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

H5N1 high pathogenicity avian influenza of clade 2.3.4.4b continues to have a significant global impact on both wild and captive birds, and occasionally mammalian species. In early 2024, the virus was detected as the cause of clinical infection in dairy cattle in the United States of America (USA) and was sporadically detected in other ruminants. Importantly, very high titres of virus in milk from infected cattle has resulted in several human infections, underpinning the significant zoonotic risk these viruses pose. Detection in US cattle has meant that countries elsewhere have increased surveillance in non-avian species to understand whether this virus might be circulating unidentified in different populations. In Great Britain (GB), infection of wild birds and both commercial and non-commercial poultry premises continue to be detected. In response to the USA situation, bulk milk tank testing is undertaken periodically, to confirm the absence of this virus in GB cattle, as well as risk-based surveillance of captive mammals co-located on an infected premises. Here we report the detection of HPAI H5N1 in a single sheep triaged in for testing on an infected premises in GB. Initial serological positivity in an asymptomatic ewe prompted further sampling; that led to repeated detection of serological positivity and H5N1 HPAIV viral RNA in milk across two sampling timepoints. This detection appeared to be limited to a single animal and it is likely that co-location of this animal with infected waterfowl and poultry resulted in this rare observation. Factors resulting from this detection are discussed.

Ashley C. Banyard1, 2, Holly Coombes1, Benjamin C. Mollett1, Jacob Terrey1, Edward Fullick3, Audra Lynne-Schlachter4, David Jorge4, Alex Nunez4, Marco Falchieri1, Joe James1, 2,, Scott M. Reid1 1 Department of Virology, Animal and Plant Health Agency (APHA-Weybridge), Woodham Lane, Addlestone, Surrey KT15 3NB, UK 2 WOAH/FAO International Reference Laboratory for Avian Influenza, Animal and Plant Health Agency (APHA-Weybridge), Woodham Lane, Addlestone, Surrey KT15 3NB, UK 3Surveillance and Laboratory Services Department, Animal and Plant Health Agency, Thirsk, UK 4 Department of Pathology and Animal Sciences, Animal and Plant Health Agency (APHA-Weybridge), Woodham Lane, Addlestone, Surrey KT15 3NB, UK

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