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Legionnaires' Disease

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Copyright 1986, 1999, 2007, 2009

Synonyms of Legionnaires' Disease

Disorder Subdivisions

General Discussion

Legionnaires' Disease is recognized as an acute respiratory pneumonia caused by the aerobic gram-negative microorganism, Legionella pneumophila, and other species. This microorganism may also affect other body systems. Afflicted patients may have pulmonary (lung and bronchi), gastrointestinal tract, and central nervous system complications. Renal insufficiency may occur occasionally and can be severe enough to require dialysis.


The primary symptoms associated with Legionnaires' Disease appear to be pneumonia including a shaking chill, sharp pain in the involved side of the chest, cough with sputum or phlegm production, fever of up to 105ยบ F, and, in some cases, rapid and painful respiration. Abdominal pain, diarrhea, neurological signs such as headache, confusion, lethargy or agitation may also be present. Laboratory data may include an abnormal liver function test, low phosphorus in the blood (hypophosphatemia), blood in the urine (hematuria), and low blood sodium (hyponatremia).


The term "legionellosis" refers to disease caused by infection with bacteria belonging to the Legionella family. Legionnaires' Disease is characterized by the development of pneumonia due to infection with the bacterium known as Legionella pneumophila. Another infectious disease, Pontiac Fever, is an acute, flulike (febrile), self-limited disease that is caused by infection with bacteria belonging to the same Legionella family. (For further information on Pontiac Fever, please see the "Related Disorders" section of this report below.)

Legionella pneumophila (L. pneumophila) is a gram-negative bacterium that thrives in the presence of oxygen (aerobic bacterium). Bacteria may be considered "gram negative" or "gram positive," depending upon the results of "Gram's stain," a testing method in which bacteria are stained with various solutions to help identify and classify the microorganisms. Such staining may be essential in identifying a specific bacterium responsible for an infectious disease and determining appropriate, effective treatments.

The L. pneumophila bacterium most easily reproduces in moist, warm areas. For example, in some outbreaks of Legionnaires' Disease, the source of infection was traced to air-conditioning or water systems. Infection with L. pneumophila results from inhalation or ingestion of contaminated water droplets.

Affected Populations

Legionnaires' Disease was initially recognized in 1976, when an outbreak of a form of pneumonia affected members attending an American Legion Convention at a hotel in Philadelphia. The bacterium responsible for the disease was later identified and named Legionella pneumophila. Researchers subsequently determined that an earlier epidemic of the disease had occurred in Minnesota in 1957.

It is estimated that as many as 10,000 to 15,000 individuals are affected by Legionnaires' Disease each year in the United States. In addition, studies suggest that the disease may account for approximately three to 15 percent of all cases of community-acquired pneumonia. Researchers also indicate that an unknown number of individuals may be infected by the L. pneumophila bacterium yet experience mild illness or develop no apparent symptoms (asymptomatic).

Certain individuals may have an increased predisposition for the disease. Risk factors may include advanced age, cigarette smoking, chronic lung disease, and a suppressed immune system (immunosuppression). Reports suggest that elderly males may be most likely to develop the disease. In addition, certain individuals may be predisposed to acquiring the infection during hospitalization (nosocomial infection), such as those who undergo surgery, particularly transplants, or infants with lung disease or immunosuppression.

As discussed above, Legionnaires' Disease may occur in outbreaks. However, the disease usually occurs in single, isolated cases. Outbreaks, though typically recognized during the summer or fall, may develop at any time of the year.

Related Disorders

Like Legionnaires' Disease, Pontiac Fever is caused by infection with bacteria belonging to the Legionella family. Associated symptoms usually include a general feeling of ill health (malaise), fatigue, fever, chills, muscle aches (myalgia), and headache. Some individuals with Pontiac Fever may also develop nausea, cough, joint aches (arthalgia), diarrhea, and abdominal pain. The pneumonia seen in Legionnaires' Disease is not present. Affected individuals usually recover without treatment (self-limiting illness) in a few days. In those who develop Pontiac Fever, the time between initial infection and symptom onset (incubation period) is typically about one to two days. In contrast, the incubation period for Legionnaires' Disease usually ranges from approximately two to 10 days.

Standard Therapies

Patients afflicted with Legionnaires' Disease may develop respiratory compromise requiring artificial ventilation and positive end expiratory pressure respirators to maintain adequate oxygenation. Erythromycin is the drug treatment of choice. In more severe cases, rifampin may be used in conjunction with erythromycin. Tetracycline may be substituted if the patient is allergic to erythromycin.

While the outbreak of Legionnaires' Disease has only occasionally been associated with a contaminated water system, appropriate treatment of the water system is recommended if this is the case.

Investigational Therapies

Information on current clinical trials is posted on the Internet at www.clinicaltrials.gov. All studies receiving U.S. Government funding, and some supported by private industry, are posted on this government web site.

For information about clinical trials being conducted at the NIH Clinical Center in Bethesda, MD, contact the NIH Patient Recruitment Office:

Tollfree: (800) 411-1222
TTY: (866) 411-1010
Email: prpl@cc.nih.gov

For information about clinical trials sponsored by private sources, contact:

Legionnaires' Disease Resources



Harrison's Principles of Internal Medicine, 14th Ed.: Anthony S. Fauci et al., Eds.: McGraw-Hill Companies, Inc., 1998. Pp. 928-33.

Cecil Textbook of Medicine, 18th ed.: James B. Wyngaarden, and Lloyd H. Smith, Jr., Eds.: W.B. Saunders Co., 1988. P. 1570.

The Merck Manual 15th ed: R. Berkow, et al: eds; Merck, Sharp & Dohme Research Laboratories, 1987. P. 665.

More Than 10 Years of Unrecognized Nososomial Transmission of Legionnaires' Disease Among Transplant Patients. J.L. Kool et al.; Infect Control Hosp Epidemiol (Dec 1998; 19(12)). Pp. 898-904. Comment in: Infect Control Hosp Epidemiol (Dec 1998; 19(12)). Pp. 893-97.

A Survey of Methods Used to Detect Nosocomial Legionellosis Among Participants in the National Nosocomial Infections Surveillance System. A.E. Fiore et al.; Infect Control Hosp Epidemiol (Jun 1999; 20(6)). Pp. 412-16.

Detecting Legionellosis by Unselected Culture of Respiratory Tract Secretions and Developing Links to Hospital Water Strains. J.R. Kohler et al.; J Hosp Infect (Apr 1999; 41(4)). Pp. 301-11.

Legionella Epidemic in the Netherlands. I.M. Hoepelman; Ned Tijdsch Geneeskd (Jun 5 1999; 143(23)). Pp. 1192-96.

Pontiac Fever at a Sewage Treatment Plant in the Food Industry. P. Gregersen et al.; Scand J Work Environ Health (Jun 1999; 25(3). Pp. 291-95.




Report last updated: 2009/04/10 00:00:00 GMT+0