Six dairy herds in Nevada have tested positive for a newer variant of the H5N1 bird flu virus that’s been associated with severe infections in humans, according to the Nevada Department of Agriculture.

Dairy cows in Nevada have been infected with a new strain of bird flu virus different from the one circulating in other herds throughout the past year, the U.S. Department of Agriculture (USDA)

A shift in the H5N1 virus has allowed it to spread more easily from birds to cows, raising concerns about its evolution.