University of Copenhagen Builds First European Early Warning System for Bird Flu in Cattle

Study published in Preventive Veterinary Medicine. Research conducted at the Department of Veterinary and Animal Sciences, University of Copenhagen.

Danish cattle farmers now have something American dairy producers desperately wished they had two years ago: advance warning of where and when bird flu is most likely to strike.

Researchers at the University of Copenhagen have built the first European risk-prediction tool for H5N1 transmission from wild birds to cattle. The model, adapted from U.S. outbreak data and calibrated to Danish conditions, identifies the country's western coastline and the island of Lolland as the highest-risk zones - with December through March representing the peak danger window each year.

The tool arrives at a moment when the threat feels less theoretical by the week. More than 1,000 cattle herds across 19 U.S. states have tested positive for bird flu since the first confirmed bovine case surfaced in Texas in 2024. The virus has infected 71 Americans, mostly poultry and dairy workers who developed eye infections. And the problem is no longer confined to North America. Dutch dairy cows have turned up with H5N1 antibodies. British sheep have tested positive. The slow creep toward Scandinavian livestock feels, to the researchers involved, like a matter of when rather than if.

How the Model Works

The Copenhagen team took a straightforward but previously untried approach. They merged detailed records of wild bird populations - species counts, seasonal movements, geographic concentrations - with cattle density maps. Using transmission data extracted from American outbreaks, they calculated the probability of spillover events at specific locations during specific times of year.

"We have combined detailed data on wild bird abundance with cattle density in the U.S. to calculate how easily the infection can be transmitted from wild birds to cattle," said You Chang, a postdoc at the Department of Veterinary and Animal Sciences who helped develop the model.

The result is essentially a heat map of risk. It does not predict outbreaks with certainty. What it does is tell a farmer in Jutland or on Lolland that during January, conditions are ripe for a wild bird carrying H5N1 to come into contact with cattle - and that heightened vigilance makes sense.

That kind of heads-up matters. In the American outbreaks, many farmers did not realize their herds were infected until milk production dropped sharply and the milk itself turned thick and abnormal. By that point, the virus had already moved through the herd and, in several cases, jumped to neighboring farms.

Denmark Sits on a Flyway

Geography puts Denmark in an uncomfortable position. The country straddles major migratory bird routes that funnel millions of waterfowl - ducks, geese, swans, and shorebirds - through its airspace and wetlands every autumn and winter. These are precisely the species most likely to carry highly pathogenic avian influenza.

Combine that flyway traffic with Denmark's dense dairy industry and you get a collision course. The western coast, where bird concentrations peak during winter migration, also hosts significant cattle operations. Lolland, a low-lying island in the south with extensive wetland habitat, presents a similar overlap.

"This is the first European study that uses outbreak data from the U.S. to assess the risk of transmission of bird flu from wild birds to cattle, and applies that data to a European context," said Beate Conrady, professor at the Department of Veterinary and Animal Sciences.

The study draws a distinction worth noting. It models the risk of initial introduction - a wild bird infecting cattle on a single farm. Whether the virus would then spread laterally between Danish farms the way it tore through American herds remains an open question that the team is still investigating. Farm-to-farm transmission in the U.S. involved multiple pathways, including shared equipment, animal transport, and possibly airborne spread over short distances. Danish farming practices differ in ways that could either help or hurt.

What Farmers Can Actually Do

The practical value of the tool lies in its specificity. A blanket warning that bird flu might someday reach Danish cattle is not particularly useful. A targeted alert telling a farmer on the west coast of Jutland that risk peaks in February gives that farmer something to act on.

"This gives Danish cattle farmers the opportunity to be alert if they know they are in a high-risk area and it's a time of year when the risk is elevated," Chang said. "Then they can keep a closer eye on whether their animals show symptoms. At the same time, the knowledge can help authorities consider targeted surveillance, such as testing milk for early detection."

Milk testing proved to be one of the most effective detection methods in the United States. Bulk tank milk samples can reveal the presence of H5N1 before individual animals show obvious clinical signs. Denmark already has robust milk-testing infrastructure for other pathogens. Plugging bird flu into that system during high-risk months would be a relatively low-cost addition to existing surveillance.

Beyond individual farm management, the tool could help Danish veterinary authorities allocate monitoring resources more efficiently. Rather than spreading surveillance evenly across the country year-round, officials could concentrate testing in the regions and seasons where the model shows elevated risk.

The American Cautionary Tale

The U.S. experience provides a stark illustration of what happens when preparedness lags behind a fast-moving pathogen. The first confirmed cattle infection appeared in Texas in early 2024. Within months, the virus had spread to herds in states as far apart as Michigan and Idaho. Some farms lost significant portions of their milk production. Workers fell ill. The economic and public health costs mounted quickly.

American authorities were caught flat-footed in part because nobody had seriously modeled the risk of H5N1 jumping from birds to cattle. The virus had been a poultry problem for years - devastating chicken and turkey flocks around the world - but its move into bovine populations was largely unexpected. Once it happened, there was no playbook.

Denmark has the advantage of watching that unfold from a distance and learning from it. The Copenhagen tool represents an attempt to build the playbook before it is needed.

"Being ready for a potential launch in Denmark is essential," Conrady said. "Preparedness should not be a luxury - it should be standard."

A European Problem Taking Shape

Denmark is not the only European country paying attention. The detection of H5N1 antibodies in Dutch dairy cattle suggests the virus may already be making quiet inroads into European livestock populations. Earlier cases in British sheep added to the concern. These scattered signals do not constitute an outbreak, but they indicate that the barrier between wild bird populations and European livestock is thinner than many assumed.

The Copenhagen model could, in principle, be adapted for other European countries. The underlying methodology - overlaying wild bird abundance data with livestock density and applying transmission parameters derived from confirmed outbreaks - is transferable. Each country would need its own bird population data and farm maps, but the analytical framework is ready.

For now, the immediate audience is Danish. The study, published in Preventive Veterinary Medicine, offers a concrete tool for a country that has so far been spared but may not stay that way. The researchers are not sounding an alarm so much as handing farmers and regulators a better pair of binoculars - a way to see trouble coming while there is still time to act.

The wild birds will migrate through Denmark again this winter, as they always do. The question is whether anyone on the ground will be watching the right fields at the right time. Now, at least, they will know where to look.