Traces of bird flu are showing up in cow milk. Here’s what to know

Genetic evidence suggests the outbreak in U.S. cattle started much earlier than thought

two people shop for milk in a grocery store dairy aisle. A cooler with gallons of white milk with blue labels and lids is in one cooler. The second cooler's door is open. It is lined with milk with red labels and lids. A person with shoulder length brown hair wearing a blue surgical mask and blue, white and black striped fuzzy sweater pulls a gallon of milk with a red lid out of a cooler. They have a white cloth bag with large red dots over their shoulder. A couple of coolers down a person with dark hair wearing a black and white plaid shirt looks over their choices. The photo was taken in 2022. Now people are worried about bird flu fragments showing up in cow milk.

The U.S. Food and Drug Administration found bits of genetic material from bird flu viruses in cow milk in U.S. grocery stores, the agency announced April 23. Pasteurized milk, like that in this 2022 photo from a grocery store in Monterey Park, Calif., is probably safe to drink.

Photo by Frederic J. Brown/AFP via Getty Images

News that bird flu has been spreading between cows for months and that fragments of the virus are even showing up in milk on U.S. grocery store shelves have fueled new worries about the risk the virus poses to people. Among the questions: Is the virus, known as highly pathogenic avian influenza H5N1, adapting to better infect mammals? And can people get bird flu from drinking that milk?

Science News went to the experts to find answers to those questions. The short answer is that, thanks to milk pasteurization and the way bird flu viruses spread, the risk to people remains low. Here’s a deeper dive into what you should know.

Are there infectious bird flu viruses in cow milk?

Probably not in pasteurized milk.

On April 23, the U.S. Food and Drug Administration announced that fragments of the bird flu’s genetic material had turned up in grocery store milk. Initial results indicate that about 1 in 5 samples contain bits of the virus’s RNA, the agency announced April 25. Samples taken from areas where infected dairy cows have been found were more likely to test positive for the bird flu virus than those with no infected herds.

That doesn’t mean that whole infectious viruses are present, says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis.

“At first blush, it would surely seem as if it should be a great concern in terms of human health. However, the good news is that while the virus is in the milk … the pasteurization process actually is very effective in killing those virus particles so that in fact, we don’t have to be concerned about ‘Are we ingesting infective material?’”

Pasteurization raises milk to a high enough temperature to kill bacteria and viruses. Milk has always had bits of dead bacteria such as E. coli and Listeria that aren’t filtered out, Osterholm says. So it’s not surprising to find genetic remains of the flu virus, he says, “but that by itself does not at all suggest a public health concern.”

H5N1 is an envelope virus. And envelope viruses — ones that wrap themselves in a blanket borrowed from a host cell’s membrane — are “just a little bit wimpier than non-envelope viruses and a little bit easier to inactivate,” says Meghan Davis, an environmental epidemiologist at the Johns Hopkins Bloomberg School of Public Health. “That means that there’s some reassurance that pasteurization ought to be working.”  

Because H5N1 has only recently been found in cattle, no studies have directly tested milk pasteurization’s ability to kill the virus, the FDA said in a statement April 23. But studies have shown that egg pasteurization, which is done at lower temperatures than milk pasteurization, inactivates the virus.

For those reasons, government officials stress that pasteurized milk is safe to drink. But the FDA recommends that people don’t drink raw milk, which has not been pasteurized.

Davis says there are many reasons to avoid raw milk and products made from it. “We know that raw milk [can contain] other infectious diseases and there have been outbreaks linked to raw milk consumption. So categorically I don’t recommend it.”

Some goats have also been infected with H5N1, so Davis suggests avoiding raw goat and sheep milk products, too.

Could people get infected with bird flu through eating or drinking?

Decades of evidence suggest that’s not likely, Osterholm says.“We have no evidence that humans have become infected from influenza A virus via ingestion.”

Some scavenger mammals have become infected with H5N1 from eating dead birds. But to get into cells, influenza viruses need to grab onto receptors, cell surface proteins studded with certain sugars. In humans, those sugars are different than the versions in scavengers. People carry the entry portals in their upper respiratory tract and the eyes. The one person in the United States who recently caught bird flu worked with cows at a farm in Texas and was diagnosed with conjunctivitis, an eye infection.

It might even be difficult for people to catch the bird flu from infected cows, Osterholm says. “If you look at the experience we’ve had in the past, even with all of the human contact that occurred with infected flocks [of] turkeys and chickens over the course of the past several years, we’ve just seen an absence of infection in humans.” Both the U.S. Centers for Disease Control and Prevention and World Health Organization have designated the virus as low risk for humans.

