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.

Bird flu (H5N1) has been spreading across the country, among both animals and people, since April of this year. As the Centers for Disease Control and

Testing of dairy workers in Michigan and Colorado shows that more workers may be catching bird flu than thought.

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 number of H5N1 bird flu cases in dairy cattle workers in the U.S. may be much higher than reported, indicates a new CDC study.