Mass mortalities caused by different genotypes of HPAIV H5N1 clade 2.3.4.4b in colony breeding Black-headed Gulls and Sandwich Terns in Denmark 2022 and 2023

Unprecedented seabird mass mortality events (MMEs) were reported in multiple European countries in 2022 and 2023. These events were attributed to Clade 2.3.4.4b H5N1 high pathogenicity avian influenza viruses (HPAIVs). In Denmark, populations of Sandwich Terns (<i>Thalasseus sandvicensis</i>) and Black-headed Gulls (<i>Chroicocephalus ridibundus)</i> were significantly impacted. Sandwich Tern mortality occurred primarily between May and June of 2022 and was caused by the HPAIV genotype EA-AB. From 2022 to 2023, Danish Sandwich Tern populations decreased by 36%. Sandwich Terns commonly nest within Black-headed Gull colonies, allowing for direct contact between the species. Despite their proximity, in 2022, neither adults nor chicks of Black-headed Gulls exhibited morbidity associated with HPAIV. During the autumn of 2022 and the winter 2022/23 a novel clade 2.3.4.4b H5N1 HPAIV, genotype EA-BB, emerged in Black-headed Gulls. The new genotype caused the death of at least 3000 adult Black-headed Gulls in Denmark (i.e., >1.8% of the Danish breeding population) in the spring of 2023. Notably, in 2023 Sandwich Tern chicks, but not adults, in contact with Black-headed Gulls were affected by mass mortality. These observations suggest that adult Terns in 2023 were less susceptible to HPAIV infection, mitigating the impact of MMEs.