Legionella Detection in Environmental Water Samples Submitted to the Gyeonggi-do Institute of Health and Environment, 2023-2025: Sample-type Differences and the Increasing Contribution of Cooling-tower Water to Positive Samples.

Objectives: Legionella spp. proliferate in artificial water systems, such as cooling towers and water supply and hot water systems of buildings, causing infection through the inhalation of contaminated aerosols. We analyzed the detection characteristics of Legionella spp. within environmental water samples submitted to the Gyeonggi-do Institute of Health and Environment, the Republic of Korea, to determine management priorities. Methods: We analyzed 9,172 Legionella spp. testing records for environmental water samples received from January 2023 through December 2025, summarized by sample type, purpose, submission route, species, and concentration. Chi-square and Cochran-Armitage trend tests were used. Results: Legionella spp. were detected in 570/9,172 samples (6.21%; 95% confidence interval, 5.74-6.73%). Detection differed significantly by sample type (p<0.001)-bathtub water: 18.30%, hot water: 11.03%, cooling-tower water: 5.53%, other water: 3.53%, and cold water: 1.28%; the ranking excluding other water was consistent across 3 years. Cooling-tower water accounted for 47.2% of positives, increasing significantly from 34.2% in 2023 to 59.6% in 2025 (p for trend<0.001); after standardization for sample-type composition, the increase was smaller (43.2% to 53.1%). Requester-collected submissions comprised 96.4% (4,691/4,865) of cooling-tower samples; detection rates were 14.94% (26/174) for officially-collected and 5.18% (243/4,691) for requester-collected samples. Cooling-tower water accounted for 55.6% of positives >10,000 CFU/L. Conclusions: Bathtub and hot water systems require preventive management given their high detection proportions, whereas cooling towers require surveillance priority given their large and increasing contribution to positives. Priorities should combine sample-type detection rates with the size and trend of positive counts. As cooling-tower testing relies largely on voluntary requests, risk-targeted official testing should complement it.