The bird flu outbreak in American dairy cattle may have started in January or as early as December, a new analysis of genetic data suggests.
The Department of Agriculture announced at the end of March that dairy cattle in Texas and Kansas had tested positive for the virus, called H5N1. Since then, cases have been reported in dozens of herds in eight states.
On Tuesday, federal health officials said they had detected fragments of H5N1 in milk samples obtained at several locations around the country. The fragments pose no threat to consumers, officials said.
It is unclear exactly how the cows became infected, but the most likely source is the feces or other secretions of a wild bird infected with the virus.
Scientists in the United States and elsewhere have criticized federal agencies for withholding key information about the outbreaks, including genetic sequences of the virus from infected cows, The New York Times reported Friday. The data could provide valuable clues about the evolution of the virus and the extent of the outbreak.
On Sunday, the department published 239 genetic sequences, but left out some details about the locations where they were obtained and the dates.
Still, the information is enough to determine that a single spillover event months ago, from bird to cow, caused the outbreak, says Michael Worobey, an evolutionary biologist at the University of Arizona.
Dr. Worobey said he worked late Monday night and early Tuesday morning to analyze the genetic information. He found that the sequences all carried the same set of mutations that allowed the virus to infect mammals – which would be unlikely if the virus had jumped from birds to cows more than once.
He said that “this really seems like just a single introduction.”
H5N1 is highly fatal in birds, but has so far caused mostly mild symptoms in livestock and in a farm worker in Texas who became infected. Infected dairy cows appear to carry large amounts of virus in their milk, which becomes thick and yellowish.
But the Department of Agriculture has released little information about other aspects of the outbreak, including how long cows stay sick and how long it takes for them to clear the virus from their bodies, said Dr. Nahid Bhadelia, director of the Boston University Center on Emerging . Infectious diseases.
“We just don’t know enough about the natural history” of bird flu in cows, she said.
Federal scientists have just begun analyzing more samples from cows without symptoms to find out whether the outbreak may be more widespread than initially thought.
“If this really is something that has been going on for so long, it seems very likely that it has already moved out of the US,” said Dr. Worobey. Canada imports livestock from the United States, he noted.
Some farmers in Texas have been reporting sick cows since February. Cows are not typically infected with this type of flu, and federal officials have tried for weeks to determine the cause of the outbreak.
But instead of examining every potential pathogen, officials could have turned to a technique called metagenomic sequencing, which can test samples for virtually all known pathogens at once.
That technique is more expensive, but would save valuable time in situations like this, Dr. Worobey said.
“I think this is one of the real missed opportunities here,” he said. “Just the fact that we don’t notice these things all over the world, and even in the US we keep getting caught with our bathing suits off when the tide goes out.”
Emily Anthes reporting contributed.