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

www.nature.com/scientificreports OPEN Trends in the incidence of severe fever with thrombocytopenia syndrome in Japan: an observational study from 2013 to 2022 Shinnosuke Fukushima1,2,3, Hidemasa Akazawa1,3, Toshihiro Koyama4 & Hideharu Hagiya3 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. Keywords Epidemiology, Severe fever with thrombocytopenia syndrome (SFTS), Tick-borne infectious disease, Joinpoint regression analysis Global warming has reportedly led to expanding habitat ranges and extended activity times for ticks, and accordingly, tick-borne infectious diseases have become a major concern worldwide1–3. Severe fever with thrombocytopenia syndrome (SFTS), caused by SFTS virus (SFTSV), or Huaiyangshan banyangvirus that is currently renamed as Dabie Bandavirus4, is an emerging tick-borne infectious disease in far eastern Asian countries5,6. The most updated systematic review and meta-analysis study demonstrated global upward trends in both notification and death rates, with the overall pooled notification rate at 18.93 (95% confidence interval [CI]: 17.02–21.05) and the overall pooled notification deaths rate at 3.49 (95% CI 2.97–4.10) per 10 million people7. While, the case fatality rate showed a significant decrease globally, with the overall pooled case fatality rate at 7.80% (95% CI: 7.01–8.69%). Comparing the three Eastern Asian countries, Japan has a substantially lower pooled notification rate at 2.46 cases per 10 million people without a statistically significant rising trend, while those in China and South Korea are 18.59 and 48.49, respectively7. However, since the first detection in Yamaguchi prefecture8, the number of SFTS cases has been increasing in Japan, mainly involving older people living in the western countryside9,10. The seasonal peak of SFTS reportedly occurs between April and September in eastern Asian countries7, with long daylight hours, shrubs, and forested areas considered risk factors for the disease10–12. However, other epidemiological features and temporal trends of SFTS in Japan remain unclear. 1Department of General Medicine, Dentistry and Pharmaceutical Sciences, Okayama University Graduate School of Medicine, Okayama, Japan. 2Department of Bacteriology, Dentistry and Pharmaceutical Sciences, Okayama University Graduate School of Medicine, Okayama, Japan. 3Department of Infectious Diseases, Okayama University Hospital, 2-5-1 Shikata-cho, Kitaku, Okayama 700–8558, Japan. 4Department of Health Data Science, Dentistry, and Pharmaceutical Sciences, Okayama University Graduate School of Medicine, Okayama, Japan. email: Scientific Reports | (2025) 15:20715 | https://doi.org/10.1038/s41598-025-07955-x 1 www.nature.com/scientificreports/ SFTS is transmitted not only by tick bites but also directly via the body fluids of infected companion animals such as cats6,13. The seroprevalence of SFTSV antibodies among Japanese veterinary staff is reported to be elevated at 4.2%13, suggesting occupational infections may occur through contact with domestic animals during care. In Japan, several prefectures have reported high positivity rates of anti-SFTSV antibodies in wild animals13–18. While anti-SFTSV antibodies were not detected in deer in Hokkaido, a northern island in Japan14, other prefectures in the western region showed high seroprevalences of anti-SFTSV antibodies across various animal species, such as deer (55–65%), boars (12–39%), and cat (33%)13,16,17. Notably, SFTSV can infect many other mammalian species7, indicating the potential threat of this zoonotic disease. Until recently, no specific treatments have been available for patients with SFTS. In the absence of effective therapy, the mortality rate of SFTS remains very high, reaching 20–35%2,3. However, recent reports have suggested the clinical efficacy of favipiravir for treating SFTS19,20, and in May 2024, the Japanese government approved the drug as the first therapeutic agent for SFTS worldwide21. Due to insufficient clinical data, the realworld effectiveness of favipiravir has yet to be determined. To further understand the clinical burden of SFTS, we analyzed the 10-year incidence trends of SFTS in Japan, with particular focus on vulnerable populations, geographic distribution, and associations with environmental factors. Methods Data source This 10-year retrospective observational study was conducted on data collected between 2013 and 2022, obtained from the Infectious Diseases Weekly Report of Japan22. Since 2013, clinical data on patients with stipulated diseases have been accumulated at the National Institute of Infectious Diseases based on the Act on the Prevention of Infectious Diseases and Medical Care for Patients with Infectious Diseases (the Infectious Diseases Control Law). SFTS is a notifiable disease classified as a category IV infectious disease that should be reported immediately after diagnosis23. Thus, when patients with the disease are diagnosed, medical practitioners are responsible for informing public health centers, and the reported data are then summarized by the National Epidemiologic Surveillance of Infectious Disease and made available to the public on an online website22. Clinically diagnosed cases of SFTS were not included because the law stipulates that only cases detected by laboratory tests, such as reverse transcription polymerase chain reaction testing, fluorescent antibody testing, and neutralizing antibody testing, are required for reporting. Details and definitions of these laboratory tests are clearly outlined in the Pathogen Detection Manual of the National Institute of Infectious Diseases24. In actual practice, since there (...truncated)