A new study published by the U.S. Centers for Disease Control and Prevention shows that the H5N1 bird flu virus is likely circulating undetected in livestock in many parts of the country and may be infecting unaware veterinarians.

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.