Environmental Distribution of Pathogenic <i>Leptospira</i> spp. in Subtropical Rivers of Japan and Implications for Human Infection

OBJECTIVES: Leptospirosis is a zoonotic disease caused by Leptospira spp. often linked to freshwater exposure in subtropical and tropical regions. Okinawa Prefecture reports the highest number of leptospirosis cases in Japan based on national surveillance data. Although recreational river activities in Okinawa are a major opportunity for exposure, direct evidence connecting environmental contamination with human infection remains limited. We investigated the spatiotemporal distribution of pathogenic Leptospira spp. in rivers on Iriomote Island, Okinawa, a subtropical region with a high leptospirosis incidence. METHODS: Ten laboratory-confirmed patients treated at the regional hospital and affiliated clinics in 2024 were assessed for demographics, onset dates and exposure histories. Water samples were collected monthly from seven major rivers (June-September and December 2024). Pathogenic Leptospira spp. were detected by nested polymerase chain reaction targeting flaB, followed by sequencing and phylogenetic analysis. RESULTS: A cluster of six cases occurred with onset on August 26-27; all reported river activities during incubation periods. In the environmental survey, highly and intermediately pathogenic genotypes were identified in all rivers (n = 84 water samples). Leptospira interrogans was frequently identified, particularly during summer. Novel sequences within the highly pathogenic group suggest potentially unreported species. Pathogenic Leptospira were detected in river water both before and after rainfall. The case cluster occurred following heavy rainfall, suggesting a possible temporal association between rainfall and increased exposure risk. CONCLUSION: Pathogenic Leptospira spp., including clinically relevant L. interrogans, were widely detected in rivers on Iriomote Island. Human case occurrence followed heavy rainfall where pathogenic Leptospira were already present in the environment. While this temporal pattern suggests that rainfall may have exacerbated exposure risk, exposures unrelated to rainfall cannot be excluded. This integrated approach may improve case detection and risk assessment in endemic settings and is adaptable to other tropical and subtropical regions with similar exposure risks.