Outbreaks of highly pathogenic avian influenza (HPAI) epidemics in Baltic Great Cormorant Phalacrocorax carbo colonies in 2021 and 2022

Capsule: Over 1,700 Great Cormorants died from H5N1 highly pathogenic avian influenza (HPAI) at 11 breeding colonies in the Baltic Sea region, and one in the North Sea, in summer 2021 and 2022. Aim: To determine the nature and impacts of the 2021/2022 HPAI outbreaks on Great Cormorants in the Baltic Sea region. Methods: We collected data on HPAI outbreaks in Baltic Great Cormorant colonies through questionnaires, national virological and serological surveys, and avian influenza surveillance databases to better understand the 2021/2022 outbreak. Results: The HPAI H5 clade 2.3.4.4b (goose/Guangdong lineage) caused mortality in five Great Cormorant breeding colonies in Denmark, three in Germany, two in Sweden and single outbreaks in Estonia and Latvia (all ground-nesting colonies). HPAI H5 monitoring of Great Cormorants in Germany since 2006 showed an increasing prevalence from 2022. Great Cormorant H5N1 viruses belonged to a genotype circulating in wild ducks since 2021. After 2021/2022 morbidity and no subsequent outbreaks, breeding colony sizes returned to above-normal levels in 2023. Conclusion: After H5N1 HPAI outbreaks in 11 exclusively ground-nesting colonies of Great Cormorants in the Baltic Sea (and one in the North Sea) in 2022, breeding numbers recovered in 2023. Lack of subsequent mortality suggested immunity, but serological investigations are needed to determine the prevalence of protective antibodies in this and other coastal birds.