In early 2024, the bird influenza that had been spreading across the globe for nearly three decades did something wholly unexpected: It showed up in dairy cows in the Texas Panhandle.
A dangerous bird flu, in other words, was suddenly circulating in mammals—mammals with which people have ongoing, extensive contact. “Holy cow,” says Thomas Friedrich, a virologist at the University of Wisconsin–Madison. “This is how pandemics start.”
This bird flu, which scientists call highly pathogenic avian influenza, or H5N1, is already at panzootic—animal pandemic—status, killing birds in every continent except for Australia. Around the world, it has also affected diverse mammals including cats, goats, mink, tigers, seals, and dolphins. Thus far, the United States is the only nation with H5N1 in cows; it’s shown up in dairies in at least 17 states.