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.

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 on Thursday called for more testing of employees on farms with bird flu after a new study showed that some dairy workers had signs of infection, even when they didn't report feeling sick.

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

Federal health authorities on Thursday broadened their response to the H5N1 avian flu outbreak based on new data suggesting that the current approach to testing and treating farmworkers may have missed cases.

A new study found that a small but not insignificant number of bird flu infections are going undetected among farm workers