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Can Humans Get Chronic Wasting Disease From Deer?

Chronic wasting disease has been spreading among deer in the United States, which has raised concerns that the fatal neurological illness might make the leap to people. But a recent study suggests that the disease has a tough path to take to get into humans.

The culprit behind chronic wasting disease, or CWD, isn't a virus or bacterium but a misfolded brain protein called a prion. A new study using miniature, lab-grown organs called organoids supports previous work, showing that CWD prions don't infect human brain tissue.

Brain organoids exposed to high doses of prions from white-tailed deer, mule deer and elk remained infection-free for the duration of the study, or 180 days, researchers report in the June 2024 Emerging Infectious Diseases. However, organoids exposed to human prions that cause a related condition, Creutzfeldt-Jakob disease, quickly became infected. The finding suggests that a substantial species barrier prevents CWD from making the jump from deer to humans.

"This was a model that could really help tell us … whether or not it was a real risk," says Bradley Groveman, a biologist at the National Institutes of Health's Rocky Mountain Laboratories in Hamilton, Mont.

In a new study exploring the risk chronic wasting disease poses to people, brain organoids — like those shown — were exposed to high doses of prions from white-tailed deer, mule deer and elk. None of the organoids were infected after 180 days, the length of the study. NIAID

But brain organoids aren't a perfect mimic of the real thing and may lack features that would make them susceptible to infection. And new prion strains can appear, perhaps including some that might help deer prions lock onto healthy brain proteins in humans.

To keep an eye on the risk to people, researchers need to keep amassing evidence and testing new prion strains on organoids or in other experiments, says Cathryn Haigh, a cell biologist also at Rocky Mountain Laboratories. "I don't think we'll ever be able to turn around and say [human infection] is impossible."

A spreading disease

Deer with CWD are doomed. There is no cure.

The prions responsible for the disease — which affects deer, elk, moose and other cervids — spur a healthy brain protein called PrP to twist into an abnormal shape. These warped proteins clump together, killing brain cells and causing symptoms such as listlessness, stumbling, lack of fear of people and drastic weight loss. Animals typically start showing symptoms around 18 to 24 months after getting infected.

Concern about the danger that deer, elk and moose prions pose to people has been rising, in part because of the disease's persistent spread across North America. On April 5, CWD was reported for the first time in Indiana, and on May 6 officials in California announced the state's first cases in two wild deer. To date, the disease has been identified in wildlife in 34 U.S. States as well as parts of Canada, South Korea and northern Europe — a considerable increase since the first known case appeared in 1967 in a captive deer from Colorado.

What's more, a higher proportion of deer are infected with the disease, says Debbie McKenzie, a prion biologist at the University of Alberta in Edmonton, Canada. "For a long time … 1 in 100, 1 in 1,000 deer would be infected…. But we're now at a time where there's deer populations where CWD prevalence is greater than 75 percent."

A spreading disease

Since chronic wasting disease was first discovered in Colorado in 1967, the disease has spread widely across North America. Prior to 2000, just Colorado and Wyoming had known cases in wild deer and elk (dark gray). Today, the disease is found in 32 other states and 4 Canadian provinces (light gray). Captive animals are also infected; some facilities have current outbreaks (red circles) and others have culled infected populations (yellow circles).

Distribution of chronic wasting disease in North America USGSUSGS

If some hunters aren't testing animals, that increases the chances infected deer meat will make it to someone's plate. (In the United States, requirements and recommendations for hunters vary by state.) And proteins aren't affected by cooking the way bacteria or viruses are, so even cooked meat could pose a risk.

Lessons from previous cases

An outbreak of mad cow disease — another prion malady — after people consumed meat from infected cattle in the 1980s and 1990s helped put a spotlight on chronic wasting disease, Haigh says. While the deer disease was discovered before mad cow, back then people largely weren't worried about the risk to humans.

But the realization that a bovine prion could infect people and cause disease "put into the consciousness that this is a possibility," Haigh says. "And now we have another disease in an animal that we eat."

Past research has suggested that prions may have a hard time jumping between certain species (SN: 4/4/14). Work done in mice tweaked to carry the human version of PrP has shown that transmission to people may be a possibility, though less transmissible than prions from cattle. Studies in macaques, a common animal stand-in for people, however hint that deer-to-human transmission of chronic wasting disease is unlikely.

Yet the prospect of transmission of faulty prions from wildlife to humans remains a big question, Groveman says, particularly because venison is on the menu in North America. Vigilance, experts say, is key to catching any potential transmission early, though there have been some jump scares.   

News reports in April put the spotlight on a case report presented at the annual American Academy of Neurology meeting in Denver that described a fatal neurological disease in two hunters. The individuals had a history of consuming deer meat from a population known to have chronic wasting disease. But both likely died of a different prion ailment, Creutzfeldt-Jakob disease, which can arise sporadically, according to both the report and the U.S. Centers for Disease Control and Prevention.

