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.

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.

A version of the H5N1 virus that has killed one person and severely sickened another has been detected in milk samples collected from Nevada dairy herds.