerous reports of myocarditis (De Lassence et al. 1994, Armengol et al.

1992, Devriendt et al. 1990), pericarditis (Lowry and Tompkins, 1993, Domingo et al. 1989), and

endocarditis (Berbari et al. 1997, Chen et al. 1996).

Table V-1. Frequency of Symptoms of Legionnaires’ Disease

Symptoms

Frequency (% of Patients)

A 1 B 2

Fever -- 99

> 38.2°C -- 71
V-3

> 39°C 70-95 --

> 39.4°C -- 79 (65)

Cough 75-95 89

Chills 59-73 78

Headache 32-75 50

Myalgias 38-75 --

Dyspnea -- 48

Neurological/Confusion 25-50 37 3

Diarrhea/Nausea 13-54 45 4

Chest Pain 30-42 5

45

1 The source of information is Davis and Winn 1987. The authors did not provide any indication

regarding the number of patients evaluated, but noted that the frequency was a "composite

estimate from published series."

2 The source of information is Ampel and Wing 1990. The authors indicated the frequency was

based on 231 patients. Numbers indicated in parentheses are exceptions.

3 Symptom was listed as "neurologic abnormalities."

4 Symptom was listed simply as "diarrhea."

5 Symptom was listed as "pleuritic pain."

The kidney is also a common extrapulmonary site. In the Philadelphia epidemic of 1976, 14 of

the 123 cases of legionnaires’ disease developed acute renal failure (Shah et al. 1992). Since 1976, at

least 53 additional cases of legionnaires’ disease complicated with acute renal failure have been reported

(Lin et al. 1995). Based on the limited histopathology that has been conducted, the acute renal failure

appears to be a result of either acute tubulointerstitial nephritis (Shah et al. 1992, Haines and Calhoon

1987) or acute tubular nephritis (Shah et al. 1992, Fenves 1985), although acute pyelonephritis (Shah et

al. 1992) and glomerulonephritis (Pai et al. 1996, Wegmüller et al. 1985) have been reported. 

Typically, extrapulmonary infections occur concurrently with pneumonia and are believed to

result from bacteremia (Stout and Yu 1997, Edelstein 1993). Where extrapulmonary infections develop

prior to the onset of pneumonia, identifying the primary site of infection may be difficult. Several cases

of infections attributed to Legionella species in the absence of pneumonia have been reported (Edelstein

1993). These infections may be the result of direct inoculation of a site with water contaminated with
V-4

Legionella bacteria. Table V-2 summarizes those extrapulmonary sites in which Legionella infections

have been implicated in the presence and absence of pneumonia. As a final note, extrapulmonary

infections tend to occur with greater frequency in immunocompromised patients or in patients with

severe cases of legionnaires’ disease (Edelstein 1993).

Table V-2. Extrapulmonary Sites of Legionella Infection (Source: Edelstein 1993)

Presence of Pneumonia Absence of Pneumonia

Blood Pericardium Blood

Brain Bone marrow Surgical wounds

Bowel Skin and fascia Bowel

Kidney Rectum Respiratory sinus

Liver Myocardium Endocardium

Spleen Thyroid Peritoneum

Hemodialysis shunt Pancreas Pericardium

Peritoneum Testes Skin and fascia

Prostate Muscle

Peripheral lymph nodes

B. Clinical Laboratory Findings

Many abnormalities in standard clinical laboratory tests have been noted in patients with

legionnaires’ disease. Some of the more common findings are summarized in Table V-3. The clinical

laboratory findings that are most frequently associated with legionnaires’ disease are hyponatremia

(Stout and Yu 1997, Roig et al. 1994, EPA 1985) and elevated levels of serum transaminase or

transpeptidase enzymes (Edelstein 1993, EPA 1985). Edelstein (1993), however, reported that in only

one of four prospective studies, patients with legionnaires’ disease showed an increased incidence of

hyponatremia compared to patients with pneumonia caused by some other agent. Furthermore,

hyponatremia was observed in only about 20 percent of the patients with legionnaires’ disease in these

studies. Edelstein also reported that only one of four prospective studies showed an increased incidence

of elevated serum enzyme levels in patients with legionnaires’ disease compared to patients with
V-5

pneumonia caused by some other agent. One study indicated the converse, and two indicated no

difference. Edelstein noted that "the most reasonable conclusion is that nonspecific test results cannot be

used to clearly distinguish between those with or without legionnaires’ disease."

Table V-3. Common Clinical Laboratory Findings in Patients with Legionnaires’ Disease 1

Leukocytosis Serum glutamic oxaloacetic transaminase

Hyponatremia Serum glutamic pyruvic transaminase

Hypophosphatemia Serum lactic dehydrogenase

Proteinuria Alkaline phosphatase

Hematuria Creatine phosphokinase

Liver function abnormalities

 1 Information was compiled from the following sources: Ampel and Wing 1990, Muder et al. 1989, Strampfer and Tu

1988, Ching and Meyer 1987.

Although investigators appear to agree that no one clinical feature or laboratory finding

distinguishes legionnaires’ disease, some have recently reported that a diagnosis of legionnaires’ disease

may be made using a multifactorial clinical model (Breiman and Butler 1998, Cunha 1998). Cunha

(1998) noted that one problem is that the "literature does not address the diagnostic significance of

characteristic signs and symptoms in concert." He recently reported a weighted point evaluation system

to aid physicians in the diagnosis of legionnaires’ disease.

The majority of patients with legionnaires’ disease exhibit abnormalities in the chest radiograph

(Muder et al. 1989). Although "all types of roentgenographic patterns are seen in cases of legionnaires’

disease" (Edelstein 1993), unilateral alveolar infiltrates, which may be segmental, lobar, or diffuse, are

typically observed in the early stages of the disease (Muder et al. 1989). These infiltrates may enlarge

and consolidate as the disease progresses (Ampel and Wing 1990). Pleural effusion is typically observed

in one-third of patients with legionnaires’ disease (Ampel and Wing 1990, Stout and Yu 1997). The

frequency, however, ranges from 6 to 63 percent and, therefore, is not a distinguishing feature (Edelstein

1993). Nodular opacities and cavitation are uncommon, except in immunocompromised patients

(Strampfer and Tu 1988, Muder et al. 1989, Stout and Yu 1997). Radiographic progression can occur

even with appropriate antibiotic therapy, and resolution is typically slow (i.e., may require one to four
V-6

months) (Muder et al. 1989, Stout and Yu 1997). Similar to clinical laboratory findings, clinicians have

concluded that "no characteristic radiographic pattern helps to distinguish any one type of pneumonia"

(Coletta and Fein 1998).

C. Mechanism of Action

The typical progression of a Legionella infection can be characterized by the following steps

(Cianciotto et al. 1989). First, Legionella is inhaled or instilled into the lower airways of the lungs. The

mechanism for evasion of the body's non-specific defenses has not been established. Second, alveolar

macrophages phagocytize the bacteria by either a conventional or coiling mechanism. The resulting

phagosome becomes studded with ribosomes within four to six hours. Intracellular survival of the

bacteria may be attributed to one or more of the following factors: reduced oxidative burst, failure of

phagosome to acidify, failure of phagosome to fuse with lysosome, and/or bacterial resistance to

lysosomal contents. Third, the bacteria undergo rapid intracellular growth. In fact, the bacterial growth

within infected macrophages has been estimated at 100- to 1000-fold within 48 to 72 hours of infection,

which is considered remarkable compared to that of other intracellular opportunistic bacteria (e.g.,

Salmonella, Mycobacterium, Listeria) (Friedman et al. 1998). Finally, the host cell dies and releases the

bacteria. Intracellular infection and bacterial growth is then escalated. At this stage, tissue damage and

induction of an inflammatory response may occur as a result of exposure to bacterial cellular

components and/or extracellular products from the bacteria.

Significant effort has been invested into elucidating the factors responsible for the pathogenesis

of Legionella. The 1985 Legionella Criteria Document described three "toxic" bacterial components: a

lipopolysaccharide (LPS) located in the outer membrane of Legionella bacteria, an extracellular acidsoluble toxin isolated from several Legionella species, and an extracellular cytotoxin isolated from L.

pneumophila. A variety of proteolytic enzymes were also recognized as potentially important factors in

the pathogenesis of Legionella. Since completion of the 1985 Legionella Criteria Document, a variety of

cellular components and extracellular products have been identified (Rechnitzer 1994). Their

involvement in the pathogenesis of Legionella, however, has not been established.
V-7

One biologically important extracellular bacterial component that has been isolated is a zinc

metalloprotease. This enzyme has exhibited proteolytic, hemolytic, and cytotoxic activity and has

received a variety of names (e.g., tissue-destructive protein (TDP), major secretory protein, cytolysin)

(Rechnitzer 1994). This enzyme has been shown to elicit the same type of pulmonary lesions that

develop in legionnaires' disease and has been found in lungs of guinea pigs infected with L. pneumophila

at a level equal to the dose of the purified protease known to cause death in experimental animals

(Conlan et al. 1988). This enzyme has also been shown to degrade two phosphorylated proteins

generated by a phosphokinase system isolated from the pulmonary cells of rabbits (Belyi 1990). 

