“We thought this was a one-off: one bird to one cow, and we wouldn’t see that again,” says Peter Halfmann, a virologist at the University of Wisconsin-Madison’s Influenza Research Institute.
Yet the more severe human cases are concurrent with the spread of a recently mutated, potentially more dangerous version of the virus called the D1.1 genotype. D1.1. is now circulating among wild birds and poultry, and it has spilled over into dairy cows at least twice in 2025, according milk testing data from the Agriculture Department. With D1.1, Halfmann explains that the threshold for cross-species transfer is “much lower than we previously thought.”