The Biden administration on Wednesday said that it would begin requiring dairy cows moving across state lines to be tested for bird flu, which has been spreading in herds for months. The new policy is part of a growing effort to stamp out the spread of a virus that federal health officials have sought to reassure Americans poses little risk to people so far.

The new order, issued by the Department of Agriculture, says that lactating cows must test negative for influenza A viruses, a class that includes bird flu, before they are transported. The owners of herds with positive tests will need to provide data on the movements of the cattle to help investigators trace the disease.

The testing will help protect the livestock industry, limit the spread of the virus and “better understand this disease,” Mike Watson, a senior Department of Agriculture official, told reporters in a press briefing Wednesday morning.

Since a highly contagious form of bird flu was detected in the United States in 2022, federal officials have sought to reassure Americans that the threat to the public remained low, even as the virus infected a growing number of mammals. Federal regulators on Tuesday announced that inactive viral fragments had been found in pasteurized milk, a suggestion that the virus was likely spreading much more widely among cattle than previously known.

Dr. Nirav Shah, the principal deputy director of the Centers for Disease Control and Prevention, told reporters on Wednesday that there were no changes in the genetic makeup of the virus that would allow it to spread easily among people. So far, Dr. Shah said, states have been monitoring 44 people who were exposed to the virus and are being monitored for infection.

As of Wednesday, the outbreak had spread to 33 herds in eight states, according to the U.S.D.A. But just one human infection has been reported, in a dairy worker in Texas who had direct contact with sick cows. The case was mild.

Federal officials on Wednesday also sought to downplay the severity of the findings of the Food and Drug Administration in recent days that inactive viral fragments had been found in pasteurized milk, including some from grocery shelves. Don Prater, an F.D.A. official, told reporters on Wednesday that regulators were conducting more advanced tests to determine whether any milk contained live virus. The agency would publish data in the coming days on the milk tests, he said.

The U.S.D.A. order will require laboratories and state veterinarians to report any positive tests from cattle to the agency. Mr. Watson said that the department would be able to process tens of thousands of tests each day, with results reported after one to three days. The agency will now reimburse dairies for testing cows without symptoms as well as those being moved.

Dr. Shah, the C.D.C. official, said that the federal government was relying on local officials and health workers to communicate with dairy producers and their workers, including veterinarians who have close relationships with people who might be hesitant to open up to strangers.

“There may be owners that are reluctant to work with public health, to say nothing of individual workers who may be reluctant to sit down with somebody who identifies themselves as being from the government in some way,” Dr. Shah said.

It is still not clear when the bird flu outbreak began, but an analysis of genetic data suggests that wild birds might have passed the pathogen to cows as early as December. Cows were not typically thought to be susceptible to bird flu, and it was not until late March that federal officials announced that the virus had been detected in sick cows in Texas and Kansas.

The U.S.D.A. order came after public health experts and dairy producers had criticized the Biden administration for the scope of its investigation into the cow outbreak and the lack of widespread testing.

While testing more cows is critical, so is reducing the risk of infection among dairy workers regularly exposed to fresh milk now thought to contain extensive virus, said Seema Lakdawala, a virologist at Emory University.

If a worker were to get splashed in the nose or eyes and become infected, that human infection would give the virus new opportunities to adapt and start spreading among people, she said.

Federal health officials said on Wednesday that they had reminded states that they could request protective equipment from the national stockpile.

But Dr. Lakdawala said the risks of worker infections were already serious enough that farms should universally implement the use of face shields. She said other steps, like a two-week ‘stay-at-home’ order for cows, could also obviate the need for even more economically disruptive measures.

Troy Sutton, a virologist at Penn State University, said the emergence of the bird flu in cattle had intensified efforts to understand the virus.

“It’s now moved into a species that humans have more contact with,” he said.