Beef tissue from a sick dairy cow has tested positive for the bird flu virus, federal officials said on Friday.
The cow had been condemned to be culled because it was sick, and the meat did not enter the food supply, according to the U.S. Department of Agriculture. The department continued to stress that the commercial food supply remained safe.
But the positive test, which came as part of an ongoing federal study of beef safety, raises concerns about whether the virus might make its way into the commercial beef supply, posing a health risk to humans.
So far, the virus, which is known as H5N1, has only been detected in dairy cattle and not in the beef cattle that are raised for meat. But experts believe that the outbreak is bigger than the official tally of 58 affected dairy herds in nine states.
“It’s evident that this is widespread and will require constant vigilance,” said Brian Ronholm, the director of food policy at Consumer Reports, an advocacy organization.
Overall, he said, he believed that the risk to consumers remained low. But, he added, “it will be important for consumers to make sure they cook meat to the proper temperature for additional assurance.”
Officials and experts have said that thorough cooking was likely to destroy any virus that might make its way into meat; preliminary lab tests of ground beef supports that idea. .
But Dr. Gail Hansen, an independent food safety and veterinary health expert who has been critical of the federal government’s response to the dairy cow outbreak, said that officials were being overly confident in the safety of beef.
“People do eat meat rare and even raw,” she said. “So once again the assurances from government agencies, before the science is in to confirm or deny the assumptions, continue to undermine the confidence by the public.”
The U.S.D.A. said the fact that inspectors had identified the ill cow, and prevented its meat from entering the food supply, was proof that its protocols were working. But some infected cows have been asymptomatic and may not be caught by such inspection systems. The agency has not found virus in ground beef samples collected from retail outlets in states where cows have tested positive.
So far, the agency’s Food Safety and Inspection Service has tested tissue samples from 96 dairy cows that had been condemned because of signs of disease. Just samples from one cow tested positive for H5N1, according to the agency, which is in the process of testing additional muscle samples.
The findings released Friday were further indication that people should take care cooking and preparing meat, said Dr. David Acheson, a former chief medical officer of the Food Safety and Inspection Service at the U.S.D.A.. Food safety experts recommend always thoroughly cooking meat to prevent infection from more common pathogens such as salmonella, listeria and E. coli.
“Those food safety recommendations were in store long before H5N1 became an issue, and they should always be our basic standard,” said Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory at the University of Wisconsin-Madison.
Earlier this month, the U.S.D.A. released results from an experimental study in which researchers added high concentrations of the virus into beef patties. The researchers found no virus present in the meat when they cooked the burger to 160 degrees Fahrenheit, the internal temperature of a well-done burger, or 145 degrees Fahrenheit, the temperature of a burger cooked to medium.
However, there was virus present in rare burgers, cooked to 120 degrees, although at greatly reduced levels; the agency said cooking to that temperature “substantially inactivated the virus.”
“All indications are: You cook your food, even if there’s virus in there, it will kill it,” said Stacey Schultz-Cherry, a virologist and influenza expert at St. Jude Children’s Research Hospital.
Experts would like to know more about the positive sample, including the levels of virus it contained, said Matthew Moore, a food science expert at the University of Massachusetts, Amherst. It’s unclear whether the virus was viable or inactivated. Whether people can contract avian flu by eating contaminated food also remains an open question. A study published on Friday found that unpasteurized milk contaminated with the virus sickened mice, heightening concerns among experts that consuming raw milk could harm humans. Several cats have also died after drinking contaminated raw milk.