The strain of bird flu is distinct from what has previously been found in dairy cattle. The finding means the virus has again spilled over from birds to cows, and it raises some worrying questions.

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

The detection indicates that distinct forms of the virus known as Type A H5N1 have spilled over from wild birds into cattle at least twice.