Identification and Phylodynamic Analysis of the Siberian Subtype of Tick-Borne Encephalitis Virus in Tick-Bitten Patients From Northeastern China

Abstract

Far Eastern (FE) tick-borne encephalitis virus (TBEV) was thought to be the only endemic subtype in northeastern China. However, in 2024, the Siberian (Sib) subtype of TBEV was found in ticks. This study investigates Sib-TBEV infection in tick-bitten patients in the region. A total of 2573 tick-bitten patients, including 506 inpatients and 2067 outpatients, were enrolled between April and September in both 2023 and 2024 and were tested for Sib-TBEV using real-time quantitative polymerase chain reaction (RT-qPCR). Seven cases were confirmed, with four among inpatients and three among outpatients. All four inpatients exhibited headaches and had hospital stays ranging from 5 to 21 days. Two inpatients with longer hospital stays showed additional symptoms: one experienced dizziness and fatigue, while the other exhibited liver injury and neurological symptoms, including facial paralysis, dysarthria, and meningeal signs. All patients recovered and were discharged without sequelae. Seven Sib-TBEV complete genomes were amplified and classified into the Sib-TBEV Zausaev and Vasilchenko lineages. Bayesian analysis estimated that the most recent common ancestor (tMRCA) of these strains dates to a period between 1898 and 1983, with possible migration pathways from Irkutsk or Zabaykalsky to the Daxing’an Mountains (DXAM) in northeastern China. We identified Sib-TBEV-infected patients in northeastern China and characterized their epidemiological and clinical features. Sib-TBEV may have been circulating in northeastern China for decades, underscoring the need for TBEV surveillance in ticks, animals, and tick-bitten patients.