Federal officials announced Friday that meat tissue from the sick dairy cows tested positive for the avian influenza virus.
The decision was made to cull the cow because of illness and its meat was not added to the food supply, according to the USDA, which continued to stress that the commercial food supply remains safe.
But the positive test, which comes as part of an ongoing federal investigation into beef safety, has raised concerns that the virus could get into the commercial beef supply and pose a risk to human health.
So far, the virus, known as H5N1, has only been detected in dairy cows, not in cattle raised for meat, but experts believe the outbreak is larger than the official tally of 58 infected dairy cows in nine states.
“It's clear that this is widespread and will require continued vigilance,” said Brian Ronholm, food policy director at Consumer Reports, a consumer advocacy group.
He said he believed the overall risk to consumers was low, but added that “for added peace of mind, it will be important for consumers to cook meat to the right temperature.”
Officials and experts have said thorough cooking likely destroys any viruses that may get into meat, and preliminary tests on ground beef support that idea.
But Dr. Gail Hansen, an independent food safety and veterinary health expert who has criticized the federal government's response to the dairy cow outbreak, said officials were being overconfident about the safety of beef.
“People eat meat rare and even raw,” she says, “so for a government agency to make assurances before there is scientific evidence to support or refute a hypothesis is yet another continued erosion of public trust.”
The USDA said the fact that inspectors identified infected cows and prevented their meat from entering the food supply is proof that the testing procedures are working. But some infected cows may be asymptomatic and not be caught by the testing system. The agency has not found the virus in ground beef samples collected from retailers 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 were culled for showing signs of disease. Only one cow's sample has tested positive for the H5N1 virus, the agency said, and it is currently testing additional muscle samples.
Dr. David Acheson, former chief medical officer for the USDA's Food Safety and Inspection Service, said the findings released Friday further suggest that people need to be careful about how they cook and prepare meat. Food safety experts recommend always cooking meat thoroughly to prevent infection with more common pathogens such as Salmonella, Listeria and E. coli.
“These food safety recommendations were in place long before H5N1 became a problem and should always be our baseline standard,” said Dr. Keith Paulsen, director of the Wisconsin Veterinary Diagnostic Laboratory at the University of Wisconsin-Madison.
Earlier this month, the USDA released the results of an experimental study in which researchers spiked beef patties with high concentrations of the virus. They found that when the burgers were cooked to 160 degrees Fahrenheit, the internal temperature of a well-done burger, or 145 degrees Fahrenheit, the temperature of a medium-cooked burger, the virus was not present in the meat.
But the virus was present in the rare burger cooked at 120 degrees, but at significantly reduced levels. Cooking at that temperature “largely inactivated the virus,” the agency said.
“All indications are that cooking food will kill the virus, even if it's present,” said Stacey Schultz Cherry, a virologist and influenza expert at St. Jude Children's Research Hospital.
Matthew Moore, a food science expert at the University of Massachusetts Amherst, said experts want to learn more about the positive samples, including the concentration of the virus. It's unclear whether the virus was live or inactivated. Whether people can get avian flu from eating contaminated food is also an open question. A study published Friday found that rats got sick after drinking unpasteurized milk contaminated with the virus, raising concerns among experts that consuming raw milk could be harmful to humans. Several cats also died after drinking the contaminated raw milk.