Minks, seals, foxes, cats – the bird flu virus H5N1 has recently repeatedly spread to mammals. There is now evidence of the pathogen in dairy cows in the USA. Since the end of March, the US Department of Agriculture has registered the virus in more than 30 dairy farms in around 10 states – including in the milk itself. The World Health Organization (WHO) currently considers the risk to people to be low, but is urging all countries to pay greater attention to possible infections in animals and humans.

Given the number of infections in numerous birds and more and more mammals around the world, researchers fear that the highly pathogenic bird flu virus will continue to change. In the case of cows – and in many other cases – it is the so-called H5N1 line

Among other things, the US Department of Agriculture ordered that from Monday (April 29th) only dairy cows with a negative bird flu test could be transported from one US state to another. “This is a drop in the ocean,” Mike Worobey from the University of Arizona told Science magazine. This limitation is comparable to that of air traffic during Covid times “long after the viruses have established themselves in a certain place.” It could simply be too late.

Infection of cows amazes experts

“I’m very surprised that cows are now infected,” says Martin Beer from the nationwide Friedrich Loeffler Institute in Greifswald. From an infection experiment in 2006, the institute concluded that cattle were “hardly at risk”. The virus is currently “maximally” adapted to birds and has low potential to infect humans. So far, the pathogen has had difficulty overcoming humans’ innate immunity to the bird flu virus. “But every new mammalian host can bring the virus a little closer to humans.”

Virologist Martin Schwemmle from the University Hospital of Freiburg says he is also surprised by the viruses in cows. And that the pathogen apparently only needed a few mutations to multiply in dairy cattle. However, Schwemmle currently believes that the spread of the virus among people in the form of an epidemic or even pandemic is “rather unlikely”. The virus has not yet adapted sufficiently to humans.

So far, only one case of transmission from cows to humans has been known in the USA, who also only developed conjunctivitis. But Beer points to many unregistered workers in the United States, “especially on cattle ranches.” Since 2021, the WHO has registered a total of 28 cases of bird flu infections in humans, around half of which were in the so-called clade

Transmission of bird flu viruses from person to person has not been reported since 2007. According to the WHO, no changes in the viruses were observed that would facilitate infection via the upper respiratory tract in humans. Transmission of the currently circulating H5N1 viruses from person to person is “unlikely” without further genetic changes.

The evolutionary biologist Worobey still sees future dangers. “We are in uncharted territory here as a mammal-adapted H5N1 virus is spreading for the first time in land mammals, with which hundreds of thousands of people come into contact every day,” he says, referring to the situation in the United States. The next pandemic virus will come from a situation very similar to this one, he suspects.

There are many unanswered questions in the USA

Exactly how the virus infected the cows is unknown, as is how the virus is transmitted from cow to cow. Experts suspect milking machines or the air. Symptoms include reduced milk production and loss of appetite. The US Food and Drug Administration (FDA) pointed out that pasteurizing milk destroys the virus. No virus was discovered in milk powder for infants either.

The WHO said it was investigating whether milk played a role in transmission. It advised people to only consume pasteurized milk and not raw milk.

Schwemmle finds it problematic that very high amounts of virus were detected in the milk of infected cows. The virus can be spread with every infectious drop of milk that gets into the environment – and thus also through the equipment used in milk production. “I think it is very difficult to get such widespread contamination under control,” says Schwemmle.

Germany is better armed

According to Schwemmle, there is still no evidence of dairy cows being infected with the virus in Europe. However, this could change at any time if it comes from the USA or develops in Europe itself.

In Germany, according to FLI researcher Beer, there is a lot of attention in this regard: “The authorities know that in the event of any unexplained illnesses in cow herds that are accompanied by a decline in milk, H5N1 must also be considered and may be tested accordingly,” he says and points out to an overall better control system. “In Germany we actually have the glass cow. Every animal is clearly marked and every animal movement can be traced via a database. That’s not the case in the USA.”

Wildlife around the globe is threatened

“Regardless of the danger to humans, bird flu is increasingly affecting global wildlife,” emphasized Beer. For example, minks and marine mammals such as sea lions are very sensitive to such viruses. “The animals often become seriously ill and a large number of them die.” In Antarctica, the virus is now on the islands and on the mainland. “It will soon be very cold there, which will lead to bird movements with possible virus spread. Here we have to look very carefully at Australia with its unique wildlife.” It is the last continent without infection with highly pathogenic bird flu viruses.