Results of Epidemiological Surveillance over West Nile Fever in the Russian Federation in 2025. Forecast of the Epidemiological Situation for 2026

An overview of the epidemiological situation on West Nile fever (WNF) in the world and the Russian Federation in 2025 is presented, a forecast for 2026 is provided. During the assessment of WNF epidemic process intensity in the Russian Federation in 2025, a three-fold decrease in the incidence (145 cases) compared to 2024 (440) and a 1,3-fold decrease compared to the long-term average values (183,2) was revealed. The incidence was registered in 22 constituent entities, including 4 previously unaffected: Republic of North Ossetia-Alania, Kirov and Kurgan Regions, Trans-Baikal Territory. Compared with long-term observation data, 2025 was characterized by a late onset of the epidemic season, a predominance of women among cases, and a significant increase in the proportion of West Nile virus infections by place of permanent residence in cities. The territorial distribution of cases, the age and social structure of the incidence and clinical manifestations of WNF were consistent with the long-term average data. In 2025, the trend of increasing volume and territorial coverage of monitoring studies continued: active detection of WNF cases was conducted in 68 constituent entities (in 2024 – 62, the long-term average – 44), epizootiological monitoring was carried out in 86 entities (81 and 68, respectively), and the immune cohort was studied in 78 (69 and 47, respectively). Enzootic circulation of West Nile virus was confirmed in 14 constituent entities, for the first time – in the Tambov Region. The proportion of positive findings in zoological and entomological material in 2025 was 0.12 % (in 2024 – 0.17 %, the long-term average – 0.18%), which may indicate a decrease in the intensity of the WNF epizootic process in Russia. In 2025, circulation of West Nile virus lineage 2 was detected in the European part of the country, with the ABB.1.1 genovariant predominating in its population structure. During the upcoming season, the Southern, Central, and Volga Federal Districts will be most actively involved in the epidemic process. With effective monitoring, incidence rates are expected to remain at the long-term average level in the medium term, in the long-run – the increase in incidence rate is predicted.

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