Although the significance of this specific system is unknown, phosphokinase systems generally are

involved in controlling intracellular metabolic processes. Therefore, this enzyme may disrupt metabolic

processes of the host cell in addition to causing necrosis.

One additional factor recently recognized that may contribute to the pathogenesis of Legionella is

the symbiotic relationship of the bacteria with amoebae. Brieland et al. (1996) investigated the effect of

intratracheal coinoculation of L. pneumophila and Hartmannella vermiformis into A/J mice. A/J mice

are recognized as an animal model for human legionnaires’ disease and have been used extensively to

investigate many aspects of Legionella infections. H. vermiformis is "the most prevalent species of

amoebae in potable water supplies in the United States and has been epidemiologically linked to

outbreaks of legionnaires’ disease." They found that coinoculation resulted in significantly increased

intrapulmonary growth of L. pneumophila, an increased severity of infection, and significant mortality

when compared to inoculation with only L. pneumophila. Furthermore, they found that coinoculation

with L. pneumophila and H. vermiformis into a resistant host (i.e., BALB/c mice) resulted in an eightfold increase in intrapulmonary bacterial growth when compared to inoculation with only L.

pneumophila.

To confirm that the amoebae were providing a niche for bacterial replication, Brieland et al.

(1997a) investigated the effect of coinoculation of A/J mice with H. vermiformis and mutant strains of L.

pneumophila that had reduced virulence for H. vermiformis. They found that the intrapulmonary

bacterial growth was not significantly increased in mice coinoculated with H. vermiformis and the

mutant strains. The authors concluded that virulence for the amoebae is necessary for increased bacterial
V-8

growth and, therefore, "inhaled amoebae may potentiate intrapulmonary growth of L. pneumophila by

providing a niche for bacterial replication."

As a final note, Brieland et al. (1997b) investigated the effect of inoculating A/J mice with H.

vermiformis infected with L. pneumophila. They found that the infected amoebae were more pathogenic

than an equal number of L. pneumophila or a mixture of L. pneumophila and uninfected amoebae. The

authors concluded that amoebae infected with L. pneumophila may be the infectious particles in

Legionella infections.

D. Immunity

Both humoral and cell-mediated immune responses to Legionella infections have been

documented (EPA 1985, Friedman et al. 1998). Based on the results of serological tests, antibodies are

produced in response to Legionella infections that interact with specific bacterial components. 

Furthermore, bacterial infection results in the activation of complement, which has been shown to occur

through both the classical and alternative pathway (Friedman et al. 1998, Mintz et al. 1992). 

Opsonization of bacteria (i.e., binding of antibodies and/or complement to the bacteria) has been shown

to increase phagocytosis by human peripheral blood monocytes and animal macrophages; however, the

ability of the bacteria to replicate within these cells does not appear to be diminished (Friedman et al.

1988). Therefore, the protection provided by specific antibodies in vivo is not currently known.

Cell-mediated immunity is recognized as the primary defense to Legionella infection (Susa et al.

1998). Research indicates that cytokines secreted by TH1 helper cells or macrophages play a primary

role in limiting bacterial replication (Friedman et al. 1998). For example, interleukin-2, which is

secreted by TH1 helper cells, appears to activate natural killer cells to lyse cells infected with Legionella,

thus limiting bacterial growth by killing the host cell (Friedman et al. 1998). 

Interferon- is also an essential component in host resistance to Legionella infection. This

cytokine, which is also secreted by TH1 helper cells, appears to activate macrophages and monocytes to

inhibit bacterial growth. In fact, bacterial growth has been shown to decrease 100-fold in activated

macrophages compared to non-activated infected macrophages (Friedman et al. 1998). The limited
V-9

growth may be the result of down-regulation of the transferrin receptors, which results in a decreased

availability of intracellular iron, an essential component in Legionella growth (Skerrett and Martin 1991,

Susa et al. 1998). 

Many studies have confirmed the important role of interferon- in the resistance to Legionella

infection. For example, based on a comparison of aged mice to young mice, Fujio et al. (1995) proposed

that the susceptibility of the elderly to Legionella infection may be the result of a decreased capacity to

produce interferon- . Finally, Heath et al. (1996b) investigated the effect of Legionella infection in

BALB/c mice (i.e., a resistant species) and in BALB/c mice in which the interferon- gene was

disrupted. Mice were inoculated intratracheally with L. pneumophila. Bacterial growth was not

observed in the BALB/c mice; however, the mutant BALB/c mice developed "persistent, replicative

intrapulmonary L. pneumophila infections with extrapulmonary dissemination of the bacteria to the

spleen." Intratracheal administration of interferon- to the mutant BALB/c mice increased clearance of

the bacteria from the lungs. The authors concluded that these results confirm the importance of

interferon- in the resistance to Legionella infection. 

One additional factor that appears to play an important role in resistance to Legionella infection

is tumor necrosis factor- , which is a cytokine secreted by macrophages. Blanchard et al. (1988)

reported that polymorphonuclear leukocytes treated with tumor necrosis factor- exhibited increased

bactericidal activity against L. pneumophila. Furthermore, mice treated with tumor necrosis factorprior to infection exhibited reduced mortality, which correlated with increased clearance of bacteria from

the lungs. Matsiota-Bernard et al. (1993) reported that treatment of human peripheral monocytes with

tumor necrosis factor- significantly inhibited the growth of L. pneumophila. Inhibition was not

observed when an inhibitor of tumor necrosis factor production or anti-tumor necrosis factor antibodies

were added to the culture medium. The mechanism by which tumor necrosis factor- inhibits bacterial

growth has not been established, although one proposal is that this cytokine may potentiate nitric oxide

release (Susa et al. 1998, Skerrett and Martin 1996).

E. Chronic Conditions
V-10

As discussed in the 1985 Legionella Criteria Document, most patients with legionnaires’ disease

recover without chronic manifestations. Ching and Meyer (1987), however, reported that fatigue and

weakness may persist for several months following treatment. Furthermore, as noted above, resolution

of infiltrates on chest radiographs is slow and may take from one to four months (Stout and Yu 1997). 

Respiratory abnormalities resulting from legionnaires’ disease occasionally occur. Gea et al.

(1988) reported the outcome of 11 patients with legionnaires’ disease followed for 53 months. Mild to

moderate ventilatory and/or gas exchange abnormalities were observed several months following

discharge from the hospital. At study termination, the majority of patients (8/11) exhibited one or more

of the following respiratory abnormalities: a restrictive ventilatory defect, a low transfer factor, and/or

hypoxemia. Because the majority of patients were smokers, some with chronic bronchitis, the authors

could not dismiss the possibility that these manifestations were the result of pre-existing conditions.

More serious respiratory abnormalities are rare. Pulmonary pathology that has been reported

includes pulmonary fibrosis, bronchiolitis obliterans, chronic vasculitis, and chronic organizing pleuritis

(Ching and Meyer 1987, EPA 1985).

F. Treatment

Early initiation of appropriate treatment is recognized today as crucial for a successful outcome

of legionnaires’ disease. Heath et al. (1996a) conducted a retrospective analysis of serologically

confirmed cases of legionnaires’ disease to determine factors associated with increased mortality. After

multiple logistic regression analysis, the only factor associated with increased mortality was a delay in

initiation of appropriate therapy. 

Retrospective analyses of early epidemics of legionnaires’ disease have helped define

"appropriate" therapy. Early studies indicated that patients treated with erythromycin had a lower

mortality rate than patients treated with aminoglycosides, -lactam antibiotics, or chloramphenicol (6%

versus 30-40%) (Roig et al. 1993). The poor response to these antibiotics has been related to their

inability to penetrate phagocytic cells. In fact, studies have indicated that clinically effective antibiotics
V-11

must have the following features: (1) superior in vitro activity against Legionella species, (2) "the ability

to enter and concentrate within phagocytic cells," and (3) "the ability to achieve high concentrations in

lung tissue and alveolar exudate" (Roig et al. 1993).

Current recommendations for antibiotic treatment of legionnaires’ disease are provided in Table

V-4. Erythromycin has historically been considered the first choice in treatment of legionnaires’ disease

(Stout and Yu 1997). Treatment with this antibiotic, however, is associated with several adverse side

effects, including transient hearing loss, phlebitis, gastrointestinal intolerance, and, more rarely,

ventricular arrhythmia (Roig et al. 1993). Newer macrolides, such as azithromycin and clarithromycin,

are attractive because they have exhibited superior activity against Legionella species and greater

intracellular penetration with potentially fewer adverse effects (Klein and Cunha 1998, Stout and Yu

1997, Roig et al. 1993). With development of intravenous formulations, these newer macrolides (e.g.,

azithromycin) may replace erythromycin as the treatment of choice (Stout and Yu 1997).

Quinolones have shown greater activity against Legionella species and higher intracellular

penetration than the macrolides (Klein and Cunha 1998, Stout and Yu 1997, Edelstein et al. 1996). 

These antibiotics have been recommended for transplant recipients with legionnaires’ disease because,

unlike the macrolides and rifampicin, they do not interfere with metabolism of immunosuppressive

medications (Stout and Yu 1997). Although successful outcomes have been reported using these

antibiotics, Baty et al. (1997) reported a case of pneumonia in an immunocompetent patient resulting

from L. jordanis that was unresponsive to ciprofloxacin and ofloxacin. These antibiotics, however, were

administered at dose levels lower than those suggested in Table V-4.

