A deadly disease stalks deer and elk. Do predators help or hurt?

In the Rockies, chronic wasting disease can devastate herds; scientists are looking for solutions.

Justin Binfet, a biologist with the Wyoming Game and Fish Department, strode down a sagebrush-covered hill in the Deer Creek Range of central Wyoming, his phone in his hand. The mid-June sun beat down as the wind picked up. Binfet hopped across a clear, rocky creek and climbed hundreds of feet up the opposite ridge. He stopped occasionally to course-correct after checking his phone, which showed dozens of dots on a map, indicating that a mountain lion had been hanging out near the area. A female lion usually sticks around for one of two reasons: She’s made a den and has kittens, or she killed something big and is taking her time dining. 

Binfet hoped for the latter. Wind and hot sun can make quick work of a carcass, so he didn’t dawdle. 


He and a few other biologists had fitted the wild cat with a GPS collar 18 months earlier so they could track her movements. Her collar collected her location every three hours, and the data was sent to a satellite daily. A complicated algorithm analyzed those pings and alerted the scientists if the lion had likely killed something. 

Binfet reached the top of the ridge, where sagebrush flowed into a few robust junipers that abutted the limestone bluffs above him. His truck sat far below along the vague two-track we’d driven in on, the only sign of humans for miles.

“I’m looking for a kill in some pretty open stuff,” Binfet said. 

Minutes later, he found it: the remains of an elk calf, baking in the sun a few feet away from a juniper as tall as a house. Binfet wanted to know whether a lion had killed this calf, and if so, whether the calf was infected with chronic wasting disease, or CWD, when it died.

Tufts of fur were buried in a bed of needles and duff beneath the tree — signs of a lion cache. The only way to tell if the calf had been infected was to examine its lymph nodes, which Binfet hoped hadn’t yet decomposed into slime. He turned to the calf, stretched blue surgical gloves over his hands and set to work, using a scalpel to dissect the several-day-old carcass that was already melting into the earth. 

Do mountain lions influence how the disease moves among other wildlife?

Any information gleaned from it would provide one small piece of a much greater puzzle — the question of whether, and how, predators affect the spread of CWD, a disease that kills every animal it infects. Binfet’s work was part of a study he’s helping to lead, to figure out if lions select their prey from the sick and weak — as many people assume — or if they choose stronger and healthier individuals, just like human hunters do. In short, do mountain lions influence how the disease moves among other wildlife?

To find out, biologists have tracked, darted and collared 26 mountain lions, then used the information to find and sift through the freshly killed carcasses of their prey. Other researchers collared newborn fawns and are tracking their movements until adulthood. Now the scientists are trying to use all this data to better understand how chronic wasting disease operates across a vast landscape and multiple species. It’s not easy. Lions don’t want to be darted and collared and often give biologists the slip. Collars stop working, complicated algorithms don’t always spit out accurate kill locations, and sometimes those locations come with a lion and kittens unhappy to see human intruders. Even when scientists can locate the lions’ prey, dead animals don’t stay fresh enough for valid disease samples in the 90-degree heat for long. That day in the field, Binfet found maggot-filled mush instead of lymph nodes.  

And then there are humans. State regulations permit unlimited mountain lion hunting in parts of central Wyoming. One winter morning, Binfet loaded GPS locations for each of the study’s collared lions onto his laptop before heading into the field. One collar — and the lion it belonged to — turned up east of Casper, off Interstate 25, in the parking lot of the Hat Six Travel Center. Likely in the back of a truck. 

Alex Boersma/High Country News

TECHNICALLY SPEAKING, chronic wasting disease is a form of transmissible spongiform encephalopathy. The condition results from microscopic misfolded proteins called prions, which cause other proteins to mutate into prions as well. This buildup of mutated proteins eventually kills cells and leaves holes in an animal’s brain, making it look like Swiss cheese under a microscope. All infected individuals die. Most wither or waste away, though because the disease slows reaction time and movements, some are hit by cars or killed by predators first. No cure exists.

CWD isn’t caused by a virus or bacteria, so it can’t be combated by antivirals or antibiotics. There’s no vaccine. The prions that cause it stick to metal, particularly stainless steel, and can only be destroyed by being doused in lye, soaked in bleach or heated to temperatures greater than 900 degrees Fahrenheit. 

