Serological evidence of influenza D virus circulation among cattle in Poland.

Introduction: The circulation of influenza D virus (IDV) has been confirmed in Europe, North America, Asia and Africa through seroprevalence and molecular studies, as well as by virus isolation. Its broad host range raises concerns about zoonotic potential, with cattle recognised as the natural reservoir. This study finds the seroprevalence of IDV in cattle, associates animal-level variables such as age, sex and origin and assesses the frequency of IDV exposure in small ruminants and horses in Poland. Material and Methods: A total of 1,029 serum samples were analysed, comprising 755 from cattle, 224 from small ruminants and 50 from horses. To assess the serological response, haemagglutination inhibition tests were performed, and samples classified as positive were subsequently tested with a virus neutralisation test. Results: The findings suggest possible circulation of IDV among cattle in Poland and give an estimated seroprevalence of 45.2%. Seroprevalence was related to age, with older cattle being more likely to test IDV seropositive. Regional differences were also observed, with the highest seroprevalence found in the Wielkopolskie voivodeship, an area known for its intensive cattle farming and one of the largest bovine populations in Poland, exceeding one million animals. No confirmation of IDV occurrence was obtained in any of the 274 sheep, goats, or horses. Conclusion: To the best of our knowledge, this is the first evidence of IDV circulation in the country, highlighting the need for further research to better understand the virus' transmission pathways.