Detection of Highly Pathogenic Avian Influenza Virus H5N1 Clade 2.3.4.4b in Great Skuas: A Species of Conservation Concern in Great Britain

The UK and Europe have seen successive outbreaks of highly pathogenic avian influenza across the 2020/21 and 2021/22 autumn/winter seasons. Understanding both the epidemiology and transmission of these viruses in different species is critical to aid mitigating measures where outbreaks cause extensive mortalities in both land- and waterfowl. Infection of different species can result in mild or asymptomatic outcomes, or acute infections that result in high morbidity and mortality levels. Definition of disease outcome in different species is of great importance to understanding the role different species play in the maintenance and transmission of these pathogens. Further, the infection of species that have conservation value is also important to recognise and characterise to understand the impact on what might be limited wild populations. Highly pathogenic avian influenza virus H5N1 clade 2.3.4.4b has been detected in great skuas (Stercorarius skua) across different colonies on islands off the shore of Scotland, Great Britain during summer 2021. A large number of great skuas were observed as developing severe clinical disease and dying during the epizootic and mortalities were estimated to be high where monitored. Of eight skuas submitted for post-mortem examination, seven were confirmed as being infected with this virus using a range of diagnostic assays. Here we overview the outbreak event that occurred in this species, listed as species of conservation concern in Great Britain and outline the importance of this finding with respect to virus transmission and maintenance.