Bird flu infections went undetected in some veternarians, CDC study says

A new study shows that bird flu has silently spread from animals to some veterinarians.

The study published Thursday by the Centers for Disease Control and Prevention echoes two smaller ones that detected evidence of infection in previously undiagnosed farmworkers. In those studies, several of the infected workers remembered having symptoms of H5N1 bird flu, while none of the veterinarians in the new paper recalled any such symptoms.

The new study is more evidence that the official U.S. tally of confirmed human bird flu infections — 68 in the last year — is likely a significant undercount, said Dr. Gregory Gray, an infectious disease researcher at the University of Texas Medical Branch in Galveston.

“This means that people are being infected, likely due to their occupational exposures, and not developing signs of illness and therefore not seeking medical care,” Gray said. He said it shows that officials cannot fully understand bird flu transmission by only tracking people who go to medical clinics with symptoms.

Study suggests bird flu cases are undercounted

CDC researchers went to an American Association of Bovine Practitioners veterinary conference in September 2024 in Columbus, Ohio. They recruited 150 vets from 46 states to fill out a questionnaire and agree to have their blood drawn. None said they had suffered red eyes or other symptoms associated with bird flu.

Testing found three of the vets, or 2%, had evidence of antibodies to H5N1 infection. All three worked with dairy cattle, as well as other animals. None said they’d worked with a herd that was known to be infected, although one had worked with a flock of infected poultry.

Gray and some colleagues did a study last year of 14 dairy farmworkers and found two, or 14%, had evidence of past infections. Both had experienced symptoms but were never diagnosed.

Another study published last year by the CDC checked 115 dairy workers. The researchers found that eight of them, or 7%, had evidence in their blood of recent infection. Half recalled feeling ill.

The studies were far too small to use as a basis to provide a solid estimate of how many undiagnosed human infections are out there, Gray said. But even just a very small percentage could translate to hundreds or thousands of Americans who were infected while working with animals, he noted.

That’s not necessarily a reason to be alarmed, said Jacqueline Nolting, an Ohio State University researcher who helped CDC with the latest study.

Available studies suggest people who are infected mount antibody responses and may develop natural immunity, which is “good news,” she said.

However, if the virus changes or mutates to start making people very sick, or to start spreading easily from person to person, that would be “a completely different story,” Nolting said.

Experts urge caution around animals

The H5N1 bird flu has been spreading widely among wild birds, poultry, cows and other animals. Its escalating presence in the environment increases the chances people will be exposed, and potentially catch it, officials have said.

Right now the risk to the general public is low, the CDC says. But officials continue to urge people who have contact with sick or dead birds to take precautions, including wearing respiratory and eye protection and gloves when handling poultry.

“No one’s really questioning that the virus has been moving around the country more than has been reported,” said Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory.

He said he expected to see stepped-up information reminding veterinarians across the country to protect themselves with gloves, masks and other equipment to halt infection.

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.

—Mike Stobbe, Associated Press

Associated Press writer JoNel Aleccia contributed to this report.

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