In a nationwide survey, the US Food and Drug Administration (FDA) found particles of the H5N1 virus in one out of every five commercial milk samples.
As per Reuters, the discovery suggests that the bird flu outbreak might be more widespread than previously believed.
The agency said there is no reason to believe the virus found in milk poses a risk to human health.
Dr Michael Osterholm, an infectious disease expert at the University of Minnesota said in a statement, noting, "This says this virus has largely saturated dairy cattle throughout the country."
Many infectious disease experts and government officials have said they believe the pasteurization process will inactivate the virus, also known as avian influenza.
However, additional testing is needed to confirm that there is no infectious virus in the milk, the agency said.
"To date, the retail milk studies have shown no results that would change our assessment that the commercial milk supply is safe," the FDA said in its latest update.
Samuel Alcaine, associate professor, of food science at Cornell University said, "I'm not worried about the milk itself. It does indicate that the virus is more widespread among dairies than we had previously thought."
He further added, "We had a little over 30 herds or farms that had been reported as having positive for avian influenza. We have just under 30,000 farms across the U.S. Thirty-three is a really small number. It makes it seem like there is definitely more spread out there."
However, human cases remain rare, with only one confirmed case reported in a Texas farm worker who experienced conjunctivitis, an eye irritation.
The U.S. Centers for Disease Control and Prevention (CDC) has not recorded any additional human cases beyond the initial confirmation.
