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.

The strain of bird flu is distinct from what has previously been found in dairy cattle. The finding means the virus has again spilled over from birds to cows, and it raises some worrying questions.