Bird flu infects many species, including wild animals, according to the United Nations. Ducks, quail and at least one eagle have been euthanized due to the virus. A global outbreak of H5 bird flu has infected millions of animals since 2020. The virus can infect chickens, cows and, more rarely, humans.

The study published Thursday by the Centers for Disease Control and Prevention echoes two smaller ones that detected evidence of infection in previously undiagnosed farmworkers. In those studies, several of the infected workers remembered having symptoms of H5N1 bird flu, while none of the veterinarians in the new paper recalled any such symptoms.

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.

With so much H5N1 circulating in the U.S., scientists worry we are a few mutations away from a potential human pandemic.