Avian flu has been detected in a commercial flock of chickens in Pennsylvania for the first time in nearly a year, the state Department of Agriculture announced Monday. The positive test for a strain of highly pathogenic avian influenza turned up in samples from a flock of 50,000 egg-laying hens on a poultry farm in Lehigh County.

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.