Highly Pathogenic Avian Influenza ravages Sandwich Tern breeding colonies in northwestern Europe

In 2022, highly pathogenic avian influenza (HPAI) A(H5N1) became enzootic, meaning it does not ease during summer months. This was a disaster for the Sandwich Tern, Thalasseus sandvicensis, and other seabird species across the northwest of Europe. A recent publication in Bird Conservation International highlights the mass mortality in breeding colonies, as thousands of individuals were found dead due to the virus. The repercussions present an ongoing challenge for people working with the Sandwich Terns and other seabirds.

The extent of the devastation from HPAI on the Sandwich Tern populations is harrowing. Within two months, approximately 20,531 adult Sandwich Terns were discovered dead, according to the publication, which represents 17% of the northwestern European breeding population. Researchers report that this likely underestimates the true depth of the catastrophe, with many dead birds likely going unnoticed and unreported. The impact within some colonies is particularly distressing, with virtually all chicks succumbing to the virus.

However, researchers highlight that mitigation efforts have shown promise, notably that carcass removal may reduce death rates within affected colonies. Further, following the outbreak's peak, a colony of late breeders showed that approximately one quarter of tested adults had some immunity to the H5 subtype, which may reduce future mortality rates. However, this brings short relief in the face of an escalating crisis for Sandwich Terns, who seem particularly vulnerable to HPAI.

The article underlines that more research is urgently needed in key areas such as identifying the sources and modes of transmission, understanding incubation times and other effects on Tern behaviour as well as further developing containment strategies.

For the Wadden Sea World Heritage Site, building on two workshops, the Trilateral Wadden Sea Cooperation published "Management guidelines - Mitigation and data collection strategies for avian influenza in bird colonies in the Wadden Sea" specifically for Sandwich tern colonies in April 2023. Further, an avian flu working group has been created on the trilateral exchange platform to commute information on the latest findings regarding Highly Pathogenic Avian Influenza (HPAI). The group is intended for site managers, researchers, conservationists, and veterinarians along the East Atlantic Flyway. To become a member of the group, registration to the platform is necessary by sending an email to wsfi@waddensea-secretariat.