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.

Dairy cattle in Nevada have been infected with a new type of bird flu that's different from the version that has spread in herds since last year. U.S.