High prevalence of positive antibodies to Legionella pneumophila among outpatients

A serologic survey of antibodies to Legionella pneumophila serogroups 1 through 6 was performed to assess the prevalence of positive antibody titers among outpatients and to evaluate the association between elevated antibody titers and previously identified risk factors for legionellosis. Subjects were recruited from outpatient clinics at the Minneapolis VA Medical Center. Each participant completed a self-administered questionnaire which asked about their general health and any recent symptoms of illness. Medical records were also audited for all participants to provide additional information on medical history and known risk factors for Legionella infection. Single samples of serum were obtained from each subject, and antibody titers to L pneumophila serogroups 1 through 6 were assayed. Three hundred ninety-six subjects were enrolled in the study. They had a mean age of 67 years, and 98 percent were male subjects. Overall, 36 percent of the subjects had positive antibody titers (≥1:128) to L pneumophila. There were no differences between those with and without elevated titers with respect to recent systemic symptoms or other previously established risk factors for legionellosis. We conclude that positive anti-Legionella antibodies are a frequent occurrence among these outpatients. This has important implications for the interpretation of single or static antibody titers from patients who are acutely ill.