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Chapter 11.3 : Detection of Legionella Antigen by Direct Immunofluorescence
Legionnaires' disease is a type of bacterial pneumonia caused by Legionella spp. ( 2 , 14 ). It is estimated that about 1 to 4% of adults with pneumonia requiring hospitalization have Legionnaires' disease and that about 20,000 to 100,000 adults with community-acquired pneumonia have this disease in the United States each year. About 5 to 20% of patients with Legionnaires' disease die of the disease, with major dependence on promptness of specific antibiotic therapy and underlying health of the patient. Legionnaires' disease occurs worldwide, in both epidemic and sporadic form, with sporadic cases being far more common. The disease is more prevalent in some geographic regions than others, for unclear reasons. The primary host risk factors for the disease include immunosuppression, cigarette smoking, and travel. Legionella pneumophila is the cause of more than 90% of cases of community-acquired Legionnaires' disease, with L. pneumophila serogroup 1 being by far the most common causative agent of the disease. Immunosuppressed patients, and those with nosocomial pneumonia, may have infections caused by other L. pneumophila serogroups and other Legionella spp., in particular L. micdadei, L. longbeachae, L. bozemanae, and L. dumoffii. Distribution of the common serogroups and species causing infection may be quite different in various geographic regions. Legionella bacteria are commonly found in the aqueous environment, including tap water and sometimes even distilled water. The preponderance of L. pneumophila as the cause of Legionnaires' disease has resulted in the development of reagents optimized for detection of this species; detection of other Legionella species can be problematic, for reasons of both test sensitivity and specificity. Rarely, organs other than the lungs and pleural space may be infected by the bacterium, causing such diseases as prosthetic heart valve endocarditis, assorted soft tissue abscesses, and systemic infection.