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Legionella
Risk Management
Review
Group Project
May 10, 2002
ENV 711
Prof. David M. Hassenzahl
Amy Klima
Jeff Geller
McClain Peterson
Rob Boehlecke
Vanessa Stevens
Abstract
Legionella refers to a group of bacteria commonly found worldwide in freshwater aquatic
environments. Legionella pneumophila is the most important member of this group
because it is commonly associated with Legionnaires’ Disease. Legionnaires’ Disease is
a severe form a pneumonia that first came to prominence in 1976, when it affected people
attending an American Legion Conference in Philadelphia, Pennsylvania. Favorable
conditions for Legionella can be found in many everyday situations including grocery
store misters, casinos, and hospitals. Anyone who is exposed to the bacteria could
contract Legionellosis, however, individuals that are immunodeficient, have chronic lung
disease, the elderly, smokers, or individuals that are receiving immunosuppressive
medication are at a greater risk of contracting Legionellosis. The population
characteristics of Las Vegas (e.g., high percentage of retired persons, heavy drinkers, and
smokers including tourists) as compared to other cities, may lead to an increased risk of
infection. Reducing the potential that susceptible persons are exposed, implementing
engineering controls, and properly maintaining water systems through such measures and
water treatment can mitigate the risk of contracting Legionellosis. Because of the many
uncertainties associated with sampling and analysis, clinical identification, susceptibility
factors, strain virulence, minimal epidemiological data, and near absence of specific
toxicological data sets there is no known model for the risk of contracting Leginellosis if
exposed. The risk of contracting Legionellosis cannot be eliminated; however, by
following the proper steps it can be reduced.
Introduction
Legionellae refers to a group of bacteria commonly found in aquatic
environments. Legionellae pneumophila is the most important member of this group
because it is commonly associated with the disease Legionellosis. There are two forms
of Legionellosis, Legionnaires’s Disease and Pontiac Fever. Legionnaires’ Disease is a
severe form of pneumonia caused by Legionellae pneumophila. Legionnaries’ Disease
first received public awareness in 1976, when it affected people attending an American
Legion Conference in Philadelphia, Pennsylvania. Currently, there are 43 species of
Legionellae and 65 serogroups. However, Legionellae pneumophila is responsible for
90% of the Legionellosis cases in the United States (Marston, et al 1994).
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The major reservoir for Legionellae is water. The bacteria are found in freshwater
environments worldwide (Steele et al 1990). Water containing the organism is
aerosolized in respirable droplets (1-5um) and inhaled by a susceptible host (CDC, 1997).
These sources can be showers, faucets, misters, spas, air conditioning units, water
fountains, and cooling towers. People may be exposed at home, hospitals or nursing
homes, work environments, grocery stores or other public places such as casinos. For
instance the following outbreaks occurred during the period between 1984 and 1898 in
the United States, see Table 1. Legionellae basically has two “lifestyles”. The
Planktonic Legionellae bacteria are free-floating and unprotected. The sessile form of the
Legionellae bacteria is attached to a surface or is considered protected (Barbaree et al,
1993). The intracellular habitat provides protection for Legionellae against the biocidal
effects of chlorine and heat (Kuchta et al, 1993).
The conditions that are required for amplification of Legionellae bacteria include
a water temperature of 32C-45C, stagnation, the presence of sediment, and the presence
of biofilms and amoebae (Yanamoto et al, 1992). In an aquatic environment, the
amoebae/protozoa feed on the biofilm and engulf the Legionellae bacteria. The
Legionellae bacteria are resistant to digestion. Inside of the amoebae the bacteria then
proliferate and break down the cell wall of the amoebae. They are then released back out
into the water (Beer, 2002). The mechanism that the Legionellae bacteria uses to infect
and multiply in protozoa is similar to the mechanism the bacteria uses to infect human
phagocytic cells (Barbaree et al, 1993)
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Overall, the population as a whole is at risk. However, individuals that are
immune deficient or have chronic lung disease, elderly, smokers, or individuals who are
receiving immunosuppressive medication are at a much greater risk of contracting
Legionellosis. Most cases that have been reported have occurred in the 40-70 year old
range. This is significant because Las Vegas has a large population of retired persons in
this age category and many of the tourists who frequent Las Vegas also fall into this age
category. Therefore, the Las Vegas region needs to have an increased awareness of
Legionnaire’s Disease.
