Bird flu makes raw milk dangerous to consume, according to mouse research

a A new study published Friday provides more evidence of the potential danger of drinking unpasteurized or raw milk that contains the H5N1 bird flu virus.

Workpublished in the New England Journal of Medicine, showed that mice fed milk from H5N1-infected cows became very ill.

The study cannot prove that what happened to the mice would definitely happen to people who drink raw milk containing the dangerous virus, but it underlines the likely risk, experts said. These types of studies cannot be conducted on humans for ethical reasons.

“There is clearly strong suspicion that there is transmission from raw milk [H5N1] for animals,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “It is still unclear what the risk is for humans. But I wouldn’t take my chances on it.”

Thijs Kuiken agreed. As a pathologist at the viroscience department of the Erasmus Medical Center in Rotterdam, the Netherlands, Kuiken has long studied the effect of H5N1 viruses on mammals. including cats. In this outbreak among dairy cattle, there have been several reports of deaths of farm cats, probably after drinking contaminated milk.

“Based on the weight of evidence of our knowledge of [this lineage of H5N1]to which this letter contributes, it is likely that people who drink raw milk from infected cows would contract a systemic disease,” Kuiken told STAT in an email.

The Food and Drug Administration has long recommended against drinking raw milk because it can contain some dangerous pathogens, such as E. coli, Salmonella and Listeria. The agency has reiterated that advice in the context of the current outbreak of H5N1 in dairy cattle. As of Thursday, 58 herds in nine states have tested positive for the virus since the outbreak was first detected in late March, and two human infections in farmworkers have been discovered.

Neither Kuiken nor Osterholm were involved in the study, which was conducted by researchers at the University of Wisconsin-Madison. The lead author was Yoshihiro Kawaoka, a leading flu virologist who has been studying H5N1 for decades.

Kawaoka told STAT in an email that the mice were euthanized on day 4 of the study “because we didn’t want mice to die before [tissue] sampling.” Studies in which animals are exposed to potentially fatal pathogens do not allow them to experience persistent deaths.

The multi-component study was conducted in early April before the FDA and other academic groups released study results showing that active virus could not be grown from dairy products that had undergone pasteurization.

In addition to the work that involved feeding raw milk to mice, the team simulated a number of different approaches to pasteurization, which uses heat to inactivate pathogens in milk, to see if they could identify active virus from the heat-treated samples to let grow. One of the approaches they used killed all the virus in the milk, while another, which involved heating for a short period of time, only reduced the amount of active virus in the milk to low levels.

The researchers cautioned that their approaches to treating the milk were not identical to what milk processors use when pasteurizing commercial milk.

Yet another aspect of the study involved storing raw milk containing the virus at refrigerator temperature for several weeks to see if the amount of virus decreased over time. They saw only a small reduction in the amount of active virus in the milk, suggesting that “the virus can therefore remain infectious for several weeks in raw milk stored at 4°C.”

Keith Poulsen, one of the authors, said the finding was not a surprise. “Flu likes it cold and wet,” said Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory and professor of large animal internal medicine at the University of Wisconsin-Madison.

Interestingly, the researchers also noticed the H5N1 virus in the mammary glands of two of the mice when they studied where the virus was found in mouse tissues after euthanization. Breast tissue from dairy cattle appears to be highly susceptible to the virus, with infected lactating cows shedding extremely high levels of virus in their milk.

This article has been updated with comments from the senior author.