Impact of highly pathogenic avian influenza virus (HPAIV) on Black-headed Gulls Chroicocephalus ridibundus population in Poland in 2023

Abstract

The highly pathogenic avian influenza virus (HPAIV) A(H5N1) has caused the most extensiveand severe epizootic event affecting both poultry and wild birds globally. This studyinvestigated the impact of HPAIV on the breeding population of the Black-headed GullChroicocephalus ridibundus, the most abundant gull species in Poland. During the 2023outbreak, this species was reported as the most frequently infected in the country. A higherthan-natural adult mortality rate (greater than 1.5% of the breeding individuals) wasobserved in 114 surveyed colonies across all regions of Poland. Laboratory tests confirmedthe presence of HPAIV in all 17 colonies sampled, with average adult mortality estimated at26.1%, and ranging from 1.7% to 77.8%. The estimated mortality rate across all surveyedcolonies was 22.2%. Extrapolations across the entire Polish breeding population (at least115,000 pairs according to the national census) indicated that approximately 51,000 adultBlack-headed Gulls might have perished due to HPAIV in 2023. The number of adults founddead was positively correlated with colony size (r = 0.733, P < 0.001). The deaths wereassociated with a single HPAIV genotype (BB) across all confirmed cases. Understanding thespread and severity of HPAIV in colonially breeding waterbirds, such as gulls, is essential forassessing the full extent of the threats this virus poses to wild bird populations.