Milk Containing Bird-Flu Virus Can Sicken Mice, Study Finds

Milk contaminated with H5N1, the bird-flu virus that has turned up in dairy herds in nine states, has been found to rapidly make mice sick, affecting multiple organs, according to a study published on Friday.

The findings are not entirely surprising: At least a half-dozen cats have died after consuming raw milk containing the virus. But the new data add to evidence that virus-laden raw milk may be unsafe for other mammals, including humans.

“Don’t drink raw milk — that’s the message,” said Yoshihiro Kawaoka, a virologist at the University of Wisconsin, Madison, who led the study.

Most commercial milk in the United States is pasteurized. The Food and Drug Administration has found traces of the virus in 20 percent of dairy products sampled from grocery shelves nationwide. Officials have not found signs of infectious virus in those samples and have said that pasteurized milk is safe to consume.

But the findings have global implications, said Dr. Nahid Bhadelia, director of the Boston University Center on Emerging Infectious Diseases, who was not involved in the work.

“If this becomes a more widespread outbreak in cows, there are other places where there isn’t central pasteurization,” she cautioned, “and there are a lot more rural communities that drink milk.”

In the study, Dr. Kawaoka and his colleagues analyzed virus from milk samples from an affected dairy herd in New Mexico. The researchers found that levels of the virus declined slowly in a sample of milk 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.

Flu viruses survive well at refrigerator temperatures, and the protein in milk also helps to stabilize them, said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital in Memphis, who was not involved in the work.

If people who drink raw milk believe that refrigeration kills the virus, “this clearly shows that’s not the case,” Dr. Webby said.

Mice that were fed the contaminated milk quickly became ill, displaying ruffled fur and lethargy. On the fourth day, the mice were euthanized, and researchers found high levels of the virus in the respiratory system and moderate levels in several other organs. Like infected cows, the mice also harbored the virus in their mammary glands — an unexpected finding.

“These mice are not lactating mice; still the virus can be found in mammary glands,” Dr. Kawaoka said. “It’s very interesting.”

It is unclear whether the presence of virus in mammary glands is a feature of this particular virus or of bird-flu viruses in general, Dr. Webby said: “We’re learning new things every single day.” Mice are common pests in farms, providing yet another potential host for the virus, and cats and birds that feed on infected mice could also become ill.

The cats that died after drinking contaminated milk showed striking neurological symptoms, including stiff body movements, blindness, a tendency to walk in circles and a weak blink response. If the mice had been allowed to live longer, they might have developed similar symptoms, Dr. Webby said.

Also unclear is what the findings mean for the course of infection in people. On Wednesday, federal officials announced that a second dairy worker had tested positive for the H5N1 virus; a nasal swab from that person had tested negative for the virus, but an eye swab tested positive.

Pasteurization kills germs by heating milk to high temperatures. In the new study, when researchers heated the milk at the temperatures and time periods typically used for pasteurization, the virus was either undetectable or greatly diminished, but it was not completely inactivated.

Dr. Kawaoka cautioned that the laboratory conditions were different from those used in commercial pasteurization, so the results did not mean that the milk on grocery shelves contains active virus.

By contrast, the findings that raw milk contains large amounts of virus is “solid,” he said.

Raw milk has become popular in recent years as wellness gurus and right-wing commentators have extolled its alleged virtues, even more so since the bird flu outbreak in dairy cows began. Some argue that it tastes better and is more nutritious than pasteurized milk. Others contend that it boosts immunity.

On the contrary, pasteurization preserves calcium, the key nutrient in milk, and adds vitamin D to help absorb it. Consuming raw milk can lead to serious complications or even death from a variety of pathogens, especially in people with weakened immune systems, according to the Centers for Disease Control and Prevention.

From 1998 to 2018, outbreaks traced to raw milk consumption led to 228 hospitalizations, three deaths and illness in more than 2,600 people.