Abstract Title
Mass mortality in colony-breeding gulls and terns in Denmark fluctuates between years
Abstract
Mass mortality was observed among sandwich terns (Thalasseus sandvicensis) in several breeding colonies in Denmark during the summer in 2022. Clade 2.3.4.4b H5N1 high pathogenicity avian influenza virus (HPAIV) was identified as the cause of the mortality. Phylogenetic analyses of the virus sequences revealed outbreaks were caused by viruses highly related to each other and to contemporary viruses from other wild bird species. Following the epidemic, the Danish sandwich tern population decreased by 40 % from 2021-22 to 2023. In 2023, mass mortality was only observed among chicks. However, in one of the affected colonies, HPAIV H5N1 was detected in apparently healthy birds (4/139 birds) in the breeding period in 2023.
In the spring 2023, mass mortality was observed among black-headed gulls (Chroicocephalus ridibundus) in breeding colonies in Denmark. The cause of mortality was identified as clade 2.3.4.4b HPAIV H5N1. Phylogenetic analyses revealed that the outbreaks were caused by viruses with a different genotype than the sandwich terns in 2022, that were closely related to contemporary viruses from European gulls. In mixed black-headed gull and sandwich tern colonies, where tern chicks were in close contact with gull chicks, sandwich tern chicks succumbed, while adult sandwich terns survived. No mass mortality was observed in gull and tern colonies in 2024.
The results suggest that the tern and gull populations can reach a certain level of immunity, thus avoiding mass mortality events. The duration of immunity is unknown and colonies may be affected in the future with devastating impact on bird conservation.
In the spring 2023, mass mortality was observed among black-headed gulls (Chroicocephalus ridibundus) in breeding colonies in Denmark. The cause of mortality was identified as clade 2.3.4.4b HPAIV H5N1. Phylogenetic analyses revealed that the outbreaks were caused by viruses with a different genotype than the sandwich terns in 2022, that were closely related to contemporary viruses from European gulls. In mixed black-headed gull and sandwich tern colonies, where tern chicks were in close contact with gull chicks, sandwich tern chicks succumbed, while adult sandwich terns survived. No mass mortality was observed in gull and tern colonies in 2024.
The results suggest that the tern and gull populations can reach a certain level of immunity, thus avoiding mass mortality events. The duration of immunity is unknown and colonies may be affected in the future with devastating impact on bird conservation.
Co-Author(s)
Charlotte K. Hjulsager (1)*, Yuan Liang (2), Thomas Bregnballe (3), Lars E. Larsen (2).
(1) Department of Virus and Microbiological Special Diagnostics, Statens Serum Institut, 2300 Copenhagen S, Denmark
(2) Department of Veterinary and Animal Sciences, University of Copenhagen, 1870 Frederiksberg C, Denmark
(3) Department of Ecoscience, Aarhus University, 8000 Aarhus C, Denmark.
*ckhj@ssi.dk
Abstract Category
Notable outbreaks, field and molecular epidemiology, and surveillance in wild birds