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Q&A on H5N1 Bird Flu


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On March 25, the U.S. Department of Agriculture announced that avian influenza had been detected in American dairy cattle. On April 23, the U.S. Food and Drug Administration reported that the agency had detected genetic material from the bird flu virus — a form of H5N1 — in the commercial milk supply.

Pasteurization has been effective at inactivating the H5N1 bird flu virus, and the viral material found in the milk has not proven to be infectious. According to the FDA, “these results reaffirm our assessment that the commercial milk supply is safe.”

However, widespread appearance of the viral genetic material in grocery store milk indicates that the virus is likely more prevalent in cows than cow testing data indicate. The virus has been detected in dairy milking cattle in 102 herds in 12 states, according to the USDA, and has been spreading among cows, both within herds and between herds as cows are transported.

Since the beginning of the cow outbreak, only one person in the U.S., a dairy worker in Texas, has tested positive for H5N1 bird flu. The virus, found in wild birds around the world, has not shown the ability to spread among humans in a sustained way. 

Update, May 31: Two additional bird flu cases in dairy workers, both in Michigan, were reported on May 22 and May 30.

The World Health Organization currently evaluates the overall public health risk from H5N1 bird flu as low, adding that the risk is low to moderate for people exposed to “infected birds or animals or contaminated environments.” The Centers for Disease Control and Prevention also currently rates the risk to public health as low.

Still, the situation with cows has renewed concerns among infectious disease experts that H5N1 bird flu — long regarded as a threat to humans — has the potential to cause a pandemic. 

“This is a virus that we are concerned that could cause human infection or could lead to an epidemic-type outbreak,” Dr. Jonathan Runstadler, chair of the department of infectious disease and global health at Tufts University’s Cummings School of Veterinary Medicine, told us. “The fact that it’s getting into an agricultural animal and seems to be spreading, and that has potential implications for contamination of the food supply — all of those are reasons to be concerned about influenza and the possibility of spillover of influenza into humans.”

But “right now really it’s more of an animal health issue,” Dr. Andrew Bowman, a veterinary epidemiologist at Ohio State University’s College of Veterinary Medicine, told us. “Certainly, keeping an eye towards public health, but with only one reported case in humans, and there’s been a growing number of animal detections and we haven’t seen the uptick in human detections, I think that’s probably a good sign for public health.”

Below, we’ve assembled answers to some basic questions about H5N1 bird flu.

How Did H5N1 Bird Flu Spread to Cows?

The H5N1 bird flu virus likely passed to dairy cows from wild birds — perhaps via contaminated food or water — and subsequently began to spread between cows. USDA analyses, described in a May 1 preprint, indicate there was just one introduction of the virus to the cows. 

The virus spreading in cows belongs to a group of H5N1 bird flu viruses that has been causing substantial disease and death in wild birds and domestic poultry since 2020.

This group of H5N1 viruses was first found in the U.S. in wild birds sampled in late 2021, and it caused the first outbreak in domestic poultry the following month. The virus spread to North America via wild birds migrating from Europe. Since then, nearly 91 million poultry in the U.S. have been affected, due to repeated introductions from wild birds. There was one human case linked to poultry in 2022, and the person recovered.

The virus also has spread from birds to mammals various times, Bowman said, but has not become established in these animals. In the U.S., these have included at least 20 species, such as seals and a dolphin, as well as foxes, raccoons and other wild animals that would be expected to eat dead birds. 

The difference now is that it “seems to be maintained amongst the cattle, which is a first from what we’ve seen in mammalian spillovers,” Bowman said. “The rest of them seem to have been dead-end hosts.”

When cows get sick with H5N1 bird flu, their milk production and appetite may decline; the milk may become thicker; and the cows may become lethargic and feverish, among other possible symptoms. Unlike birds, for which the virus can be widely lethal, the cows appear to recover from their illness. There is also evidence that some cows can carry the virus without having any symptoms

H5N1 bird flu has been detected in cats that were fed raw milk from infected cows. And cows appear to have spread the virus to poultry, wild birds and a raccoon.

