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Legionellosis' refers to a group of illnesses caused by the bacterium Legionella pneumophila. Legionellosis has two distinct forms:

Legionella pneumophila bacterium as seen in an electron micrograph. Source: CDC.
  • Legionnaires' disease, the more severe form of infection which includes pneumonia, and
  • Pontiac Fever, a milder, flu-like illness, usually without pneumonia.

Legionnaires' disease acquired its name in 1976 when an outbreak of pneumonia occurred among people attending a convention of the American Legion in Philadelphia. Later, the bacterium causing the illness was identified by the Centers for Disease Control and Prevention (CDC) and named Legionella.[1]



Legionnaire's Disease

When Legionella pneumophila infects a person, the main illness that can occur is pneumonia. Pneumonia is an infection and inflammation in the lungs. When the cause of the pneumonia is identified to be Legionella, the clinical illness is called Legionnaires' disease. Legionnaires' disease can have symptoms like many other forms of pneumonia, so the cause of the pneumonia can be hard to diagnose at first.

The incubation period (time from infection with Legionella to development of symptoms) is about 2 to 10 days. Early symptoms of the disease can be similar to the flu and include the following:

As the illness progress to pneumonia, the following symptoms can occur:

  • Cough—can range from a mild, dry cough to a very severe cough producing a lot of sputum
  • High fever
  • Sharp pains in the chest, particularly with deep breathing (pleurisy)
  • Nausea, vomiting and abdominal pain (in about 10%-20%)
  • Diarrhea (in about 25% and usually non-bloody)
  • Headache, confusion, lethargy

In severe cases of Legionnaires' disease, people can be very ill and may require assisted ventilation in the hospital. The bacterium can spread to other parts of the body via the blood stream, and this can lead to damage to many organs in the body and death. Death can occur from the illness in up to 5% to 30% of cases.

The length of illness is determined by several factors including the severity of the infection, the health of the person infected and how quickly appropriate antibiotic therapy is started. Most people who do recover from the infection are not left with any long-term disabilities.

Pontiac Fever

Legionella pneumophila can also cause a milder form of disease called Pontiac Fever. Why Legionella causes Legionnaires' disease in some people and Pontiac Fever in others is unknown. Theories include a reduced infectious dose of the microbe in people who get Pontiac Fever, or perhaps a different response of the body's immune system to the bacterium.

The incubation period of Pontiac Fever is usually 24 to 48 hours. Symptoms are generally milder than in Legionnaires' disease and can include the following:

Pneumonia does not develop in Pontiac Fever. Symptoms typically last about a week and a full recovery is the rule, even without specific treatment.


Legionella pneumophila as seen in a clinical sputum specimen from the 1976 outbreak in Philadelphia. Source: CDC.
Legionellosis is caused by a bacterium called Legionella pneumophila. There are more than 40 species in the Legionella group; L. pneumophila accounts for 90% of human infections. The organism can be difficult to grow in the hospital laboratory from clinical specimens and special growth media are required.

L. pneumophila is not a hardy microbe, but it is able to survive in the environment for long periods of time under certain conditions. Research has revealed that the organism can live in water with temperatures ranging from 0°C to 63°C, and in pH conditions from 5.0 to 8.5 (a measure of the acidity of water). The bacteria are found naturally in the environment, usually in bodies of water such as lakes and streams. The bacteria grow best in warm water, like the kind found in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings. The presence of other microorganisms and sediment in the water are necessary for optimal growth of the organism.[2]

The organism has been found in municipal drinking water, and water systems in hospitals, rehabilitation centers and long-term care facilities. Each year, between 8,000 and 18,000 people are hospitalized with Legionnaires' disease in the Unites States. However, many infections are not diagnosed or reported, so the true number may be higher. More illness is usually found in the summer and early fall, but it can happen any time of the year.


Chest X-ray of a patient with Legionnaires' disease. Source: CDC
The diagnosis of Legionellosis usually requires special tests on samples of sputum or blood. The symptoms can mimic other causes of pneumonia, but often a person's history of possible environmental exposures can lead a doctor to suspect Legionellosis.

The standard tests performed at most hospitals include the following:

  • Direct fluorescent antibody (DFA) staining is a rapid test that can be performed on samples from the secretions of the lungs. Results are possible within 2 to 4 hours, but a large number of Legionella bacteria need to be in the sample for the test to work.
  • The urine antigen test is a rapid, relatively inexpensive, and practical test for the detection of L. pneumophila antigen (a part of the microbe) excreted in the urine.
  • Culture of secretions from the lungs, lung tissue, or fluid from around the lungs (pleural effusion) can be done on special media; this method takes several days as the organism grows slowly, and is mainly used to confirm a diagnosis.

The chest X-ray shows pneumonia in virtually everyone who develops Legionnaires' disease, but the findings on X-ray are not specific to Legionella as being the cause. Sometimes clues can be seen in other laboratory tests, such as a low sodium level in the blood. This is not usually seen in pneumonia caused by other microbes.

Blood samples can also be tested for antibodies which the human body develops in response to infection. The antibodies against Legionella can take 4 to 12 weeks to appear, so the main usefulness for this test is in epidemiologic studies.


Legionnaires' disease

Legionella pneumophila tends to cause serious cases of pneumonia and most people who become ill will seek medical attention and require antibiotics. When first seen with pneumonia, patients are treated with several antibiotics that are used for Community-acquired pneumonia. Once tests have shown that L. pneumophila is the cause of the pneumonia, antibiotics can be adjusted to better cover this microbe. Typical antibiotics that are used include the macrolide antibiotics (azithromycin, clarithromycin, erythromycin), the fluoroquinolone antibiotics(gemifloxacin, levofloxacin), trimethoprim-sulfamethoxazole (Bactrim, Septra), or doxycycline.

