The disease devastating deer herds may also threaten human health

Scientists are exploring the origins of chronic wasting disease before it becomes truly catastrophic.


This story was published in collaboration with the Mountain West News Bureau, a collaboration between Wyoming Public Media, Boise State Public Radio in Idaho, KUER in Salt Lake City and KRCC and KUNC in Colorado.

Heather Swanson and Ryan Prioreschi stand in knee-high golden grass on a slope outside Boulder, Colorado, where the Rocky Mountains start slumping into the plains, at the epicenter of a now-international animal epidemic. The two ecologists, who monitor wildlife for the city, have their binoculars out, and they’re staring right at the problem.  

A fawn runs circles around the rest of the herd, with the boing of a muscular slinky toy.

He’s wired,” says Swanson, laughing. “He’s doing laps.”

A few other mule deer rear up on their hind legs and kick each other. Still others just hang out in the shade. It’s a beautiful spring morning and the animals look sleek and healthy. But all is not what it seems. This herd is harboring an infection — chronic wasting disease, or CWD.

Heather Swanson spots the mule deer herd outside Boulder, Colorado, that is known to be infected with chronic wasting disease.
Rae Ellen Bichell/Mountain West News Bureau

Scientists have called this neurodegenerative disease, which attacks deer, elk and moose, a “nightmare” and a “state of emergency.” Lately, the media’s been calling it “zombie deer disease.” Lawmakers are calling it a “crisis” and currently considering at least three bills at the national level to combat it. Researchers, resource managers and others worry it could hurt hunting, alter the landscape, or even jump across species to infect people. 

“That is buck number 46,” says Prioreschi, pointing to a deer. “He is positive.” Doe number 22, now lying in the grass, is also positive for chronic wasting disease. “Doesn’t show any symptoms,” he says. “She looks perfectly fine.”

But the mountain lions know that something is wrong. A number of years ago, Swanson and her colleagues studied which deer mountain lions prefer to attack. “The mountain lions were definitely preferentially selecting deer that had chronic wasting disease over those that were negative,” she says. “And for most of the ones that they had killed, we had not detected any chronic wasting disease symptoms yet. So certainly the lions were able to key in on far more subtle cues than we were.”

“To our eyes, they look fairly healthy, and within a number of weeks they reach that point — and then they're gone.”

Unlike us, the lions sense that while a deer might look vigorous and alert, it may actually be a ticking time bomb. That’s one of the many weird things about this disease. It isn’t like viral or bacterial illnesses. The infection can sit in a herd for years, crawling from animal to animal, before people notice anything is wrong.

Then, things can go downhill fast. “Through time (it) degrades, essentially, their brain tissue,” says Swanson. In just a few weeks, buck 46 or doe 22 could start to droop and drool, as an infection gnaws holes into the animal’s brain. “That seems to happen pretty rapidly,” she says. “To our eyes, they look fairly healthy, and within a number of weeks they reach that point — and then they’re gone.”

Elk at the National Elk Refuge in Wyoming. Conservation groups have filed suit against the U.S. Fish and Wildlife Service to phase out feeding the elk, saying the feedlots are breeding grounds for diseases including chronic wasting disease.
Tim Pratt, biology technician/USFWS

IN EARLY FEBRUARY, Wyoming Senator John Barrasso got in front of a congressional hearing to introduce one of the bills aimed at addressing chronic wasting disease. “It's highly contagious and always fatal,” said Barrasso. “Unchecked, this disease could truly be catastrophic for wildlife and for local economies.”

Barrasso’s bipartisan bill, the Chronic Wasting Disease Transmission in Cervidae Study Act, was cosponsored by senators from across the country, including Idaho, Wyoming and Colorado. It would give federal dollars to the National Academies of Sciences to identify major gaps in scientific understanding of CWD and to better identify how to keep the disease from spreading further among animals, including between Canada and the U.S.

On the same day that Barrasso addressed his colleagues in Congress, epidemiologist Michael Osterholm spoke to state lawmakers in Minnesota. “This is kind of a worst-case nightmare,” said Osterholm. It’s a nightmare that’s hard to explain. Chronic wasting disease is not your garden-variety infectious disease. It’s not bacterial, viral or even fungal. It’s caused by something we all have inside our bodies — proteins called prions. As Osterholm put it to Minnesota lawmakers, “If Stephen King could write an infectious disease novel, he’d write it about prions.”