How Is the Virus Transmitted Between Cows?

The movement of cattle from herd to herd has been linked to the spread of the virus. However, it is unclear exactly how H5N1 bird flu is spreading between dairy cows, Bowman and Runstadler both said.

The learning curve has been complicated by the fact that cow influenza isn’t exactly a research specialty. “Why cows?” Bowman said. “Because if you’d asked probably any flu expert seven weeks ago, bovine was not a primary host of influenza.”

Photo by piyaphunjun / stock.adobe.com

The virus has been primarily found in milk and mammary tissue, according to the USDA, and milk is likely implicated in its spread. The virus has not been found in significant amounts in respiratory samples. In a presentation at an event hosted by the Association of State and Territorial Health Officials on April 25, Dr. Rosemary Sifford of the USDA said that no significant viral shedding had been found from the gastrointestinal tracts of cattle, based on limited samples.

Bowman called the high levels of virus in the cows’ udders unprecedented. “As we think about influenza, for the most part we think about it as an upper respiratory pathogen in most mammalian species,” he said. However, he added that H5N1 bird flu has been more of a systemic disease than other flu types.

One theory on transmission is that contaminated milking or other dairy equipment is spreading the virus from cow to cow. Or perhaps the virus is getting aerosolized in milk and spreading between adjacent cows, or getting into the animals’ feed, Runstadler said. “There’s a lot of different possibilities in a farm situation for transmission to occur through different routes,” he said.

How Widespread Is the Virus in Cows?

It is also uncertain when the virus spread to cows and how widespread it is, Bowman and Runstadler said.

There have been reports of symptoms in dairy cows dating to late January. USDA scientists estimate that the virus spread in cows for about four months before being detected in March.

Bowman said that the lack of a precedent for significant influenza in cattle may have contributed to the illness flying under the radar at first. “As a veterinarian, if I had a cow with illness, I would have never requested an influenza test,” he said.

Sifford said during her April 25 presentation that USDA has flu surveillance systems set up in domestic pigs and poultry, as well as feral swine. But similar systems don’t exist in cows.

“This virus seems to have spilled over into dairy cattle from birds, and one of the potential reasons why we didn’t notice it until now or haven’t noticed it is that there isn’t enough public funding that’s put into surveillance for potential zoonotic disease like this,” Runstadler said, referring to the potential for animals to spread a virus to humans.

As of June 13, the USDA has reported cases in 102 herds in 12 states. But when the FDA sampled commercial milk being sold in stores, it found that 1 in 5 samples contained genetic material from H5N1. Some researchers, including Bowman, have also found genetic material from the virus in milk they bought in retail settings across various states.

Farmers are supposed to discard milk from sick cows, but the findings of viral genetic material in the milk supply indicate that this isn’t happening in all cases, perhaps because some cows are asymptomatic.

It’s hard to tell based on commercial milk testing data exactly how many cows or even farms are affected, Bowman said. However, “it gives us the indication that it’s more widespread in the cattle herd than the reported cases so far,” he said.

As of April 29, the USDA ordered that state veterinarians and laboratories report any positive test for influenza A — which would include H5N1 cases — in livestock, and that farmers get some dairy cows tested if they are going to be transported across state lines. This should likely increase testing in dairy cattle, which is otherwise left up to the discretion of the farmers.

Is It Safe to Drink Milk?

The vast majority of milk products sold in the U.S. and all milk sold via interstate commerce is pasteurized. The available evidence shows that this family of heat treatments inactivates any H5N1 that is present.

Update, June 18: A June 14 study published in the New England Journal of Medicine found that if researchers spiked milk with large amounts of H5N1, small amounts remained after the milk underwent one standard method of pasteurization. A press release accompanying the paper said the authors “stress, however, that their findings reflect experimental conditions in a laboratory setting and are not identical to large-scale industrial pasteurization processes for raw milk.”