"To date, there have been no reported cases of CWD infection in people," says epidemiologist Ryan Maddox of the CDC in Atlanta.  

Barriers to infection

While the mad cow prion sickened hundreds of people starting in 1994 — it can take a decade or more for people to show symptoms — the barrier for its transmission into people is also incredibly high, McKenzie says. Although millions of people are estimated to have been exposed to infected cattle during the outbreak in the 80s and 90s, as of 2022 there have been just 178 cases of Creutzfeldt-Jakob disease caused by infected meat in the United Kingdom, the outbreak's epicenter, and 55 in the rest of the world.

But the factors that perhaps made some people more susceptible to infection remain unclear. It's possible that infected individuals were exposed to an incredibly high dose, or the PrP protein in their brains had just the right shape to interact with cattle prions, McKenzie says. "There must have also been other things that contributed to the fact that they were susceptible."

Prion strains, which twist PrP proteins in different ways, can also come into play. How prions misfold is one factor that can prevent the proteins from infecting a new species, Groveman says. Figuring out the differences in how deer prions bend and curl could help expose which might attach to PrP from people or other animals.

But scientists have a harder time revealing what prions look like compared with normal PrP. That lack of knowledge makes it harder to explore the potential for prion diseases to make the jump from one species to another.

There are at least five prion strains that cause chronic wasting disease, for instance, each of which infects a different range of cervid species. "And we really don't understand enough about how strains are generated in an animal," McKenzie says. Lab studies suggest that some may infect other animal species, too, but researchers have never found CWD in animals other than cervids in the wild. 

So while the results in brain organoids suggest that at least some current strains don't pose a high threat to people, it's possible that new strains riskier to humans could emerge. "I still think that [chronic wasting disease] can jump into humans," McKenzie says. But in more favorable news, she thinks such cases would be rare. "I don't think it's going to be an epidemic."