“They’re just very different from traditional pathogens,” says Kaitlyn Wagner, who researches prions at Colorado State University. The prions that cause chronic wasting disease start out as normal proteins, she says, noting that all mammals have normal prions, sitting on the surfaces of our healthy cells. The difference between a good prion and a bad one is the shape. The problem is that good ones can transform into bad ones, a process that has inspired comparisons to the transformation of Dr. Jekyll into Mr. Hyde.

Mark Zabel, who is associate director of the Prion Research Center at Colorado State University, says one way to think about it is origami. Imagine that healthy proteins are shaped like origami cranes. If an abnormal origami crane, with a bent wing, say, comes along, the normal origami cranes will start to copy it. One by one, their wings will bend as well. Eventually, when a badly folded prion has, as Zabel puts it, “coerced” enough healthy proteins to get bent out of shape, they can gather in clumps, killing off cells and riddling the brain with holes, like a sponge.

In the case of other prion diseases — like bovine spongiform encephalopathy, or “mad cow disease” — the badly folded proteins tend to stay contained in the brain and nervous system. But animals infected with chronic wasting disease scatter infectious proteins all over the place. The misshapen proteins have been found in urine, feces, blood and saliva. And they can stick around for a long time.

Zabel says that a virus might be able to survive for a few hours outside its host. A bacterium might be able to make it for a week or two. A prion, on the other hand, can linger for years — decades, even.

To further complicate things, studies have shown that plants can suck up prions through their roots and harbor them in their leaves, potentially infecting the next animal that comes around for a snack.

There’s a lot that’s still unknown. What is known is that chronic wasting disease was first identified in Colorado way back in the 1960s, and has now crawled its way across the country, infecting deer, elk and moose in at least 26 U.S. states and three Canadian provinces. It’s also turned up in South Korea, Finland, Sweden and Norway.

And it all started here, in the Mountain West. Or did it?

At the Prion Research Center at Colorado State University in Fort Collins, Mark Zabel, associate director, and Kaitlyn Wagner, a Ph.D. candidate in microbiology, perform all kinds of lab work, including tests on samples collected from animals thought to be carrying chronic wasting disease.
Rae Ellen Bichell/Mountain West News Bureau

RESEARCHERS FIRST IDENTIFIED THE DISEASE in the 1960s. Soon after, Michael Miller, a senior wildlife veterinarian with Colorado Parks and Wildlife, got sucked into working on it. “Yeah, sucked into it is really right,” he says. Back then, local wildlife scientists were studying captive mule deer at a facility in Fort Collins, Colorado, trying to figure out how to help them survive harsh winters in the wild. But the animals kept getting sick and dying.

It didn’t seem to make any sense. Finally, the researchers looked at pieces of the animals’ brains and saw something disturbing: The brains were full of holes, in a pattern similar to what happens with mad cow disease — it was chronic wasting disease. And it was hard to beat.

The folks who were running these research operations decided to try to get rid of the disease, so in the mid-’80s they gathered up and killed all the captive deer and elk they had and did what, at the time, seemed like a very thorough job of cleaning up the facility grounds,” Miller says. They cleaned the pens where the animals had been kept, turned the soil, brought in a helicopter to drop chlorine onto the site, and left it alone for a full year. Then, they brought in healthy wild elk calves.

And we failed,” says Miller. Within a couple years, the disease was back. Miller and his colleagues worked hard to figure out how to contain it in northern Colorado over the next few years. “The idea at the time was that we would do what we needed to here, locally, to keep it from spreading to the Western Slope. What we didn’t realize is that it was actually more widespread. It was a really nice idea that was probably 10, 15, maybe 20 years too late,” he says.

Over the next few decades, cases kept showing up in new places, first in captive animals, then in the wild. The number of infected animals mushroomed across the U.S. and Canada. The disease even jumped continents, flying from Canada to South Korea in a shipment of infected elk. “The Panama Canal may well be a barrier to the spread of the disease,” one researcher noted in 2017. “But we can’t take that for given.”

Still, Miller says it’s unlikely that the illness will drive an entire species to extinction. “What is more likely is that we will have a deer herd that is unable to grow,” he said. A bad winter, for example, could do real damage to herds if enough animals are infected.

