United States
Environmental Protection
Agency
Office of Science and Technology
Off ice of Water
Washington, DC 20460
EPA-822-B-01-005
March 2001
www.epa.gov
&EPA
Legionella: Drinking Water
Health Advisory
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I. INTRODUCTION
The Health Advisory Program, sponsored by the Office of Water (OW), provides information on the
health effects, analytical methodology and treatment technology that would be useful in dealing with the
contamination of drinking water. Most of the Health Advisories (HAs) prepared by the Office of Water are for
chemical substances. This Health Advisory however addresses contamination of drinking water by a microbial
pathogen, examines pathogen control, and addresses risk factors for exposure and infection.
Health Advisories serve as informal technical guidance to assist Federal, State and local officials
responsible for protecting public health when emergency spills or contamination situations occur. They are not
to be construed as legally enforceable Federal standards. The HAs are subject to change as new information
becomes available.
This Health Advisory is based on information presented in the Office of Water's Criteria Document (CD)
for Legionella. Individuals desiring further information should consult the CD. This document will be
available from the U.S. Environmental Protection Agency, OW Resource Center, Room M6099; Mail Code:
PC-4100, 401 M Street, S.W., Washington, D.C. 20460; the telephone number is (202) 260-7786. The
document can also be obtained by calling the Safe Drinking Water Hotline at 1-800-426-4791.
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II. GENERAL INFORMATION
History
Legionella bacteria were discovered following a pneumonia outbreak at the 1976 American Legion
Convention in Philadelphia (Brenner 1987). The bacteria isolated from infected lung tissue and
identified as the causative agent of this pneumonia outbreak was named Legionellapneumophila,
receiving the name Legionella to honor the stricken American legionnaires andpneumophila from the
Greek word meaning "lung-loving" (Fang et al. 1989).
The symptoms exhibited in the 1976 outbreak were termed legionnaires' disease. Pneumonia occurs in
approximately 95 percent of Legionella infections (Nguyen et al. 1991). Less commonly, Legionella
bacteria cause an influenza-like infection in humans called Pontiac fever (Hoge and Brieman 1991).
Taxonomy
Although some phenotypic characteristics (i.e., gram stain, cell membrane fatty acid and ubiquinone
content, morphology, and growth on specific media) can be used to recognize Legionella bacteria at the
genus level, more specific diagnostic techniques are required to differentiate individual species
(Bangsborg 1997, Fang et al. 1989, Winn 1988). Currently, the best methods to classify Legionella
species are DNA analysis and antigenic analysis of various proteins and peptides.
Currently, the Legionella genus consists of 42 species, seven of which are 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).
The majority of human infections (70-90%) have been caused by L. pneumophila, especially serogroups
1 and 6 (Lo Presti et al. 1997).
Microbiology, Morphology, and Ecology
Legionella bacteria are small gram-negative rods. They areunencapsulated and nonsporeforming, with
physical dimensions from 0.3 to 0.9 m in width and from 2 to more than 20 m in length (Winn 1988).
Most species 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 branched-chain
cellular fatty acids and also ubiquinones with side chains of more than 10 isoprene units (Brenner et al.
1984). Legionella 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 from potting soils and moist soil samples (Fields 1996). Legionella can survive in
varied water conditions, in temperatures of 0-63 °C, a pH range of 5.0-8.5, and a dissolved oxygen
concentration in water of 0.2-15 ppm (Nguyen et al. 1991).
Symbiosis in Microorganisms
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Legionella proliferation is dependent on their symbiotic relationships with other 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 have been shown to 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).
Currently, Legionella are known to infect a total of 13 species of amoebae and two species of ciliated
protozoa (Fields 1996). Legionella also can multiply intracellularly 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 proliferate within these
symbiont hosts provides Legionella with protection from otherwise harmful environmental conditions.
Thus, Legionella are able to 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. Their enhanced resistance to water treatment has major implications for both
disease transmittance and water treatment procedures.
