Legionellosis (Legionnaires' Disease, Pontiac Fever)
Legionella is an important respiratory bacterial pathogen in the United States (US), causing between 8,000 and 18,000 cases of community-acquired pneumonias requiring hospitalization each year. Inhaling or aspirating contaminated water aerosols are the leading sources of infection. Legionellae are ubiquitous in manmade and fresh-water environments where they replicate within free-living amoebae. Warm temperatures and biofilms support bacterial growth, and hot-water and air-circulation systems, hot tubs, and decorative fountains have been implicated exposure sources in communitybased outbreaks. L. pneumophila serogroup 1 is the most frequently identified serogroup among reported cases. Most cases are now diagnosed by urine antigen, which is highly specific for L. pneumophila serogroup 1, so that disease caused by other serogroups or species is less likely to be diagnosed.
Legionellosis is associated with two clinically and epidemiologically distinct syndromes. Pontiac fever is a generally self-limited, nonpneumonic, influenzalike illness whereas Legionnaires’ disease is a common cause of serious bacterial pneumonia. The vast majority of reported legionellosis cases are Legionnaires’ disease. Although most cases occur sporadically, outbreaks have been identified in nosocomial and community-based settings. Since its addition to national outbreak surveillance in 2001, Legionella has been the most commonly reported pathogen associated with drinking water outbreaks. Persons at increased risk for legionellosis include those of advanced age and deficient immune status.