Bird Flu Is Rampant In Animals. Humans Ignore It At Our Own Peril

Mark Naniot remembers 2022 as the summer from hell.As the co-founder of Wild Instincts animal rescue in Wisconsin's Northwoods, Naniot and his team spent the season sweating in gloves, gowns, smocks and masks and going through what felt like endless rounds of disinfection as they moved between the cages of the sick and injured animals they cared for.The precautions were necessary for a trio of infectious diseases occurring with some frequency in wild animals that summer — COVID-19 was still making life difficult, and a devastating contagion called chronic wasting disease was showing up in deer in the area.Then, there was H5N1 bird flu to contend with. "It's highly, highly transmissible," said Naniot, who has been involved in animal rescue for 35 years.Since it was first discovered in birds in 1996, H5N1 has shown itself to be a Swiss Army Knife of a virus, evolving the necessary tools to break into the cells of a growing list of species. So far, it has infected and killed millions of wild and farmed birds. It's also been found in at least 26 different kinds of mammals, including, most recently in the United States — cows, cats and house mice.The voraciousness of the virus added link prompted Dr. Jeremy Farrar, chief scientist of the World Health Organization in April to call it "a global zoonotic animal pandemic."Along the way, people have been a kind of collateral damage. Humans can be infected, but we aren't really the intended targets.That could all change quickly, however."Influenza actually makes mutations, in the sense of making errors copying its genome, at a higher rate than a coronavirus like SARS-CoV2," said Dr. Jesse Bloom, a computational biologist who focuses on influenza viruses at the Fred Hutch Cancer Center in Seattle.These errors don't always work in favor of the virus. Most of the time, viruses with errors won't work or be fit enough to continue to copy and survive. But every once in a while, a random error can result in a change to the virus that give it an advantage in its environment, and that version of the virus will continue to spread and grow.If humans happen to be that environment, and H5N1 changes at the right place at the right time, suddenly the animal pandemic could become a major problem for people, too.Naniot had seen wild birds come into Wild Instincts rescue with H5N1 — bald eagles, hawks and owls — but nothing had prepared him for the red fox kits.The baby foxes were brought in stumbling and uncoordinated, making him think they might have gotten into some kind of poison. Then the seizures started."They would have these severe, severe seizures," Naniot said. "Screaming very loud, whole-body tremors."The first seizures lasted for 20 to 30 seconds at a time. "And then it would get longer and longer and longer," he said.Naniot hadn't known his young patients could get bird flu. Further research clued him in to the fact that foxes had recently joined a growing list of species that could succumb, usually after eating the flesh of infected dead birds."The severity of the seizures is something I really hadn't seen before," Naniot said. "It's a very sad thing to see, the progression of the disease."Risks to humansThough H5N1 is known to have infected nearly 900 people in the past 30 years, these infections have been sporadic and usually self-limiting. The virus can still be deadly, however: More than 50% of people who are known to have been infected with H5N1 have died.Still, the virus isn't particularly good at infecting humans. Even when virus manages to get into a person and cause symptoms, it rarely gets passed to someone else."We call these dead-end infections," said Dr. Scott Weese, a veterinarian and expert in zoonotic infections, at the University of Guelph in Canada.The way a dead-end infection happens, Weese explains, is that a person is around a large amount of the virus, or their immune system is too weak to resist, and H5N1 gets in. But it is not a virus that's well-adapted to humans, so it never really builds up in respiratory secretions — the fluid that coats the nose, throat, and lungs — which would give it a way out through coughs, sneezes or even exhaled breath.There have been at least three of these have apparently dead-end infections in dairy workers in the U.S., who worked closely with infected milk cows. Two of the workers developed conjunctivitis, or eye infections. In one case, the worker reported getting splashed with raw milk in their eyes. A third developed respiratory symptoms after close contact with cows. All were successfully treated with an antiviral medication. None developed severe symptoms or infected others.Using a strain of H5N1 from the recent cattle outbreak, scientists recently confirmed that this version of the virus is unlikely to be transmitted through the air. In experiments with ferrets, which are considered the gold standard for studying how viruses transmit in people, researchers at the U.S. Centers for Disease Control and Prevention grew a sample of the same H5N1 virus taken from a farmworker with the flu in Texas to experimentally infect six of the animals. Then, three healthy ferrets were placed in the same enclosures with three of the sick animals. These animals could touch, nose and lick the sick animals, and all of them became ill.Next, the CDC tested airborne transmission by putting three healthy ferrets into an enclosure where they could breathe the same air as sick animals but couldn't touch them. Only one of those three animals became ill, suggesting that the virus carried by cattle in the current outbreak is not well adapted to respiratory spread, the CDC wrote in a news release on the study.So far, that seems to be what's happening in the real world, too. Though more than 80 dairy herds have tested positive across at least 12 states, the number of human infections has apparently been low, though there's been little testing to confirm that.These early ferret experiments are good news, the CDC noted, because it means the virus would need to change to become an infection spread person-to-person through the airborne droplets. The agency said it plans to repeat the tests.As COVID has shown, all of this could change in the the rub of an eye or a small cough. The more opportunity the virus has to spread, the more opportunity it has to change in ways that will help it pry its way into human cells."It's really important to understand everything we know today is a snapshot of today, and these viruses can change very quickly," said Dr. Rick