Federal regulators found fragments of the bird flu virus in about 20 percent of retail milk samples tested in a nationally representative study, the Food and Drug Administration said. in an online update on Thursday.
Samples from areas of the country known to have dairy herds infected with the virus were more likely to be positive, the agency said. Regulators said there was no evidence that this milk posed a risk to consumers or that there was live virus in the milk on store shelves, an assessment that public health experts agreed with.
But finding traces of the virus in such a high percentage of samples from across the country is the strongest signal yet that the bird flu outbreak in dairy cows is more widespread than the official tally of 33 infected herds in eight states.
“It suggests there’s a lot of this virus out there,” said Richard Webby, a virologist and flu specialist at St. Jude Children’s Research Hospital.
Dr Webby said he believed it was still possible to eradicate the virus, known as H5N1, from the country’s dairy farms. But it will be difficult to design effective control measures without knowing the extent of the outbreak, he said.
The findings also raise questions about how the virus has evaded detection and where else it might be spreading silently. Some scientists have criticized the federal testing strategy as too limited to reveal the true extent of the virus’ spread.
Until Wednesday, when the Department of Agriculture announced mandatory testing of dairy cows moving across state lines, testing of cows was voluntary and focused mostly on cows with obvious symptoms.
As of Wednesday, just 23 people had been tested for the virus, while 44 people were being monitored after exposure to it, according to the Centers for Disease Control and Prevention.
A widespread outbreak in cows would pose a greater risk to farm workers, the dairy industry and public health in general. Prolonged spread among cows would give the virus more opportunities to acquire mutations that make it more contagious among humans.
The FDA did not elaborate Thursday on the number or sources of the samples.
“You’d want to go not only to places where you knew there was activity and cows — you want to go to places where at least there hasn’t been a report” of bird flu, Dr. Webby said.
Experts believe that the pasteurization process, in which the milk is briefly heated, should inactivate this bird flu virus, which is known as H5N1.
“And when you destroy the virus, it will release genetic material,” said Samuel Alcaine, a microbiologist and food scientist at Cornell University. The genetic fragments left behind are not capable of causing infection.
“It’s not surprising” to find them in milk, he added. “It doesn’t mean the milk isn’t safe.”
Federal officials are still conducting the time-consuming tests needed to determine whether any viable virus remains in the milk after pasteurization. Scientists said the prospect is highly unlikely.
Dr. Jeanne Marrazzo, director of the National Institute of Allergy and Infectious Diseases, said in a briefing Wednesday that some federally funded researchers had tested retail milk for live virus but found none, a sign that pasteurization had killed the virus. . before the milk reaches grocery store shelves.
Dr. Marrazzo cautioned that while the results were a small sample, the findings were “welcome news.”
“To really understand the scope here, we have to wait for the FDA’s efforts,” he said.
Finding traces of the virus in 20 percent of commercial milk samples does not mean 20 percent of the country’s dairy herds are infected, experts warned. “It’s too early to try to do this kind of back-of-the-napkin calculation,” Dr. Alcaine said.
Milk from different farms is usually pooled. If the virus turns up in several milk samples taken from a reservoir, it could mean that many cows are infected – or that a smaller number of infected cows are shedding large amounts of virus, Dr Alcaine said.
Even in the latter case, however, a positivity rate of 20 percent would indicate that far more than 33 herds are infected, he noted.
At Wednesday’s briefing, Dr. Prater pointed to the novelty of the research effort. No studies have ever been completed on the effects of pasteurization on the avian influenza virus in milk, he said.
Regulators were looking at milk at various points in the commercial supply chain, added Dr.