Four conditions must be met in order for an individual to contract Legionnaires’
Disease. The bacteria must be in an environment were it is able to amplify (i.e.,
reproduce). There must be a way for the Legionellae bacteria to be disseminated, such as
a showerhead or mister. The Legionella bacteria must be virulent, and the individual
must have susceptibility such as a weakened immune system.
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Table 1: Legionellae outbreak and source
Outbreak Location
Epidemic Source
Hospital
Potable water system
Hospital
Respiratory therapy device
Spa
Whirlpool
Banking Trust Building
Cooling Tower
Prison
Cooling Tower
Tourist Lodges
Potable water system
Hotel for the elderly
Evaporative condenser
Hospital
Shower heads
Hospital
Respiratory therapy device
Grocery store
Mist machine above produce
Source: Barbaree 1991
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Information and Data
There are numerous conditions and factors that make people more susceptible to
the effects of Legionnaires’ Disease. In this research we will focus on the most common
conditions and characteristics including people 40 – 70 years of age, alcoholics, smokers,
persons with human immunodeficiency virus (HIV), and individuals who suffer from
cancer (Beer, 2002). As a large city it would be expected that Las Vegas will have a
significant population that is susceptible to contracting Legionellae. Therefore, a
quantification of these individuals was necessary to ascertain how many individuals in
the Las Vegas region are most susceptible to the Legionellae bacteria.
Las Vegas is home to every age group, attracting young people with a flourishing
economy, and attracting retired people with an inviting climate and a plethora of senior
level activities. The age group that is the most susceptible to this disease, 40 – 70, is
prominent. In Clark County out of the 1,375,765 people, 617,516 (or 45%) are within
this category (USCB, 2002).
Heavy drinkers are at a higher risk for contracting the disease once they come into
contact with Legionella pneumophila. In Las Vegas there are approximately 77,042
people who are classified as “heavy drinkers” (SAMHA). These individuals, who drink
five or more drinks on the same occasion on at least five different days a month, would
be more prone to contracting this disease and experiencing its effects. This classification
of heavy drinkers is one way to quantify the at risk groups to Legionnaires’ Disease. It is
important however to note that anyone drinking alcohol is more susceptible to
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Legionnaires’ Disease because of a decrease in the immune systems performance
(NIAAA 2002).
Cigarette smoking is also a habit that makes individuals more susceptible to
Legionnaires’ Disease and its subsequent effects. In Las Vegas, 331,559 citizens or
24.1% of the total population have an increase susceptibility to the disease because of
smoking habits (Yahoo Health, 2002).
Immunodeficiency, which compromises the body’s ability to fight Legionella
pneumophila, is another factor in determining susceptibility to the disease. As with any
city, a significant portion of the population will have some disease or condition that has
comprised their body’s immunity. Human immunodeficiency virus (HIV) is one such
condition and in Las Vegas approximately 3,773 individuals suffer from HIV. Cancer is
another form of an immunodeficiency that makes individuals more susceptible to
Legionnaires’ Disease, and in Las Vegas there have been 32,076 cases of cancer reported
to the state health department since 1995 (NVDH, 2002). The health department
considers this an accurate representation of the total population inflicted with cancer.
As the primary source of revenue for the city of Las Vegas, tourism attracts
people from around the world and from various backgrounds, ethnicities, and health
related conditions. In 2001, 35,017,357 visitors came to Las Vegas (LVCVA, 2002). It
is important to include these visitors in the population of people who could be adversely
affected by Legionellae pneumophila. However, it is unknown what percentages of these
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visitors have characteristics that make them more susceptible to Legionnaires’ Disease,
but it is certain that a portion of them will.
