Bird flu likely circulated in U.S. dairy cows on a limited basis for about four months before federal officials confirmed the disease that has now spread to nine states, according to a new federally funded research paper.
The U.S. Department of Agriculture reported the first-ever H5N1 virus infection in a dairy cow in Texas on March 25, following reports of decreased milk yields in multiple states.
The USDA has said it believes wild birds, which can carry the virus, introduced H5N1 to cattle. The outbreak then expanded as cows were shipped to other states, according to the paper released on Wednesday that was funded by USDA, Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases.
"Data support a single introduction event from wild bird origin virus into cattle, likely followed by limited local circulation for approximately four months prior to confirmation by USDA," the paper said.
A team of academic scientists led by University of Arizona evolutionary biologist Michael Worobey pieced together raw genetic sequences released by USDA on April 21 without dates or locations and concluded a week ago that a single transmission event occurred in late 2023.
Scientists have criticized USDA for not releasing details of the data that would allow academic researchers around the world to trace the evolution of the virus.
One person, a Texas farm worker, has tested positive for H5N1 in the current outbreak, though the only symptom was conjunctivitis, believed to be caused by contact with cow milk. The CDC has said the general public faces a low risk for infection.
Bird flu has long been on the list of viruses with pandemic potential, and any expansion to a new mammal species is concerning to scientists.
Carol Cardona, a bird flu expert at the University of Minnesota, said the virus was able to spread during the four months it was undetected.
"By the time it was recognized, we were beyond our ability to contain the outbreak," she said.
Veterinarians observed dairy cattle displaying unexplained reductions in milk production and changes in milk quality, along with reduced feed consumption, starting in January, according to the paper. It was published an open-access preprint
server for the biological sciences called bioRxivon.
Members of USDA's network of laboratories that monitors for diseases identified influenza A virus, which includes bird flu, in milk and nasal swabs from cows at a Texas dairy, the paper said, without specifying a date.
They forwarded samples to USDA's National Veterinary Services Laboratories, which respond to animal-health emergencies, for testing as epidemiologic investigations continued elsewhere, the paper said.
USDA did not immediately respond to a request for comment.
"Overall, it's wonderful that these data have been shared," virologist Angela Rasmussen of the University of Saskatchewan's Vaccine and Infectious Disease Organization, who worked on sequencing the virus with Worobey, said in a post on X.
