Emergence of severe fever with thrombocytopenia syndrome virus in northern China: first human cases and genomic evidence from Inner Mongolia

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging tick-borne hemorrhagic fever with expanding geographic distribution in China. Although SFTS virus (SFTSV) has been detected in ticks from Inner Mongolia, no autochthonous human cases had been documented in this northern province prior to this report. We report the first detection of SFTSV RNA in serum samples from two febrile patients with a history of arthropod bites in Morin Dawa Daur Autonomous Banner, Hulunbuir City, Inner Mongolia, in 2022. Case 1 (41-year-old male) and Case 2 (44-year-old female) presented with thrombocytopenia/leukopenia and elevated liver enzymes, and both recovered after antiviral and anti-inflammatory therapy. Two SFTSV isolates (ZF53-NM-2022 and ZF96-NM-2022) were obtained and classified as genotype C3. Concurrently, tick virome sequencing of Dermacentor nuttalli from Erenhot City in 2024 identified a near-complete SFTSV genome (ELH_tick_2024, genotype C3 forming a distinct divergent lineage) from one pool, with a partial gap in the nucleoprotein open reading frame due to low viral RNA abundance. This study provides the first laboratory-confirmed evidence of autochthonous human SFTSV infection in Inner Mongolia and reports the detection of SFTSV in local ticks. These findings underscore the need for enhanced clinical awareness and expanded surveillance for SFTSV across northern China.