Legionella also grow symbiotically with the aquatic bacteria attached to the surface of biofilms (Kramer
and Ford 1994). Biofilms provide the bacteria with nutrients for growth and also offer protection from
adverse environmental conditions (including during water disinfection). The concentration of
Legionella in biofilms depends upon water temperature; at higher temperatures, they can more
effectively compete with other bacteria. Because biofilms colonize drinking water distribution systems,
they provide a habitat suitable for Legionella growth in potable water, which can lead to human
exposure.
III. OCCURRENCE
Because routine environmental monitoring for Legionella 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 section considers the worldwide occurrence or incidence of legionellosis
and outbreaks of legionellosis as well as the occurrence of Legionella bacteria in water, soil, and air.
Environmental factors influencing Legionella survival also are discussed.
Worldwide Distribution
Cases of legionellosis have been reported 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 maintained 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 artifactual due to
differences in definitions, diagnostic methods, surveillance systems, or data presentation (Bhopal 1993).
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National surveillance programs currently are conducted in the United States, 24 European countries,
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 (CDC 1994, CDC 1996, CDC 1997b). These figures
represnt passive sureillance and accounts for <1000 reported cases when compared to Marston et al.
data. Which suggest 10-15 times as many projected cases from active sureillance.
In England and Wales, annual totals of reported cases declined briefly after a peak in 1988 but have been
increasing since 1993 (Joseph et al. 1997).
In 1996, cases of legionnaires' disease were reported in 24 European countries including England, Wales
and Scotland (Anonymous 1997b). 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. This increase was
attributed mainly to a large community outbreak in Spain in 1996.
There have been 1,041 notifications of legjonellosis in Australia since 1991, with similar numbers of
cases reported each year (Anonymous 1997a).
Occurrence in Water
Legionella are considered to be ubiquitous in the aquatic environment, including both natural water
bodies and man-made waters (EPA 1985). Research has revealed that Legionella thrive in biofilms, and
interaction with other organisms in biofilms is important for their survival and proliferation in water
(Kramer and Ford 1994, Yu 1997, Lin et al. 1998a).
Natural Surface Water
Studies clearly demonstrate the widespread occurrence of Legionella in freshwater (e.g., lakes and
streams) and marine waters (EPA 1985, Ortiz-Roque and Hazen 1987, Palmer et al. 1993).
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 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.
Man-Made Waters
As noted previously, Legionella bacteria thrive 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).
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In the potable water supply, Legionella bacteria occupy niches suitable for their survival and growth
(e.g., components of water distribution systems, cooling towers, and whirlpools), which function as
amplifiers or disseminators of these bacteria (EPA 1985).
In 1980, British investigators first demonstrated that plumbing fixtures in potable water systems
contained Legionella (EPA 1985). Water distribution systems of hospitals, hotels, clubs, public
buildings, homes, and factories continue to be a major source of Legionella exposure (EPA 1998, 1985).
Studies have shown that Legionella are present in all segments of community water supplies, including
water treatment facilities (Campo and Apraiz 1988, Colbourne and Dennis 1989, Colbourne et al. 1988,
Vossetal. 1986).
Numerous outbreaks of legionellosis have been linked to heat-exchange units (e.g., cooling towers and
evaporative condensers) in hospitals, hotels, and public buildings, clearly establishing these reservoirs as
habitats for Legionella (EPA 1998, 1985). However, as knowledge and awareness of the ecology and
epidemiology of Legionella have increased, attention has shifted from heat-exchange units to potable
water distribution systems as the most important sources of human exposure and infection (Lin et al.
1998b, Yu 1997).
Whirlpools and spas also serve as ideal habitats for Legionella because they are maintained at
temperatures ideal for their growth and organic nutrients suitable for bacterial growth often accumulate
in these waters (Hedges and Roser 1991, Fallen and Rowbotham 1990, Hsu et al. 1986, Jernigan etal.
1996). In addition, whirlpools and spas can produce water droplets of respirable size that have the
potential to transmit Legionella to humans (Jernigan 1996). Other related sources where Legionella
have been identified include spring water spas and saunas (Bornstein et al. 1989a, 1989b, Den Boer et al.
