Detection of specific IgM antibody in the investigation of an outbreak of pneumonia due to Legionella pneumophila serogroup 1

Objective Management of outbreaks of pneumonia due to Legionella pneumophil a serogroup 1 (SG1) infection requires rapid and accurate diagnostic tests. Current serologic approaches, based on detection of seroconversion for tot al antibody, do not fulfil this requirement. Methods A diagnostic test based on detection of IgM antibody to L. pneumoph ila SG1 by indirect immunofluorescence was developed and used to evaluate s erum samples from patients involved in a community outbreak of L. pneumophi la SG1 pneumonia that occurred in Spain. Results Testing of samples from serologically proven, sporadic cases of pne umonia due to L. pneumophila SG1 (14) cases of atypical pneumonia due to ot her infectious agents (16) and healthy controls (100) supported the sensiti vity and specificity of the assay. On samples from the outbreak, the IgM as say recognized five of six cases with isolation of L. pneumophila SG1 from respiratory secretions or lung tissue and more than 70% of cases with confi rmed or presumptive diagnosis as determined by the current serologic criter ia. In addition, the ISM assay was positive in 23-70% of patients who fulfi lled the clinical and epidemiologic criteria of case definition but did not display diagnostically significant serologic results or who lacked a detec table antibody response in the routine assay. Among cases confirmed by the current criteria, detection of specific IgM was occasionally achieved befor e the conventional serology gave significant results. Conclusions Incorporation of IgM antibody detection in the current diagnost ic criteria for L. pneumophila SG1 infection may help to improve the manage ment of outbreaks of pneumonia due to this agent.