Epidemiology and impact of high pathogenicity avian influenza in Europe 2020-2025.

High pathogenicity avian influenza (HPAI) of H5 subtype was detected year-round for the first time in Europe in 2022 and contributed to the largest European HPAI epizootic. The switch from H5N8 to H5N1 virus subtypes heralded a substantial increase in cases in wild and domestic birds. The impact on domestic poultry production was substantial with more than 120 million birds dying or being culled to control the disease. This was partly attributed to an expansion in the host range of wild birds affected, with infection of orders of birds not previously described as susceptible. The consequence of these transmission events into naive wild bird populations was often large-scale mortality, particularly in seabirds 2022-2023. Relatively high infection pressure provided opportunity for exposure and spillover to wild mammals principally of the order Carnivora, exposed through predatory or scavenging behaviour. Other sporadic infections involved farmed fur animals and a single detection in a sheep in England, but no evidence of infection in other livestock species including dairy cattle. Human infections were rare and asymptomatic, associated with very close contact with infected domestic birds. The epizootic was sustained by continuing evolution in the virus principally through genetic reassortment with selection and fixation of genotypes of high fitness in the avian population.