Meteorological and environmental factors associated with the spatiotemporal risk of severe fever with thrombocytopenia syndrome in Japan: A prefecture-level time-series modeling study

Abstract

Background Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne zoonotic disease with a high case fatality rate and expanding geographic distribution in Japan. Comprehensive evaluations that integrate meteorological, environmental, and demographic factors remain limited. This study aimed to quantify these associations. Methods We conducted a nationwide prefecture-level time-series analysis using weekly SFTS surveillance data in Japan from March 2013 to December 2025. A generalized additive mixed model adjusting for nonlinear seasonality and long-term trends was used to assess (1) the associations between SFTS risk and meteorological, environmental and demographic factors, and (2) the value of the factor at which risk become highest. Results A total of 1237 SFTS cases were reported. Mean temperature and precipitation showed inverted U-shaped associations with SFTS risk, with peak risks observed at 21.8 °C (relative risk [RR]: 1.78, 95% confidence interval [CI]: 1.26–2.52) and 156.6 mm (RR: 1.30, 95% CI: 1.04–1.62), respectively. Atmospheric pressure and forest coverage were positively associated with SFTS risk, with maximum RRs of 2.06 (95% CI: 1.44–2.93) at 1027.1 hPa and 6.58 (95% CI: 4.95–8.74) at 0.84, respectively. Mean altitude showed an inverse association, with the highest RR observed at 45.0 m (RR: 3.99, 95% CI: 1.81–8.76). Conclusions The increasing number and geographic expansion of SFTS cases highlight the growing public health importance of this disease in Japan. Our findings improve understanding of factors associated with SFTS transmission and may inform future risk assessment and public health preparedness.