Other antibiotics that have shown variable success in treatment of legionnaires’ disease include

the tetracyclines (e.g., doxycycline, minocycline, and tetracycline) and the combination of trimethoprimsulfamethoxazole (Stout and Yu 1997, Roig et al. 1993). Rifampicin is an antibiotic 

Table V-4. Recommendations for Antibiotic Treatment of Legionnaires’ Disease1

Antibiotic Dose Route Frequency

Macrolides

 Azithrom ycin 500 mg2

oral or intravenous every 24 hours
Antibiotic Dose Route Frequency

V-12

 Clarithrom ycin 500 mg oral or intravenous3

every 12 hours

 Erythrom ycin 1000 mg

500 mg

intravenous

oral

every 6 hours

every 6 hours

Quinolones

 Levoflox acin 500 mg2oral or intravenous every 24 hours

 Ciproflo xacin 400 mg

750 mg

intravenous

oral

every 8 hours

every 12 hours

 Ofloxac in 400 mg oral or intravenous every 12 hours

Tetracyclines

 Doxycycline 100 mg2oral or intravenous every 12 hours

 Minocycline 100 mg2oral or intravenous every 12 hours

 Tetracycline 500 mg oral or intravenous every 6 hours

Trimethoprimsulfametho xazole

160 mg/800 mg

160 mg/800 mg

intravenous

oral

every 8 hours

every 12 hours

1 Source of information is Stout and Yu 1997. Recommendations are based on clinical experience rather than controlled trials. 

Specific recommendations may vary slightly depending on the source of information.

2 Doubling of the first dose was recommended by Stout and Yu (1997). Edelstein (1998), however, does not recommend this

suggested practice for azithromycin and levofloxacin.

3 Intravenous route is under investigation in United States.

that is used in combination therapy for severely ill patients and is typically administered either orally or

intravenously at dose levels of 300-600 mg every 12 hours (Stout and Yu 1997). Although rifampicin

has shown excellent in vitro and in vivo activity against Legionella species, it is not administered as a

monotherapy due to the potential of developing rifampicin-resistant strains of Legionella (Roig et al.

1993). Rifampin has been combined with many antibiotics, but some uncertainty exists regarding the

clinical efficacy of rifampicin and quinolone combinations (Roig et al. 1993, Edelstein et al. 1993). 

Furthermore, the clinical efficacy of the conventional combination of rifampicin and erythromycin has

been questioned. Hubbard et al. (1993) conducted a retrospective analysis of patients with legionnaires’

disease requiring intermittent positive pressure ventilation. Those patients receiving rifampicin in

combination with erythromycin had a significantly increased incidence of jaundice, had significantly

higher levels of bilirubin, and did not have decreased mortality compared to those patients that did not

receive rifampicin. 

For the treatment of legionnaires’ disease, the preferred route of administration of any antibiotic

therapy is intravenous (Stout and Yu 1997). This route ensures the greatest potential concentration of
V-13

antibiotic in the lung tissue. Intravenous treatment should continue until the patient becomes afebrile. 

At this point, intravenous treatment can be replaced by oral therapy. The total duration of therapy

depends on the patient history. For a patient with a mild illness exhibiting significant improvement,

therapy should continue for a period of approximately two weeks. For the severely ill or

immunocompromised patient, therapy should continue for three weeks. Newer macrolides (e.g.,

azithromycin) may allow for a shorter course of treatment.

G. Summary

Since publication of the 1985 Legionella Criteria Document, much has been learned regarding

legionnaires’ disease in humans. Although its progression has been more precisely defined, no one

symptom has been recognized that can distinguish legionnaires’ disease from other bacterial

pneumonias. Similarly, abnormalities in standard clinical laboratory tests and chest radiographs cannot

be used to distinguish legionnaires’ disease from other pneumonias. Some investigators, however, have

recently reported that a multifactorial clinical approach may be helpful in the diagnosis of legionnaires’

disease.

Although extrapulmonary diseases resulting from legionnaires’ disease are still relatively rare,

they have been reported with increasing frequency since publication of the 1985 Legionella Criteria

Document. The kidney is a common site of extrapulmonary infection; however, the heart is now

recognized as the most common site of extrapulmonary infection. These extrapulmonary infections can

occur in the absence of pneumonia. No significant new information has been located on chronic

conditions resulting from legionnaires’ disease.

Since 1985, the mechanism of bacterial replication has been more precisely defined. Briefly,

bacteria are inhaled or instilled in the lower airways of the lung and are phagocytized by alveolar

macrophages. Bacteria undergo rapid intracellular growth within the phagosome. The host cell lyses

and releases the bacteria, which escalates the bacterial infection.

Significant effort has been invested into the elucidation of factors responsible for the

pathogenesis of Legionella. One important discovery was the isolation of a zinc metalloprotease, an
V-14

enzyme that elicits pulmonary lesions similar to those that develop in legionnaires’ disease. Although

not a bacterial component or product, one factor that may affect the pathogenesis of Legionella is their

ability to infect amoebae. Recent research suggests that Hartmannella vermiformis may provide a niche

for bacterial replication in the lungs. In fact, one study suggests that amoebae infected with L.

pneumophila may be responsible for bacterial infection.

Recent research has continued to document that both humoral and cell-mediated immune

responses to Legionella infection occur. Although specific antibodies are produced, the protection that

these antibodies provide in vivo is still unknown. Cell-mediated immunity is currently recognized as the

primary defense against Legionella infection. Research also has emphasized the importance of specific

cytokines (e.g., interferon- , tumor necrosis factor- ) in host resistance to Legionella infection. Much

more research is needed to understand the host's mechanisms of resistance to these bacteria.

Since publication of the 1985 Legionella Criteria Document, many advancements in the

treatment of legionnaires’ disease have been made. Although erythromycin has historically been

considered the first choice for the treatment of legionnaires’ disease, newer macrolides

(e.g., azithromycin) are available that exhibit superior activity to Legionella and greater intracellular

penetration with potentially fewer adverse effects. Furthermore, quinolones show promising activity

against Legionella infections and are recommended for patients on immunosuppressive medication. 

Early initiation of appropriate therapy is crucial for a successful outcome to legionnaires’ disease.
VI-1

VI. Risk Assessment

Risk assessment is a tool for the synthesis of available scientific information, in both a qualitative

and quantitative manner, in order to characterize the probability of potential public health hazards

resulting from exposure to a toxic or infectious agent. The results of such an assessment can then be

employed in making informed risk management decisions. Over the past 25 years, scientists have

developed methodologies to assess risks to human health from exposure to chemicals in the

environment, foods, or drugs. The application of this methodology to the assessment of risks from

microbial pathogens is a much newer field, however. This chapter presents the relevant information,

where available, for a risk assessment of Legionella in water supplies.

A. Hazard Identification

As discussed in the preceding chapter as well as in the 1985 Legionella Criteria Document,

Legionella are opportunistic pathogens that cause a pneumonic condition known as legionnaires' disease

in some individuals. Outbreaks and sporadic cases have been reported following exposure in the general 

community and among hospitalized persons (i.e., nosocomial cases). Legionella are considered

opportunistic pathogens because, although they are highly prevalent in the environment, relatively few

people develop a clinical infection. Yu and colleagues (1993) characterized the attack rate for

Legionella as "strikingly low."

Knowledge gained from advances in laboratory identification techniques and more rigorous

epidemiological studies suggests that Legionella are responsible for a growing percentage of both

community- and hospital-acquired pneumonias. These advances have allowed a better understanding of

the relative impact of Legionella-caused pneumonia in the U.S. From a review of pneumonia patients in

Ohio, Marston and colleagues (1997) estimated that between 8,000 and 18,000 (2-4 %) of the total

485,000 community-acquired cases of pneumonia requiring hospitalization annually in the U.S. are due

to Legionella. This estimate is associated with significant uncertainty, however, because the causative

organism is identified in only 50 percent of pneumonia cases (Reynolds 1996, Marrie et al. 1996).
VI-2

Legionnaires' disease is the most serious illness caused by Legionella organisms. The clinical

course of this disease, which was described in detail in Chapter V, is quite similar for community- or

hospital-acquired infections (Petro-Botet et al. 1995). Other infections caused by Legionella are selflimiting (e.g., Pontiac fever) or are much more rare (e.g., infection of surgical incisions or other wounds)

(Lowry and Tompkins 1993). Therefore, risk assessment of this organism is focused on legionnaires'

disease as the endpoint of concern. 

