Avian flu in cows surprises virologists
Published: Monday, Apr 29th 2024, 11:01
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The bird flu virus is affecting more and more mammals worldwide. It has now reached cows in the USA and thus farm animals with which many people have contact. The World Health Organization (WHO) currently considers the risk to humans to be low, but is urging all countries to pay increased attention to possible infections in animals and humans.
In view of infections in numerous birds and more and more mammals around the world, researchers fear that the highly pathogenic bird flu virus is continuing to change. In the case of the cows - and in many other cases - it is the so-called H5N1 lineage 2.3.4.4b.
Among other things, the US Department of Agriculture ordered that from Monday (April 29) only dairy cows with a negative avian flu test may be transported from one US state to another. "This is a drop in the ocean", said Mike Worobey from the University of Arizona to the specialist magazine "Science". This limitation is comparable to that of air travel in times of Covid "long after the viruses have established themselves in a particular place." It could simply be too late.
Infection of cows surprises experts
"I am very surprised that cows are now infected," says Martin Beer from the nationally responsible Friedrich Loeffler Institute in Greifswald. At the time, the institute had concluded from an infection trial in 2006 that cattle were "hardly at risk". At present, the virus is still "maximally" adapted to birds and has a low potential to infect humans. So far, the pathogen is still finding it difficult to overcome the innate immunity of humans to the bird flu virus. "But every new mammalian host can bring the virus a little closer to humans."
Virologist Martin Schwemmle from the Freiburg University Medical Center says he is also surprised by the viruses in cows. And by the fact that the pathogen apparently only needed a few mutations to multiply in dairy cattle. However, Schwemmle also believes that the spread of the virus in humans in the form of an epidemic or even a pandemic is currently "rather unlikely". The virus has not yet adapted sufficiently to humans for this to happen.
So far, only one case of transmission from cows to a human has been reported in the USA, and the latter only contracted conjunctivitis. However, Beer points to many unreported workers in the USA, "especially on cattle farms". Since 2021, the WHO has registered a total of 28 cases of bird flu infections in humans, around half of which were so-called clade 2.3.4.4b.
No transmission among humans since 2007
There has been no known transmission of bird flu viruses from humans to humans since 2007. According to the WHO, no changes in the viruses have been observed that would facilitate infection via the upper respiratory tract of humans. Transmission of the currently circulating H5N1 viruses from person to person is "unlikely" without further genetic changes.
Evolutionary biologist Worobey nevertheless sees future dangers. "We are in uncharted territory here, as an H5N1 virus adapted to mammals 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 USA. The next pandemic virus will come from a situation very similar to this one, he suspects.
Many questions remain unanswered in the USA
Exactly how the virus infected the cows is unknown, as is how it is transmitted from cow to cow. Experts suspect it may have been transmitted via milking machines or the air. Symptoms include reduced milk production and loss of appetite. The US Food and Drug Administration (FDA) has pointed out that pasteurizing the milk destroys the virus. No virus has been detected in milk powder for infants either.
Whether milk played a role in the transmission is being investigated, the WHO announced. It advised people to only consume pasteurized milk and not raw milk.
Schwemmle finds it problematic that very high levels of the virus were detected in the milk of the infected cows. This means that the virus can be spread with every infectious drop of milk that gets into the environment - and therefore also via the equipment used in milk production. "I believe that it is very difficult to get such widespread contamination under control," says Schwemmle.
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.
©Keystone/SDA
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