1998).
Legionella also have been detected in all phases of the sewage treatment process, including treated
effluent (Palmer et al. 1993, 1995).
Occurrence in Soil
Although water is the most documented source of Legionella in the environment, these bacteria have
been isolated from mud, moist soil, and potting soil (EPA 1985, Steele et al. 1990). One species in
particular, L. longbeachae, has been shown to inhabit and thrive in potting soil. L. longbeachae was
able to persist for seven months in two potting mixes stored at room temperature (Steele et al. 1990).
Soil rather than water may be the natural habitat of this species and may be a source of human exposure.
Occurrence in Air
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 (Bollin et al. 1985, EPA 1985,
Seidel et al. 1987). Inhalation of Legionella-contaminated aerosols is an important source of human
exposure and infection (EPA 1985).
Specific Disease Outbreaks
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Human exposure to Legionella-contaminated sources can result in outbreaks of legionellosis.
Legionellosis outbreaks have been attributed most frequently to exposure to contaminated potable water,
cooling towers, or components of water distribution systems. Outbreaks of legionellosis caused by
contaminated cooling towers can be dramatic, with numerous cases occurring over a short period of time
(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 be
revealed only after active surveillance is introduced (Brady 1989, Colvilleet al. 1993, Goetz et al. 1998,
Guiget et al. 1987, 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).
Outbreaks of legionellosis are typically categorized as nosocomial (i.e., hospital-acquired), travel-
acquired, or community-acquired. Nosocomial outbreaks have been linked to hospital potable water
supplies as well as cooling towers (EPA 1998).
Travelers are usually exposed to Legionella in contaminated hotel potable water or contaminated
whirlpool spas (EPA 1998).
Community outbreaks are caused by exposure to the widest variety of sources, but potable water and
cooling towers are the most common (EPA 1998).
L. pneumophila has most frequently been implicated as the causative agent for all three types of
outbreaks (EPA 1998).
The majority of cases of legionnaires' disease are community-acquired and sporadic (i.e., non-outbreak
related) (Stout et al. 1992a).
Environmental Factors Affecting Legionella Survival
Symbiotic Microorganisms
The growth and survival of Legionella in the environment is enhanced by their ability to form symbiotic
relationships with other larger microorganisms. 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).
Because Legionella replicate rapidly intracellularly within protozoan hosts for prolonged periods of
time, amoebic vesicles can contain hundreds of Legionella cells (Berk et al. 1998, Lee and West 1991).
In addition, replication within protozoa may contribute to enhanced virulence of Legionella (Kramer and
Ford 1994).
The ability of Legionella to thrive within protozoa also allows them to survive over a wider range of
environmental conditions and to resist the effects of chlorine, biocides, and other disinfectants (Fields
1996, Kramer and Ford 1994, Paszko-Kolva et al. 1993, States et al. 1989).
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Relationships with certain algae and bacteria in biofilms also foster the growth of Legionella,
presumably due to the increased availability of nutrients and protection from disinfection (Kramer and
Ford 1994).
Water Temperature
Legionella exhibit the ability to survive in 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. Henke and Seidel (1986) claimed Legionella to be a
"thermoresistant" organism that exhibits survival in natural warm waters of up to 60°C and artificially
heated waters of 66.3°C.
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 and hot springs
(Henke and Seidel 1986, Lee and West 1991, Verissimo et al. 1991).
Colbourne and Dennis (1989) stated 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).
Other Factors
Other factors influencing the survival of Legionella in the environment include sediment accumulation
and metal content (Kusnetsov, 1993, States et al. 1987, Stout et al. 1992b, Stout et al. 1985, Vickers et
al. 1987). These factors are usually amplified by ideal water temperature or coexisting environmental
factors.
"Blind Loops", caused by the positioning of heating elements in hot water tanks, and washers,
grommets, etc. of various chemical composition can foster the growth of Legionella in plumbing
systems (Hodge et al. 1991).