Mitigation Measures
As pointed out by the American Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE) in their 1998 position paper on Legionellosis, the
most effective control for most diseases is to prevent transmission at as many points as
possible (ASHRAE, 1998). Butler et. al. also echoed this idea in stating that interrupting
the transmission from the environment to people is the foundation of prevention of
Legionellosis (Butler et.al, 1997). Prevention can be accomplished though several
methods including engineering controls on water system equipment, regular cleaning and
maintenance of water systems, addition of inhibitors into water systems, or reducing the
potential that susceptible people will be exposed.
According to Butler et.al, 1997 there are currently two schools of thought
regarding the most effective way to prevent hospital acquired Legionellosis (nosocomial
infection). The first is to routinely culture water samples for the hospitals potable water
system. The second is to maintain a high suspicion for Legionellae bacteria by using
clinical diagnostic tests on potential Legionellosis patients. When a case of disease is
confirmed, initiate an investigation for the exposure source and correct the maintenance
problem(s) that led to the exposure (Butler et.al, 1997). We will first discuss how
reducing the potential that susceptible persons are exposed to Legionellae can be aided by
sampling and analysis of both water systems and people.
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While it may be obvious that the risk of contracting Legionellosis can be
mitigated by knowing if the water supply is contaminated, it is not exactly that straight
forward due to factors such as incubation time and turnaround time for sampling and
analysis. It is also difficult to determine whether a particular person is sick due to
legionella or some other pathogen. However, in both cases it can be shown that increased
knowledge of the existence of Legionellae in a system or a patient’s body can reduce the
risk.
The risk to the individual is reduced through the identification and proper
treatment of the disease. Clinical features (e.g., symptoms) described for Legionnaires’
Disease cannot be distinguished from those due to pneumonia caused by other agents.
Therefore, serological tests are the only way to confirm Legionellosis. Most tests are run
on lower respiratory tract secretions; however, they may also be done on blood, serum
and urine samples. As reported by the EPA in 1999, early initiation of appropriate
treatment for individuals infected with Legionellae pneumophila is recognized as crucial
for the successful outcome of the disease. In fact as reported in EPA, 1999 after multiple
logistic regression analysis of seriologically confirmed cases of Legionnaires’ Disease;
the only factor associated with increased mortality in Legionellosis patients was a delay
in initiation of appropriate treatment (EPA, 1999).
Proper identification is also important to the mitigation of the risk to others in the
community as well. Additional nosocomial (hospital contracted) cases are typically
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identified once several cases have been reported (EPA, 1999). Active surveillance of
Legionella infection, especially among hospital patients at high risk is an important tool
for minimizing risk of Legionnaires’ Disease because it also allows for prompt remedial
actions (EPA, 1999). In hospitals where Legionellae was acquired, additional cases have
only been recognized after increased testing of patients who had pneumonia.
Recognition of such cases can lead to interventions to limit growth of the organism and
transmission to other patients (Butler et al, 1997). However, it should be noted that the
incubation period for Legionellae pneumophila is usually 2-10 days, therefore, a patient
must have spent greater than 10 days in a hospital prior to onset of Legionellosis, before
it can be concluded that the patient acquired the disease in the hospital (Butler, 1997).
When a case of hospital-transmitted Legionellosis is confirmed, Butler et. al (1997)
indicates that in order to support a link between a human illness and a specific source,
subtype matching between the Legionellae bacteria isolated from the patient and that
from the environmental (i.e., potential exposure source) sample should be completed.
Proper identification of non-hospital contracted Legionnaires’ Disease can also
lead to risk mitigation for other potentially exposed populations. For example, a person
goes into the hospital with pneumonia and it is proven through testing that the person
suffers from Legionnaires’ Disease, then it can be assumed the person was exposed to the
disease through exposure to a contaminated water system. Although results may not
necessarily be conclusive, sampling of potentially infected water systems may lead to
identification of the source of the exposure. The source can then be cleaned and/or
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treated thus mitigating the risk. Once the source system is identified those people who
are in high-risk groups could also be isolated from the system.
