New CDC research suggests that a small but significant percentage of dairy farmers working near infected cows have recently contracted H5N1, often without knowing.

Rising H5N1 bird flu cases in the U.S. prompted the CDC to study the virus in ferrets, revealing it may have the potential to spread and cause severe symptoms in other mammals.

The health agency now urges proactive testing and treatment for the virus among any workers who were exposed to H5N1.

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.

Following a CDC study showing bird flu exposure among dairy workers, health officials recommend increased testing and preventative measures for farmworkers, even if they show no symptoms, to prevent virus spread and mutation.

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 some dairy workers showed signs of infection, even when they didn’t report feeling sick. Blood tests from 115 farmworkers in Colorado and Michigan found eight had antibodies that indicated previous infection with the virus.