B. Dose-Response Information

The 1985 Legionella Criteria Document noted that quantitative data on the infectivity of

Legionella in humans had not been reported. Unfortunately, sufficient information is still not available

to support a quantitative characterization of the threshold infective dose (i.e., the dose required to

produce infection) of Legionella. Animal models show a great interspecies variation in susceptibility to

infection with Legionella, as described in Chapter IV. Due to the potentially serious health effects,

experiments to identify the infective dose in humans are not possible. Legionella are opportunistic

pathogens that replicate within host cells, reach target tissue via several routes (primarily inhalation or

aspiration), and exhibit a very low attack rate or virulence in the general population; therefore, it is not

surprising that definitive dose-response information continues to be elusive.
VI-3

C. Potential for Human Exposure to Legionella

1. Prevalence of Legionella in the Environment

As discussed in Chapter III and in the 1985 Legionella Criteria Document, there is clear

consensus that Legionella bacteria are widely distributed in the environment, especially in treated or

potable water supplies. Important niches or reservoirs for Legionella can occur within treated water

supply systems due to their ability to form symbiotic relationships with other microorganisms (including

biofilm formation) and theirsubsequent resistance to standard disinfection techniques (e.g.,

chlorination). Since 1985, there have been numerous studies documenting the presence of Legionella in

potable water and in water distribution systems of all types of large buildings including hospitals, office

buildings and hotels, and also smaller buildings and family residences (see Chapter III). In some cases,

Legionella occur in the absence of any reported cases of legionnaires' disease (Oppenheim et al. 1987,

Stout et al. 1992b). Through the combination of environmental sampling studies with laboratory and

epidemiological findings, a better understanding has been gained for the relative importance of various

reservoirs for transmission of Legionella to humans.

2. Mode of Transmission to Humans

Given the widespread prevalence of Legionella in the environment and their niches within

reservoirs of water supply systems, it is important to have an understanding of the circumstances under

which Legionella bacteria can reach the lower respiratory tract of humans and potentially cause serious

disease. Based on such knowledge of the important reservoirs and routes of transmission of Legionella

to humans, the most appropriate preventive measures can be selected.

Legionella are transmitted directly from the environment to humans. There is very little, if any,

evidence of human-to-human transmission, and there is no evidence of any animal reservoirs with

public health relevance for this organism. In the past decade, considerable interest and controversy have

been focused on the mechanisms by which Legionella bacteria reach the lower respiratory tract, where

they are engulfed by alveolar macrophages and commence the pathological process of infection. One

route is the inhalation of an aerosol containing respirable droplets of water (or other liquids)
VI-4

contaminated with Legionella. Alternatively, Legionella may be deposited in upper airways and

subsequently aspirated into the deeper portions of the lung. As mentioned in Chapter IV and in the 1985

Legionella Criteria Document, infection in animals following ingestion of Legionella has also been

demonstrated experimentally.

At the time of the 1985 Legionella Criteria Document, scientists believed transmission of

Legionella in the community or hospital setting occurred primarily via inhalation of infectious aerosols;

however, this assumption was based, for the most part, on circumstantial evidence. Now, with advances

in laboratory identification techniques and the availability of more rigorous epidemiological and

experimental data, there is increasing emphasis on the role of aspiration as a route of transmission. 

Thus, it follows that there is an increased focus on potable water as a primary source of infection. 

Environmental sampling from outbreaks (in communities or in hospitals) most frequently has implicated

potable water as the source (Stout and Yu 1997).

The 1985 Legionella Criteria Document reported that the most common reservoirs of

transmission for community-acquired Legionella infection are aerosols from: heat-rejection equipment

(cooling towers, evaporative condensers, steam turbine cleaning), components of plumbing systems

(showers, faucets, hot water tanks), nebulizers, humidifiers, whirlpool spas, or public fountains. Less

common sources include: ingestion of potable water, immersion in raw water, inhalation of

contaminated oil/water mixtures, and excavations (dust or soil) (EPA 1985). Additional types of aerosol

generators (e.g., grocery store mist machines) have been linked to outbreaks of legionnaires' disease

(Mahoney et al. 1992). No additional categories of sources have been identified during the period since

the 1985 EPA report, but the relative importance of contributions from some of these sources has

shifted. Although cooling towers are still a source of some community-acquired cases (e.g., Castellani

Pastoris et al. 1997, Bhopal and Fallon 1988), potable water (with subsequent inhalation or aspiration of

aerosols) is acknowledged as a much more important source (Stout and Yu 1997, Neill et al. 1985). 

There are still insufficient data to support quantification of the relative contributions from these various

sources (Bhopal 1995).

One of the most interesting and important advances made recently in the study of Legionella

transmission concerns the role of amoebae and other larger protozoa in enabling or enhancing the
VI-5

transmission of Legionella. Working with Acanthamoeba in culture with L. pneumophila (isolated from

a cooling tower), Berk et al. (1998) examined the Legionella-filled vesicles formed and expelled by the

amoebae. The vesicles were 2.1-6.4 m in diameter (i.e., respirable size), and the study authors

calculated that, based on volumes, each vesicle could contain as many as 200 bacteria. Other

investigators have estimated even higher numbers of bacteria per vesicle (Rowbotham et al. 1986 as

cited in Berk et al. 1998). Berk and colleagues also demonstrated that vesicles free in the medium were

resistant to biocide and that the biocide treatment facilitated the release of large numbers of vesicles as it

induced encystment of the amoebae. Such infectious vesicles may represent a very important vehicle of

transmission for Legionella, by protecting the bacteria from dessication while in the atmosphere and

delivering a possibly infective dose to the respiratory tract. Thus, these preliminary findings contribute

to the complexity of modeling exposure and dose-response relationships for Legionella infections in

humans.

For nosocomial cases of legionnaires' disease, there is a growing body of evidence from case

observation and experimental data that points to aerosolization of potable water (tap water) as the most

important source of transmission of Legionella. Blatt et al. (1993) analyzed 14 nosocomial cases

that occurred in a military hospital and compared them with controls. Environmental sampling for

Legionella showed colonization of 15% of potable water sites, one hot water tank, and the groundwater

supply to the hospital, while no Legionella were isolated from the hospital cooling towers, building air

intakes or other hospital air and oxygen supplies. This case-control comparison showed a negative

association between showering and acquiring legionnaires' disease, although earlier studies have

sometimes reported a positive association with showering (EPA 1985, Breiman et al. 1990, Hanrahan et

al. 1987 as cited in Blatt et al. 1993).

Potable water is now consistently identified as the most common source of Legionella in

hospitals (Yu 1993, Blatt et al. 1993, Woo et al. 1992). Observation of hospital cases indicates a high

risk of infection for patients who have received ventilation support or have been exposed to respiratory

equipment (e.g., nebulizers), suggesting a major role for aspiration as a route of transmission for

hospital-acquired legionnaires' disease. The relative significance of aspiration is also supported by the

very low infection rates (or antibody titers) among hospital personnel where nosocomial Legionella

outbreaks have occurred (Marrie et al. 1986). 
VI-6

The sources of Legionella that play an important role in transmitting the bacteria to humans have

been fairly well characterized by now. Knowledge gaps exist, however, regarding the relationship

between environmental concentrations of Legionella and the ultimate risk of infection in exposed

individuals. It is, therefore, useful to review the factors that may place an individual at increased risk for

developing legionnaires' disease.

D. Risk Factors

For opportunistic pathogens such as Legionella bacteria, identification of risk factors in

susceptible individuals is an essential element for the selection of the most appropriate control and

prevention measures. Based on the very low attack rates associated with this organism, it is clear that

the general U.S. population is quite resistant to infection by Legionella.

Certain patient populations are clearly at increased risk for contracting nosocomial legionnaires’

disease. These populations include patients who require intubation, patients who have received

ventilation assistance (including patients who have undergone surgery), and patients receiving

respiratory therapy with potentially contaminated medical equipment or whose care includes the use of

aerosol generators such as humidifiers or nebulizers (England et al. 1981, Marston et al. 1994, Stout and

Yu 1997).

Certain demographic factors are associated with an increased susceptibility to legionnaires'

disease following exposure. Subpopulations at increased risk include men over the age of 50, heavy

smokers, and heavy drinkers (Bhopal 1995, Marston et al. 1994, England et al. 1981). These findings

are based on analyses of very large series of legionnaires' disease cases. For example, Marston and

colleagues (1994) reviewed data for 3,254 patients whose cases were reported to the CDC between 1980

and 1989. The findings reported by England et al. (1981) represent the first 1,000 confirmed cases of

legionnaires' disease reported to the CDC (through September 1979).

People with certain underlying health conditions also have a significantly increased risk of

contracting legionnaires' disease. Such medical conditions include: chronic obstructive pulmonary

disease, diabetes, head or neck cancer, other malignancy, or end-stage renal disease. In addition, any
VI-7

disease state (e.g., AIDS) or medical treatment (e.g., drugs such as corticosteroids or cancer

chemotherapy, or procedures such as hemodialysis) that suppresses or depletes a patient's immune

system can lead to an increased susceptibility to opportunistic infections such as legionnaires' disease. 

Several patient populations (e.g., renal transplant patients, especially those requiring hemodialysis) are at

an extremely high risk for legionnaires' disease, as they have both an increased risk of exposure (via their

surgery and other ventilation needs), and an increased susceptibility (due to corticosteroid therapy and

dialysis) (Woo et al. 1986, LeSaux et al. 1989).

Many of these risk factors contribute not only to increased incidence of legionnaires' disease

among these groups, but also increased severity of the disease and increased mortality. Marston and

colleagues (1994) found that, among 3,254 legionnaires' disease cases reported to the CDC Legionella

surveillance system between 1980 and 1989, the following factors were significantly associated with

increased mortality attributed to legionnaires' disease: the use of steroids or other immunosuppressive

drugs; the presence of cancer, diabetes, or renal disease requiring dialysis; hospital-acquired infection;

older age; male gender; isolation of L. pneumophila subgroup 6 (Lp6); or isolation of more than one

Legionella species or L. pneumophila serogroup. More severe legionnaires' disease has also been

documented in smaller series of cases among bone marrow transplant patients (Harrington et al. 1996)

and patients receiving immunosuppressive drugs (with or without chronic disease) (Pedro-Botet et al.

