Background: In May 2005, a long-distance outbreak of Legionnairesââ?¬â?¢ disease (LD) caused by Legionella pneumophila\nserogroup 1 occurred in south-east Norway. The initial outbreak investigation without serology identified 56\nlaboratory-confirmed LD cases of whom 10 died. However, 116 patients with community-acquired pneumonia might\nbelong to the outbreak based on epidemiological investigations, but acute laboratory tests other than serology were\nnegative or not performed. To assess the true extent of the outbreak, we evaluated two serological assays in order to\nreclassify the 116 patients with indeterminate case status.\nMethods: Two polyvalent antibody tests, a serogroup 1ââ?¬â??6 immunofluorescence assay (IFA) and a serogroup 1ââ?¬â??7\nenzyme-linked immunosorbent assay (ELISA) were used. They were evaluated with cases defined as culture- or\nurinary antigen positive LD patients (n = 40) and non-cases defined as confirmed non-LD patients (n = 39) and\nhealthy control subjects (n = 62). The 116 patients, who were negative in culture, polymerase chain reaction\nand/or urinary antigen tests, were analysed by the same serological assays. Antibodies to the outbreak strain\nwere determined by immunoblotting.\nResults: In the evaluation study, the sensitivity and specificity of a ?4-fold IFA titre change was 38% and 100%,\nrespectively, with corresponding values of 30% and 99% for seroconversion in ELISA. A single high positive IFA\ntitre yielded sensitivity and specificity of 73% and 97%, respectively, with corresponding values of 68% and 96%\nfor a single high immunoglobulin (Ig) G and/or IgM in ELISA.\nBased on this evaluation, the following serological testing identified 47 more LD cases, and the outbreak thus\ncomprised 103 cases with a case fatality rate of 10%. About the same proportion (70%) of the urinary antigen\npositive and negative LD cases had antibodies to the serogroup-specific lipopolysaccharide of the outbreak\nstrain. In addition to the 103 LD cases, Legionella infection could not be verified or excluded in 32 patients\nbased on epidemiology and/or lack of microbiological sampling.\nConclusions: The acute-phase tests (culture, polymerase chain reaction, and urinary antigen) identified less than\n55% of the 103 patients in this outbreak. Serological testing thus remains an important supplement for diagnosis of\nLD and for determination of outbreak cases.
Loading....