Bright, an immunologist and former director of the U.S. Biomedical Advanced Research and Development Authority in an interview with CNN Chief Medical Correspondent Dr. Sanjay Gupta for the Chasing Life podcast."They can adapt, and they can spread very easily when they do change," said Bright, who is now CEO of Bright Global Health. Dr. Erin Sorrell, a virologist and a senior scholar at the Johns Hopkins Center for Health Security, says that while humans have been exposed to seasonal strains of the flu, and flu vaccines help build immunity H1 and H3 flu strains, H5N1 would look pretty different to our bodies."Our existing immunity to H3 and H1 is not necessarily going to protect us against exposure to an H5 virus," she said.The CDC's ferret study also had some sobering findings. In contrast to seasonal flu, which makes ferrets sick, but doesn't kill them, H5N1 killed all the ferrets that were infected."While the three cases of A(H5N1) in the United States have been mild, it is possible that there will be serious illnesses among people," the CDC wrote in its conclusions on the study.In the more than two dozen human infections with H5N1 virus worldwide since 2022, with the most recent iteration of the virus, there's been a wide spectrum of severity. Fourteen illnesses were severe or critical, seven were fatal, six were mild and eight didn't have any symptoms at all, according to the CDC.Dr. Seema Lakdawala, a microbiologist and immunologist at Emory University who specializes in the flu, thinks the difference in symptom severity may be due to previous exposure to seasonal viruses. Her experiments in ferrets suggest that our bodies wouldn't necessarily be totally defenseless. In her lab, ferrets with previous exposures to seasonal flu strains didn't get as sick when exposed to new flu viruses compared to those with no prior exposure to seasonal strains. She says she hasn't tested this with any of the strains involved in the cattle outbreak, however.So while we probably don't have any antibodies — the immune system's front-line soldiers — at the ready to fight off an H5 infection, there are memory cells in our tissues that might recognize parts of a new flu virus and respond.How much help we might get from past exposures to flu viruses is difficult to predict, however, which is why vaccination would still be important to tune up our immunity.Plans to stop the virus from spreadingThe U.S. Has vaccines against H5 viruses in its Strategic National Stockpile, and last month, government officials said 4.8 million doses are being "filled and finished" so they would be ready for use, though there's no plan to give them to anyone yet.Finland has already ordered 20,000 doses of a different H5 strain — H5N8 — which, will be used as soon as they're available to protect workers who might be vulnerable to the virus, such as scientists and those in direct contact with infected animals on mink farms, local officials told health and science news outlet, STAT News.For now, the CDC maintains its assessment that the risk to the general public from H5N1 is low, though people who work with infected animals have a higher risk and should wear protective clothing and take additional precautions to avoid getting sick. The Administration for Strategic Preparedness and Response, or ASPR, has made that protective equipment available to states for use on farms, and the USDA has made additional funding available to farms to support efforts to safeguard their livestock from disease.But so far, wearing this equipment is voluntary, and there are concerns that it might be difficult for farm workers to wear the full recommended kit, which includes coveralls, an apron, a mask, eye protection, a head covering, gloves and boots during the summer, which is again expected to break heat records.The government has also said it is working on the development of a rapid test for H5N1.Bright thinks severity of symptoms may depend on how much virus a person is exposed to when they are infected. Touching contaminated milk or the body of a dead bird and then rubbing your eyes or nose might deliver a smaller dose of the virus, and ultimately result in milder symptoms. Whereas ingesting large amount of virus — as some animals do when they scavenge for food or as humans in some countries do when consuming dishes made with duck blood — could lead to severe disease."The virus is able to infect a number of internal organs. So it doesn't just locate, say, in the lungs, as we would think most influenza viruses would," Bright said. It's also been found in "the brains and then the spleens, the intestines, and the heart and throughout the body of those animals."Dr. Richard Webby, who directs the WHO's Collaborating Centre on the Ecology of Influenza in Animals and Birds at St. Jude Children's Research Hospital, agrees."It's at the top of the list in terms of bad guy viruses," he said, noting that the virus is nerve-loving, or neurotropic. "So it goes to the brain and causes very, very severe disease."Infected animals often behave strangely or aggressively. Ducks waddle in circles, twisting their necks, writhing on the ground."I would hate to see it in humans," Webby said.So far, the virus hasn't made the changes that would enable it to become a fully human pathogen, said Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota. It's unclear whether it ever will."I've been a student of this virus. And I surely have been amazed at how it's changed over the course of the last 20-some years, but at the same time, you know, I'm looking for evidence that it is likely to become a virus infecting humans and then transmitted by humans to other humans. And we just haven't seen that yet," he added.Naniot at the Wisconsin animal rescue said they tried to save about seven infected fox kits in the summer of 2022, but all of them died.Other rescue organizations in their network had a few foxes infected with H5N1 that survived, but they ultimately went blind.While all the precautions they took to safely work with the animals were arduous, Naniot said he's grateful they were effective. They never spread the virus to any of the other animals in the facility — including themselves."Unfortunately, it's kind of like when COVID went through, you know, it first started someplace," he said.Naniot says he hasn't encountered any infected animals since 2022, but he's watching the news closely in case any cow herds become infected in Wisconsin, knowing that he could easily see H5N1 again."It spread kind of like wildfire, and it's a highly, highly contagious disease."

