Milk contaminated with the H5N1 bird flu virus, which has appeared in dairy herds in nine states, was found to rapidly make mice sick, affecting multiple organs, according to a study published Friday.
The findings weren’t entirely surprising: At least six cats died after eating raw milk that contained the virus. But new data adds to evidence that raw milk carrying the virus 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 found traces of the virus in 20 percent of dairy samples taken from grocery store shelves across the country. Officials found no signs of the infectious virus in the samples and said the pasteurized milk was safe for consumption.
But Dr. Nasheed Badria, director of the Center for Emerging Infectious Diseases at Boston University and not involved in the work, said the findings have global implications.
“If this breaks out more widely in dairy cows, there are other places that don’t have central pasteurization,” she warned, “and there are more rural communities drinking milk.”
In this study, Dr. Kawaoka and colleagues analyzed the virus in milk samples from an affected dairy herd in New Mexico. The researchers found that virus levels slowly decreased in milk samples stored at 4 degrees Celsius, suggesting that H5N1 viruses in refrigerated raw milk may remain infectious for weeks. The findings were published in the New England Journal of Medicine.
Flu viruses survive well at refrigerator temperatures, and proteins in milk also help stabilize them, said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital in Memphis who was not involved in the work. .
Dr Webby said if people who drink raw milk believe that refrigeration kills the virus, “this clearly shows that this is not the case”.
Mice fed contaminated milk quickly became sick, showing ruffled fur and lethargy. On the fourth day, the mice were euthanized, and the researchers found higher levels of the virus in the respiratory system and moderate levels in several other organs. Like the infected cows, the mice also contained the virus in their mammary glands—an unexpected finding.
“These mice are not lactating mice; they are lactating mice.” “The virus is still present in the mammary gland,” Dr. Kawaoka said. “It’s interesting.”
It’s unclear whether the presence of the virus in the mammary gland is a signature of this specific virus or avian influenza viruses in general, Dr. Webby said: “We are learning new things every day.” Rats are a common pest on farms, and for this purpose The virus provides another potential host, and cats and birds that feed on infected mice may also become sick.
Cats that died after drinking contaminated milk showed significant neurological symptoms, including stiffness of body movements, blindness, walking in circles, and weak blinking reflexes. Dr Webby said if the mice were allowed to live longer, they might develop similar symptoms.
Also unclear is what these 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; that person’s nasal swab tested negative, but an eye swab tested positive.
Pasteurization kills bacteria by heating milk to high temperatures. In the new study, when researchers heated milk at temperatures and times typically used for pasteurization, the virus was either undetectable or greatly reduced, but not completely inactivated.
Dr. Kawaoka cautioned that laboratory conditions are different from those used for commercial pasteurization, so the results do not mean the milk on grocery store shelves contains active viruses.
By contrast, the finding that raw milk contains high amounts of the virus is “robust,” he said.
Raw milk has become popular in recent years as health experts and right-wing commentators extol its purported virtues, even more so since an outbreak of bird flu in cows. Some people think it tastes better and is more nutritious than pasteurized milk. Others believe it can boost immunity.
Instead, pasteurization preserves the key nutrient calcium in the milk and adds vitamin D to aid absorption. According to the Centers for Disease Control and Prevention, consuming raw milk can cause serious complications and even death from a variety of pathogens, especially in people with weakened immune systems.
From 1998 to 2018, outbreaks caused by the consumption of raw milk resulted in 228 hospitalizations, three deaths, and more than 2,600 illnesses.