Researchers first recognized the symptoms of CWD in 1967, in captive Colorado mule deer. Years later, it appeared in wild herds in southeast Wyoming. No one knows how it started or where, exactly, it came from. Since then, the disease has crept from county to county, state to state, region to region. It has leapt to Pennsylvania and even South Korea through imported captive elk and deer. CWD has now been documented in 30 states from Texas to New York. In the West, it was most recently identified among wild deer in Montana, in 2017, and Idaho in 2021.

CWD infects deer, elk and, occasionally, moose. Other species have their own variations: Mad cow disease, for example, affects cattle and made headlines after people ate infected meat and developed a deadly variant of the disease in the United Kingdom in the ’90s. CWD has never been documented in humans, but scientists worry about the possibility of a similar transmission through eating meat from a sick animal. The most well-known prion disease that affects humans, Creutzfeldt-Jakob disease, is also incurable and fatal. As a precaution, the Centers for Disease Control and Prevention tells hunters not to eat animals that test positive for CWD. Most state wildlife agencies, including the Wyoming Game and Fish Department, offer free testing for animals killed by hunters, and some states and hunting areas either require or strongly encourage it. Right now, however, the main consequence of the disease is its devastating impact on wildlife herds.

Scientists largely agree that once CWD is established in a herd, it likely can’t be eradicated. What researchers don’t know is how to stop it. 

The best way to control the disease may be to slow its spread, wrote biochemist Sandra Pritzkow in a 2022 review paper published in the journal Viruses. CWD moves in part through feces, urine and infected carcasses, which deposit mutated proteins in the soil, where the prions remain infectious for years. In addition, CWD prions bind to plants, and might even be transmitted by earthworms as they inch through dirt. Prions also shed through saliva and nose-to-nose contact, which is how gregarious deer and elk say hello. 

“What do you do when a disease gets to be endemic?” 

It’s unclear whether animal-to-animal contact or environmental contamination is the bigger problem. But little can be done about prions lurking in the soil. So, one of the most promising — and practical — ideas involves thinning wild herds to keep animals from commingling and spreading the disease. Wyoming and Montana have tried issuing more hunting licenses in order to decrease the density of deer in certain areas. 

Another option is to allow more hunting by other predators, such as wolves, bears or lions. But humans have reduced predator numbers since pre-colonial times. It’s possible that boosting or restoring predator populations — no matter how counterintuitive it may seem — could help bolster deer numbers. A small-scale study in Colorado published in 2010 suggested that lions may prefer to prey on infected deer, but Binfet and others are diving deeper, tracking more lions and collaring deer and fawns in order to better understand whether predators can help or hinder CWD spread.

“What do you do when a disease gets to be endemic?” asked Rhiannon Jakopak, a research scientist with the University of Wyoming who is studying how CWD moves among mule deer. “Do you just watch the herd decline? Or do you try to tinker with it a little bit?” 


Alex Boersma/High Country News

JAKOPAK AND A TECHNICIAN named Erika Schwoyer stood on top of Chalk Mountain, a bluff in central Wyoming about 50 miles from Binfet’s elk carcass. They moved quietly, closing the doors of their truck with barely more than a click. Like Binfet, they had a signal to follow. Unlike Binfet, they hoped to find the animal or animals associated with it alive. 

The two researchers whispered to each other as they pulled an H-shaped radio receiver and binoculars out of their backpacks and jammed in sampling test kits — plastic bags full of syringes, vials and other gear. Then they picked their way over to a clump of pine trees near a pile of sandstone rocks. Schwoyer held the receiver in the air, hoping for a ping, but no sounds broke the stillness of the unusually windless day. 

A deer had given birth in the area the night before, and in the process had dropped a small plastic and metal vaginal implant transmitter that biologists had inserted in the spring. When the transmitter hit the ground, it sent a signal that triggered a satellite, which sent an email to Jakopak. An ultrasound performed in the field had shown that the doe was pregnant with twins, and the researchers needed to find the fawns and collar them before the spindly-legged creatures ran away. 

Newborn fawns’ best defense against predators is to hide and stay absolutely still. But, if scared enough, they will flee; Jakopak once watched a less than half-a-day-old fawn race away across a swampy field. 

