Gross pathology associated with highly pathogenic avian influenza H5N8 and H5N1 in naturally infected birds in the UK (2020–2021)

Background Multiple outbreaks with highly pathogenic avian influenza virus (HPAIV) clade 2.3.4.4b viruses, including H5N8 and H5N1, have occurred in the United Kingdom, as well as in other European countries, since late 2020. Methods This report describes the pathology among poultry species (chickens, turkeys, ducks, and pheasants) and captive birds (Black Swans, a whistling duck and peregrine falcons) naturally infected with HPAIV from 22 cases of HPAIV H5N8 and two cases of HPAIV H5N1 outbreaks investigated between October 2020 and April 2021. Results On gross examination, pancreatic necrosis was easily identified and most commonly observed in galliformes infected with both subtypes of HPAIV but rarely in anseriformes. In addition, splenic necrosis was also frequently observed in chickens and turkeys infected with HPAIV H5N8. Other less common lesions included cardiac petechiae, serosal haemorrhages and ascites in a variety of species. Conclusion Given the widespread dissemination of HPAIV infection in susceptible avian species during autumn/winter 2020–2021, these data, when evaluated along with clinical information, is a valuable first step for both veterinarians and field services to evaluate gross pathology at post‐mortem to support the diagnosis of HPAIV infection.