Federal regulators said Tuesday that pasteurized milk samples from across the country tested positive for inactive residues of the avian influenza virus that has infected dairy cows.
The viral fragments pose no threat to consumers, officials said. “To date, we have seen nothing that would change our assessment that the commercial supply of milk is safe,” the Food and Drug Administration said in a statement.
Over the past month, the avian influenza virus known as H5N1 has been detected on more than 30 dairy farms in eight states. The virus is also known to have infected a farmer, whose only symptom was pink eye.
Scientists have been critical of the federal response, saying the Agriculture Department has been too slow to share important data and has not adequately pursued testing cattle for the infection.
Finding viral fragments in milk from the commercial supply chain is not ideal, but the genetic material poses little risk to consumers who drink milk, said David O'Connor, a virologist at the University of Wisconsin-Madison.
“The risk of becoming infected from milk containing viral fragments should be zero,” he said. “Genetic material cannot replicate on its own.”
Officials did not say how many pasteurized milk samples tested positive for viral fragments or where those samples came from. These are key questions, experts say. About a third of the samples tested positive, according to two people familiar with the data who were not authorized to speak publicly.
If the fragments were present in many samples of the commercial milk supply, it would suggest that the outbreak is likely to be much more widespread than previously believed.
Last week, the New York Times reported that the virus had also been detected in a herd of North Carolina dairy cows that had no symptoms of illness.
“The dairy cow problem could be much bigger than we know,” Dr O'Connor said. “That would be the concern, not that the milk itself would pose a risk.”
The FDA said it was studying milk samples from several sources, including infected cows, the milk processing chain and grocery store shelves. Federal officials are still waiting for the results of experiments aimed at determining whether milk samples might contain active virus, according to two people familiar with the ongoing federal reviews.
These tests take much longer than so-called PCR tests that determine whether viral material is present in the milk supply.
Federal officials have repeatedly reassured consumers that the commercial milk supply is safe, stressing that dairy producers are required to keep milk from sick animals out of the human food supply.
Nearly all milk produced on U.S. farms is pasteurized, a process designed to kill pathogens with heat. Pasteurization should also inactivate flu viruses, which are known to be fragile and sensitive to heat, experts say.
Only recently did the FDA test the effectiveness of pasteurization on H5N1. The risk of contracting the virus from unpasteurized dairy products remains unknown, but regulators have long warned consumers that raw milk poses a variety of disease risks.
The Centers for Disease Control and Prevention has been tracking flu test data and flu-related emergency room visits. “We haven't seen anything elevated to date at the local, state or national level,” said Dr. Nirav Shah, the agency's principal deputy director.
The discovery of viral fragments in milk has raised considerable concern in the White House about how to avoid raising undue alarm about the dairy supply, according to people familiar with the internal deliberations who were not authorized to speak publicly.
Federal officials are expected to address the findings at a news conference in the coming days.
Brian Ronholm, director of food policy at Consumer Reports, an advocacy organization, said it would be “very critical” for officials to clearly communicate the findings and educate consumers about what they mean.
Milk from farms is already collected in thousands of liters which would greatly dilute any virus present. Pasteurization further reduces the levels of virus present.
Soon after the September 11 attacks, Michael Osterholm, an infectious disease expert at the University of Minnesota, helped the federal government plan a possible bioterrorist attack that could use botulism neurotoxin, a highly lethal pathogen, to contaminate the milk.
Working with the dairy industry, he and others identified pasteurization conditions that would inactivate the neurotoxin. This experience reassures him that H5N1 is highly unlikely to pose any problems, Dr. Osterholm said.
“With a virus like this, I would have to believe that even if I had the highest levels of viral activity that you could ever imagine in actual milk from the udder of an infected cow, it would be diluted millions of times during pasteurization. “
Ingested milk is also broken down by the body's digestive and immune systems, so “I wouldn't worry about residue,” said Dr. Nahid Bhadelia, director of Boston University's Center for Emerging Infectious Diseases.
“As long as it's not a live virus, there's unlikely to be any health risks,” he said.
Infected people – and cows – may carry remnants of viral genetic material with them long after the active infection has cleared. That's why PCR tests for Covid sometimes produce positive results after a person has recovered from the disease.
Affected cows appear to have large amounts of the virus in their milk, suggesting it may take time to clear the virus, Dr. Bhadelia said.
“But the interesting part is that we have no idea how long it takes cows to clear the virus,” he said.