Federal regulators said Friday they had yet to find live bird flu virus in the first batch of retail milk samples they tested, a reassuring sign that milk on store shelves remains safe despite an outbreak of the virus in dairy cows.
In one electronic updatethe Food and Drug Administration said an initial set of tests that looked for live virus, not just genetic fragments, suggested that the pasteurization process effectively kills the pathogen.
“These results confirm our assessment that the commercial milk supply is safe,” the FDA wrote in the update, adding that testing efforts are ongoing.
Officials also tested infant and toddler formula, which incorporates powdered dairy, and did not detect the virus, the agency wrote.
The FDA began a national survey of milk samples shortly after the bird flu virus, called H5N1, was discovered among dairy cows. Government scientists have tested 297 samples of retail dairy products from 38 states, a swath of the country that covers areas well beyond the nine states known to have infected herds.
The first type of test regulators conducted, a form of polymerase chain reaction, or PCR, is relatively quick, but only detects genetic traces of the virus and does not tell researchers whether the live pathogen is present.
On Thursday, the FDA said those tests showed that about one in five retail milk samples nationwide contained fragments of the bird flu virus, suggesting it is spreading among cows much more widely than previously known.
Samples containing genetic fragments are then tested for avian influenza virus in live birds, which, if present, could pose a widespread health threat.
The test for live virus, called egg inoculation, is the most sensitive of its kind, but it takes time. The process involves injecting a portion of the dairy product into chicken eggs, waiting for the virus to grow in the egg, and then looking for signs of infection.
Chicken eggs are effective vessels for the growth of influenza viruses. even rare quantities will thrive there. For that reason, the FDA’s new results strongly suggest that the samples tested did not contain infectious virus and that pasteurization is working, the scientists said.
The negative results reported Friday came from a “limited set of geographically targeted samples,” according to FDA officials who did not specify where the samples came from.
“The answer at this point seems pretty convincing that pasteurized milk is safe,” said Samuel Scarpino, a professor of health sciences practice at Northeastern University. “The fact that it comes back negative is really strong evidence that, at least in the samples they tested, there is no live virus.”
Raw milk is never safe to drink, experts say, and poses additional risks amid a bird flu outbreak in cattle. Almost all milk produced on US farms is pasteurized, a process that kills pathogens with heat. Influenza viruses are known to be fragile and sensitive to heat.
The scientists stressed that the federal government should test more milk samples and continue to test them as the outbreak continues. Some blamed officials for not acting sooner.
“The FDA should have performed these tests six weeks ago when we first heard about it,” Dr. Scarpino said, referring to the outbreak among cattle.
Dr. Scarpino also urged the government to conduct experiments inoculating eggs with milk containing various concentrations of viral genetic material. These tests, he said, could provide reassurance that even pasteurized milk containing copious amounts of genetic fragments remains safe to drink.
In addition to pasteurization, other existing safety procedures require that milk from apparently symptomatic cows be kept from commerce. While more studies are needed, Dr. Scarpino said, “you start layering these things on top of each other and it becomes unlikely that there really is a problem.”
Andrew Bowman, a veterinary epidemiologist at Ohio State University who studied 150 retail milk samples he collected around the Midwest, said the FDA’s findings mirror the results of ongoing testing he has done for live virus.
Analysis by the FDA on Friday showed that the multiplying virus was still unlikely to show up in retail milk samples anywhere.
“I have a gallon of milk in my fridge that can be used tonight,” he said.