Legionellosis

Legionellosis is a respiratory tract infection with considerable morbidity and mortality that is being recognized more commonly as an important cause of community-acquired pneumonia as well as nosocomial pneumonia. As with all severe respiratory infections, an aggressive diagnostic approach is warranted for legionellosis. Clearly the most rapid diagnostic measures available should be employed in the diagnosis of these infections. However, rapidity is of no value with tests that are low in specificity and may provide ambiguous results. The standards in Legionella diagnosis are culture, DFA, and urinary antigen tests. The methods have been improved, but the principals have remained the same for the past two decades. Culture and antigen testing are highly specific, while DFA and urine antigen testing provide rapid results. Newer rapid tests emploring genetic detection are being developed, and will be increasingly incorporated by clinical microbiologists. Therapy, however, should not be dictated by the results of the diagnostic efforts as a delay in instituting appropriate therapy for legionellosis significantly increases the morbidity and mortality of this disease. Accordingly, empirical therapy for Legionella spp. should be included in the treatment of severe community-acquired pneumonia. Although intravenous erythromycin has historically been the drug of choice, newer macrolides such as azithromycin as well as fluoroquinolones have superior in vitro activity, greater intracellular and pulmonary tissue penetration, and are available in intravenous form. Parenteral therapy should be given until there is a clear clinical response. Rifampin is also highly active against Legionella spp. and may be combined with a newer macrolide or a fluoroquinolone in cases of severe illness. Therapy can be switched to oral dosing after a clinical response and should be continued for three weeks in immunocompromised patients, two weeks in others.