Mark Naniot remembers 2022 as the summer from hell.

As the co-founder of Wild Instincts animal rescue in Wisconsin's Northwoods, Naniot and his team spent the season sweating in gloves, gowns, smocks and masks and going through what felt like endless rounds of disinfection as they moved between the cages of the sick and injured animals they cared for.

The precautions were necessary for a trio of infectious diseases occurring with some frequency in wild animals that summer — COVID-19 was still making life difficult, and a devastating contagion called chronic wasting disease was showing up in deer in the area.

Then, there was H5N1 bird flu to contend with. "It's highly, highly transmissible," said Naniot, who has been involved in animal rescue for 35 years.

Since it was first discovered in birds in 1996, H5N1 has shown itself to be a Swiss Army Knife of a virus, evolving the necessary tools to break into the cells of a growing list of species. So far, it has infected and killed millions of wild and farmed birds. It's also been found in at least 26 different kinds of mammals, including, most recently in the United States — cows, cats and house mice.

The voraciousness of the virus added link prompted Dr. Jeremy Farrar, chief scientist of the World Health Organization in April to call it "a global zoonotic animal pandemic."

Along the way, people have been a kind of collateral damage. Humans can be infected, but we aren't really the intended targets.

That could all change quickly, however.

"Influenza actually makes mutations, in the sense of making errors copying its genome, at a higher rate than a coronavirus like SARS-CoV2," said Dr. Jesse Bloom, a computational biologist who focuses on influenza viruses at the Fred Hutch Cancer Center in Seattle.

These errors don't always work in favor of the virus. Most of the time, viruses with errors won't work or be fit enough to continue to copy and survive. But every once in a while, a random error can result in a change to the virus that give it an advantage in its environment, and that version of the virus will continue to spread and grow.

If humans happen to be that environment, and H5N1 changes at the right place at the right time, suddenly the animal pandemic could become a major problem for people, too.

