The FDA on Tuesday announced that samples of pasteurized milk taken from grocery store shelves tested positive for bird flu, also known as Highly Pathogenic Avian Influenza (HPAI) or H5N1.
The FDA used quantitative polymerase chain reaction (qPCR) testing to check the milk.
Although the finding is concerning, it doesn't necessarily mean the milk contains a live virus that could cause an infection, explained Nam Tran, a professor and senior director of clinical pathology at UC Davis Health.
"With qPCR tests, the genetic material, not necessarily the whole active or infectious virus, is what is detected. In the case of food, the genetic material, the RNA found in the grocery store milk samples, may not be the infectious H5N1 virus, but fragments from it," Tran said.
We have been working closely with the Division of Infectious Diseases on H5N1 preparations since before the COVID-19 pandemic. If there are any suspected cases of bird flu in humans, UC Davis Health has the tests ready."-Nam Tran, senior director of clinical pathology
Pasteurization kills pathogens
Based on currently available information, the FDA says commercial milk is safe.
Commercially available milk is pasteurized, a process that kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time. The FDA noted the viral particles detected by highly sensitive qPCR tests were likely to have been remnants of viruses killed during the pasteurization process.
"To date, we have seen nothing that would change our assessment that the commercial milk supply is safe," the agency said.
To determine if any active, infectious virus remain in the milk samples, the FDA will perform egg inoculation tests, considered the gold standard for determining viability.
"With these tests, they inject the sample into the egg to see if the virus replicates or not," Tran said.
While this provides the most sensitive results, it takes longer to complete than other methods.
The FDA said the results from multiple studies will be available in the next few days to weeks.
Spread to dairy cattle detected in March
Bird flu is very contagious and often fatal in bird populations. It was first detected in dairy cattle in the U.S. in March. To date, 33 outbreaks of bird flu have been confirmed in dairy cattle in eight states. Since the outbreak, the FDA has been evaluating milk from affected animals, the processing system and grocery store shelves.
On Thursday, the USDA ordered mandatory bird flu testing for any cattle transferred between states. No bird flu has been detected in California's estimated 1.7 million dairy cows.
Two human cases of H5N1 have been confirmed in the United States. The Centers for Disease Control and Prevention (CDC) considers the risk to the general public to be low.
Dean Blumberg is the chief of pediatric infectious diseases at UC Davis Health. "Bird flu rarely spreads person-to-person. It is more commonly spread from animal to human," Blumberg said. "There has been no evidence of sustained human-to-human spread, although there is always a chance the virus may evolve to transmit among humans more easily."
Some groups of people with job-related or recreational exposures to birds or other infected animals are at greater risk of infection. They should take appropriate precautions to protect against bird flu.
Regarding the safety of milk, Blumberg notes the primary risk would be from raw milk. "Pasteurization results in a greater than 99.9999% reduction in infectious virus in contaminated milk, likely eliminating transmission, but there may be a risk of transmission if unpasteurized milk from an infected animal is ingested."