Where in the United States is bird flu spreading now?

The U.S. Department of Agriculture first announced finding a strain of bird flu in dairy cattle in Kansas and Texas on March 25 (SN: 4/3/24). Since then, the virus has been found in dairy cows in eight states. The same version of the virus infecting cows — called clade — has also been found in poultry in five states, the USDA said in a federal order. That order announced mandatory testing for all dairy cows before they can be moved to another state, and also requires tracing the previous movements of cows from infected herds. The order will go into effect on April 29.

H5N1 may have jumped into cattle only once and then spread from cow-to-cow, evolutionary biologist Michael Worobey of the University of Arizona in Tucson posted on X (formerly Twitter) after he and colleagues analyzed 239 bird flu viruses from cattle and other species. The jump may have happened in late December 2023, months earlier than thought.

These findings are still preliminary and haven’t been independently verified by other scientists, Davis cautions. But they do suggest that bird flu in cattle is “probably more widespread than what we’ve identified based on the reported cases on the USDA dashboard. What we’re looking at is probably circulation, potentially cow-to-cow, but … we don’t yet know the extent.”

“At the moment we have both concern and reassurance,” she says. Despite the low risk, “we have the concern that this particular H5N1 has had staying power and has been around globally since 2020. It has been in the U.S. since 2022.” The virus continues to cause major outbreaks in birds and has jumped into many species of mammals. That pattern is different from what was seen with earlier iterations of H5N1 avian influenza. “It would flash up, we’d have cases and then it would die off by the next year. But that’s not what we’re seeing here.”

Do cows get sick with bird flu?

Yes, but cases tend to be fairly mild. H5N1 infections in cows can cause a drop in appetite and milk production, abnormal feces, tiredness, fever and other symptoms.

Davis, a former dairy veterinarian, says that vets often get calls that cows are “off feed,” meaning the animals aren’t eating well or giving as much milk as usual. Those symptoms are not specific to any particular disease, so cases of H5N1 influenza may easily have been overlooked.

Some cows may not have any symptoms. Lung tissue taken from one cow with no symptoms tested positive for the virus, the USDA announced April 24.

The virus has also been found in cats on dairy farms in Texas and in Poland, South Korea and France. At least one cat died, according to the World Organization for Animal Health.

For poultry such as chickens and turkeys, the virus can be deadly, destroying flocks in a matter of days.

Could H5N1 adapt to infect people more easily?

Possibly, Osterholm says. “In our business, we almost sleep with one eye open, just because on any one given day there could be a change in the virus from mutation or reassortment.”

Reassortment happens when flu strains swap part of their genetic instructions. Reassortment generated the H1N1 flu strain that caused a pandemic in 2009. In a pig, bits from the 1918 pandemic flu strain combined with pieces from a virus that was already a mish-mash of a swine flu virus and an avian flu virus. The resulting virus eventually infected people (SN: 5/22/09; SN: 2/12/10). The current H5N1 resulted from the reassortment of avian influenza viruses from poultry and wild birds.

“What we are concerned about is adaptation of the virus to better suit some of the receptors that humans might have, which is why any transmission out of bird populations into any mammal is initial cause for concern,” Davis says. People working on dairy farms, those in the dairy production chain and anyone else who may come into contact with raw milk, cows or poultry should wear protective equipment to avoid possible infection. She also advocates for testing farm workers, their families and others in their communities.

Both Davis and Osterholm are concerned about the virus infecting other food animals.

“The challenge that I see right now on U.S. farms is a virus getting into hogs,” Osterholm says. Pigs carry receptors similar to the ones found in both humans and birds, making swine a hog-heaven for bird flus that have potential to become a pandemic. Meanwhile, Davis says, “if this virus can infect both cows and goats, sheep are on my suspicion list for another potential species of concern.”

H5N1 avian influenza viruses have been found in birds since 1996 and have infected more than 880 people globally since 1997. In the United States, a poultry worker tested positive for H5N1 in 2022. The dairy farmworker with conjunctivitis was only the second person in the United States with a documented bird flu infection.

So far, H5N1 has not developed the ability to spread easily from person-to-person, which would make it a pandemic-potential virus. “Lightning could strike tomorrow,” Osterholm says, “but at least based on the track record so far, there’s been very little evidence to support that this is going to infect humans [and] then be transmitted by people to other people.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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