Naniot had seen wild birds come into Wild Instincts rescue with H5N1 — bald eagles, hawks and owls — but nothing had prepared him for the red fox kits.

The baby foxes were brought in stumbling and uncoordinated, making him think they might have gotten into some kind of poison. Then the seizures started.

"They would have these severe, severe seizures," Naniot said. "Screaming very loud, whole-body tremors."

The first seizures lasted for 20 to 30 seconds at a time. "And then it would get longer and longer and longer," he said.

Naniot hadn't known his young patients could get bird flu. Further research clued him in to the fact that foxes had recently joined a growing list of species that could succumb, usually after eating the flesh of infected dead birds.

"The severity of the seizures is something I really hadn't seen before," Naniot said. "It's a very sad thing to see, the progression of the disease."

Risks to humans

Though H5N1 is known to have infected nearly 900 people in the past 30 years, these infections have been sporadic and usually self-limiting. The virus can still be deadly, however: More than 50% of people who are known to have been infected with H5N1 have died.

Still, the virus isn't particularly good at infecting humans. Even when virus manages to get into a person and cause symptoms, it rarely gets passed to someone else.

"We call these dead-end infections," said Dr. Scott Weese, a veterinarian and expert in zoonotic infections, at the University of Guelph in Canada.

The way a dead-end infection happens, Weese explains, is that a person is around a large amount of the virus, or their immune system is too weak to resist, and H5N1 gets in. But it is not a virus that's well-adapted to humans, so it never really builds up in respiratory secretions — the fluid that coats the nose, throat, and lungs — which would give it a way out through coughs, sneezes or even exhaled breath.

There have been at least three of these have apparently dead-end infections in dairy workers in the U.S., who worked closely with infected milk cows. Two of the workers developed conjunctivitis, or eye infections. In one case, the worker reported getting splashed with raw milk in their eyes. A third developed respiratory symptoms after close contact with cows. All were successfully treated with an antiviral medication. None developed severe symptoms or infected others.

Using a strain of H5N1 from the recent cattle outbreak, scientists recently confirmed that this version of the virus is unlikely to be transmitted through the air. In experiments with ferrets, which are considered the gold standard for studying how viruses transmit in people, researchers at the U.S. Centers for Disease Control and Prevention grew a sample of the same H5N1 virus taken from a farmworker with the flu in Texas to experimentally infect six of the animals. Then, three healthy ferrets were placed in the same enclosures with three of the sick animals. These animals could touch, nose and lick the sick animals, and all of them became ill.

Next, the CDC tested airborne transmission by putting three healthy ferrets into an enclosure where they could breathe the same air as sick animals but couldn't touch them. Only one of those three animals became ill, suggesting that the virus carried by cattle in the current outbreak is not well adapted to respiratory spread, the CDC wrote in a news release on the study.

So far, that seems to be what's happening in the real world, too. Though more than 80 dairy herds have tested positive across at least 12 states, the number of human infections has apparently been low, though there's been little testing to confirm that.

These early ferret experiments are good news, the CDC noted, because it means the virus would need to change to become an infection spread person-to-person through the airborne droplets. The agency said it plans to repeat the tests.

As COVID has shown, all of this could change in the the rub of an eye or a small cough. The more opportunity the virus has to spread, the more opportunity it has to change in ways that will help it pry its way into human cells.

"It's really important to understand everything we know today is a snapshot of today, and these viruses can change very quickly," said Dr. Rick Bright, an immunologist and former director of the U.S. Biomedical Advanced Research and Development Authority in an interview with CNN Chief Medical Correspondent Dr. Sanjay Gupta for the Chasing Life podcast.

"They can adapt, and they can spread very easily when they do change," said Bright, who is now CEO of Bright Global Health.

Dr. Erin Sorrell, a virologist and a senior scholar at the Johns Hopkins Center for Health Security, says that while humans have been exposed to seasonal strains of the flu, and flu vaccines help build immunity H1 and H3 flu strains, H5N1 would look pretty different to our bodies.

"Our existing immunity to H3 and H1 is not necessarily going to protect us against exposure to an H5 virus," she said.

The CDC's ferret study also had some sobering findings. In contrast to seasonal flu, which makes ferrets sick, but doesn't kill them, H5N1 killed all the ferrets that were infected.

"While the three cases of A(H5N1) in the United States have been mild, it is possible that there will be serious illnesses among people," the CDC wrote in its conclusions on the study.

In the more than two dozen human infections with H5N1 virus worldwide since 2022, with the most recent iteration of the virus, there's been a wide spectrum of severity. Fourteen illnesses were severe or critical, seven were fatal, six were mild and eight didn't have any symptoms at all, according to the CDC.

