Legionella pneumophila was first recognized as the etiology of an outbreak of pneumonia among attendees at a 1976 American Legion convention. Pneumonia caused by Legionella, known as Legionnaires’ disease or legionellosis, is a relatively common cause of community-acquired pneumonia in adults and is a cause of healthcare-associated pneumonia and pneumonia in immunocompromised adults and children. Legionella is also associated with Pontiac fever, an uncommon, short incubation, self-limited influenza-like illness that primarily affects adults, presenting in epidemic form with symptoms of fever, malaise, myalgia, chills, headache, and pleuritic pain.
PATHOGENESIS AND EPIDEMIOLOGY
Although more than 50 Legionella species have been identified, less than one-half have been isolated from humans. L pneumophila, comprised of 14 serogroups, 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. 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.
Infection with Legionella results from inhalation of contaminated aerosols from environmental or aquatic sources, except for L longbeachae that is transmitted via exposure to soil. Legionella species 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 grow at temperatures between 30°C (86°F) and 54°C (129°F) and are killed at temperatures above 60°C (140°F). Legionella species are facultative intracellular pathogens, surviving and multiplying in both mammalian macrophages and monocytes and in free-living amoebae in the environment. Most cases are sporadic and presumably related to exposure to aerosols of contaminated water in the home including water heaters, particularly electric water heaters; workplace; or public places. Community outbreaks, almost all due to L pneumophila, 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, whirlpool spas, and humidifiers also have been associated with outbreaks. Healthcare-associated infections and outbreaks also occur and are most commonly traced to the water supply, particularly the hot water supply.
Legionella are not transmitted from person to person. Most cases of Legionnaires’ disease occur in susceptible elderly or middle-age adults and 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 14 days, with a median of 7 days. The main risk factors in adults are chronic lung disease; immunosuppression, especially associated with corticosteroid treatment, anti–tumor necrosis factor therapy, or organ transplantation; and cigarette smoking.