Legionellosis is an acute bacterial disease caused by Legionella pneumophila, a poorly staining, Gram-negative bacillus commonly found in water. Of the fourteen currently recognized serogroups of L. pneumophila bacteria, serogroup 1 is most commonly associated with outbreaks and sporadic cases of legionellosis. Legionellosis may occur in two primary forms: Legionnaires' disease and Pontiac fever. Legionnaires' disease is the most common form of legionellosis.
Legionella bacteria typically live in water; hot tubs, shower heads, water faucets, air conditioning systems, water cooling towers, and hot water heaters have all been implicated as sources of aerosolized water droplets contaminated with the bacteria.
Legionella bacteria were first recognized in association with an outbreak of pneumonia in 1976 that occurred among attendees of an American Legion convention in Philadelphia. Since then, more than 39 species and 61 serogroups of Legionella bacteria have been identified. While more than half of these species/serogroups have been associated with human disease, L. pneumophila, the first Legionella species identified, accounts for approximately 90% of infections, with illness most frequently associated with serogroups 1, 4, and 6.
Legionella can cause Pontiac Fever, an often undiagnosed and generally mild and self-limiting "flu-like" illness. They also cause Legionnaires' disease, a potentially severe bacterial pneumonia that is accompanied by cough, fever, and fatigue. Legionellosis is used to describe both diseases. The incubation period for Legionnaires' disease is usually 2-10 days. Based on studies in several parts of the country, Legionnaires' disease may account for 5-15% of all community-acquired pneumonias. Without appropriate antibiotic therapy, infection can cause serious complications and even death. Patients with Legionnaires' disease have signs and symptoms that resemble other bacterial pneumonias, and the diagnosis generally cannot be made by a physician in the absence of specialized laboratory testing.
Certain populations are clearly at greater risk than others for developing severe Legionella infections. The most important host risk factors for developing illness include 1) immunosuppressive therapy (anti-rejection therapy to prevent graft rejection in bone marrow and solid organ transplant patients), 2) chemotherapy for neoplastic disease, current steroid therapy (>20 mg/day for more than 14 days;), and 3) chronic underlying illnesses such as hematologic malignancies or end-stage renal disease. There is a moderately increased risk of illness among the elderly (age > 65 years), smokers, and people with chronic lung disease, diabetes, or congestive heart failure. The disease is extremely rare in children.