Dr. Seema Lakdawala, a microbiologist and immunologist at Emory University who specializes in the flu, thinks the difference in symptom severity may be due to previous exposure to seasonal viruses. Her experiments in ferrets suggest that our bodies wouldn't necessarily be totally defenseless. In her lab, ferrets with previous exposures to seasonal flu strains didn't get as sick when exposed to new flu viruses compared to those with no prior exposure to seasonal strains. She says she hasn't tested this with any of the strains involved in the cattle outbreak, however.

So while we probably don't have any antibodies — the immune system's front-line soldiers — at the ready to fight off an H5 infection, there are memory cells in our tissues that might recognize parts of a new flu virus and respond.

How much help we might get from past exposures to flu viruses is difficult to predict, however, which is why vaccination would still be important to tune up our immunity.

Plans to stop the virus from spreading

The U.S. Has vaccines against H5 viruses in its Strategic National Stockpile, and last month, government officials said 4.8 million doses are being "filled and finished" so they would be ready for use, though there's no plan to give them to anyone yet.

Finland has already ordered 20,000 doses of a different H5 strain — H5N8 — which, will be used as soon as they're available to protect workers who might be vulnerable to the virus, such as scientists and those in direct contact with infected animals on mink farms, local officials told health and science news outlet, STAT News.

For now, the CDC maintains its assessment that the risk to the general public from H5N1 is low, though people who work with infected animals have a higher risk and should wear protective clothing and take additional precautions to avoid getting sick. The Administration for Strategic Preparedness and Response, or ASPR, has made that protective equipment available to states for use on farms, and the USDA has made additional funding available to farms to support efforts to safeguard their livestock from disease.

But so far, wearing this equipment is voluntary, and there are concerns that it might be difficult for farm workers to wear the full recommended kit, which includes coveralls, an apron, a mask, eye protection, a head covering, gloves and boots during the summer, which is again expected to break heat records.

The government has also said it is working on the development of a rapid test for H5N1.

Bright thinks severity of symptoms may depend on how much virus a person is exposed to when they are infected. Touching contaminated milk or the body of a dead bird and then rubbing your eyes or nose might deliver a smaller dose of the virus, and ultimately result in milder symptoms. Whereas ingesting large amount of virus — as some animals do when they scavenge for food or as humans in some countries do when consuming dishes made with duck blood — could lead to severe disease.

"The virus is able to infect a number of internal organs. So it doesn't just locate, say, in the lungs, as we would think most influenza viruses would," Bright said. It's also been found in "the brains and then the spleens, the intestines, and the heart and throughout the body of those animals."

Dr. Richard Webby, who directs the WHO's Collaborating Centre on the Ecology of Influenza in Animals and Birds at St. Jude Children's Research Hospital, agrees.

"It's at the top of the list in terms of bad guy viruses," he said, noting that the virus is nerve-loving, or neurotropic. "So it goes to the brain and causes very, very severe disease."

Infected animals often behave strangely or aggressively. Ducks waddle in circles, twisting their necks, writhing on the ground.

"I would hate to see it in humans," Webby said.

So far, the virus hasn't made the changes that would enable it to become a fully human pathogen, said Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota. It's unclear whether it ever will.

"I've been a student of this virus. And I surely have been amazed at how it's changed over the course of the last 20-some years, but at the same time, you know, I'm looking for evidence that it is likely to become a virus infecting humans and then transmitted by humans to other humans. And we just haven't seen that yet," he added.

Naniot at the Wisconsin animal rescue said they tried to save about seven infected fox kits in the summer of 2022, but all of them died.

Other rescue organizations in their network had a few foxes infected with H5N1 that survived, but they ultimately went blind.

While all the precautions they took to safely work with the animals were arduous, Naniot said he's grateful they were effective. They never spread the virus to any of the other animals in the facility — including themselves.

"Unfortunately, it's kind of like when COVID went through, you know, it first started someplace," he said.

Naniot says he hasn't encountered any infected animals since 2022, but he's watching the news closely in case any cow herds become infected in Wisconsin, knowing that he could easily see H5N1 again.

"It spread kind of like wildfire, and it's a highly, highly contagious disease."

