The Biden administration said Wednesday it would begin requiring dairy cows crossing 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 eradicate the spread of a virus that federal health officials have tried to assure Americans so far poses little risk to people.

The new order, issued by the Department of Agriculture, says lactating cows must test negative for the influenza A virus, a class that includes bird flu, before being transported. Owners of herds with positive tests will be required to provide data on livestock movements to help researchers track the disease.

The tests 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 at a news conference Wednesday morning.

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

Dr. Nirav Shah, principal deputy director of the Centers for Disease Control and Prevention, told reporters 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 USDA. But only 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 seriousness of the Food and Drug Administration’s findings in recent days that inactive viral fragments had been found in pasteurized milk, including some on supermarket shelves. Don Prater, an FDA official, told reporters Wednesday that regulators were conducting more advanced testing to determine whether the milk contained live virus. The agency will publish data on milk testing in the coming days, he said.

The USDA order will require state laboratories and veterinarians to report any positive livestock tests to the agency. Watson said the department could 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 that are moved.

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

“There may be owners who are reluctant to work with public health, not to mention individual workers who may be reluctant to sit with someone who identifies in any way as belonging to the government,” Dr. Shah said.

It is still unclear when the bird flu outbreak began, but an analysis of genetic data suggests that wild birds could have transmitted the pathogen to cows as early as December. Cows were not normally 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 USDA order came after public health experts and dairy producers criticized the Biden administration for the scope of its investigation into the cow outbreak and 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 that is now believed to contain extensive viruses, said Seema Lakdawala, a virologist at Emory University.

If a worker splashed their nose or eyes and became infected, that human infection would give the virus new opportunities to adapt and start spreading among people, he said.

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

But Dr. Lakdawala said workers’ infection risks were already serious enough that farms should universally implement the use of face shields. He said other measures, such as 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 bird flu in livestock had intensified efforts to understand the virus.

“Now it has become a species that humans have more contact with,” he said.