The Food and Drug Administration announced Tuesday that samples of pasteurized milk taken from grocery store shelves had tested positive for bird flu, also known as Highly Pathogenic Avian Influenza (HPAI) or H5N1. On Thursday, the agency announced that one in five milk samples nationwide showed genetic traces of the virus. Milk samples from areas with infected herds were more likely to test positive.

The FDA used quantitative polymerase chain reaction (qPCR) testing to check the milk.

Although the findings are 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.

Beate Crossley, a professor of clinical diagnostic virology at the UC Davis School of Veterinary Medicine, agreed. “The PCR tests target the genetic material of a virus. The finding of genetic material does not give any information about the viability status of the virus,” Crossley said. “Or put another way, PCR can detect dead and live viruses.”

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.

“Virus isolation propagates viruses, and needs a live virus particle to start with,” Crossley said. “A virus isolation positive result of a sample would indicate a live, infectious virus is present in the sample.”

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 Wednesday, 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.”

According to the CDC, symptoms of bird flu in humans range from mild, such as conjunctivitis (pink eye), and flu-like upper respiratory symptoms, to severe, such as high fever and pneumonia requiring hospitalization. A full list of bird flu symptoms in humans is available here.

Humans infected with bird flu can be effectively treated with the antiviral oseltamivir (Tamiflu) according to Blumberg.

Although the jump into cattle is recent, bird flu has been on Tran’s radar for years. The first human cases were reported in China in 2020.

“We have been working closely with the Division of Infectious Diseases on H5N1 preparations since before the COVID-19 pandemic,” Tran said. “If there are any suspected cases of bird flu in humans, UC Davis Health has the tests ready.”

• Updates on Highly Pathogenic Avian Influenza (FDA)
• Current U.S. Bird Flu Situation in Humans (CDC)
• Prevention and Antiviral Treatment of Bird Flu Viruses in People
• Avian influenza (bird flu)
• How bird flu virus fragments get into milk sold in stores, and what the spread of H5N1 in cows means for the dairy industry and milk drinkers