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Legionella Infections

by Larry M. Bush, MD, Maria T. Perez, MD

Legionella pneumophila most often causes pneumonia with extrapulmonary features. Diagnosis requires specific growth media, serologic testing, or PCR analysis. Treatment is with doxycycline, macrolides, or fluoroquinolones.

The first appearance of this organism was in 1976 at a convention of the American Legion in Philadelphia, Pennsylvania—thus, the name legionnaires’ disease. This disease is the pneumonic form of an infection usually caused by Legionella pneumophila serogroup 1. Nonpneumonic infection is called Pontiac fever, which manifests as a febrile, viral-like illness

The organisms are often present in soil and freshwater. Amebas present in freshwater are a natural reservoir for these bacteria. A building’s water supply is often the source of a Legionella outbreak. Legionella organisms are embedded in a biofilm that forms on the inside of water pipes and containers. The infection is usually acquired by inhaling aerosols (or less often aspiration) of contaminated water (eg, as generated by shower heads, misters, whirlpool baths, or water cooling towers for air-conditioning). Nosocomial infection usually involves a contaminated hot water supply. The infection is not transmitted from person to person.

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