Unpasteurized milk contaminated with the H5N1 avian influenza virus, which has been identified on dairy farms in nine states, can rapidly make mice sick, affecting multiple organs, according to a study published Friday.
The findings aren't entirely unexpected — at least six cats have died after drinking virus-containing raw milk — but the new data adds to growing evidence that virus-containing raw milk may not be safe for other mammals, including humans.
“The message is: Don't drink raw milk,” said Yoshihiro Kawaoka, a virologist at the University of Wisconsin-Madison who led the study.
Most commercially available milk in the United States is pasteurized. The Food and Drug Administration found traces of the virus in 20 percent of dairy products sampled from grocery store shelves nationwide. The agency found no signs of infectious virus in the samples and said pasteurized milk is safe to drink.
But Dr. Nahid Bhadelia, director of the Center for Emerging Infectious Diseases at Boston University, who was not involved in the study, said the findings have global implications.
“If this infection spreads further among cows, there are other places that don't have central pasteurization facilities,” she warned. “There are many more rural areas that drink milk.”
In the study, Dr. Kawaoka and his colleagues analyzed the virus in milk samples from infected dairy farms in New Mexico. The researchers found that virus levels declined slowly in milk samples stored at 4 degrees Celsius, suggesting that H5N1 in refrigerated raw milk may remain infectious for several weeks. The findings were published in the New England Journal of Medicine.
The flu virus can survive refrigerator temperatures, and proteins in milk also help stabilize the virus, said Richard Webby, an influenza expert at St. Jude Children's Research Hospital in Memphis, who was not involved in the study.
If raw milk drinkers believe refrigeration kills the virus, “this clearly shows that this is not the case,” Dr. Webby said.
Mice fed the contaminated milk quickly became ill, their fur stood on end and they became lethargic. After four days, the mice were euthanized, but the researchers found high levels of the virus in their respiratory systems and moderate levels in several other organs. As in infected cows, the virus harbored itself in the mammary glands of the mice, an unexpected finding.
“These mice are not lactating mice, but we still find the virus in their mammary glands,” Dr. Kawaoka said. “That's very intriguing.”
It's unclear whether the presence of the virus in the mammary gland is a feature of this particular virus, or of avian influenza viruses in general, Dr. Webby says. “We're learning something new every day.” Mice are common farm pests and can serve as reservoirs for the virus, which can then sicken cats and birds that feed on infected mice.
The cats that died from drinking the tainted milk showed pronounced neurological symptoms, including stiffness, blindness, pacing and a weakened blink response, and Dr Webby said the rats might have shown similar symptoms if they had lived longer.
It's also unclear what the findings mean for the course of human infection. On Wednesday, federal officials announced that a second dairy worker had tested positive for the H5N1 virus. A nasal swab from that person tested negative for the virus, but an eye swab came back positive.
Pasteurization involves heating milk at high temperatures to kill bacteria. In this study, when researchers heated milk at temperatures and times typically used for pasteurization, the viruses were either undetectable or greatly reduced, but not completely inactivated.
Dr Kawaoka cautioned that the results do not mean milk on grocery store shelves contains the active virus, because laboratory conditions are different from those used in commercial pasteurization processes.
In contrast, he said the findings were “robust” that raw milk contains large amounts of the virus.
Raw milk has become increasingly popular in recent years, with health experts and right-wing commentators extolling its benefits, especially since the start of the avian flu epidemic in dairy cows. Some claim it tastes better and is more nutritious than pasteurized milk. Others say it boosts the immune system.
Conversely, pasteurization preserves milk's key nutrient calcium and adds vitamin D, which aids absorption. According to the U.S. Centers for Disease Control and Prevention, consuming raw milk can lead to serious complications and death from a variety of pathogens, especially in people with weakened immune systems.
Between 1998 and 2018, outbreaks linked to the consumption of raw milk resulted in 228 hospitalizations, three deaths, and more than 2,600 illnesses.
