What is Legionnaires disease?

Updated: Mar 11, 2021
  • Author: Chinelo N Animalu, MD, MPH; Chief Editor: Michael Stuart Bronze, MD  more...
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Legionnaires disease (LD) is the pneumonia caused by Legionella pneumophila. LD also refers to a more benign, self-limited, acute febrile illness known as Pontiac fever, which has been linked serologically to L pneumophila, although it presents without pneumonia. Pontiac fever is usually self-limiting and typically does not require antibiotics administration. 

L pneumophila is an important cause of nosocomial and community-acquired pneumonia (CAP) and must be considered a possible causative pathogen in any patient who presents with atypical pneumonia. Empiric antibiotic coverage for community acquired pneumonia usually includes classes of antibiotics that have legionella coverage such as fluroquinolones, macrolides and tetracyclines.

The Legionella bacterium was first identified in the summer of 1976 during the 58th annual convention of the American Legion, which was held at the Bellevue-Stratford Hotel in Philadelphia. Infection was presumed to be spread by contamination of the water in the hotel's air conditioning system. The presentation ranged from mild flulike symptoms to multisystem organ failure. Of the 182 people infected, 29 died.

Although Legionella was not identified until 1976, L pneumophila was subsequently found in a clinical specimen dating to 1943 and, according to retrospective analysis, may have been responsible for pre-1976 pneumonia epidemics in Philadelphia; Washington, DC; and Minnesota.

Legionnaires disease is the term that collectively describes infections caused by members of the Legionellaceae family.

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