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IN THE SUMMER OF 1976, an outbreak of pneumonia at an American Legion convention in Philadelphia, Pa., was traced to contaminated water in the convention hotel's air conditioning system. About 180 people became ill and 29 died. The responsible pathogen, a Gram-negative bacterium, was isolated and named Legionella pneumophila. The most severe form of the infection it causes, Legionnaires' disease, is characterized by pneumonia. Legionella also causes Pontiac fever, a less severe illness characterized by fever and muscle aches, but not pneumonia. Most people recover without treatment. Currently, 18 groups of L. pneumophila and 35 related Legionella species have been recognized, but L. pneumophila serogroup 1 is the type most often associated with disease. It can reproduce in high numbers only in a narrow temperature zone—from 90° F to 105° F (32° C to 41° C)—so it thrives in warm, stagnant water found in certain plumbing systems, hot water tanks, and spas. Legionella is transmitted when someone inhales aerosolized water vapor from the air around contaminated water. Disease (legionellosis) develops when a susceptible person inhales enough bacteria from a contaminated source, such as a shower, a bedside humidifier, or aerosolizing respiratory therapy equipment. The percentage of people who become ill after exposure is low: 0.1% to 5%. Those most susceptible to disease include the elderly, cigarette smokers, people with chronic lung or other disease, and those who are immunocompromised. According to the Centers for Disease Control and Prevention, 23% of Legionnaires' disease cases are nosocomial. The infection doesn't spread person to person, so isolation isn't necessary. Assessing signs and symptoms Incubation for Legionnaires' disease is 2 to 10 days. Initial symptoms are anorexia, malaise, myalgia, and headache. Within a day, the patient may experience a rapidly rising fever, chills, and a cough that may be either dry or productive. Some patients also experience abdominal pain and diarrhea. A chest X-ray will show patchy consolidative pneumonia. Severe disease may progress to respiratory or multiorgan failure. Legionnaires' disease is commonly diagnosed from a culture of sputum or bronchial washing specimens, blood serology for antibody titer, and urine antigen. Another option is immunofluorescence antibody testing of lung tissue taken by biopsy, respiratory secretions, or pleural fluid. How to treat and prevent disease Erythromycin is the antibiotic of choice; rifampin may be added to treat severe cases (it shouldn't be used alone to treat the disease). The newer macrolides, clarithromycin and azithromycin, may also be effective, but penicillin, cephalosporins, and aminoglycosides aren't. When caring for a patient with Legionnaires' disease, administer antibiotics as ordered, maintain the airway, support ventilation and oxygenation, rehydrate the patient as indicated (especially if she has diarrhea), and monitor for signs and symptoms of complications, such as respiratory failure, syndrome of inappropriate antidiuretic hormone, disseminated intravascular coagulation, and bacteremia. To help prevent nosocomial Legionella infections, use only sterile water (not distilled or tap water) in respiratory therapy equipment and other aerosolizing and misting devices. If a patient develops Legionnaires' disease, report it to the Health Department if required in your state. Investigators may look for contaminated water sources the patient may have been exposed to, particularly if they receive more than one report of disease within 6 months. If Legionella is found in a facility's water supply, the system may need to be cleaned by hyperchlorination or superheating. But because the bacteria are ubiquitous and may eventually recolonize the system, these measures are recommended only if facility-related human cases occur. SELECTED WEB SITES U.S. Department of Labor, Occupational Safety and Health Administration http://www.osha.gov/dts/osta/otm/legionnaires/faq.html Centers for Disease Control and Prevention http://www.cdc.gov/ncidod/dbmd/diseaseinfo/legionellosis_g.htm Last accessed on November 2, 2004. Barbara Wyand Walker is an infection control/employee health nurse at Greenbrier Valley Medical Center in Ronceverte, W.Va.