Meanwhile, people in Canada, the U.S. and Nordic countries are scrambling to keep the disease under control. Idaho is trying to tighten rules about moving animals across its borders. In Wyoming, environmental groups are suing the U.S. Fish and Wildlife Service for feeding elk in the winter, a practice they believe could contribute to the spread of CWD. Colorado, where 57% of deer herds and 37% of elk herds are infected, just came out with its latest management plan for chronic wasting disease. It includes testing animals, in some cases thinning out overly infected herds, warning hunters and taxidermists about how to handle tainted entrails and potentially investing in more incinerators to dispose of infected carcasses.

A sign at the Prion Research Center in Fort Collins, Colorado, warns of the hazards in the lab.
Rae Ellen Bichell/Mountain West News Bureau

“We’re not talking about going in and annihilating deer over large tracts of land,” says Miller about the plan in Colorado. They already tried that, he says, and it didn’t work. Once again, the infection was more entrenched than biologists realized: “It isn’t something that lends itself to a quick fix, and we don't need to do draconian things, but we need to do something.”

Historically, a lot of agency plans have rested on one big assumption: That the disease started here in the Mountain West and then slunk its way across the world. But Mark Zabel, the prion researcher at Colorado State University, says that could be wrong in a big way.

“Most of the outbreaks in the U.S. can be traced back to movement of animals on the game farms from the Front Range to places like Saskatchewan in Canada to the Midwest and Wisconsin to South Dakota, repopulation in Arkansas,” he says. “But then there are some that have no known connection.”

Scientists were mystified in spring 2016, for example, when the disease unexpectedly showed up in Norway. Researchers were checking on wild reindeer when they noticed one animal had been left behind by a fleeing herd. “It was lying on the side, flat out — not very common for reindeer. And it had all these bubbles and stuff coming out of his mouth,” says Roy Anderson, a research technician with the Norwegian Institute for Nature Research. Just three or four minutes later, he says, the reindeer was dead. Samples from the sick animal eventually made their way over to the laboratory of Sylvie Benestad of the Norwegian Veterinary Institute. “And it was really strongly positive,” she says. “That started the problem.”

The Norwegians sent sharpshooters out in helicopters and snowmobiles to kill all the reindeer where the infection had been found, about 2,400 of them. But just two months later, the disease showed up in a couple of elderly moose. And this month, neighboring Sweden detected its first case of chronic wasting disease. “We still don't know how it came to Norway,” says Benestad.

And yet another mystery has arisen: Benestad and other researchers have concluded that the chronic wasting disease in Norwegian moose is not the same as the one circulating in North America. Upon closer inspection, the disease in reindeer is different, too. Scientists are trying to figure out what all this means. Are there multiple kinds of chronic wasting disease? And where are they coming from, anyway? “I think this question of what's going on is kind of opened up again,” says Kaitlyn Wagner.

Wagner and Zabel have suggested a possible answer: Perhaps, they say, there is not just one chronic wasting disease, but rather a bunch of different strains of it. And those different strains could be emerging at different times across the globe.

One day in late February, in their laboratory in Fort Collins, Colorado, Wagner and Zabel compared the prions from the brains of CWD-infected deer in Texas with those of elk in Colorado. They want to know if the proteins were all mangled in the same way, or not. “If they are different, this would suggest that we have different strain properties, which is evidence as we're building our case that we might have multiple strains of CWD circulating in the U.S.,” says Wagner.

Step one is to see if they’re equally easy to destroy using a chemical called guanidine. The shape of a prion dictates everything, including the way it interacts with an animal’s cells and the ease with which chemicals can unfold it.

“Moment of truth,” said Wagner, as she and Zabel huddled around a computer, waiting for results to come through. When they did, Zabel was surprised.

“Wow,” he said. “Unlike anything we've seen before.”

The prions from the Texas deer were a lot harder to destroy than the ones from the Colorado elk. In fact, the guanidine barely damaged them at all. “We’ve never seen that before in any prion strain, which means that it has a completely different structure than we've ever seen before,” says Zabel. And that suggests that it might be a very different kind of chronic wasting disease. The researchers ran the same test on another Texas deer, with the same results.

Now, these are only the preliminary results from a few animals. Wagner and Zabel have a lot more experiments to do. But if future tests come to the same conclusion, it would support their hypothesis that there are multiple strains of chronic wasting disease out there, all with different origins. That, in turn, could mean that this disease will become even trickier to manage than it already is.

