Section I: What is Legionnaires' disease?
Legionnaires' disease is a common name for one of the several illnesses caused by
Legionnaires' disease bacteria (LDB). Legionnaires' disease is an infection of the lungs and is a form of pneumonia. More than
43 species of Legionella have been identified and more than 20 linked with human diseases. Legionellosis is the term for the diseases
produced by LDB. In addition to Legionnaires' disease, the same bacteria also cause a flu-like disease called Pontiac fever.
- Legionnaires' disease bacteria (LDB)
- Legionnaires' disease/Pontiac fever
What are LDB?
The Centers of Disease Control and Prevention (CDC) first identified Legionella pneumophila in 1977 as the cause of an outbreak of pneumonia that caused 34 deaths at a 1976 American Legion Convention in Philadelphia. L. pneumophila had undoubtedly caused previous pneumonia outbreaks, but the organism's slow growth and special growth requirements prevented earlier discovery.
The species of Legionella that have
been associated with cases of
Legionnaires' disease are called
Legionnaires' disease bacteria
| Fig. 1: Legionella pneumophila Bacterium
- L. pneumophila is a gram-negative rod that exists in a number of distinguishable serogroups.
- Other species of Legionella cause
legionellosis but L. pneumophila causes the majority of
- L. pneumophila is also implicated in wound infections, pericarditis, and endocarditis without the presence of pneumonia.
Additional information (App I:A) on serogroups and subtypes is also available.
| What are the sources of exposure and transmission?
sources may include almost any warm water
system or device including man-made
or natural, that disseminates water,
particularly as aerosols, sprays or
mists and provides favorable
conditions for LDB growth and
|Fig. 2: Levels of LDB
are low in natural
- LDB are widely present at low levels in lakes, streams, rivers, freshwater ponds, and mud.
However, the levels of LDB that are found in the natural environment are so low that it is unlikely that an individual will contract the disease from these sources.
- The risk of exposure increases when high concentrations of the organism grow in water systems.
- Legionnaires' disease only occurs in the presence of a contaminated water source.
- Domestic (potable water) plumbing systems, cooling towers, and warm, stagnant water can provide ideal conditions for the growth of the organism.
Disease transmission is most likely to occur via:
| Fig. 3: Transmission may
occur via inhalation or
- Inhalation: of aerosols, fine sprays, mists or other
microscopic droplets of water contaminated with LDB, providing direct access into the lungs.
- Aspiration: such as may occur when choking or
spontaneously during the drinking, ingesting, swallowing process. This allows oral fluids
and particles to by-pass natural gag reflexes and enter into the respiratory tract and lungs
instead of the esophagus and stomach.
- There is no evidence that the diseases are transmitted from one person to another.
| What are the symptoms?
|Legionellosis is associated with two
distinct illnesses: Legionnaires' disease, which is characterized by
fever, myalgia, cough, pneumonia, and Pontiac fever, a milder illness without pneumonia.
Legionnaires' disease has an incubation period (the time from exposure to the onset of symptoms) of 2 to 10 days. Severity ranges from a mild cough and low fever to rapidly progressive pneumonia, coma, and death. Not all individuals with Legionnaires' disease experience the same symptoms.
Pontiac fever is a non-pneumonia disease with a short incubation period of one to three days. Full recovery usually occurs in two to five days without medical intervention and no deaths have been reported.
| Fig. 4: Early symptoms
may include fever,
headache, and tiredness
- Early symptoms include slight fever, headache, aching joints and muscles, lack of energy or tiredness, and loss of appetite.
- Later symptoms include:
- High fever (102° to 105° F, or 39° to 41° C)
- Cough (dry at first, later producing phlegm)
- Difficulty in breathing or shortness of breath
- Chest pain
- Common gastrointestinal symptoms include vomiting, diarrhea, nausea, and abdominal pain.
- Pontiac fever produces flu-like symptoms that may include fever, headache, tiredness, loss of appetite, muscle and joint pain, chills, nausea, and a dry cough.
- Pontiac fever has been associated with exposure to non-viable LDB and may be a hypersensitivity response to
bacterial or other antigens rather than an infection.
What are the incidence rates and risk factors?
The likelihood of contracting Legionnaires' disease depends on the level of contamination in the water source, the susceptibility of the person exposed, and the intensity of exposure.
Unlike Legionnaires' disease, which occurs in approximately 5 percent or less
of people who are exposed, Pontiac fever will occur in approximately 90
percent of those exposed. The factors that cause the same organism
to produce two illnesses with major differences in "attack rate" (the
fraction of exposed persons who become infected) and severity are not known.
Fig. 5: People with an
underlying illness or
weakened immune system are
most at risk
Legionnaires' disease is characterized as an "opportunistic" disease that attacks individuals who have an underlying illness or weakened immune system. The most susceptible people include:
- In the United States,
Legionnaires' disease is fairly
common and serious. LDB are one of the top three causes of
non-epidemic, community-acquired pneumonia.
- It is estimated that over 25,000 cases of the illness occur each year and cause more than 4,000 deaths.
- The fatality rate is similar to that of other forms of pneumonia, approximately 15 percent.
- It is difficult to distinguish this disease from other forms of pneumonia; so many cases go unreported.
For additional information and current statistics, see CDC Patient Facts: Learn More about Legionnaires' disease.
- The elderly, smokers, and those on immunosuppressive therapy.
- Individuals with chronic obstructive pulmonary disease (COPD), organ transplant patients, and people taking corticosteroid therapy.
- It is important to emphasize that relatively healthy individuals can be at risk of contracting disease.
How is Legionnaires' disease diagnosed and treated?
Legionnaires' disease is difficult to diagnose because the pneumonia caused by LDB is not easily distinguished from other forms of pneumonia.
Fig. 6: Macrolides are the
current drug of choice
disease treatment requires the use
of antibiotics. Early treatment reduces the severity of symptoms and improves chances of recovery.
- The Centers
for Disease Control and Prevention
(CDC) defines a confirmed case of Legionnaires' disease
as a clinically compatible case that is
confirmed by a laboratory. A confirmed case requires a physician's diagnosis of pneumonia based on a chest x-ray and positive laboratory test results. A laboratory test is necessary for confirmation because the symptoms and x-ray evidence of Legionnaires' disease resemble those of other types of pneumonia.
- The CDC laboratory criteria for diagnosis are:
- Isolation of LDB from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluids,
- Demonstration of a fourfold or greater rise in the reciprocal immunofluorescence antibody (IFA) titer to greater
than or equal to 128 against Legionella pneumophila serogroup 1 between paired acute- and convalescent-phaseserum specimens,
- Detection of L. pneumophila serogroup 1 in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody testing,
- Demonstration of L. pneumophila serogroup 1 antigens in urine by radioimmunoassay or enzyme-linked immunosorbent assay,
- For current information on laboratory criteria for diagnosis, see CDC: Legionellosis (Legionella pneumophila).
Additional information (App I:B) on diagnostic methods, including culture, direct fluorescent antibody (DFA) staining, and serology is also available.
- The drugs of choice belong to a class of antibiotics called
macrolides. They include azithromycin, erythromycin, and clarithromycin.
More: OSHA recommends a level one investigation when
there is evidence of exposure to LDB. See
Section III:A. Investigation Protocols: Level One.
A single antibody titer of 256 or higher indicates that a
person previously was exposed to Legionella. Such individuals may
have had an illness clinically compatible with Legionnaires' disease or may have had milder
or no symptoms. OSHA recommends a level two investigation when
more than one confirmed or possible case of Legionnaires' disease has been reported at a
facility. See Section III:B. Investigation Protocols: Level Two.
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