Serological Assays Define Extent of Legionnaires’ Disease Outbreak

The isolation of Legionella spp. by culture is considered the gold standard for diagnosing LD, but the sensitivity is low, and the urinary antigen test (UAT) has become the most performed test in diagnosing LD because of its easy performance and rapidity.

Scientists at the Ostfold Hospital Trust (Sarpsborg, Norway) investigated a long-distance outbreak of LD caused by Legionella pneumophila serogroup 1 that occurred in south-east Norway. The initial outbreak investigation without serology identified 56 laboratory-confirmed LD cases of whom 10 died. However, 116 patients with community-acquired pneumonia (CAP) might belong to the outbreak based on epidemiological investigations, but acute laboratory tests other than serology were negative or not performed.

To assess the true extent of the outbreak, they evaluated two serological assays in order to reclassify the 116 patients with indeterminate case status. The two commercial polyvalent serological L. pneumophila assays were used for evaluation of the methods and the outbreak investigation: a serogroup 1–6 immunoglobulin (Ig) G/IgM/IgA immunofluorescence assay (IFA) (Meridian Bioscience Europe; Milan, Italy) and a serogroup 1–7 enzyme-linked immunosorbent assay (ELISA) (Serion ELISA classic, Institut Virion/Serion GmbH; Würzburg, Germany) with separate levels of IgG and IgM antibodies measured in an ELISA robot (DSX Automated System, Dynex Technologies; Inc., Chantilly, VA, USA). Antibodies to the outbreak strain were determined by immunoblotting.

In the evaluation study, the sensitivity and specificity of an equal to or greater than four-fold IFA titre change was 38% and 100%, respectively, with corresponding values of 30% and 99% for seroconversion in ELISA. A single high positive IFA titre yielded sensitivity and specificity of 73% and 97%, respectively, with corresponding values of 68% and 96% for a single high immunoglobulin (Ig) G and/or IgM in ELISA.

The serological testing identified 47 more LD cases, and the outbreak thus comprised 103 cases with a case fatality rate of 10%. About the same proportion (70%) of the urinary antigen positive and negative LD cases had antibodies to the serogroup-specific lipopolysaccharide of the outbreak strain. In addition to the 103 LD cases, Legionella infection could not be verified or excluded in 32 patients based on epidemiology and/or lack of microbiological sampling.

The authors concluded that the acute-phase tests including culture, polymerase chain reaction, and urinary antigen identified less than 55% of the 103 patients in this outbreak. Serological testing thus remains an important supplement for diagnosis of LD and for determination of outbreak cases. The study was published on March 28, 2015, in the journal BMC Infectious Diseases.

Related Links:
Ostfold Hospital Trust
Meridian Bioscience Europe 
Dynex Technologies