Jakopak wanted to know if infected mothers pass CWD to their fawns at birth or shortly after, or whether some fawns escape the disease completely. If they aren’t infected, she wondered, why not?

We scrambled down the rocky hillside, pausing every few minutes to listen for pings from the dropped transmitter and scan the sagebrush-covered valley below. Jakopak has spent years searching Wyoming’s mountains and plains for baby animals. She knew the fawns would be impossible to see from a distance, so she watched for movement from the mother; the newborns would be nearby. 

Midway down the hill, we heard the first ping. The tempo of the receiver’s “beep, beep, beep” began to speed up as we reached the valley below, indicating that the transmitter was close. Moments later, Schwoyer found it in a patch of green grass. 

Now the search became even trickier and more urgent. We hadn’t spied the mother, which meant the fawns were either hiding or had taken off, likely never to be found. Then Jakopak waved to us and pointed to the ground a couple of yards away.

We scrambled down the rocky hillside, pausing every few minutes to listen for pings from the dropped transmitter and scan the sagebrush-covered valley below.  

Curled up in an almost perfect circle was a tiny brown fawn marked with white spots. She lay motionless in a divot next to a knob of sagebrush — out in the open, yet completely camouflaged. She barely breathed; even her wide-open eyes stayed still.

Jakopak gently wrapped her hands around the fawn, its legs shooting out in all directions in a last-ditch attempt to escape. She carefully wrangled the spidery limbs, then placed the baby in a gray cloth grocery bag and hooked the bag’s handles to the bottom of a scale. 

“Eight pounds,” she said to Schwoyer, who noted the weight on a clipboard. Jakopak scooped the fawn out of the bag and placed her back on the ground, keeping her hand gently on the newborn’s chest so she couldn’t run away. 

Schwoyer wrote down the length of the deer’s body and the bottom section of one of her back legs as Jakopak called out the measurements. Jakopak filled several vials with blood for potential CWD testing later. (A blood test for CWD is currently awaiting USDA approval.) Finally, she wriggled an elastic GPS collar around the fawn’s neck. As the deer grows, thin pieces of thread constraining the collar will break, allowing it to expand so researchers can track the fawn to adulthood. A battery pack attached to the GPS unit may last for upwards of nine months. If either dies — the battery or the fawn — a satellite will email Jakopak, letting her know. 

While Binfet and his team concentrate on lions, Jakopak focuses on deer. She isn’t sure how much predators curb the spread of CWD, but she knows any reduction means more time to find a cure or a better management strategy. Even a small difference, she said, could be important — saving just one deer from this awful disease would be worth it.

“You can tell when they have died from CWD,” she said, though she noted that a formal diagnosis requires lab testing. “It looks like they were standing and pushed over, but pushed over by nothing. Sometimes they get scavenged, and your heart sinks. You can see their ribs sticking out, and hipbones. They look like they were skin and bones, and they toppled over.”

Jakopak is especially interested in the fawns’ teenage years. Males tend to wander off by nine months or so, while females might roam around a little before settling somewhere nearby. Ecologists rarely focus on this period — after the fawns have left their mom and before they settle — but it could be critical in understanding CWD contagion. During this time, young deer likely interact with other family units or even join different herds, exchanging saliva — and possibly CWD. That kind of contact, along with time in new environments, are prime opportunities for spreading disease. But researchers don’t know exactly where teenage fawns go.

“That’s one of the missing pieces,” Jakopak said.

She’s also tracking how and why the fawns or their moms die. They’ve collared 68 adult female deer so far and lost nearly a third of them. Of those that died and have been tested, 11 had confirmed CWD; eight died from it, while predators killed the other three. Still, CWD likely contributed to their deaths, Jakopak said. One was killed by a coyote, which rarely kill adult mule deer.

Once the fawn was collared, Jakopak returned her to the notch where she’d found her. The fawn pressed herself into the ground, then stilled.

“Science is so incremental,” Jakopak said. “We’re trying to control for as much as we can and explore one little piece of the puzzle. And, hopefully, after someone’s entire career, we might have the bottom left corner.”


