Suspicions that there may be more bird flu going around undetected were backed by a voluntary serosurvey among veterinary practitioners who work with cattle, CDC investigators said. Out of 150 participating veterinary practitioners across the country,

The report is the latest evidence that the outbreak in dairy herds is spreading undetected in cows, and the spillover into people at highest risk of exposure is going unnoticed.

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.