# Highly Pathogenic Avian Influenza A (H5N1) Caused Mass Death Among Black‐Legged Kittiwakes (Rissa tridactyla) in Norway, 2023

## TL;DR

A deadly bird flu outbreak in Norway killed thousands of black-legged kittiwakes, causing a major drop in their population.

## Contribution

This study documents the first large-scale HPAI H5N1 outbreak in black-legged kittiwakes, revealing severe neurological and systemic disease.

## Key findings

- More than 15,000 dead kittiwakes were reported, reducing the population by at least 50%.
- Necropsies showed severe brain and pancreatic lesions with viral RNA and antigen.
- Viral spread may occur through blood vessels due to α2,3-linked sialic acid expression.

## Abstract

In 2023, highly pathogenic avian influenza (HPAI) heavily affected gulls in Europe. In July, a mass mortality event was reported in the black‐legged kittiwake (Rissa tridactyla) breeding colony at Ekkerøy in Northern Norway. The cause was confirmed to be infection with the HPAI H5N1 clade 2.3.4.4b virus, genotype EA‐2022‐BB. We describe the outbreak in kittiwakes, including pathological and virological investigations, and discuss the management and zoonotic potential. With more than 15,000 dead birds reported, we estimate that the outbreak caused a reduction in the kittiwake population at Ekkerøy of at least 50%. Diseased birds exhibited neurological signs. Necropsies of 10 birds revealed a peracute fatal systemic disease, with severe lesions in the brain and pancreas co‐localizing with viral RNA and antigen. Vascular expression of α2,3‐linked sialic acids (SAs) and viral RNA/antigen may reflect hematogenous viral spread. Further studies should investigate the long‐term impact of HPAI on kittiwake populations.