[Legionnaires' disease outbreak caused by an electric fireplace-like interior product generating ultrasonic aerosols]

Objectives　An epidemiological study was conducted of 43 cases of Legionnaires' disease reported to the Tokyo Metropolitan Tama-Kodaira Public Health Center and other health centers between September 30 and November 30, 2024. This report describes the methods used to identify an electric fireplace in a restaurant as the source of infection for some patients and the subsequent administrative measures implemented.Method　The epidemiological investigation involved mapping and case interviews. The restaurant identified as the suspected source of the outbreak was inspected on-site, and water samples collected from the vapor tray of an electric fireplace installed as interior decor were submitted to the Tokyo Metropolitan Institute of Public Health for Legionella isolation testing. Environmental isolates obtained from the restaurant and clinical isolates from patients' sputum were analyzed using sequence-based typing (SBT) and pulsed-field gel electrophoresis (PFGE). For the patient group without common behavioral histories, places of residence and living areas were analyzed.Results　Initially, no clusters of Legionnaires' disease were identified in specific residential areas, and no shared exposure risks were apparent. However, detailed interviews revealed that several patients had dined at the same restaurant equipped with an electric fireplace. This device generated water vapor using a mechanism similar to ultrasonic mist generators. Legionella pneumophila SG1 and L. feeleii SG1 were detected at a total concentration of 3 million CFU/100 mL in water samples. PFGE analysis demonstrated indistinguishable patterns between L. pneumophila environmental isolates and clinical isolates, all of which were assigned to sequence type 23 by SBT. Twenty-one patients developed Legionnaires' disease after visiting the restaurant, had no other shared behavioral history, and showed concordant genotyping results, indicating that the outbreak was caused by aerosols generated by the electric fireplace. Among patients without common behavioral histories, 95% resided or lived within a 3 km radius.Conclusions　To our knowledge, this is the first reported outbreak of Legionnaires' disease associated with an electric fireplace in Japan. Prompt tracing investigations and on-site inspections following identification of the suspected source, combined with immediate confirmation of device use and issuance of administrative guidance, prevented additional cases. Repeated educational initiatives addressing the etiology of Legionnaires' disease and appropriate hygiene management of products with similar functions are necessary to prevent future occurrences.