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.

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.