1998).

People immunocompromised due to HIV infection are also at risk of developing more severe

legionnaires' disease, but Legionella infections (in the absence of other pneumonia-causing pathogens) in

this population are relatively rare. This may be due, in part, to exposure to other more common (and

more virulent) pathogens in the environment and, in part, to increased infection control vigilance

(including concern for waterborne pathogens) when patients with AIDS are hospitalized. Marston and

colleagues at the CDC (1994) reported an increased prevalence of legionnaires' disease among AIDS

patients compared to the general U.S. population (8 people with AIDS among 2,575 legionnaires' disease

cases; 0.19 expected). Bangsborg et al. (1990) reported that among 180 AIDS patients with pneumonia,

only six had Legionella infection, but four of these six patients were also infected with the fungus

Pneumocystis carinii. A high rate of coexistent pulmonary infection (again, with P. carinii) was also

reported by Blatt et al. (1994): among seven HIV-infected individuals who had legionnaires' disease, six
VI-8

were also infected with P. carinii. Of bacterial pneumonias reported in persons with AIDS, other

species that are more pathogenic and hardier than Legionella are reported most frequently, including

Haemophilus influenzae, various Streptococci, and Branhamella catarrhalis (Chaisson 1998).

Another population that may be at increased risk of contracting Legionella infection is neonates,

due to their underdeveloped immune systems, intensive ventilation procedures, and corticosteroid

therapy. Nosocomial cases of legionnaires' disease have been reported, albeit infrequently, in this

population (Holmberg et al. 1993, Horie et al. 1992). Older infants and children who have the risk

factors identified for adult populations (e.g., are receiving corticosteroid therapy or are undergoing

mechanical ventilation) are also at increased risk of contracting legionnaires' disease (Carlson et al.

1990). But even though pneumonia (all types/sources) is common in the general pediatric population,

reports of legionnaires' disease in otherwise healthy children is extremely rare (Abernathy-Carver et al.

1994, Carlson et al. 1990, Famiglietti et al. 1997).

E. Quantification of Potential Health Effects

The 1985 Legionella Criteria Document reported that our understanding of the mechanisms of

transmission of and infection by Legionella was inadequate for quantification of the potential health

effects or the development of specific recommendations for control (EPA 1985). Although

improvements in laboratory isolation and identification techniques for Legionella, along with important

experimental, epidemiological, and ecological study results, have greatly expanded our understanding of

Legionella infections in humans, the current state of the science still does not allow estimation of the

probability of the potential adverse health effects caused by Legionella. Estimation of the infective dose

(i.e., the dose required to produce infection) is necessary for completing a risk assessment of a microbial

pathogen. Legionella are opportunistic pathogens with widespread environmental occurrence and a very

low attack rate in the general population. Legionella survive and thrive inside vesicles after being

ingested by amoebae in water reservoirs, but much more information is needed on the implications of

this potential vehicle for enhanced transmission and infectivity. More complete information is also

needed concerning the conditions under which a population is likely to be exposed to the infective dose,

with models that accommodate aspiration and inhalation routes, as well as the variability introduced by

the bacteria's potential replication within host cells.
VI-9

Despite deficiencies in understanding several of the factors necessary to determine the risk of

infection by Legionella, the current state of knowledge is sufficient to support specific recommendations

to control and prevent legionnaires' disease.

F. Minimizing Risk

In the 22-year period since the sentinel outbreak in Philadelphia of what is now known as

legionnaires' disease, a great deal of knowledge has been gained on the behavior and occurrence of

Legionella. Based on this knowledge, efforts to minimize the risks of Legionella infection have been

instituted, especially for the protection of susceptible or high-risk individuals.

Because there is little if any person-to-person transmission of Legionella and no vaccine is

available to prevent infection, risk minimization efforts are focused on breaking the chain of

transmission between environmental sources of Legionella and human hosts. Approaches used for

controlling the growth of Legionella in treated water, frequently used in combination, include heat,

chlorination, ultraviolet light, copper-silver ionization, and ozone treatment. These various treatment

options are detailed in Chapter VII of this report. For hospitals and other health care settings, regular

environmental surveys of both hot water systems and distal sites should be conducted; some health

departments have issued mandates for such testing (Allegheny County Health Department 1997). In

health care institutions, these environmental surveys can also serve to raise awareness and the index of

suspicion of health practitioners for consideration of Legionella as the causative agent in nosocomial

pneumonia cases (Yu 1997).

Active surveillance for Legionella infection, especially among hospital patients at highest risk of

acquiring nosocomial infection (i.e., transplant patients, immunocompromised patients, or patients with

certain chronic underlying health conditions) is also an important tool for minimizing risk of

legionnaires' disease because it allows for prompt remedial actions and rapid diagnosis and treatment of

confirmed cases. As discussed in Chapter V of this document, earlier treatment is associated with

reduced severity of disease and reduced mortality. Both the control measures and active surveillance for

cases can be expensive, however, and ultimately require cost-benefit decisions. Several recent

publications have outlined some of the important considerations in making such cost-benefit decisions.
VI-10

Shelton and colleagues (1993) developed a system for determining when Legionella detection

may justify preventive or remedial actions. Analysis of samples from buildings with reported

legionnaires' disease outbreaks and with no reported cases (a total of 900 samples) revealed a strong

association between amplified Legionella levels and legionnaires' disease outbreaks. Their system

matches action levels (scale of 1 to 5) with specific environmental concentrations of viable Legionella

detected in three types of sources: cooling towers and evaporative condensers; potable water; and

humidifiers/foggers. The concentrations corresponding to the action levels vary depending on the

environmental source. For example, the highest level of concern (Hazard Level 5) corresponds to

Legionella concentrations above 1,000 organisms/mL in cooling towers, above 100/mL in potable water,

or above 10/mL in a humidifier/fogger. For Hazard Level 5, the authors recommend immediate

disinfection of equipment. Hazard Level 3 represents "a low but increased level of concern" and

corresponds to Legionella concentrations above 10/mL in cooling towers and above 1/mL in potable

water or a humidifier/fogger. The study authors noted, however, that among the 900 samples, some of

the samples rated Hazard Level 5 were obtained in buildings without any reported legionnaires' disease

cases. Such findings illustrate the lack of a direct relation between detected environmental levels of

Legionella and risk of disease. Nevertheless, preventive or remedial actions may be warranted when

Legionella concentrations exceed certain limits.

In 1997, the Centers for Disease Control (CDC) for the first time included information on

Legionella infections in their revised "Guidelines for Nosocomial Pneumonia" (CDC 1997a). For

hospitals without any identified cases, the CDC outlined two primary prevention measures: (a) routine

culturing of the potable water system, with initiation of active surveillance (i.e., increasing the use and

availability of diagnostic laboratory tests for Legionella) when 30 percent or more of environmental

samples are positive for Legionella; or (b) utilizing diagnostic laboratory tests for high risk patients with

nosocomial pneumonia, with routine maintenance of potable water supplies (i.e., with sufficient heat and

chlorination), and initiation of an environmental investigation once one definite or two possible cases of

legionnaires' disease have been identified. 

The CDC Guidelines also outline secondary prevention measures for hospitals where nosocomial

legionnaires' disease cases have been identified, with a caveat that full-scale environmental

investigations and decontamination measures may not always be indicated in all hospitals. The decision
VI-11

to initiate such measures depends on the level of risk for infection and mortality from Legionella in the

given patient population. Nonetheless, the CDC also cautions that a low threshold for initiating an

environmental investigation may be appropriate because nosocomial cases have typically been

underdiagnosed, and additional recent or ongoing nosocomial cases typically are identified once several

cases have been confirmed. The Guidelines (CDC 1997) describe five important steps in conducting

such an environmental investigation: (1) review of medical records; (2) active surveillance to identify

recent or ongoing nosocomial cases; (3) identification of risk factors for infection and comparison of

cases and controls, through the collection of information on environmental exposures (e.g., showering or

the use of respiratory therapy equipment); (4) collection of water samples from the implicated sources

and other potential aerosol sources; and (5) subtype matching of patient isolates and the environmental

samples. Decontamination or replacement of the identified environmental sources must also take place. 

Clearly, these secondary prevention measures can require extensive resources. 

G. Summary

Given that legionnaires' disease is the most serious infection caused by Legionella, risk

assessment of these organisms should be focused on legionnaires' disease as the endpoint of concern. 

Legionella are opportunistic pathogens with widespread distribution in the environment but a very low

rate of infection in the general population. The sources of transmission of Legionella to humans have

been well characterized, and almost all of these sources (with the exception of contaminated medical

equipment) involve the aerosolization of water contaminated with Legionella and subsequent inhalation

or aspiration. Potable water, especially in hospitals and other buildings with complex hot water systems,

is considered to be the most important source of Legionella transmission.

