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.

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.

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.