Legionnaires' Disease Investigation, Sam Roi Yot District and Hua Hin District In Prachuap Khiri Khan Province, 27–29 March 2024

Abstract

Background: On 20 March 2024, the Joint Investigation Team (JIT), Department of Disease Control (DDC), received a report from the Office of Disease Prevention and Control Region 5, Ratchaburi Province, regarding five confirmed cases of Legionnaires' disease in foreign nationals in Prachuap Khiri Khan Province. These cases occurred between 14 February–18 March 2024. The risk of environmental transmission in the affected area remained unclear. In response, the JIT investigated on 27–29 March 2024. The objectives were to conduct a descriptive epidemiological study, confirm the diagnosis and outbreak, assess the extent of the issue, identify the infection source, evaluate environmental transmission risk, and recommend control measures.

Methods: A descriptive epidemiological study was carried out to examine Legionnaires’ disease cases in Prachuap Khiri Khan Province over the past 5 years. Diagnosis of confirmed cases was verified through medical records, including clinical signs, symptoms, and laboratory results. Active case finding was conducted by interviewing individuals residing in the same households or buildings as confirmed patients. An environmental survey was performed to identify potential infection sources and assess the risk of transmission. This included inspecting the municipal water supply system in Hua Hin City, assessing accommodations where patients had stayed, and collecting water samples and surface swabs from sanitary facilities. All samples were analyzed based on guidelines for environmental investigation of Legionnaires’ disease in hotel settings.

Results: Five confirmed cases were identified, all foreign nationals aged 10–81 years. Four patients had severe symptoms, and 1 fatality was reported. The onset of illness occurred between 8 January–3 March 2024. All patients tested positive for Legionella pneumophila from sputum samples by real-time PCR. The distribution of cases over an extended period. This suggests that the patients were likely exposed to contaminated environmental sources or water systems that were repeatedly contaminated in different locations, without requiring direct contact between patients. However, no epidemiological links between the five patients were identified, as no common exposure sites could be determined. Environmental testing of 20 water sources revealed that 17 (85%) had free chlorine residuals below the standard levels. However, Legionella spp. was not detected in any of the water samples sent for laboratory analysis.

Conclusion and Discussion: This investigation confirmed an outbreak of Legionnaires' disease in Prachuap Khiri Khan Province with a higher number of cases than the past 5 years. However, a definitive source of infection could not be identified. The findings highlight the need for improvements in water quality management and surveillance in tourist accommodations and related public utility systems. Ongoing monitoring and surveillance of Legionnaires' disease are essential to prevent future outbreaks.