IV. HEALTH EFFECTS IN ANIMALS
Although Legione lla 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 (EPA 1985).
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 and dogs (EPA
1985).
Experimental animals have been used primarily as hosts for the isolation of 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,
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and possible therapeutic approaches (EPA 1985). Guinea pigs have been studied extensively due to
similarities between the natural legionnaires' disease in humans and the experimental disease in guinea
pigs (EPA 1985).
There are varying degrees of susceptibility ioLegionella infection among animal species In comparison
to guinea pigs, other species such as rats, monkeys, marmosets and mice are more resistant to infection
by Legionella aerosols (EPA 1985). Gerbils are highly susceptible toLegionella infection by the
intraperitoneal route.
The disease process in the lungs of susceptible guinea pigs is characterized by multiplication of the
Legionella in alveolar macrophages with eventual destruction of the macrophages, release of toxic
cellular products, and the accumulation of bacterial and cellular debris in the alveoli that may eventually
results in impaired respiratory function and hypoxia (Davis et al. 1983). Clinical features include weight
loss, fever and seroconversion (Berendt et al. 1980).
The LD50 for guinea pigs exposed to L. pneumophila by the aerosol route is somewhat less than 105 cells
(Baskerville 1984, Huebner et al. 1984).
The long-term effects of Legionella-induced pneumonia are pulmonary fibrosis and functional
impairment of the lung (Baskerville et al. 1983, Parent! et al. 1989).
IV HEALTH EFFECTS IN HUMANS
Symptoms and Clinical Manifestations
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 (EPA 1985).
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.
The course of legionnaires' disease has been fairly precisely defined (Davis and Winn 1987, Ampel and
Wing 1990; Nguyen et al. 1991, Stout and Yu 1997, WHO 1990). The incubation period is two to ten
days, although incubation periods exceeding ten days have been reported. Malaise, myalgia, anorexia,
headache, and fever typically occur within 48 hours. A dry cough is typically present in the early stages
of the illness. 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, dyspnea, and respiratory distress may occur.
No single symptom has been recognized that can distinguish legionnaires' disease from other bacterial
pneumonias (Edelstein 1993, Roig et al. 1994, Stout and Yu 1997).
Extrapulmonary diseases resulting from Legionella infection are relatively rare but can occur. The heart
is the most common site of extrapulmonary infection (Armengol et al. 1992, Berbari et al. 1997, Chen et
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al. 1996, De Lassence et al. 1994, Devriendt et al. 1990, Domingo et al. 1989, Lowiy and Tompkins
1993, Stout and Yu 1997). The kidney is another site of extrapulmonary infection (Fenves 1985, Haines
and Calhoon 1987, Linet al. 1995, Pai et al. 1996, Shah et al. 1992, Wegmuller et al. 1985). These
extrapulmonary infections can occur in the absence of pneumonia.
Clinical Laboratory Findings
Many abnormalities in standard clinical laboratory tests have been noted in patients with legionnaires'
disease. 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). However, abnormalities in
standard clinical laboratory tests cannot be used to distinguish legionnaires' disease from other bacterial
pneumonias (Edelstein 1993).
Similarly, clinicians have concluded that no characteristic radiographic pattern helps to distinguish
legionnaires' disease from other bacterial pneumonias (Coletta and Fein 1998).
Mechanism of Action
The typical progression of a Legionella infection can be characterized by the following steps (Cianciotto
et al. 1989). 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 phagosomes. The host
cells lyse 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 enzyme that elicits
pulmonary lesions similar to those that develop in legionnaires' disease (Conlan et al. 1988).
Although not a bacterial component or product, another 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 (Brieland et al. 1996, 1997a, 1997b). One
study suggests that amoebae infected with L. pneumophila may be responsible for bacterial infection.
Immunity
Both humoral and cell-mediated immune responses to Legionella infection have been documented (EPA
1985, Friedman et al. 1998).
Although specific antibodies are produced, the protection that these antibodies provide in vivo is still
unknown (Friedman et al. 1998).
