Circulation of an emerging neurotropic tick-borne phenuivirus in brown bears and wildlife in northern Japan

Tick-borne phenuiviruses are a group of zoonotic pathogens causing severe, sometimes lethal, diseases in humans. While several novel phenuiviruses have been identified in ticks, especially in East Asia, their mammalian infectivity largely remains uncharacterized. Here, we report the identification and successful isolation of an emerging tick-borne phenuivirus, Toyo virus (TOYV), from Hokkaido brown bears (Ursus arctos yesoensis) during wildlife surveillance in northern Japan. TOYV was isolated from bear samples using type I and II interferon receptor-knockout (AG129) mice and Hep3B cells. Intracerebral inoculation of suckling AG129 mice induced neurological signs, and viral replication was confirmed in mouse neuroblastoma cells. Detection of TOYV RNA in brown bears and Haemaphysalis species ticks, together with seropositivity in brown bears, raccoons, and sika deer, demonstrated active tick-borne circulation of TOYV among diverse wildlife hosts. This study provides the first evidence of tick-borne phenuivirus infections in an ursid species and the neuroinvasive and neurotropic potential of TOYV. Our findings underscore the critical role of wildlife surveillance in early detection of viruses with zoonotic potential and accelerating responses to emerging viral epidemics.