Trends in the incidence of severe fever with thrombocytopenia syndrome in Japan: an observational study from 2013 to 2022.

We aimed to determine the 10-year trend in the incidence of Severe fever with thrombocytopenia syndrome (SFTS) in Japan. This retrospective observational study used a publicly available national database. Trends in the incidence of SFTS with annual percent changes (APC) were examined using Joinpoint regression analysis with stratification by patient age, season, and region. The association between disease incidence and environmental factors was investigated using Spearman's rank correlation. Between 2013 and 2022, there were 803 notified cases (397 males and 406 females) of SFTS, with 79.5% aged ≥ 65 years. The annual incidence rate increased continuously with an APC of 9.6%. The incidence peaked between May and June, with 80.8% of cases observed between May and October. The incidence was predominantly higher in western Japan, and the mean annual incidence rate was the highest in Miyazaki prefecture, with 0.89 per 100,000 people. Correlations between the SFTS incidence rates and environmental factors were observed in western Japan, with forest area (correlation coefficient, 0.80), followed by agricultural population rate (0.70). SFTS incidence is continuously increasing in Japan, especially among the elderly population. Environmental factors such as broader forest areas and increased agricultural population were possibly associated with the incidence.