The CDC study provides the largest window to date into how the bird virus first detected in March in dairy cows may be spreading to people. It suggests that the virus has infected more humans than the 46 farmworkers identified in the U.S. as of Thursday. Nearly all were in contact with infected dairy cows or infected poultry.

Federal officials said Thursday that farmworkers exposed to bird flu should be tested for the virus even if they are not showing any symptoms, especially if they were not wearing the proper personal protective equipment.

The CDC said Thursday it is updating its recommendations for testing, treatment and protection for workers who may be exposed animals infected with bird flu.

Federal health officials are recommending expanded testing of dairy farm workers after a new study found evidence of previously undetected cases of bird flu in humans. Why it matters: The study is leading officials to recommend the testing of all workers at farms where the H5N1 virus is detected,

Blood tests of 115 workers on farms in Michigan and Colorado showed that eight workers had antibodies that indicated previous infection.