A Second Dairy Worker Has Contracted Bird Flu, C.D.C. Reports

A farmworker in Michigan has been diagnosed with bird flu, state officials announced on Wednesday, making it the second human case associated with the outbreak in cows.

Officials said that the individual became infected with the virus, called H5N1, after exposure to infected livestock. The individual had only mild symptoms and has fully recovered, officials said. They did not provide additional details in order to protect the privacy of the farm and farmworker, they said.

In 2022, a person in Colorado with direct exposure to infected poultry became the first confirmed human case of H5N1 in the United States. Last month, the Centers for Disease Control and Prevention reported an H5N1 infection in a Texas dairy farm worker — the first case associated with the outbreak in cows.

The detection of this latest case did not suggest that bird flu was widespread in people, officials said, adding that the risk to the general public remained low.

“This virus is being closely monitored, and we have not seen signs of sustained human-to-human transmission at this point,” Dr. Natasha Bagdasarian, Michigan’s chief medical executive, said in a statement.

Still, the case suggests that as more herds are infected, farmworkers continue to be at elevated risk of bird flu. “This case was not unexpected,” said Dr. Nirav Shah, principal deputy director of the C.D.C.

A nasal swab from the individual had tested negative for the H5N1 virus, but an eye swab the agency received on Tuesday tested positive, Dr. Shah said. The Michigan and Texas patients only reported experiencing eye symptoms, although in the Texas case the eye and nose swabs both tested positive.

The C.D.C. recommends that clinicians collect both nasal and eye swabs from people exposed to the virus. “This is why making sure samples are taken in full compliance with C.D.C. guidelines is really important,” Dr. Shah said.

Veterinarians have reported that some farmworkers have developed flulike symptoms, but few farmers and farmworkers have agreed to be tested for the cause. In Michigan, farmworkers exposed to infected animals have been asked to report even mild symptoms, and testing for the virus has been made available, Dr. Bagdasarian said. As of Wednesday, the C.D.C. had tested only about 40 people, agency officials said.

The Michigan farmworker was being monitored because they worked on a farm with herds known to be infected, and received a daily text message from Michigan’s health department asking if they were experiencing any symptoms.

“When the worker indicated that they were, public health sprang into action, culminating in the results that we are reporting today,” Dr. Shah said.

“We found this case because we were looking for this case, and we were looking for it because we were prepared,” he added.

The virus has been detected in 52 dairy herds in nine states, according to the U.S. Department of Agriculture. But the outbreak is thought to be much more widespread. Michigan officials have reported four infected herds that were not included in the federal count. The Food and Drug Administration has found traces of the virus in 20 percent of dairy products sampled from grocery shelves in 17 states.

The virus was probably transmitted from wild birds to dairy cows in a single spillover event in the Texas Panhandle as early as last December, scientists have said. Federal officials did not confirm the first infections until late March.

Cows were not thought to be susceptible to the virus, and the virus spread undetected among them with no visible symptoms, studies have found. The virus has been detected in some dairy herds with no known links to affected farms, suggesting that the virus spread among asymptomatic cows and that there are infected herds that have not yet been identified.

The virus may have spread between cows largely through contaminated milk, which contains high levels of virus. The virus has also spread from dairy farms to poultry farms, possibly through the movement of contaminated vehicles or equipment. The transport of infected cows from Texas may have spread the infection as far as North Carolina and Michigan.

During the outbreak, the virus has acquired dozens of new mutations, including some that may make it more adept at spreading between species.

The Texas farmworker diagnosed in April had severe conjunctivitis, but recovered fully, C.D.C. officials reported earlier this month. The officials were unable to test the individual’s social contacts.

Farm owners have been reluctant to test their workers or cattle, fearful of the financial consequences. And barring extraordinary circumstances, federal and state officials cannot compel farmers or farmworkers to get tested.

To prepare for the possibility of more cases, federal agencies have begun to fill and finish approximately 4.8 million doses of a vaccine that is well matched to the currently circulating strain of H5N1, officials said.

To identify how the virus is evolving, C.D.C. scientists are sequencing the virus isolated from the infected individual, and will compare it to virus isolated from infected cows in Michigan and the infected dairy farmworker in Texas, Dr. Shah said.

Noah Weiland contributed reporting.