A bird flu outbreak in US dairy cattle may have started in January or even December, according to a new analysis of genetic data.
The Department of Agriculture announced in late March that dairy cattle in Texas and Kansas had tested positive for the virus, called H5N1. It has since reported outbreaks in dozens of herds in eight states.
On Tuesday, federal health officials said they had discovered fragments of H5N1 in milk samples taken at various locations around the country. The fragments do not pose a threat to consumers, officials said.
It is not clear exactly how the cows became infected, but the most likely source is faeces or other secretions from 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 may contain valuable clues about the evolution of the virus and the extent of the outbreak.
On Sunday the department published 239 genetic sequencesbut omitted some details as to the localities from which they were obtained and the dates.
But the information is enough to gather that a single diffusion event months ago, from a bird to a cow, triggered the outbreak, said Michael Worobey, an evolutionary biologist at the University of Arizona.
Dr. Worobey said he worked late Monday night and early Tuesday morning analyzing the genetic information. He found that all the sequences carried the same set of mutations that allow the virus to infect mammals – something that would be unlikely if the virus jumped from birds to cows more than once.
He said “this really looks like a single import”.
H5N1 is extremely deadly in birds, but so far it has led to mild symptoms mostly in cattle and a farmer 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 to clear the virus from their bodies, said Dr. Nahid Bhadelia, director of the Boston University Center for the Emerging. Infectious diseases.
“We just don’t know enough about the natural history” of bird flu in cows, he said.
Federal scientists are just beginning to analyze more samples from asymptomatic cows to learn whether the outbreak may be more widespread than first thought.
“If this is something that’s been going on for that long, it seems pretty plausible that it’s already moved out of the US,” Dr. Worobey said. Canada imports cattle from the United States, he noted.
Some farmers in Texas have been reporting sick cows since at least February. Cows are not usually infected with this type of flu, and federal officials spent weeks trying to identify the cause of the outbreak.
But instead of investigating every possible pathogen, officials could turn to a technique called metagenomic sequencing, in which samples can be tested for nearly all known pathogens at once.
This technique is more expensive, but would save valuable time in situations like this, Dr. Worobey said.
“I think it’s one of the real missed opportunities here,” he said. “Just the fact that we don’t have eyes on these things around the world, even in the U.S., we keep getting caught in our bathing suits when the tide goes out.”
Emily Anthes contributed to the report.