Laboratory diagnosis of legionnaires’ disease

Abstract

Legionnaires’ Disease is a pulmonary infection caused by Legionella pneumophila . Most sporadic and epidemic-forms of the disease are due to serogroup 1 strains. Legionella of the other serogroups and other legionella-like organisms can cause opportunistic infections in some individuals. Laboratory diagnosis is most simply achieved by demonstrating legionella in pulmonary tissue or respiratory secretions. Special stains are necessary since legionella stain poorly or not at all in the Gram stain. Direct fluorescent antibody staining is the most sensitive and specific test for delecting organisms, but the range of sera and their availability are limited. Tissue and or secretions should be injected into guinea pigs and cultured on to charcoal yeast extract (CYF.) agar or a suitable alternative. Organisms infecting the guinea pigs can be isolated on CYE agar by plating splenic tissue. Serological testing of acute and convalescent sera to demonstrate a significant rise in antibody by the indirect fluorescent test (IFA) can be used to make a retrospective diagnosis. Most patients show increasing litres of specific IgM antibody in the second week of their illness. No single level of antibody can be considered diagnostic. At present only a limited range of strains (serogroups 1 -4) are available as antigen. A negative serological test does not exclude infection with one of the other serogroups or with other /egione/la-Yikc organisms which are antigenically distinct from those used in this test.