A new study shows that bird flu has silently spread from animals to some veterinarians. 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.

Bird flu infections were detected among veterinarians tested last fall, including two people with no known exposure to animals with the virus, according to a report released by the U.S. Centers for Disease Control and Prevention on Thursday.

CDC finds H5 bird flu antibodies in 3 veterinary practitioners, highlighting need for enhanced surveillance and safety protocols.

A new CDC report found evidence of previous bird flu infection in veterinary practitioners who work with cattle and who did not know they had been exposed.

The new study is more evidence that the official U.S. tally of confirmed human bird flu infections—68 in the past year—is likely a significant undercount.

The vets had no symptoms, and one worked only in states where no dairy infections had been reported. Three dairy veterinarians, including one who worked only in states with no known bird flu outbreaks in cows,