The presentation, so brief as to be little more than a public service announcement, signals an abandonment of efforts to prevent emergence of a wider avian flu pandemic.

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

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 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 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.