Field survey of ticks following cases of severe fever with thrombocytopenia syndrome in companion animals in Japan

Severe fever with thrombocytopenia syndrome (SFTS), primarily endemic to East Asia, is a high-fatality tick-borne disease caused by Dabie bandavirus, commonly called SFTS virus (SFTSV). Although several cases in companion animals have been reported across the endemic regions, no studies have examined SFTSV infection rates among questing ticks collected from the vicinity of SFTS-positive companion animals shortly after case occurrence. We conducted three field surveys (A, B, and C) of ticks within 14-22 days after the onset of SFTS in companion animals in Nagasaki Prefecture, Japan. A diverse range of tick species were collected: Haemaphysalis kitaokai adults were dominant in survey A (83%) and Haemaphysalis formosensis nymphs in surveys B (74%) and C (38%). Genetically-detected SFTSV positivity rates among pooled samples were 0%, 12%, and 21% in surveys A, B, and C. Notably, an exceptionally high positivity rate (4/5 pools, 80%) at a single sampling point in survey C indicated a highly heterogeneous spatial distribution, suggesting localized SFTSV small-scale foci (microfoci). In a follow-up survey conducted at the spots with positive ticks in survey C 3 months later, the tick population had markedly shifted (the predominant species was larval Haemaphysalis hystricis), and no SFTSV-positive ticks were detected. These findings suggest substantial variability in SFTSV positivity in ticks, even around confirmed SFTS cases, indicating the presence of transient microfoci after an SFTS case. Further investigation is needed to clarify the ecological and environmental factors governing the emergence and persistence of these microfoci and their implications for local risk assessment.