Federal regulators said Tuesday that samples of pasteurized milk 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.
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 reported.
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.”
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.
And 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 discovery of viral fragments in milk has raised significant 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 about it.
Federal officials are expected to address the findings at a news conference in the coming days.
This developing story will be updated.