A dairy worker in Nevada has reportedly been infected by the killer variant of H5N1 bird flu; analysis shows changes in new strain that makes it more lethal in mammals.

An emerging strain of highly pathogenic avian influenza has spilled over from wild birds to dairy cows in the state of Nevada, and officials at the US Department of Agriculture (USDA) and some scientists are concerned by a mutation they are seeing.

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.