Molecular survey of Rickettsia japonica in ticks and wild boars in Ibaraki Prefecture: a newly recognized endemic area of Japanese spotted fever in eastern Japan.

Japanese spotted fever (JSF) is a tick-borne disease caused by Rickettsia japonica, a member of the spotted fever group rickettsiae. Recently, Japan has experienced a marked increase in JSF cases, with endemic regions expanding from western to eastern areas. However, the specific tick species carrying R. japonica in these newly affected eastern regions, as well as the factors influencing this geographic dissemination, remain unclear. In this study, we investigated the tick species carrying R. japonica and assessed the potential involvement of wild boars (Sus scrofa) in the dispersal of these ticks in Ibaraki Prefecture, a newly recognized JSF-endemic area in eastern Japan. A total of 4549 questing ticks were collected and pooled into 747 samples; 289 samples (38.7%) tested positive for Rickettsia DNA. Among these positive samples, R. japonica was detected only in Haemaphysalis hystricis. Nineteen wild boars captured near the tick sampling sites exhibited heavy infestations with H. hystricis, suggesting that wild boars are important blood-meal hosts for this tick species. Additionally, H. hystricis carrying R. japonica were collected from wild boars. In contrast, R. japonica was not detected in any biological samples collected from wild boars. These results suggest that H. hystricis is a potential vector of R. japonica in Ibaraki Prefecture and that wild boars may potentially transport H. hystricis carrying R. japonica. However, this study found no evidence that wild boars act as reservoirs or amplifying hosts. Given the recent increase and geographic expansion of wild boar populations in the prefecture, continued surveillance of the distribution of H. hystricis is warranted.