Siberian Subtype of Tick-Borne Encephalitis Virus: Molecular Epidemiology, Clinical Features, and Modern Paradigms of Specific Prevention

Abstract

Relevance . The Southern Urals is an endemic region for tick-borne encephalitis (TBE), where the Siberian subtype of the virus has historically been dominant, represented by the Zausaev lineage. The 2025 identification of the Baltic lineage marks a shift in the established epidemiological paradigm. Aim . To investigate the molecular epidemiology of the Siberian genotype of TBE virus in the Southern Urals, the clinical polymorphism of the disease, and the effectiveness of various prevention strategies in the context of the identified genetic diversity of the pathogen. Materials and Methods. A continuous longitudinal study was conducted on a series of verified TBE cases among patients hospitalized in the Infectious Diseases Department of City Clinical Hospital No. 8 in Chelyabinsk between 2024 and June 30, 2026. To genetically characterize the agent, sequencing of a fragment of the viral protein E gene was performed, followed by phylogenetic analysis. Results and Discussion. The proportion of focal forms was 11.3 %, and the case fatality rate was 5.0 %. Among the 4 deceased patients, 3 received post-exposure immunoglobulin. The Baltic lineage of the Siberian subtype was detected in one fatal case. In the vaccinated, there were no focal forms of tick-borne encephalitis or fatal outcomes. Conclusion . The study revealed an expansion of the Baltic lineage in the region. While emergency post-exposure prophylaxis with immunoglobulin did not prevent severe disease and fatal outcome upon infection with highly virulent strains, vaccination was found to provide reliable protection.