Serological and molecular assessment of severe fever with thrombocytopenia syndrome virus infection in wild boars from two adjacent endemic prefectures in Japan.

Severe fever with thrombocytopenia syndrome virus (SFTSV) is a tick-borne zoonotic virus endemic to western Japan. Miyazaki and Oita Prefectures are neighboring regions on Kyushu Island; however, the total number of reported human SFTS cases in 2013-2025 was nearly double in Miyazaki compared with Oita. To assess whether this difference is reflected in wildlife, we conducted a serological and molecular survey of SFTSV infection in wild boars. A total of 1,506 serum samples collected between November 2022 and May 2025 were analyzed by enzyme-linked immunosorbent assay (ELISA) and screened for viral RNA using quantitative reverse transcription PCR (qRT-PCR). The ELISA positivity rate was significantly higher in Miyazaki (34.1%, n=1,045) than in Oita (11.9%, n=461) (P<0.001). A randomly selected subset of 150 samples was also examined by an 80% plaque reduction neutralization test (PRNT80). In contrast, although both the PRNT80 positivity rate (44.2%, n=104 vs. 37.0%, n=46) and the viral RNA detection rate (0.57%, n=1,045 vs. 0%, n=461) were higher in Miyazaki, neither difference reached statistical significance. Furthermore, regional differences in ELISA seropositivity within each prefecture broadly corresponded with the distribution of human SFTS cases, suggesting that ELISA-based surveillance in wild boars may be useful for assessing broad regional patterns of SFTSV transmission.