[Association between the stringency of local ordinances and regulations governing public bathhouse facilities and the occurrence of legionellosis in Japan].

Abstract

Objective　Strengthening local ordinances and regulations is considered an important strategy for preventing legionellosis associated with bathing facilities. However, no nationwide study has comprehensively examined the relationship between regulatory stringency and legionellosis occurrence. This study aimed to assess the association between the stringency of local ordinances and regulations governing public bath facilities, measured using an ordinance score, and the occurrence of legionellosis across Japan's 47 prefectures.Methods　The ordinance score comprised four domains: water quality testing, facility and equipment standards, hygiene management, and management systems. The score was developed from 31 items identified as relevant to legionellosis occurrence and extracted from Ministry of Health, Labour and Welfare guidelines for hygiene management of public bath facilities. Each item was assigned a score of 1 when prefectural requirements met or exceeded guideline standards and 0 otherwise. Ordinance scores were calculated using prefectural ordinances and regulations available on official websites as of February 2026. Publicly available data were used to calculate 5-year average values for all indicators. Reported legionellosis cases per 100,000 population and the proportion of the population aged ≥50 years were derived from annual data (2020-2024). The number of public bath facilities per 100,000 population, mean annual relative humidity, and annual precipitation were derived from fiscal data (2020-2024). Associations between ordinance scores and reported legionellosis cases per 100,000 population were evaluated using Spearman's rank correlation coefficient. Univariable and multivariable negative binomial regression analyses were conducted with adjustment for selected covariates.Results　A significant negative correlation was observed between ordinance scores and reported legionellosis cases per 100,000 population (Spearman's ρ = －0.33, P = 0.022). Higher ordinance scores were significantly associated with fewer reported legionellosis cases per 100,000 population in both univariable and multivariable negative binomial regression analyses. In the multivariable model, the incidence rate ratio was 0.80 (95% confidence interval, 0.70-0.93). None of the covariates showed a statistically significant association with reported legionellosis cases per 100,000 population.Conclusion　Prefectures with more stringent ordinances and regulations tended to report fewer legionellosis cases per 100,000 population, suggesting that stronger regulatory frameworks may help prevent legionellosis. This study provides the first nationwide evidence linking regulatory stringency with legionellosis occurrence in Japan. Although strengthening local ordinances and regulations may represent an effective public health strategy, its implementation should balance potential benefits against the operational burden imposed on facility operators.