In order to identify the exposure source, samples must be collected and analyzed.
ASHRAE states that currently the most reliable method of testing for viable (the bacteria
must be alive to cause the disease) Legionellae in a system is culturing (ASHRAE, 1998).
This is supported by the CDC, which considers bacteria culturing the only reliable
method for detecting viable organisms (Beer, 2002).
However, culture-based
techniques require 10-14 days from the time of sample collection until results are
available (CTI, 2000). In their paper CTI states this period is too long a time for
sampling and analysis to serve as an effective tool for treatment control. Most of the
documentation supports this stance and indicates routine culturing of water systems
should not be performed (Yu, 1997; Cohen, 1997; ASHRAE, 2000) because the
concentration of Legionellae bacteria in a system may not be predictive of risk
transmission (ASHRAE, 2000).
ASHRAE (2000) and other documentation (e.g., Butler et.al, 1997) indicate that
routine culturing cannot be directly correlated with the risk of contracting the disease
because of several factors. The first is that the presence of the organism cannot be
directly related to risk of infection (ASHRAE, 2000). Among a study reported by EPA
(1999) the study authors noted that among 900 samples from building water systems,
some of the samples with the highest hazard level were obtained from a building without
any reported Legionnaires’ Disease cases. The second is that interpretation of results is
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complicated by use of different collection and analysis methods and fluctuations of
results within samples collected from the same source (ASHRAE, 2000). Also the risk of
illness is influenced by a number of factors as previously discussed, and finally, test
results only represent the counts at the time the sample was collected (ASHRAE, 2000).
Results of a specific test must be interpreted cautiously because the concentration of
organisms can increase substantially in a short period of time (e.g., days) (ASHRAE,
1998). Butler et.al (1997) adds several other factors influencing the rate of infection
including strain virulence and how efficiently the bacteria are aerosolized.
In an earlier position paper on Legionellae, ASHRAE stated that there is currently
not enough information to adopt a position on what is a “safe” or “unsafe” number of
Legionellae bacteria or how long that number would be valid. In some cases, small
numbers caused disease and in others large numbers did not result in a disease outbreak
(ASHRAE, 1998). Then why sample water systems at all?
Even those opposed to routine sampling state that there is some utility in water
system sampling. Butler et.al (1997) states, “culturing in a water system may be
appropriate if performed to evaluate a suspected source of infection, as part of an
outbreak investigation, to assess the effectiveness of treatment or decontamination, or to
evaluate the potential for transmission in health care facilities with high risk patients.”
ASHRAE also indicates “culturing for Legionellae may be appropriate if carried out for a
specific purpose, such as tracing the source of an infection, evaluating the potential
amplifier or transmission sources, verifying the effectiveness of a water treatment system,
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verifying that decontamination procedures have been effective, or at health care facilities
with patients at high risk.” Yu (1997) indicates that the presences of Legionellae within a
hospital water supply system invariably leads to infections and recommends routine
environmental culturing in hospitals to raise the index of suspicion.
Water system sampling and analysis can never be a substitute for sound
maintenance and water treatment (ASHRAE, 2000). A variety of engineering controls
are recommended to help reduce the risk of Legionellae transmission from common
systems such as cooling towers, evaporative condensers, humidifiers, potable water
heaters and holding tanks, pipes containing stagnant warm water, showerheads, faucet
aerators and whirlpool baths can foster Legionellae growth (ASHRAE, 1998).
Controlling temperature in any of the many potable water supply systems
mentioned above helps to minimize growth of the bacteria (ASHRAE, 2000). Studies
have indicated that temperature plays a potential role as an amplifier. Temperature
ranges between 68 F and 131 F can produce favorable conditions for Legionellae to
colonize. Therefore, plumbing in any water system must be routinely checked and have
devices installed that control temperature. Insulation should also be installed, especially
in colder regions. Another simple engineering practice that can reduce the ability of the
Legionellae bacteria to colonize is to remove any dead pipes that do not receive
continued flow.
