Epidemiological Situation on Tick-Borne Viral Encephalitis in the Russian Federation in the Context of a Changing Climate (2016–2025) and Incidence Forecast for 2026

The aim of this review was to characterize the epidemiological situation regarding tick-borne viral encephalitis (TBVE) in the Russian Federation between 2016 and 2025 and to provide a forecast of TBVE incidence for 2026 for federal districts and constituent entities of the highest epidemiological risk. During 2016–2025, the average annual number of individuals affected by tick bites in the RF was 498575.1. In 2020–2021, an upward trend in this indicator was noted in certain entities of the Volga and Northwestern Federal Districts, among others. Climate warming has shifted the onset of Ixodidae tick activity to the last week of March. A lengthening of the epidemic season is observed. In 2025, 1,807 TBVE cases were identified, corresponding to an incidence rate of 1.24 per 100,000 population (compared to the long-term average of 1.17 per 100,000). The five entities with the highest incidence are Kirov Region, Krasnoyarsk Territory, Tuva Republic, Khakassia Republic, and Perm Territory. A total of 44 fatal outcomes were recorded (case fatality rate – 2.4 %). Based on the data from 2016–2025, 17 entities at highest epidemiological risk have been identified, eight of which are located within the Siberian Federal District. The average tick infection rate in the Russian Federation for 2025 (tested via PCR and ELISA) does not differ from the long-term average. A comparison of the genetic structure of tick-borne encephalitis virus strain samples from 1937–1989 and 1990–2025 has showed an increase in the proportion of the Siberian subtype from 54.8 to 89.7 %, while the ratio of individual genetic lineages within the Siberian subtype remained invariable. The volumes of specific and non-specific TBVE prevention measures in 2025 increased compared to 2024 and corresponded to the target figures in all federal districts. According to the forecast, the TBVE incidence in the Russian Federation in 2026 is projected to be 1.14 per 100,000 population, with a confidence interval from 0.95 to 1.32 per 100,000. An increase in morbidity is possible in the Volga and Far Eastern Federal Districts, while the epidemiological situation in the remaining districts is not likely to change.