“At this point every indication is that milk is safe to drink,” Bowman said. “We’re just picking up viral genetic material. We’re not picking up viable virus.”

The FDA strongly recommends against consuming raw milk, which isn’t pasteurized, under ordinary circumstances and has affirmed this recommendation. Researchers told STAT that this would be a particularly bad time to drink raw milk, citing the high levels of virus found in udders of infected cows, and the high levels of inactivated viral material found in commercial pasteurized milk. The FDA bans interstate raw milk sales and reports that “20 states explicitly prohibit intrastate raw milk sales in some form and 30 allow it.”

The FDA is looking at the effects of pasteurization on H5N1 using various methods and in various dairy products, Dr. Donald Prater of the FDA said during the April 25 presentation. FDA testing has since shown, for instance, that viral fragments in cottage cheese and sour cream are inactive. Prater also said that the FDA is evaluating H5N1 risk in aged cheese made with raw milk, which can be sold in interstate commerce.

Is the Virus Spreading in Beef Cattle?

So far, there’s no evidence the virus has affected beef cattle, according to the USDA. In her April 25 presentation, USDA’s Sifford said the virus likely originally spilled over from wild birds into dairy cattle due to “the management of dairy cattle being in much more confined spaces.” The access to feed and water in a confined space may have attracted wild birds.

Therefore, when looking to see whether the virus might spread into beef cattle, it would make sense to look at a more confined setting where the birds could access food and water, such a feedlot, she said.

The USDA reported no evidence of H5N1 in 30 samples of ground beef bought in retail settings in areas where H5N1 has been reported in dairy cows. USDA experiments also indicated that cooking ground beef patties to 160 F — the recommended temperature — inactivated the virus. In medium burgers (145 F), the virus was also inactivated, according to the USDA, and in rare burgers (120 F), the virus was “substantially inactivated.”

While most beef comes from dedicated beef cattle, a small portion comes from dairy cattle. USDA testing of slaughtered dairy cows found genetic material from H5N1 bird flu in 1 out of 109 cow muscle samples. These were cows whose meat had been condemned and was not destined for the food supply. The cow that tested positive had shown signs of sickness during standard inspection after death, according to the USDA, leading USDA personnel to prevent “the animal from entering the food supply.”

“USDA is confident that the meat supply is safe,” the agency says on its website.

What Is the Risk of H5N1 Bird Flu Spreading to Humans?

As we have written, WHO and the CDC say the current risk to public health is low. The virus would need to undergo various changes to easily infect and replicate in humans, and to spread between them. But the circulation of H5N1 bird flu in dairy cows raises a few concerns.

One is that the cows put a new population of workers and animals in contact with H5N1 bird flu. Flu viruses trade sequences with each other, in a process called reassortment, and they also mutate over time. Contact with humans and livestock, such as pigs, may give the virus more chances to mix with other flu viruses or develop mutations that would allow it to better spread between humans.

“It’s one thing to have a virus circulating out in wild birds that might be a risk,” Runstadler said. “It’s another thing to have it in a domestic agricultural animal population if it might be a risk.”

“Not only is this in a new host species, but then this is a different set of farm workers,” Bowman said, explaining that pig and poultry farmers are well versed in flu protection measures but that occupational health recommendations for dairy workers have not historically included such measures. 

Given that it’s uncertain exactly how the cows are transmitting H5N1, it’s unclear what precautions dairy workers need to take, Bowman said. But the CDC has recommended new procedures for people working in the cattle industry, particularly those exposed to infected cows.

Another concern is that H5N1 bird flu may become better adapted to mammals as it circulates in cows, which could in turn help the virus gain a foothold in people or other mammals. Researchers are monitoring H5N1 from cows to better understand if it has developed mutations “that would suggest that it’s better adapted to causing disease in a mammalian host,” Runstadler said.

Vivien Dugan, director of the influenza division at the CDC, told STAT that “we’ve not seen anything that would be concerning to us for mammalian adaptation, at this point.”

