CDC officials said the study suggests asymptomatic workers who engage in jobs that put them at high risk for exposure at poultry and dairy farms might be at heightened risk. The CDC had recommended testing be limited to workers who had symptoms after exposure to infected animals.

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.

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

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.

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.

Seven percent of tested workers on dairy farms where cows were infected with bird flu caught the virus themselves, according to a new study.

Farm workers who have been exposed to animals with bird flu should be tested for the virus even if they do not have symptoms, the U.S. Centers for Disease Control and Prevention said on Thursday.