“Preventing avian influenza starts with understanding”

What role does the Dutch Wildlife Health Centre play in monitoring avian influenza among wild animals in the Netherlands? “The disease spreads widely throughout wildlife. New species keep being affected. We very much want to gain a better understanding of this.”

“Besides living animals, dead animals have always interested me,” says ecologist and forest ranger Leon Kelder. He has worked for Staatsbosbeheer for 22 years, spends a great deal of time out in nature, and is often called by hikers and farmers when they come across a carcass. Sometimes it concerns individual animals, in other cases groups. For example, he remembers the large die-off among eider ducks in the 1990s. “But in recent years something really different has been happening with avian influenza,” he says. “The disease spreads very broadly through wildlife, and new species keep being affected. We would very much like to gain more insight into that.”

When the Dutch Wildlife Health Centre (DWHC) called for extra attention to dead scavengers and predators at the end of 2025, Kelder immediately sprang into action. “I always take requests from the DWHC seriously and activate my entire network. When a dead fox was recently found in the Schoorl Dunes, after consultation with the DWHC it went straight away—securely packaged—by courier to Utrecht.”

Carnivores and scavengers

The Dutch Wildlife Health Centre of the Faculty of Veterinary Medicine is the national centre that detects and investigates diseases in wild animals in the Netherlands. The DWHC works closely with colleagues from other organisations. For monitoring avian influenza, dead wild birds suspected of infection are sent directly to the laboratory of Wageningen Bioveterinary Research (WBVR) in Lelystad. Carnivorous mammals and scavengers that showed abnormal neurological behaviour before death are reported to the Netherlands Food and Consumer Product Safety Authority (NVWA), which may decide to send them directly to Lelystad as well.

If this is not the case—as with the fox found by forest ranger Kelder—pathological examination takes place in Utrecht under strict safety protocols. Researchers wear protective clothing and breathe clean air through special masks.

Researchers first take swabs from the animal’s nasal and throat cavity and from the intestines. If, during the examination, they find signals that could indicate highly contagious highly pathogenic avian influenza—such as inflammation in the brain—they contact the NVWA. The swabs and samples from the lungs and brain are also sent to WBVR, where they are tested for the presence of the avian influenza virus.

As a final safeguard, samples from all carnivorous mammals and scavengers without clear signs of avian influenza are later still tested by Erasmus MC. The DWHC also collects blood from the animal and stores it for antibody research by WBVR and Erasmus MC. Antibodies show whether an animal has ever come into contact with the avian influenza virus, even if it was not ill at the time.

Jumping to mammals

According to pathologist and DWHC director Judith van den Brand, monitoring avian influenza in mammals is essential. Avian influenza is an infectious disease of birds, but the virus can sometimes jump to mammals, including humans. “Every time the virus moves from a bird into a mammal and infects that animal, it has the opportunity to change,” says Van den Brand. “That is precisely why it is important to follow how the virus behaves.”

Every time the virus moves from a bird into a mammal and infects that animal, it has the opportunity to change

So far, the highly contagious highly pathogenic avian influenza virus has only occasionally been observed in wild mammals. In the Netherlands the virus has been detected in species including foxes, polecats, stone martens and seals. Research into antibodies in carnivores has shown that infections may occur more frequently and that monitoring is therefore very important.

In January, antibodies against avian influenza were detected for the first time in the Netherlands (and Europe) in dairy cows. This does not pose direct risks to public health, and milk is safe for human consumption after pasteurisation.

As far as is known, transmission from mammal to mammal has not yet occurred. But researchers do not want to miss the first signals of such an adaptation. Early detection makes it possible to warn people who work with animals, tighten safety guidelines and incorporate information about possible viral changes into the development of vaccines and medicines.

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Antibodies in the fox

The DWHC is now focusing particularly on scavengers and carnivorous mammals. Van den Brand explains: “These are animals that may eat carcasses of birds infected with avian influenza. That makes them a higher-risk group.”

At the beginning of this year, a study on foxes and avian influenza in Friesland was completed. Commissioned by the Ministry of Agriculture, Fisheries, Food Security and Nature (LVVN), WBVR and the DWHC examined fifty foxes that had been shot as part of wildlife management and damage control. Friesland was chosen because large numbers of birds have died there from avian influenza in recent years.

Almost half of the examined foxes were found to have antibodies against avian influenza. This indicates that these animals had previously been infected with variants of the avian influenza virus. Van den Brand says: “In two foxes the highly pathogenic virus was actually detected in the nasal and throat swabs. Fortunately, there is no evidence of large-scale spread among mammals—at least not yet.”

Close collaboration

Monitoring avian influenza in mammals will now continue within the European research project Strong1H, Van den Brand explains. In this project, led by the National Institute for Public Health and the Environment (RIVM), the NVWA, WBVR, Erasmus MC, Royal GD and the DWHC work together.

Among other things, the project examines how avian influenza spreads from birds to wild scavengers, carnivorous mammals and the environment. In 2025, researchers temporarily placed cameras at sites where bird mortality occurred, collected water samples and gathered carcasses.

To gain better insight into the risk factors for infection with avian influenza in scavengers and carnivorous mammals, the DWHC—together with these partners—will examine as many foxes, polecats, stone martens, pine martens, raccoon dogs, stoats, weasels, raccoons and badgers as possible in the first half of 2026. This includes not only animals found dead, but also roadkill and animals obtained through hunting and damage control.

Wild species whose role in avian influenza has not yet been well mapped will also be tested in the project, such as rats (Erasmus MC) and wild boar (WBVR). The findings will be shared across Europe.

Van den Brand: “I am very pleased with this collaboration. Preventing avian influenza starts with understanding. Wild mammals and scavengers are not a structural part of national monitoring, as birds are. Through this national and international collaboration we gain more insight into how we can best organise surveillance of avian influenza in wild mammals and we can more quickly see whether the virus is taking new steps. And of course thanks to the cooperation of so many partners and volunteers. I am extremely grateful to people like forest ranger Leon Kelder!”

Text: Maarten Post | Images: Bas Niemans (marten examination) & iStock (fox)

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