Despite many advances in laboratory isolation and identification techniques and the availability

of findings from recent epidemiological and experimental studies, the current state of the science does

not allow for quantification of the potential risks caused by Legionella in water supplies. Nevertheless,

important preventive (or remedial) actions have been identified that can minimize the risks of Legionella

infection, especially for the protection of high-risk or susceptible individuals.
VI-12

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VI-13

VII. Analysis and Treatment

A. Analysis of Samples 

Legionella can survive in a wide range of conditions including variable temperatures, pH-levels,

and dissolved oxygen concentrations. In addition, algae and other water bacteria can promote their

growth (Nguyen et al. 1991). Detection of Legionella contamination in potable water and plumbing

fixtures, as well as in biological samples, is a major concern, particularly of hospitals experiencing cases

of legionnaire’s disease. The 1985 Legionella Criteria Document discusses collection (disassembly,

swabbing and scraping, centrifugation, filtration), isolation (culturing), and detection techniques (Direct

Fluorescent Antibody (DFA), Indirect Fluorescent Antibody (IFA), monoclonal antibodies, and

radioimmunoassay). However, more recent studies and additional data on the collection, isolation, and

detection of Legionella in both water and biological samples have been published and are described

below.

1. Collection of Legionella

Most outbreaks of legionellosis come from warm waters, as higher temperatures generally

stimulate the growth of these organisms. It is difficult to culture Legionella in waters below 20°C

(Colbourne et al. 1988). Test samples for Legionella typically come from anthropogenic sources such as

faucets, sink outlets, taps, filters, and showerheads, which are usually sampled by disassembling,

swabbing, and scraping to obtain Legionella-bearing debris or scale (Stout et al. 1992b, Helms et al.

1988, Stout and Yu 1997, Barbaree et al. 1987). As reported in the 1985 Legionella Criteria Document,

the most effective manner of obtaining the sample is by insertion of sterile cotton swabs into the interior

surface of the water source. Ta et al. (1995) found that swabbing recovered greater concentrations of

Legionella organisms than two other methods (water sampling before swabbing and water sampling after

swabbing), exposing an average of 30.2 CFU in each swab sample while the concentration of Legionella

in the water samples averaged 4.7 CFU. Swab sampling is also the preferred sampling method because

the swab is easier to transport and requires less processing time than straight water samples (Ta et al.

1995).
VI-14

The study by Ta et al. (1995) also concluded that concentration of the water sample, either by

filtration or centrifugation, greatly improved the ability to detect Legionella in the samples. Filtration

was proven more effective than centrifugation, recovering 77 percent of the expected organism count

while centrifugation recovered only approximately 34 percent (Ta et al. 1995). 

The 1985 Legionella Criteria Document provided summaries of several studies that used

filtration or centrifugation to concentrate Legionella, and recommended heat and acid wash treatment to

isolate Legionella from environmental specimens. Because Legionella can survive at high temperatures,

heating (at 60°C for 1-2 minutes) was found to reduce the strains of other bacteria contaminants (e.g.,

Pseudomonas aeruginosa) by 98 percent while leaving the Legionella unaffected. 

Acid wash treatment is used to isolate Legionella because unlike most bacteria, Legionella

strains are acid resistant (Nguyen et al. 1991). Ta et al. (1995) showed that although acid buffer

treatment did not enhance the recovery of L. pneumophila bacteria, it was in fact required for an optimal

yield of other strains. A detailed procedure for isolation of Legionella from environmental water

samples by acid treatment was described by Bopp et al. (1981) and summarized in the 1985 Legionella

Criteria Document. Water samples were pretreated, either concentrated by centrifugation or not

concentrated, with an HCl-KCl buffer mixture at pH 2.2 for periods of 5-60 minutes. The greatest

quantity of isolations were obtained by acid treatment of centrifuged samples for 5 minutes (Bopp et al.

1981). More current studies have shown that samples treated with acid for three minutes can minimize

the development of competing bacteria (Ta et al. 1995). 

Following the collection and pretreatment steps, the samples are plated onto appropriate media. 

Legionella do not grow on standard culture media. They have complex nutritional requirements,

featuring an unusually high iron requirement. The 1985 Legionella Criteria Document described various

media that can be used for culturing Legionella including a charcoal yeast extract (CYE) medium, which

was improved to the buffered charcoal yeast extract (BCYE) that is presently used to successfully isolate

these organisms. This medium is ACES buffered charcoal yeast extract (BCYE) agar supplemented with

-ketoglutarate (BCYE ), a Krebs-cycle intermediate that is readily catabolized by these bacteria

(Edelstein 1987). An incubation period of two to six days ensues when Legionella are cultured on this

medium (Grimont 1986). The buffer maintains the pH within a range that is critical for Legionella
VI-15

(around pH 6.9) while the -ketoglutarate stimulates growth. Growth is further enhanced by the

addition of L-cysteine, keto acids, and ferric ions. Antimicrobials such as glycine (inhibitor),

cefamandole, polymyxin B, vancomycin (antibacterials), and anisomycin and cyclohaxamide

(antifungals) are added to inhibit or prevent the overgrowth of contaminants (Nguyen et al. 1991). 

Selective media containing dyes, glycine, vancomysin, and polymyxin (DGVP) is used for

environmental sampling (Lin et al. 1998). 

2. Detection of Legionella in Environmental and Biological Samples

An array of serological tests have been used for detecting Legionella in water, sputum, blood,

serum, and urine samples. Kohler (1986) reports that antigens can be detected in the urine of

approximately 80 percent of patients with L. pneumophila serogroup 1 pneumonia, and that the

specificity of these assays is greater than 99 percent. Most tests are used on lower respiratory tract

secretions, specifically tissue specimens, bronchial and tracheal secretions, and sputum. Sputum

specimens are pretreated with acid and cultured on selective media, similar to the pretreatment of

environmental samples. Tracheal aspirate specimens on culture media can provide a yield of 90 percent

sensitivity (Nguyen et al. 1991).

The two main serologic tests performed on bacteria are direct and indirect fluorescent assays,

which are applicable to both environmental and clinical specimens. Fluorescent organic compounds are

attached to antibody molecules that are bound to a cell or tissue’s surface antigens, and then these tags

are detected by a fluorescent microscope. In the direct method, the antibody against the organism is

fluorescent, while the indirect method has the fluorescent antibody detected against a nonfluorescent

antibody on the surface of the cell. The Direct Immunofluorescence Assay (DFA) and the Indirect

Immunofluorescence Assay (IFA), which were examined in the 1985 Legionella Criteria Document, are

described further below. Other serologic tests described in the 1985 Legionella Criteria Document and

discussed below are enzyme-linked immunosorbent assays, monoclonal antibodies, and

radioimmunoassay. Serologic tests that are currently being used for detection of Legionella antigens or

antibodies, including Polymerase Chain Reaction, and nucleic acid and DNA probes, are also discussed

below. The serologic tests differ primarily in sensitivity, specificity, predictive value, and complexity.
VI-16

Direct Immunofluorescence Assay (DFA)

The most common and rapid test for Legionella is the DFA. According to Nguyen et al. (1991),

the exhibited sensitivity of DFA tests ranges anywhere from 25 to 85 percent. Sputum, lung specimens,

and bronchial and tracheal secretions are excellent samples to test by the DFA method (Grimont 1986). 

Kohler (1986) reports that as long as proper quality control exists, the specificity of DFA testing in

respiratory specimens is greater than 90 percent. However, he cites examples of DFA accuracy results

for serogroup 1 infections of 50 percent, 47 percent, 68 percent, and 33-47 percent, where sensitivity and

specificity were not ideal in testing respiratory specimens. There are also DFA tests that use speciesspecific monoclonal antibodies that are particularly useful for lower respiratory tract samples and tissues. 

The monoclonal reagent is optimal for detection of L. pneumophila in respiratory specimens due to its

ability to detect multiple serogroups of L. pneumophila and decreased non-specific fluorescence of the

specimen and other bacteria. It may not be useful in the detection of environmental specimens (Grimont

1986). 

Indirect Immunofluorescence Assay (IFA)

Legionella bacteria, and antibodies in patient sera, are detected through IFA. Heat-fixed antigens

are commonly used in the United States, but formolized antigens, used primarily in Europe, are said to

actually be more specific than heat-fixed antigens. Because seroconversion only occurs after a rather

long time period in humans, the IFA test is often used in conjunction with other tests (Kohler 1985). A

series of serological tests are typically conducted to test for antibodies, and they are most often run in

conjunction with the IFA (Colbourne et al. 1988, Grimont 1986, Edelstein 1987, Kohler 1985, Ehret et

al. 1986, Kashuba and Ballow 1996).

Enzyme-Linked Immunoabsorbent Assays (ELISA)

Enzyme-Linked Immunosorbent Assays (ELISA), radioimmunoassay (RIA), and agglutination

assays have also been used to detect Legionella antibodies. These methods employ enzymes and

radioisotopes to detect antibody molecules. The ELISA method is used to detect Legionella antibodies

in patient sera, but it has also been used to detect Legionella antigens in urine. The RIA method has also
VI-17

been used for the detection of Legionella antigens in urine, but is no longer commercially available. The

agglutination method has been used to detect antibodies in serum and antigens in urine. These tests are

all extremely sensitive because radioactivity and enzyme reaction products can be measured in very

small amounts. Enzyme-linked methods are preferred over radioactively tagged methods of discovery to

eliminate the problem of radioactive material disposal even though a longer incubation period is required

for these types of tests. With ELISA, preheating of the specimen is required to avoid false positives. 

