[Public health responses and preventive measures for legionellosis outbreaks in Japan: Lessons from past cases]

Objective　Several outbreaks of legionellosis in Japan have been attributed to baths. This study aimed to analyze past cases to inform outbreak responses and preventive measures.Methods　This study was conducted as part of a narrative review. The target cases were 36 major outbreaks that occurred in Japan between 2000 and 2023, and the sources of infection were bathing facilities, excluding baths on cruise ships and ferries. Relevant literature was searched using publicly available information, including J-STAGE, Google Scholar, and the Ministry of Health, Labour and Welfare, as well as related governmental sources, with keywords such as "date of occurrence," "prefecture," and "legionellosis." The extracted literature was descriptively analyzed, focusing on facility characteristics, deficiencies in sanitary management, and local government responses and their rationale.Results　Nineteen cases were analyzed. Regarding facility characteristics, 52.6% (10/19) used hot spring water and 78.9% (15/19) had at least one additional facility such as open-air baths, recirculating systems, or bubble-generating devices. Regarding deficiencies in sanitary management, 63.2% (12/19) had inadequate control of the free residual chlorine concentration in bath water and 47.4% (9/19) had deficiencies in self-management manuals and checklists. Regarding local government responses and their rationale, based on the date of the first reported case, voluntary suspension requests were issued on average 3.9 days later, public announcements 12.0 days later, and business suspension orders 16.7 days later. Voluntary suspension requests were based on either or both inspection results and multiple patient reports in 81.8% (9/11) of the cases, public announcements on culture test results in 85.7% (6/7), and business suspension orders on genetic matches between environmental and clinical strains in 90.0% (9/10).Conclusion　These findings suggest that the use of hot spring water and presence of additional facilities are risk factors for legionellosis outbreaks. Prevention requires adequate control of the free residual chlorine concentration in bath water and proper implementation of self-management manuals and checklists, with the latter being verified before or immediately after the opening of the facility. The rationale for local government intervention appears to be the inspection results and multiple patient reports for voluntary suspension requests, culture test results for public announcements, and genetic matches between environmental and clinical strains for business suspension orders. To accelerate responses, reference criteria for each intervention should be established in advance, along with institutional measures including ordinance revisions, guideline development, and routine preparedness exercises.