So far, there aren’t any signs that the virus is spreading widely among humans. The CDC has flu surveillance systems that detect influenza A, the general family of flu that H5N1 bird flu is a part of. The rate of influenza A in health care settings is decreasing as flu season ends. CDC also does surveillance for unusual subtypes of flu, and routine testing for flu should also detect if an unusual type of flu is circulating.

However, it is difficult to know how many other cases there are beyond just the single one detected in the Texas dairy worker, Bowman said. “I think we generally accept the idea that there are more cases than reported cases, always,” he said.

There are anecdotes of sick dairy workers who did not get testing. There are multiple known barriers that could keep workers from getting testing, such as low wages and poor benefits, undocumented status, or speaking a language other than English as a first language. 

How Dangerous Is H5N1 for Humans?

Bowman emphasized that H5N1 comprises a broad group of viruses that has changed over time. Looking at people who have gotten H5N1 bird flu since 2003, mainly via animal exposures, the virus has a case fatality rate of more than 50%, representing 463 deaths out of 888 known cases. But this doesn’t necessarily apply to the current situation.

“The H5N1 viruses that were historically responsible for that 50% case fatality rate — this is a different version,” Bowman said. 

Thus far, the one American known to be infected by cows only suffered from pinkeye and recovered. The other American who got H5N1 bird flu since it began spreading among wild birds in the U.S. in late 2021 also had mild disease and recovered. 

Update, May 31: A second dairy worker who tested positive for bird flu also presented with an eye infection and recovered. A third dairy worker with bird flu reported upper respiratory tract symptoms, as well as eye-related symptoms.

Case fatality rates simply show what percentage of people known to be infected died. They don’t show the percentage of the total number of infected people who die. People who are asymptomatic or only mildly ill may not get testing.

Finally, it’s unknown what other changes H5N1 might undergo if it did develop the ability to spread well in humans. It could become more lethal, or it could become less lethal.

If H5N1 did begin spreading between humans, some treatments and vaccines would likely be available. The CDC has confirmed that the version of H5N1 bird flu that infected the Texas dairy worker should be susceptible to available antiviral flu medications and recommends treating people who are exposed or infected with oseltamivir, otherwise known as Tamiflu.

The U.S. has a stockpile of vaccines previously developed to protect against H5 influenza viruses. The CDC says these are expected to provide some protection against the version of the virus currently spreading in cattle, should they become necessary. However, there would likely be challenges in scaling up vaccine production to vaccinate people around the world. 

People also may have some immunity to part of the virus from the H1N1 swine flu pandemic virus, as well as some vaccines, but the degree to which this would be helpful is unclear.

What Can I Do to Reduce Risks?

As we’ve said, the CDC recommends specific precautions for people working with livestock, including people with backyard birds, such as chickens. These include wearing appropriate gear when dealing with potentially infected animals or materials.

The CDC also recommends a few general precautions. These include keeping one’s distance from wild birds and avoiding contact with substances like bird poop that could be contaminated. People should also avoid contact with sick or dead animals, whether domestic or wild, and look into policies for reporting dead birds in their area. Researchers suggest keeping pets away from animals that could be infected, including in urban areas.

The CDC also recommends following normal food safety practices, such as properly cooking poultry, eggs and beef, and avoiding raw milk and raw milk products.

Finally, the agency says that it’s important to get annual seasonal flu shots. If a person is infected with an ordinary human flu virus and becomes infected with H5N1 at the same time, the viruses could mix together and undergo reassortment, potentially leading to a virus better positioned to replicate in people.

Update, June 18: The virus has been detected in 102 herds in 12 states as of June 13, according to the USDA. We have updated this article accordingly.

Update, May 31: This story has been edited to reflect new data from USDA experiments on the effectiveness of cooking to inactivate H5N1 in ground beef, as well as a new USDA finding of H5N1 genetic material in the tissue of a condemned, slaughtered dairy cow.


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