The purpose of this study was to examine the risk of Legionella pneumophila (Lp) infection due to the use of a 24-h hot water bath (24HHWB), which is popular for bathing in Japan because of its water conservation through the use of recycled bath water, and to investigate other possible factors that may be associated with Legionella infection. The risk of anti-Lp serum antibody formation associated with the 24HHWB use and various bio-psycho-socio-ecological factors was estimated based on the logistic model among 204 employees (161 men, 43 women) who were either 24HHWB users or non-users. There was a significantly higher rate of anti-Legionella antibody titers (≥8) in the 24HHWB users than in the non-users, particularly against Lp serogroup (SG) 5 (odds ratio, 16.3; 95% Cl, 2.1-126.5; p<0.01) and Lp SG 6 (odds ratio, 2.9; 95% Cl, 1.3-6.5; p<0.01). However, most of the titers were <64, and no clinical cases of Legionnaires' disease among the 24HHWB users were found. No significant correlations were observed between the duration of the 24HHWB use and the increased antibody titers. The impossibility of relieving stress, a decrease in age, the use of artificial marble bathtubs, and a decrease in the bathing frequency per week were also related to the increased antibody titers against Lp SG 6 (odds ratio, 5.9; 95% Cl, 1.6-22.1; p<0.01), Lp SG 3 (odds ratio, 0.9; 95% Cl, 0.8-1.0; p<0.01: odds ratio, 10.5; 95% Cl, 1.4-76.9; p<0.05), and Lp SG 1 (odds ratio, 1.9; 95% Cl, 1.2-3.2; p<0.05), respectively. The anti-Legionella antibodies were considered to be IgM dominant, because most of the antibodies were sensitive to 2-mercaptoethanol treatment. In conclusion, the use of a 24HHWB was associated with Legionella sensitization, but not with clinically apparent legionellosis. Psychological, environmental, and lifestyle factors also seemed to have causal relations to antibody formation.
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