Less reports but bird flu still prevalent

During the breeding seasons of 2022 and 2023, highly pathogenic avian influenza (HPAI) caused increased or even mass bird mortality among wild birds in the Netherlands. Certain waterbird species that breed closely together in colonies, such as gulls and terns, were particularly affected. To the relief of bird researchers and conservationists, HPAI did not cause problems in these colonies during the past breeding season, although the virus was still detected a few times.

In total, only 149 dead waterbirds were reported in the second quarter of 2024 through the Dutch Wildlife Health Centre (DWHC) and Sovon, compared to 5609 in the same period in 2023 and 4516 in 2022. In both previous years, the mortality peak occurred in June and mainly affected colonies of gulls and terns, with fatalities among both adult birds and chicks. This year, breeding sites were closely monitored, partly from the air using drones, to detect increased mortality early and take action if necessary. Fortunately, this was not required. None of the colonies reported unusual mortality.

Some positive cases

In the past quarter, 60 dead birds (reported to Sovon and DWHC) were sent in for bird flu research at Wageningen Bioveterinary Research (WBVR). One of these—a Great Black-backed Gull found in Domburg in June—tested positive for HPAI H5N1. Sampling projects focusing primarily on live wild birds also revealed a few positive cases after research by Erasmus Medical Center (EMC). These included a live Lesser Black-backed Gull in Lelystad in early May and three Herring Gulls in IJmuiden in mid-June. In the latter case, it involved three dead chicks from the same nest.

Situation in neigboring countries

Across Europe, HPAI among both captive and wild birds has also been found less than in previous years. The most notable incident was on July 2 when an HPAI H7N5 virus was detected at a laying hen farm near Bad Bentheim (Germany), just across the border from the Netherlands, near Enschede. The origin of this virus remains unclear but is most likely due to mutation from a low-pathogenic virus within a poultry flock. The virus is not related to highly pathogenic H7 viruses detected during outbreaks elsewhere globally, and highly pathogenic H7 strains have never been found among wild birds. Currently, there is no evidence that the virus detected at Bad Bentheim is circulating among wild birds.

The coming period

After the breeding season, many bird species enter their molting period. During this time, they seek safe and sheltered locations with sufficient food where they are less noticeable to predators. Molting requires significant energy, so birds minimize movement to conserve energy. Some species form large dense groups at suitable molting sites. Although there is currently no increased mortality due to HPAI, it seems that the virus has not completely disappeared. Vigilance for bird flu-related deaths at locations with concentrations of waterbirds remains important during this period.

Please keep reporting

We would like to thank everyone who has taken the time and effort to report or send dead birds for research. Naturally, reports remain welcome during this period as well. They provide essential insights into what is happening in nature.

Recent publication

In a recent publication in the scientific journal Pathogens, De Vos & Elbers (2024) showed that airborne transmission of bird flu via droppings from wild waterbirds to housed poultry does not play a significant role. The authors hypothesize that other risk factors—such as occasional failures in strict and consistent adherence to biosecurity measures on poultry farms—may be more important for introducing HPAI viruses.

Roy Slaterus, Sovon Vogelonderzoek Nederland

Valentina Caliendo, Dutch Wildlife Health Centre