Bird Flu Is Rampant In Animals. Humans Ignore It At Our Own Peril

Mark Naniot remembers 2022 as the summer from hell.As the co-founder of Wild Instincts animal rescue in Wisconsin's Northwoods, Naniot and his team spent the season sweating in gloves, gowns, smocks and masks and going through what felt like endless rounds of disinfection as they moved between the cages of the sick and injured animals they cared for.The precautions were necessary for a trio of infectious diseases occurring with some frequency in wild animals that summer — COVID-19 was still making life difficult, and a devastating contagion called chronic wasting disease was showing up in deer in the area.Then, there was H5N1 bird flu to contend with. "It's highly, highly transmissible," said Naniot, who has been involved in animal rescue for 35 years.Since it was first discovered in birds in 1996, H5N1 has shown itself to be a Swiss Army Knife of a virus, evolving the necessary tools to break into the cells of a growing list of species. So far, it has infected and killed millions of wild and farmed birds. It's also been found in at least 26 different kinds of mammals, including, most recently in the United States — cows, cats and house mice.The voraciousness of the virus added link prompted Dr. Jeremy Farrar, chief scientist of the World Health Organization in April to call it "a global zoonotic animal pandemic."Along the way, people have been a kind of collateral damage. Humans can be infected, but we aren't really the intended targets.That could all change quickly, however."Influenza actually makes mutations, in the sense of making errors copying its genome, at a higher rate than a coronavirus like SARS-CoV2," said Dr. Jesse Bloom, a computational biologist who focuses on influenza viruses at the Fred Hutch Cancer Center in Seattle.These errors don't always work in favor of the virus. Most of the time, viruses with errors won't work or be fit enough to continue to copy and survive. But every once in a while, a random error can result in a change to the virus that give it an advantage in its environment, and that version of the virus will continue to spread and grow.If humans happen to be that environment, and H5N1 changes at the right place at the right time, suddenly the animal pandemic could become a major problem for people, too.Naniot had seen wild birds come into Wild Instincts rescue with H5N1 — bald eagles, hawks and owls — but nothing had prepared him for the red fox kits.The baby foxes were brought in stumbling and uncoordinated, making him think they might have gotten into some kind of poison. Then the seizures started."They would have these severe, severe seizures," Naniot said. "Screaming very loud, whole-body tremors."The first seizures lasted for 20 to 30 seconds at a time. "And then it would get longer and longer and longer," he said.Naniot hadn't known his young patients could get bird flu. Further research clued him in to the fact that foxes had recently joined a growing list of species that could succumb, usually after eating the flesh of infected dead birds."The severity of the seizures is something I really hadn't seen before," Naniot said. "It's a very sad thing to see, the progression of the disease."Risks to humansThough H5N1 is known to have infected nearly 900 people in the past 30 years, these infections have been sporadic and usually self-limiting. The virus can still be deadly, however: More than 50% of people who are known to have been infected with H5N1 have died.Still, the virus isn't particularly good at infecting humans. Even when virus manages to get into a person and cause symptoms, it rarely gets passed to someone else."We call these dead-end infections," said Dr. Scott Weese, a veterinarian and expert in zoonotic infections, at the University of Guelph in Canada.The way a dead-end infection happens, Weese explains, is that a person is around a large amount of the virus, or their immune system is too weak to resist, and H5N1 gets in. But it is not a virus that's well-adapted to humans, so it never really builds up in respiratory secretions — the fluid that coats the nose, throat, and lungs — which would give it a way out through coughs, sneezes or even exhaled breath.There have been at least three of these have apparently dead-end infections in dairy workers in the U.S., who worked closely with infected milk cows. Two of the workers developed conjunctivitis, or eye infections. In one case, the worker reported getting splashed with raw milk in their eyes. A third developed respiratory symptoms after close contact with cows. All were successfully treated with an antiviral medication. None developed severe symptoms or infected others.Using a strain of H5N1 from the recent cattle outbreak, scientists recently confirmed that this version of the virus is unlikely to be transmitted through the air. In experiments with ferrets, which are considered the gold standard for studying how viruses transmit in people, researchers at the U.S. Centers for Disease Control and Prevention grew a sample of the same H5N1 virus taken from a farmworker with the flu in Texas to experimentally infect six of the animals. Then, three healthy ferrets were placed in the same enclosures with three of the sick animals. These animals could touch, nose and lick the sick animals, and all of them became ill.Next, the CDC tested airborne transmission by putting three healthy ferrets into an enclosure where they could breathe the same air as sick animals but couldn't touch them. Only one of those three animals became ill, suggesting that the virus carried by cattle in the current outbreak is not well adapted to respiratory spread, the CDC wrote in a news release on the study.So far, that seems to be what's happening in the real world, too. Though more than 80 dairy herds have tested positive across at least 12 states, the number of human infections has apparently been low, though there's been little testing to confirm that.These early ferret experiments are good news, the CDC noted, because it means the virus would need to change to become an infection spread person-to-person through the airborne droplets. The agency said it plans to repeat the tests.As COVID has shown, all of this could change in the the rub of an eye or a small cough. The more opportunity the virus has to spread, the more opportunity it has to change in ways that will help it pry its way into human cells."It's really important to understand everything we know today is a snapshot of today, and these viruses can change very quickly," said Dr. Rick Bright, an immunologist and former director of the U.S. Biomedical Advanced Research and Development Authority in an interview with CNN Chief Medical Correspondent Dr. Sanjay Gupta for the Chasing Life podcast."They can adapt, and they can spread very easily when they do change," said Bright, who is now CEO of Bright Global Health. Dr. Erin Sorrell, a virologist and a senior scholar at the Johns Hopkins Center for Health Security, says that while humans have been exposed to seasonal strains of the flu, and flu vaccines help build immunity H1 and H3 flu strains, H5N1 would look pretty different to our bodies."Our existing immunity to H3 and H1 is not necessarily going to protect us against exposure to an H5 virus," she said.The CDC's ferret study also had some sobering findings. In contrast to seasonal flu, which makes ferrets sick, but doesn't kill them, H5N1 killed all the ferrets that were infected."While the three cases of A(H5N1) in the United States have been mild, it is possible that there will be serious illnesses among people," the CDC wrote in its conclusions on the study.In the more than two dozen human infections with H5N1 virus worldwide since 2022, with the most recent iteration of the virus, there's been a wide spectrum of severity. Fourteen illnesses were severe or critical, seven were fatal, six were mild and eight didn't have any symptoms at all, according to the CDC.Dr. Seema Lakdawala, a microbiologist and immunologist at Emory University who specializes in the flu, thinks the difference in symptom severity may be due to previous exposure to seasonal viruses. Her experiments in ferrets suggest that our bodies wouldn't necessarily be totally defenseless. In her lab, ferrets with previous exposures to seasonal flu strains didn't get as sick when exposed to new flu viruses compared to those with no prior exposure to seasonal strains. She says she hasn't tested this with any of the strains involved in the cattle outbreak, however.So while we probably don't have any antibodies — the immune system's front-line soldiers — at the ready to fight off an H5 infection, there are memory cells in our tissues that might recognize parts of a new flu virus and respond.How much help we might get from past exposures to flu viruses is difficult to predict, however, which is why vaccination would still be important to tune up our immunity.Plans to stop the virus from spreadingThe U.S. Has vaccines against H5 viruses in its Strategic National Stockpile, and last month, government officials said 4.8 million doses are being "filled and finished" so they would be ready for use, though there's no plan to give them to anyone yet.Finland has already ordered 20,000 doses of a different H5 strain — H5N8 — which, will be used as soon as they're available to protect workers who might be vulnerable to the virus, such as scientists and those in direct contact with infected animals on mink farms, local officials told health and science news outlet, STAT News.For now, the CDC maintains its assessment that the risk to the general public from H5N1 is low, though people who work with infected animals have a higher risk and should wear protective clothing and take additional precautions to avoid getting sick. The Administration for Strategic Preparedness and Response, or ASPR, has made that protective equipment available to states for use on farms, and the USDA has made additional funding available to farms to support efforts to safeguard their livestock from disease.But so far, wearing this equipment is voluntary, and there are concerns that it might be difficult for farm workers to wear the full recommended kit, which includes coveralls, an apron, a mask, eye protection, a head covering, gloves and boots during the summer, which is again expected to break heat records.The government has also said it is working on the development of a rapid test for H5N1.Bright thinks severity of symptoms may depend on how much virus a person is exposed to when they are infected. Touching contaminated milk or the body of a dead bird and then rubbing your eyes or nose might deliver a smaller dose of the virus, and ultimately result in milder symptoms. Whereas ingesting large amount of virus — as some animals do when they scavenge for food or as humans in some countries do when consuming dishes made with duck blood — could lead to severe disease."The virus is able to infect a number of internal organs. So it doesn't just locate, say, in the lungs, as we would think most influenza viruses would," Bright said. It's also been found in "the brains and then the spleens, the intestines, and the heart and throughout the body of those animals."Dr. Richard Webby, who directs the WHO's Collaborating Centre on the Ecology of Influenza in Animals and Birds at St. Jude Children's Research Hospital, agrees."It's at the top of the list in terms of bad guy viruses," he said, noting that the virus is nerve-loving, or neurotropic. "So it goes to the brain and causes very, very severe disease."Infected animals often behave strangely or aggressively. Ducks waddle in circles, twisting their necks, writhing on the ground."I would hate to see it in humans," Webby said.So far, the virus hasn't made the changes that would enable it to become a fully human pathogen, said Dr. Michael Osterholm, who directs the Center for Infectious Disease Research and Policy at the University of Minnesota. It's unclear whether it ever will."I've been a student of this virus. And I surely have been amazed at how it's changed over the course of the last 20-some years, but at the same time, you know, I'm looking for evidence that it is likely to become a virus infecting humans and then transmitted by humans to other humans. And we just haven't seen that yet," he added.Naniot at the Wisconsin animal rescue said they tried to save about seven infected fox kits in the summer of 2022, but all of them died.Other rescue organizations in their network had a few foxes infected with H5N1 that survived, but they ultimately went blind.While all the precautions they took to safely work with the animals were arduous, Naniot said he's grateful they were effec