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.

The variant of bird flu that killed a Louisiana resident has now been found in dairy cows. Until now, the human cases of bird flu traced to cattle have been mild, mostly eye infections. But the U.S. Department of Agriculture confirmed that a strain previously found only in wild birds has turned up in dairy herds in Nevada ‒ raising concerns that future human infections could be more serious.