New experiments confirm that milk from H5N1-infected cows can make other animals sick and raise questions about flash pasteurization


New laboratory experiments with milk from cows infected with H5N1 influenza, known as bird flu, confirm that it is contagious, especially when left raw or untreated, and potentially even when Instant pasteurized.

The researchers, from the University of Wisconsin School of Veterinary Medicine, are part of a federally funded program called Centers of Excellence for Influenza Research and Response, or CEIRR. This network has been conducting rapid investigations to answer urgent questions about the H5N1 outbreak in dairy cattle.

In a research letter published online Friday in the New England Journal of Medicine, researchers describe the results of experiments using milk from four infected cows: two from New Mexico and two from Kansas.

Because the H5N1 virus is considered a select agent, it was handled in a high-security biosafety level 3 laboratory at the University of Wisconsin, using strict safety protocols.

First, they confirmed that the raw milk was teeming with the H5N1 virus. They then stored some of the raw milk at refrigerator temperature to see if the virus levels in the milk decreased over time. Over five weeks, the viral levels in the raw milk decreased a little, but not by much.

“It’s concerning that it hasn’t deteriorated over time,” said Dr. Seema Lakdawala, an associate professor of immunology and microbiology at Emory University, who is also part of the CEIRR network but was not involved in the study.

In another series of tests, the researchers checked which types of pasteurization might work best to inactivate the virus.

They heated small samples of milk to times and temperatures used in two types of pasteurization: low-temperature, long-term or vat pasteurization; and high-temperature rapid pasteurization or flash pasteurization, which is the most common method used in the U.S. today, according to the International Dairy Foods Association.

Heating milk to 63 degrees Celsius, or 145 degrees Fahrenheit, for intervals of between 5 and 30 minutes (the vat pasteurization method) reduced the virus to undetectable levels.

Heating the milk to 72 degrees Celsius, or 181 degrees Fahrenheit, for 15 to 20 seconds (conditions that approximated flash pasteurization) greatly reduced levels of the virus in the milk, but did not completely inactivate it.

Milk samples heated for 15 or 20 seconds could still infect incubated chicken eggs, a test the U.S. Food and Drug Administration has called the gold standard for determining whether viruses remain infectious in milk.

“But we emphasize that the conditions used in our laboratory study are not identical to large-scale industrial treatment of raw milk,” he said in an email.

That’s a good reason not to panic about the study’s findings, Lakdawala said.

Lakdawala said commercial flash pasteurization involves a preheating step, which was not done here. It also involves homogenization, a process that emulsifies the fat globules in the milk so that the cream does not separate. Both steps would likely make it more difficult for the virus to survive, but he adds that the results of this study suggest that the entire commercial flash pasteurization process should be done “with all steps in place.”

Recent testing of 297 dairy products purchased at retail stores by the FDA found traces of genetic material from the H5N1 virus in about 1 in 5 milk samples, and additional testing confirmed that the viral fragments were inert and could not make anyone sick.

The U.S. Centers for Disease Control and Prevention says the risk to the general public is low, but people should not eat or drink raw milk or products made with it.

To date, H5N1 influenza has been found in 58 dairy herds in nine states. Michigan has now surpassed Texas as the state with the highest number of infected herds.

To test whether raw milk could infect other animals, the researchers also sprayed some milk. in the mouths of mice. The animals showed signs of illness the next day.

By day four, the mice had not died from their infections, but were euthanized so the researchers could see which parts of their bodies had become infected. Scientists found the virus throughout the body, with high viral loads in the lungs and respiratory tract. They also found viruses in the mice’s mammary glands, even though they were not producing milk at the time.

Taken together, their findings confirm that raw milk can infect susceptible animals, the researchers said, and that could also indicate a risk to humans.

At a recent news conference, U.S. Department of Agriculture officials said that, to their knowledge, no raw milk from farms known to have H5N1 infections was being sold to consumers.

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However, not all dairy farms are testing their cows and some of the cows infected in the current outbreak have not shown any symptoms.

“Raw milk is not safe at any rate, and the idea that one could protect against H5 infection by consuming it is flawed. There are much safer ways to protect yourself, namely avoiding raw milk,” CDC Principal Deputy Director Dr. Nirav Shah said in a briefing.

When asked if the raw milk in the new experiments looked different than normal, Kawaoka said the researchers only got a small number of samples, but some had more waste than milk from healthy cows. That would be leaked before the sale, she said. Some of the milk appeared yellowish, but he points out that milk from healthy and infected cows would likely be mixed together in a large vat, making it appear normal, and it would be impossible to tell if the milk is safe to drink just by looking at it. .

“This is good quality research with conclusions supported by solid data,” said Dr. Ruth Harvey, deputy director of the World Influenza Center at the Francis Crick Institute in London, in a comment on the study provided to the nonprofit organization. Science Media Centre.

“The limitations of the experimental setup for the heat inactivation portion of the research are clearly stated: that the conditions used are not identical to large-scale industrial treatment of raw milk,” Harvey said.

“The findings that viable virus was found in milk samples after 5 weeks of storage at (4 degrees Celsius), and that mice could become infected by drinking milk containing the virus, support the conclusions that drinking raw milk could represent a risk. Harvey added.

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