Genetic material from a particularly virulent strain of bird flu virus has been found in 1 in 5 samples of pasteurized milk, according to an April 25 update from the Food and Drug Administration. The tested milk came from a nationally representative sample and the positive results came from milk in areas with herds of dairy cows where Highly Pathogenic Avian Influenza (HPAI) or H5N1 infections have been detected. The FDA’s new test results indicate that the virus has spread further among dairy cows than previously indicated.
As of April 25, bird flu had been detected in 33 herds in Idaho, Kansas, Michigan, New Mexico, North Carolina, South Dakota, Ohio, and Texas. This particular virus strain has caused a devastating outbreak in wild and commercial birds since 2021. It first spread to mammals in 2022 and can occasionally infect humans. Only two human cases of HPAI have been reported in the United States, according to the Centers for Disease Control (CDC).
The FDA used quantitative polymerase chain reaction (qPCR) testing to inspect the milk samples. While the findings are concerning, it does not necessarily mean that the milk was contaminated with live virus–which can cause infection.
[Related: Bird flu detected in dairy cow milk samples.]
“With qPCR tests, the genetic material, not necessarily the whole active or infectious virus, is what is detected,” clinical pathologist Nam Tran said in a statement. “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.” Tram is a professor at the University of California Davis and senior director of clinical pathology at UC Davis Health.
The FDA believes that the commercial milk supply remains safe, since the testing only revealed small genetic traces of bird flu and not live virus that causes infections. The virus itself was first detected in dairy cows in the US in March and the FDA announced on April 23 that it had found viral fragments in commercially sold milk.
Milk sold in grocery stores is pasteurized. This process kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time. The viral particles detected by highly sensitive qPCR tests were likely the remnants of viruses that had already been killed during the pasteurization process.
To determine if any active, infectious virus remains in the milk samples, the FDA is going to perform egg inoculation tests. These rests are often considered the “gold standard” for determining a virus’ viability. In these tests, scientists will inject the virus sample into a raw chicken egg to see if it replicates or not. This test provides the most sensitive results, but it will take longer than other methods.
“Virus isolation propagates viruses, and needs a live virus particle to start with,” UC Davis School of Veterinary Medicine professor of clinical diagnostic virology Beate Crossley said in a statement. “A virus isolation positive result of a sample would indicate a live, infectious virus is present in the sample.”
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The World Health Organization has also urged public health officials to prepare for a potential spillover to humans in the future. While cases of humans getting infected and seriously ill from bird flu are rare, the more it spreads among mammals, the easier it will be for the virus to evolve to spread.
Health officials continue to believe that commercial milk is unlikely to help spread the virus to humans and that pasteurization is the best line of defense. Consuming raw or unpasteurized milk is dangerous, no matter what the internet says. Raw milk has no added nutritional benefits and it can be contaminated with harmful germs. The CDC even considers raw milk one of the riskiest foods a person can consume.
As an evolving situation, the USDA and FDA will continue to share updates.