Pontiac Fever

Pontiac Fever is usually a self-limited illness that does not require any specific antimicrobial treatment. Supportive therapy for control of fever, headaches and muscle pain are usually sufficient to keep people comfortable until the illness has resolved.


There is currently no vaccine for the disease.

Regular cleaning and disinfecting of cooling towers and air conditioning units along with applications of chemicals that can control the growth of bacteria can help to decrease the spread, although some studies question the effectiveness of these methods.[3]

Chances of Developing Legionellosis

Risk Factors

People most at risk of getting sick from the bacteria are older people (usually 65 years of age or older), as well as people who are smokers, or those who have a chronic lung disease (like emphysema).

People who have weak immune systems from diseases like cancer, diabetes, or kidney failure are also more likely to get sick from Legionella bacteria. People who take drugs to suppress (weaken) the immune system (such as after a transplant operation or chemotherapy) are also at higher risk.

A person diagnosed with Legionnaires’ disease in the workplace is not a threat to others who share office space or other areas with him or her.

Recently, the CDC has listed travel as a risk factor for catching Legionellosis. About 21% of all cases of Legionnaires' disease reported to the CDC between 1980 and 1999 was travel-related, and that number is rising.[4]

How Legionellosis is Spread

People get Legionnaires' disease when they breathe in a mist or vapor (small droplets of water in the air) that has been contaminated with the bacteria.[5] One example might be from breathing in the steam from a whirlpool spa that has not been properly cleaned and disinfected. Another example of how Legionellosis is spread was reported when a community-wide outbreak occurred from a grocery store misting machine.[6]

The bacteria are NOT spread from one person to another person. In addition, showers are not a risk for transmission

Outbreaks are when two or more people become ill in the same place at about the same time, such as patients in hospitals. Hospital buildings have complex water systems, and many people in hospitals already have illnesses that increase their risk for Legionella infection. Studies have found Legionella in the water supply of some hospitals.[7]

Other outbreaks have been linked to aerosol sources in the community, or with cruise ships and hotels, with the most likely sources being whirlpool spas, cooling towers (air-conditioning units from large buildings), and water used for drinking and bathing.

Related Problems


Most people who have Legionnaires' disease only develop pneumonia. But some can have involvement of other areas of the body, particularly if they have a weakened immune system, are debilitated from another illness, or are elderly. L. pneumophila has been associated with the following conditions:


Newspaper headlines during the outbreak of Legionnaires' disease. Source: CDC.
In 1976, there was an outbreak of pneumonia in people attending an American Legion conference in Philadelphia.[1] A total of 182 people became ill and 34 died. Investigators from the CDC isolated a bacterium from lung tissues taken from those who died. The bacterium was named Legionella pneumophila. It took a while to discover the cause of the outbreak and there was a great deal of skepticism in the lay press as to what the cause was, or whether the disease was even real.

Using techniques to measure antibodies in stored blood samples, researchers have found that there were outbreaks of Legionnaires' disease that predated the outbreak in Philadelphia. An outbreak of pneumonia in people attending an Oddfellows convention in 1974 at the same hotel in Philadelphia was found to be Legionnaires' disease.[8] Other outbreaks traced to Legionella were found in a psychiatric hospital in Washington, D.C., in 1965, and in a meat-packing plant in Minnesota in 1957.[9]


  1. 1.0 1.1 Fraser DW, Tsai TR, Orenstein W, et al. Legionnaires' disease: description of an epidemic of pneumonia. N Engl J Med. 1977 Dec 1;297(22):1189-97. Abstract
  2. Stout JE, Yu VL, Best MG. Ecology of Legionella pneumophila within water distribution systems. Appl Environ Microbiol. 1985 Jan;49(1):221-8. Abstract | PDF
  3. Vickers RM, Yu VL, Hanna SS, Muraca P, Diven W, Carmen N, Taylor FB. Determinants of Legionella pneumophila contamination of water distribution systems: 15-hospital prospective study. Infect Control. 1987 Sep;8(9):357-63. [1]
  4. CDC. Surveillance for Travel-Associated Legionnaires Disease --- United States, 2005--2006. MMWR 2007 Dec 7;56(48):1261-3. Full Text
  5. Blatt SP, Parkinson MD, Pace E, et al. Nosocomial Legionnaires' disease: aspiration as a primary mode of disease acquisition. Am J Med. 1993 Jul;95(1):16-22. Abstract
  6. Mahoney FJ, Hoge CW, Farley TA, et al. Communitywide outbreak of Legionnaires' disease associated with a grocery store mist machine. J Infect Dis. 1992 Apr;165(4):736-9. Abstract
  7. Alary M, Joly JR. Factors contributing to the contamination of hospital water distribution systems by legionellae. J Infect Dis. 1992 Mar;165(3):565-9. Abstract
  8. Terranova W, Cohen ML, Fraser DW. 1974 outbreak of Legionnaires' Disease diagnosed in 1977. Clinical and epidemiological features. Lancet. 1978 Jul 15;2(8081):122-4. Abstract
  9. Osterholm MT, Chin TD, Osborne DO, et al. A 1957 outbreak of Legionnaires' disease associated with a meat packing plant. Am J Epidemiol. 1983 Jan;117(1):60-7. Abstract

External Links

Centers for Disease Control and Prevention (CDC)

World Health Organization (WHO): Legionellosis

The European Working Group for Legionella Infections

Medpedia-logo.gif The basis of this article is contributed from Medpedia.com These articles are licensed under the GNU Free Documentation License It may have since been edited beyond all recognition. But we thank Medpedia for allowing its use.
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