Since 2020, the UK and Europe, have experienced annual epizootics of high pathogenicity avian influenza virus (HPAIV). The first during autumn/winter 2020/21 involved the detected with six H5Nx subtypes although H5N8 HPAIV dominated in the UK. Whilst genetic assessment of the H5N8 HPAIVs within the UK demonstrated relative homogeneity, there was a background of other genotypes circulating at a lower degree with different neuraminidase and internal genes. Following a small number of summer detections of H5N1 in wild birds over the summer of 2021, autumn/winter 2021/22 saw another European H5 HPAIV epizootic, that has dwarfed the prior epizootic. This second epizootic was dominated almost exclusively by H5N1 HPAIV, although six distinct genotypes were defined. We have used genetic analysis to evaluate the emergence of different genotypes and proposed reassortment events that have been observed. The existing data suggests that the H5N1 circulating in Europe during late 2020, continued to circulate in wild birds throughout 2021, with minimal adaptation, but has then gone on to reassort with AIVs in the wild bird population. We have undertaken an in-depth genetic assessment of H5 HPAIVs detected in the UK, over the last two winter seasons and demonstrate the utility of in-depth genetic analyses in defining the diversity of H5 HPAIVs circulating in avian species, the potential for zoonotic risk and whether incidents of lateral spread can be defined over independent incursion of infection from wild birds. Key supporting data for mitigation activities.

This focus article has been prepared by Marco Falchieri, Scott M. Reid, Craig S. Ross, Joe James, Alexander M. P. Byrne, Madalina Zamfir, Ian H. Brown and Ashley C. Banyard of the APHA; Glen Tyler and Emma Philip of NatureScot; and Will Miles of Scottish Oceans Institute, School of Biology, University of St Andrews.