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bioRxiv · 10.1101/2025.09.17.676714

Outbreak dynamics of high pathogenicity avian influenza virus H5N1, clade 2.3.4.4b euBB, in black-headed gulls and common terns in Germany in 2023

Knief, U.·Ahrens, A. K.·Allendorf, V.·Behringer, C. J.·Bertram, J.·Bouwhuis, S.·Fiedler, W.·Globig, A.·Guenther, A.·Herrmann, C.·Knauf, S.·Marchowski, D.·Piro, S.·Pohlmann, A.·Rollins, R. E.·Staubach, C.·Harder, T.
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Abstract

Since winter 2022/23, high pathogenicity avian influenza virus (HPAIV) H5N1 clade 2.3.4.4b, genotype euBB, has caused extensive mortality among wild birds. This genotype emerged in France in spring 2022 through reassortment between a gull-adapted low-pathogenicity virus and HPAIV H5N1. Phylogeographic and spatiotemporal analyses show that transmission into German breeding colonies of black-headed gulls (Chroicocephalus ridibundus) and common terns (Sterna hirundo) involved multiple independent incursions, likely via black-headed gulls returning from wintering grounds in the Netherlands, Belgium, and France. It spilled over into common terns, which breed in shared colonies with black-headed gulls, and led to high adult mortality in both species in 2023 (at least 8,137 black-headed gulls and 614 common terns; >3% of the breeding population), followed by significant breeding pair declines in 2024 (-15.9% in black-headed gulls, -5.8% in common terns). Increased immunity, at least in common terns, may have contributed to the apparent fade-out of genotype euBB. These findings highlight how integrating ornithological, epidemiological, and virological data can aid our understanding of viral transmission routes and population-level impacts, while also stressing that HPAIV should be added to the growing list of pressures on seabirds, a group that was already the most threatened among all bird taxa globally. Research Highlights- HPAIV H5N1 euBB caused mass mortality in gulls and terns in Germany in 2023 - Multiple independent viral incursions occurred into different parts of Germany - Serology showed survival and rising H5-seroprevalence in common terns - Outbreaks reduced the black-headed gull breeding populations by up to 24%

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Knief, U., Ahrens, A. K., Allendorf, V., Behringer, C. J., Bertram, J., Bouwhuis, S., Fiedler, W., Globig, A., Guenther, A., Herrmann, C., Knauf, S., Marchowski, D., Piro, S., Pohlmann, A., Rollins, R. E., Staubach, C., Harder, T.. 2025-09-21. Outbreak dynamics of high pathogenicity avian influenza virus H5N1, clade 2.3.4.4b euBB, in black-headed gulls and common terns in Germany in 2023. https://doi.org/10.1101/2025.09.17.676714

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