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.

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.

A new study revealed more farmworkers had signs of infection, even though they reportedly did not display symptoms of illness.