Cell-mediated immunity is currently recognized as the primary defense against Legionella infection
(Susa et al. 1998). Research also has emphasized the importance of spedfic cytokines (e.g., interferon-
, tumor necrosis factor- ) in host resistance ioLegionella infection (Blanchard et al. 1988, Friedman et
al. 1998, Skerrett and Martin 1996, Skerrett and Martin 1991, Susaetal. 1998).
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Chronic Conditions
Fatigue and weakness are two chronic conditions that may persist for several months following treatment
(Ching and Meyer 1987). Most patients with legionnaires' disease recover without any chronic
manifestations (EPA 1985).
Mild respiratory abnormalities (e.g., restrictive ventilatory defect and/or hypoxemia) resulting from
legionnaires' disease occasionally occur (Gea et al. 1988). More serious respiratory abnormalities are
rare. Pulmonary pathology that has been reported includes pulmonary fibrosis and chronic vasculitis
(Ching and Meyer 1987, EPA 1985).
Treatment
Early initiation of appropriate therapy is considered crucial for a successful outcome to legionnaires'
disease (Heath et al. 1996).
Erythromycin (a macrolide antibiotic) has historically been considered the first choice for the treatment
of legionnaires' disease (Stout and Yu 1997). However, newer macrolides (e.g., azithromycin) are
available that exhibit superior activity to Legionella and greater intracellular penetration with potentially
fewer adverse effects compared to erythromycin (Klein and Cunha 1998, Stout and Yu 1997, Roig etal.
1993). With development of intravenous formulations, these newer macrolides 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, they do not interfere with metabolism of immunosuppressive medications (Stout and Yu
1997).
Other antibiotics that have shown variable success in treatment of legionnaires' disease include
tetracyclines(e.g., doxycycline, minocycline, and tetracycline) and the combination of trimethoprim and
sulfamethoxazole (Stout and Yu 1997, Roig et al. 1993).
For the treatment of legionnaires' disease, the preferred route of administration of any antibiotic therapy
is intravenous (Stout and Yu 1997). Intravenous treatment should continue until the patient's fever
subsides. At this point, intravenous treatment can be replaced by oral therapy. The total duration of
therapy depends on the individual patient's history.
IV RISK ASSESSMENT
Hazard Identification
Given that legionnaires' disease is the most serious infection caused by Legionella, risk assessment of
these organisms should be focused on legionnaires' disease as theendpoint of concern.
Dose-Response Information
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Sufficient information is not available to support a quantitative characterization of the threshold
infective dose (i.e., the dose required to produce infection) of Legionella.
Potential for Human Exposure to Legionella
Legionella are opportunistic pathogens with widespread distribution in the environment but a very low
rate of infection in the general population.
Legionella are transmitted directly from the environment to humans (EPA 1985). 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.
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 (Blatt et al. 1994, Stout and Yu 1997, Woo et al. 1992, Yu 1993).
Risk Factors
The very low attack rates associated with this organism suggest that the general U.S. population is quite
resi stant 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).
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 an increased incidence of legionnaires' disease among
these groups, but also increased severity of the disease and increased mortality (Harrington et al. 1996,
Marston et al. 1994, 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 (Bangsborg et al. 1990, Marston et al. 1994).
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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). Even though pneumonia (of all types/sources) is common in
the general pediatric population, reports of legionnaires' disease in otherwise healthy children are
extremely rare (Abernathy-Carver et al. 1994, Carlson et al. 1990, Famiglietti et al. 1997).
Quantification of Potential Health Effects
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.
Minimizing Risk
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 (Lin et al. 1998a, 1998b).
For hospitals and other health care settings, where the lethal dose has been established 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.
Both the control measures and the 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 (CDC 1997a, Shelton et al. 1993).
IV ANALYSIS AND TREATMENT
Analysis of Samples
Collection of Legionella
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The examination of water for the presence ofLegionella is best done by taking swab samples of the
medium over which the water flows (EPA 1985, Ta et al. 1995).