And, Zabel adds, there’s something else. “If it's still evolving, it may still evolve into a form that could potentially, eventually affect humans,” he says.

Zabel is not the only one worried about that possibility.

U.S. Geological Survey

OSTERHOLM, THE EPIDEMIOLOGIST from Minnesota, is also concerned. He directs the Center for Infectious Disease Research and Policy at the University of Minnesota, and is serving a one-year stint as a “Science Envoy for Health Security” with the U.S. State Department. In February, he told Minnesota lawmakers that when it comes to chronic wasting disease, we are playing with fire. You are going to hear from people that this is not going to be a problem other than a game farm issue. You're going to hear from people that it's not going to transmit to people, and I hope they're right, but I wouldn't bet on it,” he said. “And if we lose this one and haven’t done all we can do, we will pay a price.”

If that wasn’t warning enough, he added: “Just remember what happened in England.”

He was talking about mad cow disease. Decades ago, Osterholm got involved in studying the potential for the newly emerging condition — bovine spongiform encephalopathy, or BSE for short — to be transmitted to humans.

At that point, researchers had yet to document a prion disease in animals that could infect people. They did, however, have a few pieces of the puzzle. For one, work in Papua New Guinea had shown that people could transmit prion diseases to each other if they practiced cannibalism, especially of the brain-eating variety. They also knew that BSE was spreading quickly between cattle. Osterholm says he and others worried that the more widespread it became, the more chances it might have to change into something that could sicken people.

A lot of people thought that it was an overreaction,” says Osterholm. “Then, of course, in 1996, 10 years later, we recognized that in fact transmission had occurred.” Variant Creutzfeldt-Jakob disease, as the illness is called when it appears in human beings, has infected about 230 people worldwide. Osterholm says he feels like he’s having déjà vu, except that instead of mad cow, now it’s chronic wasting disease that’s spreading in animals, with the potential to cross the species barrier to infect humans. 

But some view Osterholm’s statements as pure fear-mongering.“To say that without any concrete proof — I think that's irrational,” says Daniel Schmidt, the editor-in-chief of Deer & Deer Hunting. He says Osterholm needs to lower the fear flag. “If CWD is a threat, it is more to the lifestyle of the hunting public in America,” says Schmidt. “If you scare people enough in America, they're going to stop doing something.

Schmidt and his family, who live in Wisconsin, eat wild venison almost every day, and he says they don’t give chronic wasting disease a second thought. If you need something to worry about, he says, how about climate change, or pesticides in your strawberries? “This is not a zombie apocalypse, and the hamster wheel of fear-mongering is nothing short of sensationalism, in my opinion,” Schmidt says.

So, who’s right? Could chronic wasting disease present a public health crisis? Or are we, as Schmidt put it, merely hamsters spinning the wheel of fear?

The answer to that question may largely depend on Stefanie Czub, a professor of veterinary medicine with the University of Calgary. Czub runs the Canadian Food Inspection Agency lab that tests for mad cow disease, and everyone is waiting for results from her decade-long study of chronic wasting disease in macaque monkeys, which is scheduled to end in March 2020.

“At this point, what we would like to stress — my collaborators and I — is that we have some evidence that it might infect non-human primates.”

While Czub cautions that the project isn’t yet complete, she does have some preliminary results: “At this point, what we would like to stress — my collaborators and I — is that we have some evidence that it might infect non-human primates.”

Czub and her collaborators exposed 18 macaque monkeys to chronic wasting disease prions. Some had the prions inserted straight into their brains. Some ate infected venison, while others were exposed via blood transfusion. And some were given little cuts that were wrapped in infected deer brain, which was meant to model how a hunter might be exposed to infectious viscera after getting nicked during field dressing. There were also three control animals, which were exposed to healthy deer and elk tissue.

So far, four out of the 18 monkeys developed what Czub calls “subtle and transient” symptoms that “could be indicative” of chronic wasting disease. Two of those animals had received CWD straight into their brains. Two had eaten infected meat.

Those four lost weight and became anxious. “Anxiety is a very common clinical expression in animal prion diseases,” says Czub. “It is one of the main symptoms in bovine spongiform encephalopathy, and that is the reason why some people decided to call it ‘mad cow disease.’ The animals are not mad, they are scared to death.” In monkeys, that involves crouching in the farthest corner of the cage. Czub says they shivered and had difficulties picking up pieces of food. One monkey lost a third of its body weight in just six months.

