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

The study proved that more workers were catching bird flu after contact with infected animals than the numbers reflected in official counts, something veterinarians working these farms had warned about since the outbreak began in March. The research was led by disease detectives at the U.S.

Centers for Disease Control and Prevention in partnership with state health departments in Colorado and Michigan. It was published Thursday in the CDC’s Morbidity and Mortality Weekly Report. In response to the findings, the agency is expanding its recommendations for testing and treatment of workers on affected farms.

Previously, the CDC recommended testing of only those workers who had symptoms after exposure to infected cows. Now, officials say all workers on farms with infected cows should be tested, even if they don’t have symptoms. The CDC is also recommending that workers without symptoms receive the antiviral medication Tamiflu if they’ve had a high-risk exposure to infected animals when they weren’t wearing adequate personal protective equipment.

An example of a high-risk exposure would be a splash of raw cow’s milk to the face, CDC Principal Deputy Director Dr. Nirav Shah said. Previously, only workers with symptoms were offered Tamiflu.

Every infection gives the virus an opportunity to change and become more dangerous, so th.