Epidemiological characteristics of Japanese encephalitis virus in Taiwan: Insights from long-term monthly mosquito surveillance, 2009–2025

BACKGROUND: Japanese encephalitis (JE) remains a vaccine-preventable, mosquito-borne disease of concern in Asia, including Taiwan. Both residents and international travelers are at risk during seasonal epidemics. While vaccination and personal protection against mosquito bites are key preventive measures, limited information is available on the virological and epidemiological features of JE virus (JEV) in mosquitoes following the emergence of genotype I (GI) in Taiwan. METHODS: Since 2009, we have conducted JEV surveillance of mosquito populations monthly at 3 sites in Taichung and annually across 13 sites in 8 counties/cities in Taiwan. Surveillance results were linked with laboratory-confirmed JE cases to describe the virological, temporal, and geographic patterns of JEV activity. RESULTS: From 2009 to 2025, we collected 200,168 female mosquitoes and identified 147 JEV GI-positive pools, exclusively from Culex tritaeniorhynchus and Cx. annulus. Phylogenetic analysis suggested that GI viruses detected in Taiwan were locally maintained rather than imported. JEV-positive mosquitoes and JE-confirmed cases were detected across the island between May and November, and the number of JEV-positive pools was significantly associated with JE human cases. CONCLUSIONS: Our findings indicate that GI JEVs are locally maintained throughout Taiwan and pose a recurring risk of infection between May and November. These results highlight the need for continuous vector surveillance, timely public health alerts, and reinforcement of vaccination recommendations for local populations and travelers visiting Taiwan during the transmission season.