Farmworkers who are exposed to infected poultry or dairy cattle as part of their jobs should be tested for bird flu even if they show no symptoms, particularly if they did not wear protective equipment, said the Centers for Disease Control on Thursday. The CDC revised its safety guidelines after researchers reported that 7% of unprotected dairy workers carried antibodies in their blood indicating infection by the H5N1 avian flu virus.

“The less room we give this virus to run, the fewer chances it has to cause harm or to change,” Nirav Shah, CDC principal deputy director, told reporters. Antiviral medication should be offered to workers who did not wear protective equipment during a high-risk exposure but who do not appear ill, he said. “Again, we’re expanding our recommendation to include workers who were exposed and who do not have symptoms, particularly those who were not wearing appropriate PPE [personal protective equipment].”

Shah said the CDC would adjust its PPE recommendations so that “the higher-risk activities will call for more PPE use.” Culling infected poultry flocks or working in a milking parlor with infected cows were examples of tasks where PPE use was critical, he said.

In a study of 115 dairy farmworkers in Colorado and Michigan, researchers found that eight — 7% of the group — were infected, according to blood tests that looked for antibodies. All eight milked cows or cleaned milking parlors. During interviews, four of them “recalled being ill around the time the cows were ill.” The other four did not. The blood samples were drawn from 15 to 90 days after bird flu was found in the herds.

“This finding supports the need to identify and implement strategies to prevent transmission among dairy cattle to reduce worker exposure and for education and outreach to dairy workers concerning prevention, symptoms, and where to seek medical care if the workers develop symptoms,” said the researchers in an article published in the CDC’s Morbidity and Mortality Weekly Report.

“None of the workers with HPAI A(H5) virus antibodies reported using the PPE recommended for working with HPAI A(H5)–infected animals, and use of recommended PPE was low among all workers,” said the MMWR article. “Only one of the persons whose test results indicated antibodies to HPAI A(H5) virus reported working with known HPAI A(H5) virus–infected cows, supporting the need for additional education and outreach to employers and farmworkers once HPAI A(H5) is identified in herds.”

The CDC recommends the use of PPE such as masks, goggles, gloves, and water-resistant clothing. Compliance has been low, in part due to hot and cramped working conditions.

To date, 46 people in six states have been diagnosed with bird flu, all but one of them a livestock worker. California, Washington, and Colorado account for 41 of the cases. Some 446 dairy herds in 15 states have tested positive for bird flu; 236 of them are in California.