U.S. dairy farm worker infected as bird flu spreads to cows in five states : Unexpected H5N1 outbreaks in cattle raises difficult questions about how to protect herds and people

An experiment published in 2006 demonstrated that a different H5N1 clade could infect calves. Virologist Martin Beer of the Friedrich Loeffler Institute in Germany, who led that study, says “very special circumstances” might explain the spread of the virus in U.S. cattle, including the way the animals are milked or as-yet undetected genomic adaptations in the virus. “We need to wait for much better epidemiological data,” Beer says.

The good news is that so far, mammals infected with clade 2.3.4.4b have rarely seen sustained transmission, Webby notes. “Putting it into context, I'm not sure at this stage that [the cattle infections are] much different than any other mammal infections,” he says.