Molecular Epidemiology and Seroepidemiology of Oz Virus Infection in Ticks and Wild Boars in Ibaraki Prefecture, Japan

Oz virus (OZV), an emerging negative-sense single-stranded RNA virus classified under the family <i>Orthomyxoviridae</i> and genus <i>Thogotovirus</i>, was first isolated from <i>Amblyomma testudinarium</i> ticks in Ehime Prefecture, Japan, in 2013. Moreover, a single fatal case in an elderly individual, suspected to be associated with OZV infection, was reported in Ibaraki Prefecture in 2023. Given these circumstances, this study was conducted to investigate the molecular epidemiology and seroepidemiology of OZV in Ibaraki Prefecture, Japan. From April to November 2023, a total of 2430 ticks were collected at 19 sites. The OZV RNA was detected in one <i>A. testudinarium</i> nymph. Additionally, among 934 wild boar serum samples collected between 2019 and 2023, one sample tested positive for OZV RNA. Neutralizing antibody assays revealed that 243 samples (26.0%) obtained from wild boars were seropositive, indicating widespread exposure among wild boars. Antibody prevalence and titers were highest in the central-western mountainous region, suggesting an active transmission hotspot. Sequence analysis of the OZV viral RNA detected from one tick sample and one wild boar serum revealed that a 212 bp fragment of segment 4 and a 261 bp fragment of segment 5 were 100% identical to a human-derived strain isolated in the same prefecture, suggesting the circulation of a single viral lineage within the local environment. These findings represent the first report demonstrating the circulation of OZV in the natural environment in Ibaraki Prefecture, implicating <i>A. testudinarium</i> as the principal vector and wild boars as a potential source of OZV infection These results suggest that OZV should be considered a potential emerging zoonotic pathogen. Further seroepidemiological studies among residents are warranted to assess the risk of human infection in the region.