“I think this sort of changes the whole ecology,” says bird flu researcher Richard Webby of St. Jude Children’s Research Hospital. “From a dairy industry perspective, this, to me, signals that they’re in it for the long term.” He says the U.S. government needs to do more to monitor and contain the virus, and that cattle may need to be vaccinated to control the outbreaks. Otherwise, “We’re going to be playing whack-a-mole,” says evolutionary biologist Michael Worobey of the University of Arizona.
The H5N1 infections in cows were the latest surprise move of a highly pathogenic avian influenza strain called clade 2.3.4.4b that has ravaged wild bird and poultry populations since 2020. It soon became clear the virus was confined to the udders of lactating dairy cows. Initially, Worobey and others speculated a particular set of mutations in the viral genotype, named B3.13, might have made the jump possible. But veterinary virologist Martin Beer and colleagues at the Friedrich Loeffler Institute showed that inoculating cow udders with a different version of H5N1 from European birds also led to infection. And the spillovers announced in January and February were both with yet another genotype called D1.1.
If cows are easily infected, it’s puzzling that spillovers have not been reported from Europe, where H5N1 outbreaks in poultry and wild birds have gone on much longer. Jürgen Richt, a veterinarian at Kansas State University, believes spillovers probably do occur on the continent, but that differences in farming practices prevent them from resulting in large outbreaks. More than 50,000 lactating milk cows were moved between U.S. farms every week before the outbreak, he says, in part because cattle operations are much larger and often span several states. “That doesn’t happen in Europe and it makes a huge difference.”
Kuiken agrees: “There appears to be something different between the U.S. dairy industry and the dairy industry of other countries that allows this to happen.”
A surprising new infection supports that view. On 24 March, U.K. officials announced that recently introduced testing of livestock at properties with infected birds has picked up an H5N1 infection in a sheep—the first ever in that species. Other sheep in the flock were not infected. Details are sparse, but “it fits with the idea that such spillovers occur more frequently, but don’t lead to extended outbreaks,” Kuiken says.
On the other hand, if Europe has the occasional spillover into cattle, some cows there would have antibodies against H5N1, a sign of past infection. Testing in thousands of cows in Italy, Germany, Sweden, Norway, the United Kingdom, and the Netherlands has not found antibodies or active infections. If spillover is happening in Europe, “it must be a very rare event,” Beer concludes.
He thinks it is more common in the U.S. because the poultry outbreak there, though it lagged Europe’s, is now much larger. “There is a huge viral pressure,” Beer says. More than 160 million poultry birds have been killed by H5N1 or were culled since late 2021, far more than in Europe. The fact that U.S. cattle often roam outside may also increase the risk. “In Europe at most of the dairy farms I’ve seen, the animals are inside and rather controlled,” Richt says.
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