Bird flu in cows: should we be worried?

For a long time, the bird flu virus H5N1 circulated almost exclusively among wild birds and poultry, but about five years ago, new, highly contagious variants of the virus took over. Birds started dying in much larger numbers and mammals were observed getting sick too, such as foxes, badgers and seals that ate infected birds or came into contact with bird droppings. Infections and deaths were also seen in domestic cats. ‘Those were individual incidents and relatively rare,’ says Mónika Ballmann, a virologist at Wageningen Bioveterinary Research (WBVR). ‘Until recently, we hadn’t seen evidence of mammals infecting other mammals.’ As head of the National Reference Laboratory for Avian Influenza, Ballmann is responsible for diagnostic testing for bird flu.

The situation changed in 2024, when there was an outbreak of infections among cattle in the US. ‘That was the first major outbreak in ruminants and it was caused by an American strain of avian influenza,’ says WBVR researcher Luca Bordes, who specialises in bird flu infections of mammals. He discovered that the European H5N1 strains can also reproduce in the respiratory-tract epithelial cells of cattle. It soon became clear that cows infected with the virus generally only experience mild flu symptoms. The infection can result in inflammation of the udder, with the virus being secreted into the milk. The milk undergoes a standard heat treatment, which renders the virus harmless to consumers.

However, it was not only cows that became infected in the US; humans were too. To date, around seventy people have become infected. As with cows, most people only have mild symptoms; they often get inflammation of the eye. There are now increasing concerns worldwide that the bird flu virus is adapting to mammals. That means that more wild mammals, farm animals and humans could become infected. ‘If that happens, the virus is one step closer to causing a pandemic,’ says Bordes.

In January 2026, the Netherlands was shocked by the news that bird flu antibodies had been found in a cow. Ballmann and Bordes answer some of the questions this new development raises.

How was the bird flu virus infection in this Dutch cow discovered?

Ballmann: ‘It happened because of a sick cat that had access to both the field and barn used by the cows. When this cat died from flu-like symptoms in December 2025, bird flu was suspected. A PCR test confirmed the diagnosis. The cat may have got avian flu from eating a dead bird. This Frisian farm had one cow that had exhibited symptoms of udder inflammation shortly before. Its milk was also thicker than normal and somewhat yellowish. Neither the farmer nor the vet had found that particularly alarming as other diseases can cause inflamed udders. But whenever bird flu is confirmed to be present in poultry, we also test other vulnerable species such as dairy cows.

‘Initially, the milk samples were examined. They tested negative, which meant there was no active virus infection at that point. But one sample tested clearly positive for antibodies to the H5N1 virus, pointing to an infection in the past. There were a couple of borderline cases, so we took a second series of samples. We took blood samples from all the cows and samples of the milk from all the milking cows. This showed that several cows had been infected.’

How can cows catch the virus?

Ballmann: ‘Around that time, the Netherlands saw a marked peak in bird flu cases among wild birds. The owner of the farm had indeed seen a lot of dead wild birds in the surrounding area. It was therefore highly likely that there were virus particles in the local environment. There are various ways that the virus could get into a barn. It could be carried in on shoes or in the fur of rats and mice. It could also be transmitted through the feed and the drinking water. But in this particular case, the most likely scenario is that the cows were infected through contact with bird droppings or dead birds in the field. The cows on this farm had been kept in the field until the end of November.’

Bordes: ‘We know from experiments that cows can also become infected through their udders. Therefore, milking could play a key role in transmission. The udders, milk and milking equipment are in constant contact with one another during milking. All these factors can facilitate the spread of the virus among cows. We are still investigating which route is most important in practice. We are doing that in the high-security labs of WBVR’s High Containment Unit in Lelystad.’

Is the bird flu virus now spreading among cows in the Netherlands?

Ballmann: ‘We haven’t found any more active virus infections on the farm in question, nor are there any other farms where cows have tested positive for antibodies to the virus. If an infection is found in poultry, we also always test the dairy cattle in the locality. So far, this is the only case of infected cows outside the USA.’

The virus is spreading between cows in America. How does the situation there differ from the Netherlands?

Ballmann: ‘Like here, America has an H5N1 virus, but its genetic makeup is different to the European variant. A European strain ended up in America in 2021, spreading rapidly and exchanging genes with local American strains. Bird flu viruses often do this; it’s an accelerated form of evolution. This process resulted in virus variants that can spread easily among cows. We haven’t seen these variants in Europe yet.’

Bordes: ‘It has really taken off in America. To date, over a thousand dairy farms have become infected, in eighteen states. It should be noted that farming is different in America, where animals are frequently traded between farms. Here, animals are tracked throughout their lifetime and we try to avoid transporting the cows where possible for animal welfare reasons. Anyway, dairy farms are smaller in the Netherlands, with about one to two hundred cows.’

Humans became infected in the US as well as cows, and there have even been fatalities in Asia. Are humans as susceptible to bird flu as cattle?

Bordes: ‘The cases of the disease in the US were probably caused by intensive contact between cows and people, and in Asia between chickens and people. But the virus hasn’t adapted to function optimally in mammal cells. There are no indications that humans can infect one another. If that does happen, the people running the greatest risk of becoming seriously ill will be young children, the elderly and people with weakened immune systems – as with any form of influenza. Not everyone who becomes infected will get sick.’

Could a bird flu virus develop that is capable of being transmitted between humans?

Ballmann: ‘Now that bird flu is circulating among cows in America, there’s a greater risk of the virus adapting to mammalian cells. The virus will then be able to infect mammals more efficiently and be transmitted more easily. That scenario is the main cause for concern in the light of current developments.’

Bordes: ‘The really tricky thing is that the virus is constantly adapting. The avian influenza virus is also very similar to the viruses that cause flu in humans and pigs. When bird flu, swine flu, and human flu end up infecting the same host, gene segments can be exchanged, creating an entirely new virus with potentially dangerous characteristics. That’s precisely how the major flu pandemics of the past started, such as the Spanish flu.’

All over the world, people are in close contact with animals. What can we do to prevent outbreaks in humans?

Ballmann: ‘It’s difficult to say how the virus will develop, which is why we continue to monitor the situation closely. In the Netherlands, wild birds that are found dead are tested so that we can detect any changes in the virus at any early stage. What is more, the Dutch Centre for Avian Migration & Demography and Erasmus Medical Center monitor live wild birds. These two monitoring systems give us a better picture of the virus variants circulating in nature. On top of that, we’re now screening cows in various parts of the Netherlands to see whether there have been other cases of infection. Apart from that, we are relying on our rapid diagnostic testing every time there is a suspicion of bird flu, whether in birds or in mammals.’