Detection of clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus in a sheep in Great Britain, 2025.

Clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus (HPAIV) continues to pose a significant global threat, affecting wild and domestic avian, and mammalian species. In early 2024, H5N1 HPAIV was detected in dairy cattle in the United States of America, where it has continued to circulate, with sporadic detections also reported in other ruminant species. The detection of high viral loads in milk from infected cattle, resulted in several human infections, underscoring the zoonotic potential of these viruses. In response, several countries have intensified surveillance in non-avian species to evaluate the potential for undetected viral circulation in captive mammals. In Great Britain, bulk milk tank testing of cattle and targeted surveillance of captive mammalian species on an infected premises is undertaken in accordance with the outcome of a rapid risk assessment. This assessment is undertaken to determine epidemiological links between the poultry and captive mammals. A result of this testing was the first recorded detection of clade 2.3.4.4b H5N1 HPAIV in a sheep in March 2025, identified on an infected poultry premises housing ducks, chickens, turkeys and geese in Great Britain. An initial seropositive result in a single ewe triggered further investigation, confirming serological positivity across repeated sampling and the presence of viral RNA in milk samples. This detection was confined to a single animal and is likely attributable to proximity to infected poultry and a presumed heavily contaminated environment. The implications of this detection in a ruminant host are discussed in the context of interspecies transmission and surveillance strategies.