The specimen should then be concentrated by filtration and treated with an acid buffer to enhance
Legionella recovery (Bopp et al. 1981, EPA 1985, Nguyen etal. 1991, Ta et al. 1995). Acid wash
treatment is used to isolate Legionella because unlike most bacteria, Legionella strains are acid resistant
(Nguyen etal. 1991).
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 (EPA 1985). Selective buffered charcoal yeast extract (BCYE)
medium is most commonly used to culture Legionella (Edelstein 1987).
Detection ofLegionella in Environmental and Biological Samples
The most common and rapid test for Legionella is the Direct Immunofluorescence Assay (DFA).
Sputum, lung specimens, and bronchial and tracheal secretions are excellent samples to test by the DFA
method; however, it may not be useful in the detection of environmental specimens (Grimont 1986).
More recently, monoclonal antibody test have been developed and have been found to eliminate false
positive results due to cross reactivity with non-Legionella organisms (Stout and YU 1997).
Monoclonal antibody test are effective due to their high specificity for a single antigenic determinant.
Legionella bacteria, and antibodies in patient sera, can be detected using the Indirect
Immunofluorescence Assay (IFA). Because seroconversion only occurs after a rather long time period in
humans, the IFA test is often used in conjunction with other tests (Kohler 1986). 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 1986, Ehret et al. 1986, Kashuba and
Ballow 1996).
Enzyme-linked immunosorbent assays (ELIS A), radioimmunoassays (RIA), and agglutination assays
have also been used to detect Legionella antibodies (EPA 1985). 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
been used for the detection ofLegionella antigens in urine, but is no longer commercially available. The
agglutination method has been used to detect antibodies in serum and antigens in urine.
The Polymerase Chain Reaction (PCR) test uses two disparate primers: one that is specific for
Legionella species and one for L. pneumophila only (Fricker and Fricker 1995). PCR is a relatively
new method not yet available commercially designed to rapidly multiply DNA target genes in a
laboratory setting to yield detectable quantities for testing.
Disinfection as a Water Treatment Practice
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. Because some methods have not always proven
completely successful or have not provided permanent protection from recolonization, a combination of
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these methods may be the most effective way of managing water systems and preventing future
outbreaks.
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). In cases of outbreaks, thermal disinfection can be quickly implemented. No
special equipment is needed, and it is relatively inexpensive (Stout and Yu 1997, Muraca etal. 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).
Hyperchlorination of water distribution systems requires the installation of a chlorinator. Shock
hyperchlorination involves the addition of chlorine to a water system, raising chlorine levels throughout
the system for one to two hours (Lin et al. 1998a). Continuous hyperchlorination entails the addition of
chlorinated salts to the water (Stout and Yu 1997, Muraca et al. 1990). 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 m Legionella recolonizing the system
(Nguyen et al. 1991). Hyperchlorination may cause human health problems. Levels of trihalomethanes
tend to increase in the hot water system when chlorine levels exceed 4mg/L (Helms et al. 1988, Muraca
etal. 1990).
Copper-silver ionization distorts the permeability of the Legionella cell, denatures proteins, and leads to
lysis and cell death (Nguyen et al. 1991, Miuetzner et al. 1997, Muraca et al. 1990). A commercial
system can be easily installed to perform this ionization. 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 ablackish
color, which can stain porcelain (Lin et al. 1998a).
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. 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 mustbe 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).
Ozone, which can be created using ozonators, can be used to kill L. pneumophila. 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
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protection. Also, ozonation is more expensive than hyperchlorination, and a large amount of space is
required for the air preparation equipment or oxygen tanks and contacting tank (Muraca et al. 1990).
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 i s not disinfected (Muraca et al. 1990). In addition, any Legionella that may have
colonized the system downstream of the heater will be unaffected.
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 and 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 RESEARCH NEEDS
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. In addition, Legionella are widely distributed in the environment, including treated water supplies.
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.
More comprehensive data on the occurrence of Legionella in groundwater, especially as it relates to
potable water supplies.
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.
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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 ofLegionella 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 posed by 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.
Development or rapid diagnostic test are needed to detect infections caused by Legionella.
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.
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