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.

A version of the avian influenza virus different from the one rampaging through herds since last spring has been detected in dairy cows in Nevada, USDA says.