The role of indoor and outdoor occupations in the seroepidemiology of Legionella pneumophila.

Exposure to disturbed soil has been postulated as a source of L. pneumophila in relation to outbreaks of Legionnaires' disease [1]. Using serum samples from 588 healthy individuals engaged in indoor and outdoor occupations in Ohio, we performed indirect fluorescent antibody tests for L. pneumophila serogroups 1 and 2. Reciprocal and geometric mean titers were compared between indoor and outdoor occupational groups. We found 19.2 of the outdoor workers to be positive (titer, greater than or equal to 1:128) for serogroups 1 and 2, whereas 20.7% and 16.3% of the indoor workers were positive for serogroups 1 and 2, respectively. The differences in seropositivity were not statistically significant between occupations when the data were evaluated by sex, race, geographic area, and age. An analysis of the variance and a comparison of the two proportions were used to test the data and geometric mean titers were tested by Student's t-test. All of the differences were not significant at a level of P less than or equal to 0.05. Combining the data from the two groups of workers resulted in 19.9% and 17.9% seropositivity in the total study population for serogroups 1 and 2, respectively. These findings demonstrated that the presence of antibodies to L. pneumophila serogroups 1 and 2 in this non-epidemic-related study was comparable to or higher rates in epidemic-related studies [1]. The present study defines what may be an endemic level of L. pneumophila in Ohio and demonstrates geographic variations in the seropositivity rates [2]. Our results also support the findings of Muldoon et al., who found that seroconversion may occur early in children and also may be maintained in adults over a long interval[3].