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Bathwater-associated cases of legionellosis in Japan, with a special focus on Legionella concentrations in water.

To evaluate the relationship between the incidence of legionellosis and Legionella concentrations in bathwater, we sent a questionnaire to 76 prefectural and municipal public health laboratories in Japan and found that 35 had encountered cases of legionellosis and had implemented investigations to determine the sources of the infections. Based on the results of the questionnaire, we were able to analyze various characteristics of the patients, of the facilities that were thought to be associated with the cases, and of the species and serogroups of the isolates and concentrations of Legionella. Ninety-six cases were included in this study. The median age was 67 years (range, 13-89 years). The most prevalent underlying medical condition among patients was diabetes, and the second most prevalent was high blood pressure. Concentrations of Legionella in bathwater ranged from 10 to 160,000 CFU/100 ml. Ten episodes were selected in which causative strains were found in the suspected source environment, and were then confirmed by pulsed-field gel electrophoresis analysis, enabling us to provide an estimated infectious concentration range of Legionella of 90 to 140,000 CFU/100 ml. It was thus suggested that the current Japanese regulatory safety level for Legionella in bathwater, which is set below the detection limit of culture techniques (10 CFU/100 ml), should be appropriate to prevent bathwater-associated legionellosis. In tandem with the above-mentioned research, a review of literature concerning bathwater-associated legionellosis and typical cases was undertaken.