Federal health officials called Thursday for more testing of farm workers with bird flu after a new study showed some dairy workers showed signs of infection even if they didn’t report feeling sick.

Farmworkers in close contact with infected animals should be tested and offered treatment even if they have no symptoms, said Dr. Nirav Shah, senior director of the U.S. Centers for Disease Control and Prevention.

The new guidelines come after blood tests on 115 farmworkers in Michigan and Colorado showed that eight workers – or 7% – had antibodies indicating previous infection with the virus known as H5N1 influenza type A .

“The purpose of these actions is to ensure the safety of workers, limit the transmission of H5 to humans and reduce the possibility of the virus evolving,” Shah told reporters.

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The CDC study offers the largest window yet into how the avian virus first detected in March in dairy cows could spread to humans. This suggests the virus has infected more humans than the 46 farmworkers identified in the United States as of Thursday. Almost all of them have been in contact with infected dairy cows or poultry.

Outside experts said it was notable that the study prompted the CDC to take further action. Previous recommendations called for testing and treating workers only when they show symptoms.

“This is a significant step forward in assessing that these H5N1 viruses represent a higher risk than the CDC has previously estimated,” said Dr. Gregory Gray, an infectious disease researcher at the Medical Branch of the University of Texas at Galveston.

Each additional infection in animals or humans gives the virus the opportunity to change in potentially dangerous ways, said Angela Rasmussen, a virus expert at the University of Saskatchewan in Canada.

“This shows once again that we are not responding effectively to the H5N1 virus outbreak in humans or animals and if we continue to let this virus spread and jump from one species to another, our luck will run out by burning out,” Rasmussen said in an email.

The CDC study included 45 workers in Michigan and 70 in Colorado tested between June and August. Of the eight workers whose blood tests were positive, four reported no symptoms. All eight milking parlors were cleaned and none used respiratory protection such as face masks. Three of them reported using protective glasses.

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High levels of the virus were found in the milk of infected cows, increasing the risk of exposure and infection, the researchers said.

Researchers said efforts to monitor dairy worker illnesses have been hampered by several obstacles, including the reluctance of farm owners and farmworkers to allow testing.

Rasmussen and others have criticized the federal response to the outbreak as too slow and “lackluster.”

“These studies should have been done months ago and should have been prioritized,” she said.

The virus has been confirmed in at least 446 cattle herds across 15 states. Last week, the Department of Agriculture said a pig on an Oregon farm was confirmed to have avian flu, the first time the virus has been detected in a American pork.