Agglutination assays, which clump organisms to other particles, are simpler and faster assays that are

generally easier to perform than the others (Kohler 1986).

Monoclonal Antibody

Monoclonal antibody tests, tests with antibodies formed from a single clone of cells, have been

found to be more accurate than polyclonal tests due to the suppression of background fluorescence. 

Also, false positive results from cross-reactivity with non-Legionella organisms are eliminated (Stout

and Yu 1997). Monoclonal antibody tests are effective due to their high specificity for a single antigenic

determinant. Monoclonal antibodies can be produced to react only with a particular species, or even

strain, of bacteria. According to Kohler (1986), numerous laboratories have asserted that antibodies for

L. pneumophila have been developed to be species-specific and even serogroup-specific. These

monoclonal antibodies can be aimed against subsets of a specific serogroup and then used for antigen

detection by ELISA (Kohler 1986).

Polymerase Chain Reaction (PCR)

The Polymerase Chain Reaction (PCR) test uses two disparate primers. One is specific for

Legionella species, and the other is specific for L. pneumophila only. The primers are specific for the

gene sequences of the 5S ribosomal RNA gene. The PCR was converted into a kit called the

EnviroAmp® Legionella Kit; however, the kit is no longer commercially available. PCR is a relatively

new method designed to rapidly multiply DNA target genes in a laboratory setting to yield detectable

quantities for testing. A study done by Fricker and Fricker (1995) compares this technique to standard

culture methods for water samples. The positive results of the PCR matched those of the culture in all

but 4 of 87 cases, where the PCR reaction was apparently inhibited. Generally, the PCR was found to be
VI-18

a very useful screening test because it is both fast and accurate. The main problem with the PCR method

is that it identified several negative cultures as positive. This issue is being investigated, but it is either

due to false positive results, discovery of dead Legionella, or detection of viable but non-culturable

Legionella (Fricker and Fricker 1995).

Murdoch et al. (1996) studied the ability of PCR tests to detect Legionella DNA in urine and

serum samples of pneumonia patients. There was a 64 percent detection rate in the urine and/or serum

samples, with this figure rising to 73 percent if testing was done within four days of the onset of

symptoms. 

Nucleic Acid and DNA Probes

Nucleic acid and DNA probes can also be used to detect Legionella. With these methods, probes

are marked with RNA or DNA sequences that are specific to a particular species or strain of bacteria. 

Nucleic acid probes require that the nucleic acids of the bacteria become accessible and prepared to react

with the tagged probe. Detection using this method has been reported to be anywhere from 5 to 100

percent, with L. pneumophila giving the highest values (Grimont 1986). According to Edelstein (1987),

a probe kit generally has a sensitivity of 75 percent and a specificity of 100 percent “if certain samples

are excluded from the analysis.” Although it is more sensitive to L. pneumophila detection, it will still

quickly recognize all Legionella species (Edelstein 1987). This new detection method, however, as

reported by Nguyen et al. (1991), has yet to be clinically validated and is rather insensitive and costly. 

Urinary Antigen

There is a commercially available enzyme immunoassay (EIA) test for the Legionella antigen in

urine from the company Binax, Inc. in Portland, Maine. Nguyen et al. (1991) reports that the test

exhibits 99 percent specificity and greater than 90 percent sensitivity and that it is relatively inexpensive. 

The main drawback to this urinary antigen test is that it only detects antigens of L. pneumophila

serogroup 1. However, since this species accounts for upwards of 80 percent of all legionellosis

infections, this weakness is rather slight (Nguyen et al. 1991). 
VI-19

B. Disinfection as a Water Treatment Practice

Legionella are found in natural aquatic environments, artificial aquatic environments such as heat

rejection devices (cooling towers and evaporative condensers), and water distribution systems (Muraca

et al. 1990). Water distribution systems in hospitals, hotels, institutional buildings, and domestic homes,

as well as personal respiratory therapy equipment, freestanding room humidifiers in hospitals, industrial

cutting oil/water emulsions, and communally used whirlpools and spas have been shown to be reservoirs

for Legionella (World Health Organization 1990, Moreno 1997). Legionella colonization is promoted

by temperatures below 50°C (122°F), scale and sediment accumulation, stagnation (which prevents

disinfectant from reaching the bacteria), and design of the hot water tank (see Chapter III, Section F for

further discussion of factors affecting Legionella survival) (Muraca et al. 1990). The 1985 Legionella

Criteria Document indicated that Legionella surviving initial water treatment may colonize pipe joints,

cul-de-sacs, and corroded areas or adhere to the surface or sediment of storage tanks, especially those

constructed of wood (EPA 1985). New distribution systems may also be a source of Legionella

contamination; the 1985 document cited cases in which Legionella outbreaks have occurred in new

distribution systems (EPA 1985).

There are several control methods available for disinfection of water distribution systems. These

include thermal (super heat and flush), hyperchlorination, copper-silver ionization, ultraviolet light

sterilization, ozonation, and instantaneous steam heating systems. These disinfection methods are

discussed below. The use of heat, chlorine, ultraviolet sterilization, and ozone were discussed in the

1985 Legionella Criteria Document, however, recent studies have been conducted that provide updated

information. Because one methodology may not be sufficient, a combination of these techniques may be

more effective in eradicating Legionella from the system and preventing recolonization (Yu et al. 1993). 

Thermal Disinfection

Thermal disinfection is a common practice for water distribution systems in hospitals, hotels, and

other institutional buildings. The hot water temperature is elevated to above 70°C (158° F), and distal

sites, such as faucets and showerheads, are flushed for thirty minutes (Nguyen et al. 1991, Miuetzner et

al. 1997, Stout and Yu 1997). L. pneumophila is killed at temperatures above 60°C (140°F). At 70°C
VI-20

(158°F), it takes ten minutes to eliminate L. pneumophila from water, and at 60°C (140°F) L.

pneumophila are eradicated in 25 minutes (Muraca et al. 1990). In cases of outbreak, thermal

disinfection can be quickly implemented. No special equipment is needed, and it is relatively

inexpensive (Stout and Yu 1997, Muraca et al. 1990, Nguyen et al. 1991). The disadvantages to this

method are the potential for scalding and the fact that many personnel are required to monitor distal

sites, tank water temperatures, and flushing times (Nguyen et al. 1991, Muraca et al. 1990). In addition,

recolonization will occur within months because disinfection using this method is only temporary (Lin et

al. 1998).

In state development centers for mentally and physically handicapped people, hot water tanks

positive for Legionella were heated to 71°C for 72 hours followed by flushing for 15 minutes. In one

center, Legionella reoccurred after three months. Consequently, a quarterly heating schedule was

established in both centers (Beam et al. 1984).

Hyperchlorination

Hyperchlorination of water distribution systems requires the installation of a chlorinator. Shock

hyperchlorination involves the addition of chlorine to a water system, raising chlorine throughout the

system to a concentration of 20 to 50 mg/L. The chlorine levels of the system are returned to 0.5 to 1

mg/L after one to two hours (Lin et al. 1998). Continuous hyperchlorination entails the addition of

chlorinated salts (e.g., calcium hypochlorite (solid) or sodium hypochlorite (aqueous)) to the water at

concentrations ranging from 2 to 6 mg/L (ppm) (Stout and Yu 1997, Muraca et al. 1990). Domestic

residual levels are typically 1 mg/L (ppm) (Muraca et al. 1990). The 1985 Legionella Criteria

Document suggests using chlorine levels of 1-2 mg/L (ppm), however, recent studies have shown that

using chlorine levels of 3-5 mg/L is more effective (Helmes et al. 1988). The chlorinator will maintain a

set level of chlorine throughout the system, which should completely eliminate Legionella.

Unfortunately, this method is relatively expensive, and it does have some drawbacks. This method leads

to corrosion of the pipes of the system after five to six years of operation, and eventually parts of the

system may be destroyed. Corrosion can be reduced by the use of a silicate coating on the water pipes

(Nguyen et al. 1991). In addition, mechanical failure of the chlorinator, if not detected, could result in

Legionella recolonizing the system (Nguyen et al. 1991). Human health problems are another result of
VI-21

hyperchlorination. High levels of trihalomethanes develop in the hot water of the system when chlorine

levels exceed 4 mg/L (Helmes et al. 1988, Muraca et al. 1990). Trihalomethanes are potentially

carcinogenic, and can be reduced by maintaining the concentration of the chlorine below 4 mg/L

(Muraca et al. 1990).

Ezzeddine et al. (1989) describes disinfection in a hospital where 6 ppm of free residual chlorine

was used in a heating tank during a 6-hour period of time. Legionella was eliminated from the tank;

however, chlorination of the mixer tank, where the temperature was 45°C, was not successful even when

chlorine levels were raised to 6 ppm over 48 hours. 

Helmes et al. (1988) combined the hyperchlorination method with an elevated water temperature

at a University of Iowa hospital after a 1981 outbreak of nosocomial legionellosis. Chlorine levels were

set at 3-5 mg/L, while temperatures were raised to 60-70°C. After six months of hyperchlorination,

Legionella was no longer detected in samples.

