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.

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.