Three dairy veterinarians, including one who worked only in states with no known bird flu outbreaks in cows, had recent, undetected bird flu infections, according to a new study from the Centers for Disease Control and Prevention. The results are based on antibody testing of 150 veterinarians working in 46 U.S. states.

It also could mean there are more “virus–infected dairy cattle in states where infection in dairy cattle has not yet been identified,” the CDC says. Human cases of bird flu still remain rare, with only 68 cases confirmed in the United States by the CDC.