Additional tests of retail dairy products from around the country have shown no signs of the bird flu virus, bolstering the consensus that pasteurization protects consumers from the threat, federal health and agriculture officials said at a news conference Wednesday.
However, the scope of the bird flu outbreak in cattle remains unclear because dairy herds are not routinely tested for infection, scientists and other experts noted.
Just one human infection, which was mild, has been reported in a dairy worker in Texas who had direct contact with sick cows. But scientists fear there may be many more undetected infections, particularly among farm workers.
Just two dozen people have been tested for bird flu, federal officials said in the briefing. There have been no unusual increases in flu cases across the country, even in areas with infected cows, they added.
But Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, said farms are not required to screen employees, many of whom are migrant workers who are reluctant to cooperate with state health officials.
“How much do we ignore because of anxiety and fear of what will happen if you don’t get an answer you like?” said Dr. Poulsen.
Until last week, potentially contaminated dairy products appeared to be the most immediate threat to the public. Federal regulators last week announced early test results of about 95 retail milk samples: About one in five were found to contain genetic fragments of the virus, which health officials said did not pose a threat to consumers.
More advanced tests later in the week found no live virus in the samples, a relief to federal regulators.
On Wednesday, Dr. Donald A. Prater, deputy director of the Food and Drug Administration’s Center for Food Safety and Applied Nutrition, said federal scientists tested an additional 201 commercial dairy samples, including milk, cottage cheese and cream.
So far scientists have found no evidence of a potentially infectious virus. “Findings from US government partners, as well as academic researchers, do not change our assessment of the milk’s safety,” Dr. Prater said.
Dr. Prater said the FDA still strongly advises against consuming raw, unpasteurized dairy products. Federal scientists are still reviewing data on whether the virus in raw milk could be infectious, he added.
As of Wednesday, the outbreak had spread to 36 herds in nine states, according to the Department of Agriculture. Scientists have criticized the Biden administration for not conducting more animal testing to determine the extent of the outbreak.
Some dairy farms have been difficult to access, and owners are sometimes reluctant to allow government workers into production facilities, federal officials said.
“There are a lot of farms out there that don’t report,” said Dr. Poulsen, the Wisconsin expert. “They don’t report because they’re really afraid of what will happen if they’re not negative.”
The Department of Agriculture has decided that lactating cows must test negative for influenza A viruses, a class that includes bird flu, before being transported across state lines. The rule also requires owners of herds with positive tests to provide data on the cattle’s whereabouts to help investigators track down the disease.
But further Directive released last week revealed that farmers only have to test 30 cows in a group, potentially allowing infected cows in larger herds to move between states undetected.
Dr. Rosemary Sifford, a senior USDA official, defended the scope of the order, saying 30 cows was a “statistically significant number to be able to determine the condition of the lot.” The department now requires laboratories and state veterinarians to report any positive tests from cattle to the agency.
The USDA has also turned its attention to meat. Colombia last week became the first country to ban beef and beef products from some US states because of the bird flu outbreak.
Dr. José Emilio Esteban, senior food safety official at the USDA, said in the briefing that the beef was safe to eat, but that the agency conducted three studies to “strengthen our scientific knowledge to make sure we have additional points data”.
The department is testing ground beef from grocery stores, as well as offal from slaughtered animals, in states known to have infected dairy cattle, Dr. Esteban said. The agency is also testing the effectiveness of cooking in killing the virus by heating beef patties to three different temperatures.
Officials are also looking at possible ways to compensate dairy farmers for their “cooperation and adoption of additional biosecurity practices,” Dr. Sifford said.
Underlying much of the concern about the cattle outbreak is scientists’ fear that the bird flu virus is adapting to mammals. Dr. Sifford said at the briefing that federal scientists have not identified any changes in the virus that would allow it to spread more easily between people.
Dr. Dimitris Daskalakis, a senior official at the Centers for Disease Control and Prevention, acknowledged that only about 25 people had been tested for infection, roughly the same number reported last week.
More than 100 people are being monitored for symptoms. Dr Daskalakis said the number of people being tested and monitored was “dynamic”, in part because the monitoring period for individuals ends as infected herds recover.
Emily Anthes contributed reporting.