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 finding indicates that the virus, known as H5N1, has spilled from birds into cows at least twice — leading to these two sets of infections — and that it could continue to do so. It also suggests that the virus may pose a persistent risk to cows and to the people who work closely with them.

A version of the avian influenza virus different from the one rampaging through herds since last spring has been detected in dairy cows in Nevada, USDA says.

At least four cattle herds in Nevada have tested positive for a strain of H5N1 bird flu never before seen in cows, state agriculture officials confirmed Wednesday, and respiratory symptoms like coughing and sneezing have been reported.