Copper-Silver Ionization

Copper-Silver Ionization distorts the permeability of the Legionella cell, denatures proteins, and

leads to lysis and cell death. A commercial system can be easily installed to perform this ionization. 

This system sends an electrical current to copper/silver electrodes, which generate positively charged

ions. These positively charged ions electrostatically bond to the negatively hypercharged sites on the

cell walls of the microorganisms (Nguyen et al. 1991, Miuetzner et al. 1997, Muraca et al. 1990). The

Legionella are then killed, making it unlikely that recolonization will occur. Copper-silver ionization is

less expensive than hyperchlorination and provides residual protection throughout the water distribution

system (Nguyen et al. 1991, Muraca et al. 1990). A disadvantage of this approach is that the system’s

performance will suffer unless scale is removed regularly from the electrodes and the pH of the system is

maintained below 8. Also, extremely high concentrations of copper and silver ions will turn the water a

blackish color, which can stain porcelain (Lin et al. 1998). Another disadvantage is that over an

extended period of time, human consumption of the water from this system may result in accumulation

of copper and silver and toxic effects (Muraca et al. 1990). However, because copper and silver ions are

typically only added to hot water recirculating lines, human exposure would be minimal since
VI-22

consumption of large amounts of water is unlikely (Lin et al. 1998). In addition, the levels of ions in hot

water are maintained below the EPA recommended levels for cold drinking water which are 1.3 ppm

copper and 100 ppb for silver (a secondary minimum contaminant level).

 

Miuetzner et al. (1997) used a flow-through cell containing two sets of four copper-silver

electrodes. A single cell was installed in each of three hot water circuits of a hospital. The copper-silver

ionization system significantly reduced the amount of L. pneumophila recovered from the faucets from

72 percent to 2 percent within one month. Control of Legionella was maintained for at least 22 months

after the ionization treatment.

Ultraviolet Light Sterilization

Ultraviolet light kills Legionella by disrupting cellular DNA synthesis (Muraca et al. 1990). An

ultraviolet light sterilization system can be installed easily. It can be positioned to disinfect the incoming

water, or it can be installed at a specific place in the pipe system that services a designated area. The UV

system consists of low-pressure mercury lamps in quartz sleeves. Sterilization is most effective at UV

energy wavelengths of 254 nm and temperatures of 40°C (104°F) (Muraca et al. 1990). A filter should

be used to remove particulates from the water to keep UV light transmission optimal (World Health

Organization 1990). No chemical by-products are produced, and the taste and odor of water from a

water distribution system containing a UV sterilizer are not affected (Muraca et al. 1990). The UV

sterilization system requires continuous maintenance in order to prevent scale from coating the UV

lamps. The system does not provide residual protection, so distal areas must be disinfected (Nguyen et

al. 1991, Muraca et al. 1990). Operational problems, such as electrical malfunction and water leaks, are

possible, in which case experienced technicians are needed (Muraca et al. 1990). 

Ozonation

Ozone can be used to kill L. pneumophila. It can be created using ozonators, which electrically

excite oxygen (O2) to ozone (O3). Ozone instantaneously inactivates Legionella; however, it has a short

half-life and decomposes quickly back to oxygen. A second form of disinfection may be required in the

distribution system for residual protection. Also, ozonation is more expensive than hyperchlorination, 
VI-23

and a large amount of space is required for the air preparation equipment or oxygen tanks and contacting

tank (Muraca et al. 1990). Ozonation was described in the 1985 Legionella Criteria Document as a

possible method of eliminating Legionella from a water distribution system. At the time, few studies

had been conducted and the results were inconclusive.

Muraca et al. (1987) recommend using a 1-2 mg/L ozone residual for treatment of domestic

water. They demonstrated that a 1-2 mg/L ozone residual caused a 5 log decrease in a L. pneumophila

population of 107

 CFU/mL over five hours within a model plumbing system. 

Instantaneous Steam Heating

Instantaneous Steam Heating systems entail flash heating water to temperatures greater than

88°C (190°F) and then blending the hot water with cold water to attain a designated water temperature

(Nguyen et al. 1991, Muraca et al. 1990). These systems are often cost-effective because specialized

personnel are not needed to operate them; maintenance can be performed by regular building staff. The

maintenance is, however, more complex than the maintenance of a conventional hot water tank. 

Instantaneous Steam Heating systems work best when installed as the original system of a building

rather than when the building has already been contaminated by Legionella. Another drawback to this

system is that it can only be used to control Legionella in the hot water supply system. The cold water

portion of the distribution system is not disinfected (Muraca et al. 1990). Any Legionella that may have

colonized the system downstream of the heater will be unaffected. In order for disinfection to be

complete, the hot water temperature at outlet sites must exceed 60°C. These heaters may not have the

ability to flush large amounts of outlets with superheated water for thirty minutes (Lin et al. 1998).

C. Summary

The examination of water for the presence of Legionella is best done by taking swab samples of

the medium over which the water flows. The specimen should then be concentrated by filtration, treated

with an acid buffer to enhance Legionella recovery, and cultured on a BCYE agar medium. Legionella

can be detected in environmental and biological samples by a number of tests, the most common of

which are direct and indirect immunofluorescence assays.
VI-24

Contamination by Legionella has occurred in the water distribution systems of many hospitals,

hotels, and other buildings. Various means of disinfection have been established and utilized. Some

methods have not always proven completely successful or have not provided permanent protection from

recolonization. A combination of these methods may be the most effective way of managing water

systems and preventing future outbreaks. Yu et al. (1993) defines two categories of disinfection, focal

and systemic. Focal disinfection is directed at a specific portion of the system and would include

ultraviolet light sterilization, instantaneous heating systems, and ozonation. Systemic methods, such as

thermal, hyperchlorination, copper-silver ionization, disinfect the entire system. Selecting a combination

of focal and systemic disinfection techniques would ensure eradication of present Legionella colonies

and prevent recolonization of the water distribution system. 
VIII-1

VIII. Research Requirements

From all of the information presented in the previous chapters, it is clear that Legionella bacteria

are an important cause of community- and hospital-acquired pneumonia, and they can be associated with

serious morbidity and mortality, especially when the infection is not rapidly diagnosed and treated. 

Legionella are widely distributed in the environment, including treated water supplies. In the past 13

years (i.e., during the time since publication of the 1985 EPA Criteria Document on Legionella),

dramatic advances have been achieved in our understanding of the behavior and transmission of

Legionella, including information on: special ecological niches occupied by these organisms, including

their presence in biofilms and their symbiotic relationships with larger microbes such as amoebae;

improved techniques for the clinical isolation (e.g., culture techniques) and characterization (e.g., PCR

technology) of these organisms; improved methods for identifying patients recently or currently infected

with these organisms (e.g., urinary antigen assay); factors important for understanding the epidemiology

of legionellosis infection; and effective measures for eradicating these organisms from treated water

supplies.

Despite the important advances in the 13 years since the previous EPA Legionella Criteria

Document, additional information is needed to institute optimal prevention and control measures and to

minimize the morbidity and mortality associated with Legionella. Specific information gaps include the

following:

! The relative influence of the symbiotic relationship between Legionella organisms and larger

microbes on Legionella survival, transmission, virulence, and susceptibility to disinfection.

More information is also needed on the implications of the intracellular replication of Legionella

inside host microbes.

! Key environmental factors promoting the growth of Legionella in biofilms. Additional

information is needed about the structure and physiology of biofilms, and in particular, the

effects of changing environmental conditions on their ecology.
VIII-2

! More comprehensive data on the occurrence of Legionella in groundwater, especially as it relates

to potable water supplies.

! Further information on the relative importance of various reservoirs of the organism (and thus the

allocation of expenditures for disinfection); in particular, the diminishing role of cooling towers

and the increasing prominence of potable water distribution systems as reservoirs for Legionella.

! The nature of the dose-response relationship for this organism, including the development of

models, particularly for exposures from potable water. An effort should be made to determine

the predictive value of Legionella concentrations found in a given reservoir. Research is also

needed to establish the minimal infectious dose for high-risk populations.

! A clearer definition of the important factors involved in transmission of this infectious agent

from a specific source, which would be facilitated by more accurate identification of legionellosis

cases, especially of sporadic cases, and the corresponding improved epidemiological and

environmental analyses.

! The further characterization of risk factors for acquiring legionellosis, particularly for

community-acquired, sporadic cases. Many cases of legionellosis undoubtedly still go

unrecognized. Information indicating patients at greatest risk of Legionella infection should also

be disseminated more widely to clinicians, with the hope of more accurately and rapidly

identifying (and treating for) Legionella as the causative agent, thus reducing morbidity and

mortality associated with these organisms.

! The risk for development of legionnaires' disease from Legionella present in residential water

systems (single family or multi-family dwellings).

! Identification of the most effective (and most cost-effective) biocidal treatments for a given

source of Legionella.
VIII-3

! Delineation and development of specific design and operational/physicochemical modifications

for building water supply systems, in order to minimize colonization by Legionella and symbiont

hosts, including biofilm eradication.

Given the potentially high costs of surveillance for, and eradication of, Legionella from treated

water supplies, new information that fills some of these gaps will be of great value in identification and

institution of the best strategies for prevention of legionellosis.
VIII-4

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IX-1

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