U.S. dairy cattle tested positive for a strain of bird flu that previously had not been seen in cows, the U.S. Department of Agriculture said on Wednesday, ramping up concerns about the persistent spread of the virus.

The U.S. Department of Agriculture has detected a second bird flu strain in dairy cattle that previously had not been seen in cows. Until now, all 957 bird flu infections among dairy cow herds had been caused by the same strain of the virus.