ScienceAdviser: Why do U.S. cows keep catching avian flu?

When the US reported a year ago that the H5N1 avian influenza virus had infected dairy cattle, it seemed some rare confluence of factors had allowed the dangerous virus to enter a cow’s udder and infect the cells there.

Then it happened again.

And again.

One year into its cow flu outbreak, the U.S. has detected at least three separate instances of the virus passing from birds to cows. Researchers are wondering how exactly this happens—and why no such spillover has been reported in Europe, where H5N1 outbreaks in poultry and wild birds have gone on much longer. “There appears to be something different between the U.S. dairy industry and the dairy industry of other countries that allows this to happen,” says wildlife pathologist Thijs Kuiken.

One possibility: Spillovers happen in Europe with a similar frequency, but differences in farming practices prevent them from resulting in large outbreaks. For instance, tens of thousands of lactating milk cows were routinely shipped around the U.S. before the outbreak, something that does not happen in Europe, says veterinarian Jürgen Richt. Another possibility: Spillovers are much more likely to occur in the U.S., either because the virus is much more common in the environment there than it was in Europe or because some farming practices give the virus an easy way to jump species.

Figuring out what exactly is different is crucial to avoid future outbreaks and reduce the risk of the virus mutating or recombining in cows and touching off a human pandemic. Without the knowledge, there is no clear endgame. Even if the US can get rid of the viruses currently spreading in herds, new ones will keep coming, says evolutionary biologist Michael Worobey. “We’re going to be playing whack-a-mole.”