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Another way to reduce the transmission potential of Legionellae is to avoid
aerosolization via water droplets since this is the primary means by which Legionellae
are transmitted to humans (Butler et.al, 1997). Therefore, regulating climatic conditions
around the potential locations of the bacteria, such as conditions in the air, humidity and
harsh solar radiation would help to minimize risk of exposure.
Other engineering controls may include maintenance and routine inspections of
air filters, especially those associated with the air intake of cooling towers (ASHRAE,
2000). Designing cooling towers that do not discharge their air near the buildings air
intake should be a must. This would further reduce the chance of distributing
Legionellae into other parts of the building.
Finally, there is speculation that a variety of water treatment measures could be
effective in the control of Legionellae. Ozone, chlorine, ultraviolet light and heavy metal
ions have been shown to be effective in some cases; nonetheless, further research needs
to determine if this is true (Butler et.al, 1997). Chlorine treatment is recommended in
emergency situations using 1 ppm to 2 ppm free chlorine at the tap for several hours
(ASHRAE, 1998). However, chlorine treatment should not be used as a long-term
solution to an area heavily contaminated. Use of chlorine is the current method used by
the Southern Nevada Water Authority for treating potable water. However, an ozone
treatment facility is currently being built. Ozone is the current treatment of choice for
controlling the Legionellae bacteria.
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Conclusion
Clearly, Legionellosis is a potentially preventable disease but there is still a great
deal of uncertainty in why outbreaks occur. Most if not all lakes and streams harbor
Legionellae naturally and it would be impossible to eradicate the bacteria entirely from
the outdoor environment (ASHRAE, 1998), not to mention that eradication could have
negative effects. There could be some societal benefit to the Legionellae bacteria, where
it fights off other potentially more harmful microorganisms. However, more speculation
will only lead to more debate amongst the scientific community.
The engineering controls and water treatment mitigation measures discussed in
this paper can limit the amount of bacteria in our water systems. Culturing water samples
can be useful to pinpoint the origin of an exposure and thus limit further exposures. In
addition, if we can maintain a high degree of suspicion for Legionellosis when
diagnosing human illness, exposed people can get the proper treatment and reduce their
individual risk of succumbing to the direst consequences of infection.
For the Las Vegas region it is important that a more comprehensive evaluation of
the associated risks be completed. We must know who in the valley is at risk for
exposure to this bacteria and whether or not appropriate measures are being taken to
protect these individuals. We must also evaluate our water sources for possible
contamination points and evaluate the treatment facilities currently employed. In
addition, we must know whether the Las Vegas climate and yearly temperatures make it
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more susceptible to Legionella contaminations and whether the increased use of cooling
towers and misting systems provides the valley with an increased risk.
One must remember that the risk of Legionellae pneumophila can never be
eradicated completely, we can only reduce the risk. Furthermore, a threshold infective
dose has not been established and animal models show great interspecies variation in
susceptibility to infection with Legionellae (EPA, 1999). In order to increase our
understanding of why and when outbreaks occur, government and industry need to
develop sampling standards and acceptable infectious dose levels. More information is
also needed on the virulence characteristics, of specific subtypes and of the Legionellae
bacteria itself. Finally, the usefulness of non-culture methods such as antigen detection or
polymerase chain reaction for detecting viable Legionellae needs to be defined (Butler
et.al, 1997).
Local Update!
An outbreak of Legionnaires’ Disease at the Polo Towers in 2001 is currently being
investigated. This incident is still under evaluation by the CDC and the final report will
not be public record until the end of May. Currently, the Southern Nevada Water
Authority conducts weekly sampling to determine Legionellae and other microbial levels.
However, their sampling technique is to analyze bulk water samples and not biofilm
where there is a greater likelihood for Legionellae to occur.
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ASHRAE, see American Society of Heating, Refrigerating and Air-Conditioning
Engineers
American Society of Heating, Refrigerating and Air-Conditioning Engineers. 1998.
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American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2000.
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approved by the ASHRAE Standards Committee February 5, 2000. Atlanta, GA.
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