Bird flu in cattle: how worrying is that?
In december 2025 the avian influenza virus infected a cow in the Netherlands for the first time in Europe. During the bird flu wave among cattle in the US, humans are also getting infected. Can that happen in Europe too? Can people infect each other? Is there a risk of a pandemic?
By Pauline Van Schayck interviewing to the experts Monika Ballmann and Luca Bordes
For a long time, the H5N1 bird flu virus affected almost exclusively wild birds and poultry, but around five years ago new, highly contagious variants of the virus took over. Mortality among birds increased considerably, but mammals also began to contract the disease, such as foxes, badgers and seals that ate infected birds or came into contact with bird droppings. Infections and deaths among domestic cats were also recorded. ‘All those cases are isolated and relatively rare. Until recently, we saw no mammal-to-mammal transmission,’ says Mónika Ballmann, virologist at Wageningen Bioveterinary Research (WBVR). As head of The Netherlands National Reference Laboratory for Avian Influenza, she is responsible for the diagnostics of bird flu.
In 2024 the picture changed, when a wave of infections spread among cattle in the US. ‘That was the first large-scale outbreak among mammals, caused by an American bird flu strain,’ says researcher Luca Bordes, who at WBVR focuses on bird flu infections in mammals. He discovered that European H5N1 strains can also replicate in the airway epithelial cells of cattle. It quickly became clear that cattle usually only develop mild flu symptoms from the virus. Milk undergoes a standard treatment that renders the virus harmless to consumers.
In the US, however, not only cattle but also humans were infected. The number of infections now stands at around seventy. Just as with cattle, the disease in most people follows a relatively mild course. They often develop an eye infection. Worldwide, concerns are now growing that bird flu is increasingly adapting to mammals. This means that more wild mammals, farm animals and humans could be affected. ‘If that happens, a pandemic is lurking,’ says Bordes.
The Netherlands was alarmed for the first time in January 2026 by the news that antibodies against bird flu had been found in a cow. Ballmann and Bordes answer some of the questions this raises.
How was it discovered that this Dutch cow was infected with the bird flu virus?
Ballmann: ‘That happened through a sick cat that had access to the pasture and the barn of the cattle. When this cat died of flu symptoms in December 2025, bird flu was suspected. A PCR test confirmed this. The cat may have contracted bird flu by eating a dead bird. On this Frisian farm, one cow had also recently shown symptoms of mastitis. The milk from this cow was also thicker than normal and yellowish in colour. That was not an alarm signal for the farmer or the vet, because mastitis is also frequently caused by other diseases. But as soon as bird flu is confirmed in birds, we also test other susceptible animal species, such as dairy cattle.
‘Initially, milk samples were examined. These tests were negative, meaning there was no active viral infection at that moment. But one sample tested clearly positive for antibodies against the H5N1 virus. This points to a prior infection. There were a few borderline cases, so a second sampling followed. We collected blood samples from all the cattle, and milk samples from all lactating cows. This showed that multiple cows had been infected.’
‘So far this is the only infection in cattle outside the US.’ — Mónika Ballmann, virologist, Wageningen Bioveterinary Research
How can cattle contract the virus?
Ballmann: ‘The Netherlands was experiencing a clear peak in bird flu cases in wild birds during that period. The farm owner had also seen many dead wild birds in the surroundings. So the likelihood was high that viral particles were present in the environment. These can enter the barn through various routes. The virus can hitch a ride on shoes or, for example, in the fur of rats and mice. Transmission is also possible via feed and drinking water. But in this case it is more likely that the cattle were infected through contact with bird droppings or with dead birds in the pasture. The cattle on this farm had been out in the field until the end of November.’
Bordes: ‘We know from experiments that infection in cattle can also occur via the udder. The milking machine probably plays an important role in this. During milking, udders, milk and equipment are constantly in contact with each other. All these factors can contribute to the spread of the virus between cattle. We are still investigating which route is most important in practice. We do this in the strictly secured laboratories of the High Containment Unit of WBVR in Lelystad.’
Is the bird flu virus now spreading among cattle in the Netherlands?
Ballmann: ‘We have found no further active viral infection on the affected farm, and there are as yet no other farms where cattle are testing positive for antibodies against the virus. When there is an infection in poultry, we also test dairy cattle in the vicinity. So far this is the only infection in cattle outside the US.’
In America the virus is spreading among cattle. How does the situation there differ from that in the Netherlands?
Ballmann: ‘The bird flu in America is also caused by an H5N1 virus, just like here, but the genetic composition differs from the European variant. In 2021, a European strain reached America, where it spread widely and began exchanging genes with American strains. Bird flu viruses do this regularly; it is a rapid form of evolution. This has given rise to viral variants that spread easily among cattle. We have not yet seen these variants in Europe.’
Bordes: ‘In America it spread incredibly fast. More than a thousand dairy farms have now been infected, spread across eighteen states. The farm structure in America is also different. A lot of trading goes on between farms there. Here we try to limit the transport of animals as much as possible, for ethical reasons. Dutch farms are also smaller, with roughly one hundred to two hundred cattle.’
In the US both cattle and humans were infected, and in Asia deaths have even occurred. Are humans just as susceptible to bird flu as cattle?
Bordes: ‘In the US the cases of illness were probably caused by intensive contact between cattle and humans, and in Asia between chickens and humans. But the virus is not optimally adapted to mammalian cells. There are as yet no indications that humans can infect one another. If that does happen, young children, the elderly and people with a weakened immune system have a greater chance of more serious illness, just as with any flu. Not everyone who becomes infected falls ill.’
‘The virus is not optimally adapted to mammalian cells.’ — Luca Bordes, researcher, Wageningen Bioveterinary Research
Is it possible that a bird flu virus emerges that can jump from person to person?
Ballmann: ‘Now that bird flu is spreading among cattle in America, the risk increases that the virus will adapt further to mammalian cells. It could then infect mammals more efficiently and become more easily transmissible. That scenario is the primary concern with current developments.’
Bordes: ‘What makes it especially difficult is that the virus is constantly changing. Furthermore, the bird flu virus is largely similar to the viruses that cause influenza in humans and pigs. If bird flu, swine flu or human flu infect the same host, gene segments can be exchanged, giving rise to a completely new virus with potentially dangerous properties. Previous major flu pandemics began in this way, such as the Spanish flu.’
‘Now that bird flu is spreading among cattle in America, the risk increases that the virus will adapt further to mammalian cells. That scenario is the primary concern.’ — Mónika Ballmann, virologist, Wageningen Bioveterinary Research
People and animals come into close contact with each other everywhere in the world. What can we do to prevent outbreaks in humans?
Ballmann: ‘It is difficult to predict how the virus will develop. That is why we continue to monitor the situation closely. In the Netherlands, dead wild birds are examined to detect changes in the virus at an early stage. Living wild birds are also monitored by the Bird Ringing Centre and Erasmus MC. Through both methods we get a better picture of the viral variants circulating in nature. In addition, we are in the process of screening cattle from various areas of the Netherlands to find out whether more cases of infection have occurred. And beyond that, we must rely on our rapid diagnostics in every suspected case of bird flu, in birds and mammals alike.’
Source:
Wageningen World, issue 1/2026. Text: Pauline van Schayck. More information: www.wur.nl/vogelgriep
To learn more:
-. Vaccination in poultry against avian influenza
-. Vaccination of workers on poultry farms at NeXusPoultry.com
-. Avian Influenza vaccine shows promising results in laying hens
-. AI vaccination does not have to be an all-or-nothing strategy
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