Serological studies on the presence of antibodies against West Nile virus in wild birds of the order Passeriformes in Ukraine

West Nile fever is a very dangerous zoonotic viral disease of animals and humans. It is a naturally occurring focal disease, the natural cycle of which involves a natural reservoir of pathogens, such as wild birds, and vectors, such as mosquitoes, ticks, etc. Today, the problem of West Nile fever is becoming increasingly relevant from an epidemiological point of view. Natural foci of this disease pathogen have been present in Ukraine for a long time in the southern and eastern regions, but now, due to climate change, there are changes in the ecology of both natural carriers and vectors, which significantly changes the epidemiological risks to humans. Over the past few years, in particular, in 2024, an increase in human cases, including fatalities, has been recorded in Ukraine. At the same time, there is a lack of up-to-date information on the circulation of West Nile virus and other flaviviruses (Usutu virus, etc.) in the natural reservoir and among vectors in Ukraine. Our research aimed to conduct serological monitoring in Ukraine among wild forest birds, which are one of the main natural reservoirs of the West Nile virus. During 2023-2024, 268 blood samples and 9 egg yolks of wild Passeriformes (families buntings, finches, true sparrows, tits, bushtits, shrikes, wagtails, Hirundinidae) and Piciformes were collected in Kharkiv, Kyiv, Poltava, Odesa, and Khmelnytsky regions. Blood serum and egg yolks were tested in the ELISA ID.Vet - ID Screen West Nile. It was found that antibodies to WN virus were present in blood samples from Great Tits (seroprevalence from 20% to 100% depending on the region), Song Thrush (60–100%), Blackbird (93%-100%), Chaffinch (100%), Goldfinch (100%), House Sparrow (100%), Greenfinch (10% and 100%), hawfinch (100%), Jays (50%), Field Sparrow (20% and 25%), and Great Reed warbler (40%), reed warbler (33.3%), Сommon whitethroat (25%), Robin (100%), Yellowhammer (100%), Blackcap (100%), Spotted flycatcher (100%), Pied flycatcher (100%), Thrush nightingale(100%). No antibodies to the WN virus were detected in the barn Swallow, Sedge Warbler, Paddyfield Warbler, Collared Flycatcher, Reed Bunting, Spanish Sparrow, Common Woodpecker, Goldfinch, Red-backed Shrike. Seropositivity was found in wild birds from all regions studied. The highest percentage of seropositive birds was found in Poltava (86%, 58%) and Khmelnytsky (67%) regions, and the lowest in Kyiv (9%) and Odesa (17.1%) regions. We also found a difference in seropositivity in different years. Thus, in 2023 it was 27.4%, and in 2024 it was 50.5%