Dynamics of high pathogenicity avian influenza virus infection with multiple introductions in a crow flock in an urban park in Hokkaido, Japan

Since 2021, high pathogenicity avian influenza viruses (HPAIVs) of the H5N1 clade 2.3.4.4b has been circulating globally, not only in domestic poultry but also in wild birds, both migratory and resident species. In March to May 2022, March to April 2023, and January to April 2024, crow die-offs were reported in an urban garden in Hokkaido, Japan, raising suspicions of HPAIV infection. Since August 2022, all dead carcasses were investigated for HPAIV detection and isolation. Phylogenetic analysis of the H5 hemagglutinin gene revealed that all detected HPAIVs belonged to clade 2.3.4.4b, a dominant lineage in Hokkaido since early 2022. Two distinct subgroups were identified: G2d (in 2022-2024) and G2a (in 2024). A maximum clade credibility tree, based on concatenated nucleotide sequences of the isolates, suggested that multiple distinct types of HPAIVs were introduced into the garden in rotation during the winters of 2022-2023 and 2023-2024. Infectious HPAIVs were isolated not only from the lungs and brains but also from the rectal contents of the dead crows, with no apparent difference in viral titers between the two subgroups. The case reproduction numbers of HPAIV infection in the crow flock ranged from 0.52 and 1.57 in the spring of 2022 and from 0.55 to 1.78 in the spring of 2023, suggesting that the contiguous HPAIV infections in the crows were due to multiple introductions into the flock. Crow can play a key role of potential spread to other animals, poultry and wildlife in urban areas or humans in rural areas.