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.