Outbreak of highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus in cats, Poland, June to July 2023

Background: Over a 3-week period in late June/early July 2023, Poland experienced an outbreak caused by highly pathogenic avian influenza (HPAI) A(H5N1) virus in cats. Aim: This study aimed to characterise the identified virus and investigate possible sources of infection. Methods: We performed next generation sequencing and phylogenetic analysis of detected viruses in cats. Results: We sampled 46 cats, and 25 tested positive for avian influenza virus. The identified viruses belong to clade 2.3.4.4b, genotype CH (H5N1 A/ Eurasian wigeon/Netherlands/3/2022-like). In Poland, this genotype was responsible for several poultry out-breaks between December 2022 and January 2023 and has been identified only sporadically since February 2023. Viruses from cats were very similar to each other, indicating one common source of infection. In addition, the most closely related virus was detected in a dead white stork in early June. Influenza A(H5N1) viruses from cats possessed two amino acid substitu-tions in the PB2 protein (526R and 627K) which are two molecular markers of virus adaptation in mam-mals. The virus detected in the white stork presented one of those mutations (627K), which suggests that the virus that had spilled over to cats was already par-tially adapted to mammalian species. Conclusion: The scale of HPAI H5N1 virus infection in cats in Poland is worrying. One of the possible sources seems to be poultry meat, but to date no such meat has been iden-tified with certainty. Surveillance should be stepped up on poultry, but also on certain species of farmed mammals kept close to infected poultry farms.