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Legionnaires’ disease

Legionnaires’ disease was first identified in 1976 during an American Legion convention in Philadelphia. This disease is a primarily a pneumonia caused by bacteria of the genus Legionella which are found in water supplies such as plumbing systems and decorative pools where it is symbiotically present in aquatic amoebae. The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease. L. pneumophila comprises approximately 80% of reported human Legionella infections. L. pneumophila is an intracellular pathogen replicates within alveolar macrophages and epithelial cells. Each year, 8,000–18,000 people in the United States are hospitalized with Legionnaires’ disease.  Smokers, the elderly, immunocompromised patients and patients with chronic lung disease are susceptible to serious life-threatening infections.


Legionella is not spread from one person to another. It is acquired from contaminated water supplies. Hot tubs that are not cleaned and sufficiently disinfected can become contaminated with Legionella. A person can get infected with Legionella when they breathe in steam or mist from a contaminated hot tub.


Laboratory diagnosis is performed by detecting the Legionella bacteria in sputum or Legionella antigens in urine samples. Serological confirmation of infection can be performed by detecting a rise in levels antibodies to Legionella in two blood samples taken 3 to 6 weeks apart.


Legionnaires’ disease is usually treated successfully with antibiotics, but the mortality rate of is between 5% and 30%.  Symptoms can mimic a number of other respiratory infections such as influenza and, therefore, prompt and accurate diagnosis of the disease is critical to help prevent exposure of other individuals to contaminated water sources in the home, workplace or hospital.

Recommended Reading
Kumpers P., Tiede A., et al. Legionnaires disease in immunocompromised patients: a case report of Legionella longbeachae pneumonia and review of the literature. J Med Microbiol 2008, 57:384-387.

U.S. Centers for Disease Control and Prevention website