Legionnaires' disease: clinical, epidemiological, and public health perspectives

Legionnaires' disease is a modern environmental infectious disease. It stems from the capacity of the causative agent, Legionella, to multiply within amoebae in warm water and the use, during the 20th century, of devices that maintain water at warm temperatures and produce aerosols. When contaminated with Legionella, aerosols consisting of respirable droplets place the bacteria in juxtaposition with alveolar macrophages, which, as with amoebae, they may parasitize, resulting in illness in susceptible persons. The disease is much more common than previously appreciated with at least 13,000 cases estimated to occur per year in the United States, based on prospective studies. Two highly specific tests, urinary antigen detection and sputum culture, are available for diagnosis during illness. With 60% to 80% sensitivity, urinary antigen tests rapidly detect antigens of Legionella pneumophila serogroup 1, which are responsible for 70% of the cases of legionnaires' disease; results can be available within a few hours. Culture of sputum is 50% to 60% sensitive, but several days are required for growth, and many patients do not produce sputum. Serologic testing, although useful for epidemiologic studies when convalescent-phase antibody titers can be compared with acute-phase titers, is not helpful for clinical decision making because of the low positive predictive value of commercially available acute-phase serologic tests. Erythromycins, intravenous azithromycin, and levofloxacin are currently approved by the US Food and Drug Administration for treatment of legionnaires' disease. However, clarithromycin and several other fluoroquinolones are active against Legionella and may also provide effective therapy. Recent recommendations from the Centers for Disease Control and Prevention's Hospital Infection Control Practices Advisory Committee should be helpful in reducing nosocomial legionnaires' disease. Recommendations are in place or are being developed to minimize the risk of disease in a variety of other settings.