New H5N1 Genotype Killed at Least 3,000 Danish Black-Headed Gulls
A newly emerged H5N1 genotype killed at least 3,000 adult black-headed gulls (Chroicocephalus ridibundus) in Denmark in spring 2023, according to a study in the Canadian Journal of Microbiology. The losses represented more than 1.8% of the country’s breeding population.
The researchers compared outbreaks in colonies of black-headed gulls and sandwich terns (Thalasseus sandvicensis) during two consecutive breeding seasons. Sandwich terns often nest within gull colonies, bringing the two species into close contact. That made the two-year contrast unusually informative: in 2022 the severe outbreak was centred on sandwich terns, whereas in 2023 it shifted strongly toward black-headed gulls and tern chicks living among them.
In May and June 2022, severe mortality among sandwich terns was linked to the highly pathogenic H5N1 genotype EA-AB. The Danish sandwich tern population fell by 36% between 2022 and 2023. Despite sharing colonies with the terns, neither adult black-headed gulls nor their chicks showed influenza-associated illness in 2022.
EA-BB carried a different combination of influenza gene segments
Influenza A viruses have eight gene segments. The H5N1 subtype is named for the H5 haemagglutinin and N1 neuraminidase surface proteins, while a genotype describes the particular combination of all eight segments. EA-BB first appeared in France in spring 2022 and contained several segments most closely related to low-pathogenicity H13 influenza viruses of gull origin. The authors suggest that this genomic background may have made EA-BB better adapted to gulls than EA-AB, which lacked gull-origin segments and had circulated widely in waterfowl.
A different pattern in 2023
EA-BB spread northward during the second half of 2022 and was detected in Danish black-headed gulls by the following breeding season. At least 3,000 adult gulls died in Denmark in spring 2023, and the authors estimate that at least another 600 chicks died in the eight colonies visited during the chick period. Because most Danish black-headed gull colonies were not systematically surveyed for mortality, the true number of deaths was almost certainly higher than the recorded adult total.
Of 89 dead black-headed gulls tested in 2023, H5N1 was detected in 65 adults and 11 chicks. Whole-genome sequences were obtained from viruses in 29 birds representing all ten breeding colonies with major mortality, and every one was genotype EA-BB. Most of the 76 H5N1-positive carcasses were emaciated; haemorrhages and congestion of internal organs were also common, although the visible lesions varied among birds.
Genetics linked infections in gulls and tern chicks
Sandwich tern chicks living in contact with black-headed gulls also suffered mass mortality in 2023, while unusual mortality was not recorded among adult terns. In active surveillance of 135 live sandwich terns, H5N1 was detected in four chicks and in none of the adults. Whole-genome sequences from three infected chicks were EA-BB.
Those tern viruses formed a tight genetic group with EA-BB viruses from black-headed gulls, indicating a recent common ancestor and supporting transmission of the same virus between the species in mixed colonies. The authors suggest that adult sandwich terns were less susceptible in 2023, while chicks lacked comparable protection. The contrast with 2022 — when EA-AB caused severe tern mortality but black-headed gulls sharing the colonies showed no influenza-associated disease — points to genotype-dependent differences in which colonial seabirds were most strongly affected.
Disease, parasites & microbiology
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