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H5N1 Drove Local Outbreaks Among Mute Swans Across Southern Norway

Mute swans gathered on Østensjøvannet in Oslo
Research · Disease, parasites & microbiology

Several local outbreaks of highly pathogenic avian influenza H5N1 affected mute swans (Cygnus olor) in southern Norway during the winter of 2025/2026. The pattern is documented in a status report from the Norwegian Veterinary Institute published on 25 March 2026.

The report covers detections from 21 October 2025 to 20 March 2026. During that period, the Veterinary Institute tested 313 wild birds and confirmed highly pathogenic avian influenza in 65 individuals. Waterfowl were the most affected group, and several local outbreaks involved illness and mortality among swans, especially mute swans.

New cases continued from January into March

H5N1 continued to be detected in mute swans after the turn of the year. In January, three new infected mute swans were recorded in Oslo and Drammen. An outbreak around Bergen followed in early February, with H5N1 confirmed in six mute swans in Bergen and one in Øygarden. Three additional mute swans tested positive in Stavanger in March.

The report notes that many affected birds were sick or found dead. Swans in the local outbreaks often showed signs involving the central nervous system, which the authors say indicated that the virus had affected the brain. Many detections occurred around lakes, coastal areas and harbours in or near densely populated places where waterbirds congregate.

One H5N1 variant dominated

Nearly all characterised Norwegian H5N1 viruses in the reporting period belonged to the EA-2024-DI.2.1 variant, which had dominated in Norway and much of Europe since autumn 2025. The report also notes that the virus could be detected in apparently healthy birds, alongside the visibly sick and dead birds found through passive surveillance.

The winter outbreaks show that mute swans were one of the wildlife species most visibly affected by H5N1 in Norway during this period. Although the number of new detections had begun to decline by March, the Veterinary Institute concluded that the virus was still circulating, particularly among waterfowl, and recommended continued surveillance and vigilance as spring migration progressed.

Sources and image credit

  1. Granstad, S., Ytrehus, B., Helberg, M., Åkerstedt, J., Gjerset, B., Bøe, C. A., Madslien, K., Meling, S., Fjermestad-Eie, B., Bergfeldt, A., Fjeldheim, I., Stålcrantz, J., & Tønnessen, R. (2026). Høypatogen aviær influensa i Norge, Europa og globalt: Statusrapport for perioden 21. oktober 2025–20. mars 2026 (Rapport 11/2026). Veterinærinstituttet.Primary national status report published 25 March 2026. It covers 21 October 2025–20 March 2026: 313 wild birds were tested and HPAI was confirmed in 65; waterfowl were the most affected group, with several local outbreaks involving swans, especially mute swans, and nearly all characterised H5N1 viruses belonged to EA-2024-DI.2.1.

Image: Ryan Hodnett — mute swans at Østensjøvannet in Oslo, Norway, 20 May 2021; Wikimedia Commons; representative Norwegian image, not birds sampled in the H5N1 report; cropped to 16:9 · CC BY-SA 4.0 · Source

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