A study published Thursday by the Centers For Disease Control and Prevention suggests bird flu infections in humans are being significantly undercounted in the U.S.

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.

Blood testing of large-animal veterinarians suggests that H5N1 bird flu has spread more widely than US surveillance of the virus is capturing, according to a new study by federal and state disease detectives.