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Legionella pneumophila was first recognized as the etiology of an outbreak of pneumonia among attendees at a 1976 American Legion Convention.1,2 Pneumonia caused by Legionella, known as Legionnaires disease or legionellosis, is a common cause of community-acquired pneumonia in adults and is a cause of health care-associated pneumonia and pneumonia in immunocompromised adults and children. L pneumophila may also cause Pontiac fever, an uncommon, short incubation, influenzalike illness that primarily affects adults with symptoms of fever, malaise, myalgia, chills, headache, and pleuritic pain.3-5

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Epidemiology

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Although at least 50 Legionella species have been identified, only one half have been isolated from humans.6-8L pneumophila, comprised of 14 serotypes, is the most virulent species, accounting for the majority of human infections. L pneumophila serogroup 1 causes 50% to 90% of human infections, whereas L pneumophila serogroup 6, other L pneumophila serogroups, and Legionella longbeachae, Legionella bozemanii, Legionella dumoffi, and Legionella micdadei cause most of the remainder of human infections.8-10 These bacilli are nutritionally fastidious, aerobic rods that, after recovery on artificial media, stain as gram negative. L micdadei is unique among Legionella species in that it can be visualized in specimens using a modified acid-fast stain.

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Infection with Legionella results from inhalation of contaminated aerosols from environmental or aquatic sources.11-15 Legionella are not transmitted from person to person. Legionella spp are ubiquitous in natural freshwater habitats such as lakes, rivers, and groundwater. From these sources, they gain entry into water systems of buildings, including hospitals. These bacteria thrive at temperatures between 30°C (86°F) and 54°C (129.2°F) but are killed at temperatures above 60°C (140°F). Legionella spp are facultative intracellular pathogens and may replicate in nature within various protozoa, including amoeba. Community outbreaks, almost all due to L pneumophila,8 have occurred and have been linked to aerosol-generating machinery such as cooling towers and evaporative condensers; however, showers, respiratory therapy devices, air conditioners, ultrasonic mist machines for vegetables,13 whirlpool spas,15 and humidifiers have also been associated with outbreaks. Health care-associated infections and outbreaks also occur and are most commonly traced to the water supply, particularly the hot water supply. Sporadic cases have often been linked to home water heaters, particularly electric water heaters.16

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Most cases of Legionnaires disease occur in susceptible elderly or middle-age adults. Legionella are responsible for 1% to 15% of community-acquired pneumonias in adults that require hospitalization. The incubation period has been estimated to range from 2 to 10 days, with an average of 7 days.1 The main risk factors in adults are chronic lung disease; immunosuppression,17,18 especially associated with corticosteroid treatment or organ transplantation; and cigarette smoking.1,13

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Legionellosis is uncommon in the pediatric age group. Only 40 (1.4%) of 2766 cases of legionellosis reported to the Centers for Disease Control and Prevention in 2006 were in persons 24 years of age ...

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