Legionnaires’ disease is due to infection with Legionella pneumophila, an aerobic gram-negative bacillus found in aquatic environments such as reservoirs, cooling towers, water distribution systems, and humidifiers. It often results in a severe pneumonia with a high mortality. It is characterized by malaise, myalgia, headache, abdominal and chest pain, nausea, vomiting, diarrhea, high fever, rigors, dyspnea, and cough. The cough is usually productive, and can be associated with hemoptysis. Complications include endocarditis, sinusitis, brain abscess, and pancreatitis. Since the organism is difficult to culture from sputum, diagnosis is usually established serologically by an indirect fluorescent antibody test, as it was in this case. The radiographic appearance usually begins with a peripherally situated patchy consolidation. It can progress to involve more than one lobe, and is bilateral in about half the cases. Pleural effusion can result, and later did in this case (image not shown) Radiographic improvement often lags behind clinical improvement.

The Legionella Urinary Antigen EIA has been shown to be a sensitive and specific method to detect the presence of Legionella pneumophilis serogroup 1 soluble antigen in human urine. L. pneumophilia antigenemia can occur 2-3 days after infection and may persist for prolonged periods after treatment in some patients. A negative test result does not rule out the possibility of infection due to other L. pneumophilia serogroups or L. micdadei or L. longbeache.