After the four symptomatic animals were euthanized, Czub and her colleagues ran a bunch of tests, which Czub says “suggested the presence of CWD.” But there are a number of factors that make this complicated. First off, three of the four sick monkeys also happened to have diabetes. “And it's really important to mention that, because diabetes — uncontrolled diabetes — really does induce wasting, so we need we need to be super careful in the interpretation of wasting,” says Czub.

Czub has presented her preliminary results at conferences, but they have not yet gone through the true scientific ringer: peer-reviewed publication. That’s a crucial step, because where one researcher might see an unusual level of anxiety, another might just see an animal in captivity and under stress. Even the results from more technical evaluations, like analyzing slices of the brain for neuron death, could be interpreted in different ways. “We'd like to see them published so we can get a better idea of how strong the data really is to support transmission,” says Brent Race, a staff scientist at Rocky Mountain Laboratories in Montana, which is part of the National Institutes of Health’s National Institute of Allergy and Infectious Diseases.

Race and his colleagues ran an experiment similar to Czub’s, in which they gave macaques deer and elk brain tainted with chronic wasting disease, in some cases injected into the monkeys’ brains and in others delivered to the stomach through a tube. In behavior and biochemical tests, the animals appeared no different from those in the control group.“We watched our macaque monkeys for over 13 years in some cases, and we were unable to find any evidence of transmission of chronic wasting disease,” says Race. A later study on “humanized” mice spliced with a human gene also showed no strong proof of transmission, despite the fact that Race and his colleagues tried to make it as easy as possible for the infection to take hold.

In a different study, however, Race’s research concluded that a different type of primate — squirrel monkeys — were highly susceptible to chronic wasting disease. Thirteen squirrel monkeys were exposed to the disease directly in their brains. Every one of them developed symptoms, including severe weight loss, tremors, drooling and weakness, after an average of about 4 years. The researchers fed another group of 12 squirrel monkeys infected meat and found that 11 of them developed chronic wasting disease an average of about half a dozen years after exposure. A systematic review of 23 studies cited the squirrel monkey findings as a reason that human infection “cannot be entirely ruled out.”

Still, human transmission remains uncertain. As the researchers noted in the journal Emerging Infectious Diseases, macaque monkeys are biologically much closer to humans than squirrel monkeys. “Aside from a few benchtop assays and the unpublished macaque study from Canada, news has been very encouraging,” says Race.

Christina Sigurdson, a professor of pathology at UC San Diego and UC Davis, did a study that hunting enthusiasts have pointed to as a reason not to worry about chronic wasting disease. It showed that a certain part of human prions makes it hard for chronic wasting disease prions to guide them into misfolding, kind of like how a zipper just won’t zip if there’s a pebble stuck somewhere in its teeth. “It suggested that this region was a barrier — at least, a partial barrier — for blocking infection,” says Sigurdson.

But only a partial barrier — and even then, it’s only against the particular versions of chronic wasting disease that Sigurdson tested from Colorado deer and elk. “We need more research to find out how many strains there are, how different are these different strains and would there potentially be some strains in the U.S. that could be infectious for people,” she says.

The Centers for Disease Control and Prevention has not yet found any evidence of chronic wasting disease in people, despite researchers actively looking for it. Epidemiologists in states like Colorado and Wyoming have also been watching for an elevated rate of prion disease in hunters — hunters like researcher Brent Race.

“I’m an avid hunter myself, and my entire family eats it,” Race says. “Actually, we raise cattle and we sell all of our cattle and eat deer and elk instead.” But Race wouldn’t go so far as to eat meat that hasn’t been tested for chronic wasting disease. That feeling is shared by pretty much every person in this story: If you’re hunting in an area with chronic wasting disease, get the animal tested before it ever hits your plate, and don’t eat meat that tests positive.

“Otherwise,” says Osterholm, “I wish you well and hope you enjoy your venison.”

A deer herd in Roxborough State Park south of Boulder.
David Parsons/Istock

Rae Ellen Bichell is a regional radio reporter with the Mountain West News Bureau. She’s based at KUNC in northern Colorado. She frequently covers science and health. Email High Country News at [email protected] or submit a letter to the editor.