Alex Boersma/High Country News

IN MIDWINTER, Binfet, riding shotgun in a small white pickup, bumped down a winding dirt road in the central Wyoming’s Deer Creek Range. Snow-covered red clay, sagebrush and junipers surrounded the truck, and a few deer bounded up a nearby hill. Binfet was looking for a collared mountain lion whose GPS signal kept fading in and out. The truck’s driver, Ryan Rohrer, who owns a large ranch in the area, talked about his appreciation for lions — unlike most Wyoming ranchers, who generally loathe the large predators, partly because they occasionally kill livestock, mainly sheep. 

“Mountain lions are one of the coolest creatures on the landscape, in my opinion,” he said. “They’re just so elusive, and it’s a 150-pound athlete that makes a living off killing things four times its size, you know? And you never see them. They’re just — they’re just super cool.”

Rohrer has never had issues with lions eating his cattle or horses. In early February, in fact, a collared female spent a few days behind his horse barn, near 300 weaned calves. Rohrer’s horses were there, too, eating hay. The cat killed a deer and a skunk but left the livestock alone. Perhaps, he said, mountain lions don’t need domestic animals in an area with such a rich supply of wild food.

So when Binfet called Rohrer a couple of years ago asking if he could study lions on his land, Rohrer agreed. Much of the West — more than 600 million acres, including roughly half of Wyoming — is public, but everything else is private. That means researchers often need the cooperation of private landowners to get much done. 

For most of Binfet’s career, he managed deer herds with CWD. About a dozen years ago, he worked on a project testing deer from a herd just south of Douglas, Wyoming, for the disease. The head researcher, Melia DeVivo, later published a paper suggesting that the herd could decline by as much as 50%. One worst-case scenario predicted that CWD, if left unchecked, could wipe it out completely. Today, the herd is relatively stable, but still just half the size that the Wyoming Game and Fish Department believes the landscape can support.

DeVivo also noted that 20 of the deer in the study with CWD were killed by mountain lions, making the big cats the primary source of mortality for CWD-positive deer. Binfet wondered if he could use the ponderosa- and juniper-covered rolling mountains of central Wyoming to measure the impact of predators on deer numbers in an area already saturated with the disease. Members of the public tended to call for killing predators whenever deer numbers waned, but in the presence of CWD, that might only worsen the situation. Binfet also wanted to know if anecdotal stories about lions and coyotes avoiding the carcasses of deer killed by CWD were true, or just that — stories.

So he sat down with a Game and Fish large-carnivore biologist, Justin Clapp, to develop the current project: a plan to spend almost four years tracking dozens of mountain lions to see what’s on their menu. 

 EARLY ONE MORNING, my phone buzzed with a message from Jakopak. We were supposed to meet for coffee to discuss how many fawns in the study had died, among other topics. But at the last minute, she emailed to say she had tested positive for COVID-19. Her housemates had been sick for the last 10 days. She’d quarantined with them, testing negative for the duration, then woken up sick. It didn’t make sense. But the last three years of this pandemic have demonstrated how confounding diseases can be, and how much can remain unknown — even after years of global efforts.

The Wyoming study hopes to address some of CWD’s unknowns. But even so, it’s unclear what wildlife officials will do with the data. Would they be willing to impose lion-hunting quotas in an area where the culture is steeped in anti-predator ideologies?

Potentially, said Dan Thompson, the Game and Fish Department’s large carnivore section supervisor, if the data is clear enough. “We’re open to what we will learn from this study,” he said. “I think it would only make sense if the results suggest there’s something we can do from the lion perspective that would be beneficial for mule deer.

“A lot of people are using the predation standpoint for their agendas. Some groups say predation will fix everything and some say predators ruin everything, and until we have answers that are well quantified, we’re playing the middle game.”

Regardless, every sample or drop of blood taken by Jakopak, Binfet and others will add to the growing body of work on CWD from across the world. 

“Some groups say predation will fix everything and some say predators ruin everything, and until we have answers that are well quantified, we’re playing the middle game.

It’s also possible that modern wildlife managers have been looking at this in the wrong way from the beginning, said Jason Baldes, an Eastern Shoshone tribal member and executive director for the Wind River Tribal Buffalo Initiative. “In Western thought, we silo everything and think it should be broken down in pieces,” Baldes said. “But when you look at it whole, it’s not siloed. It’s one system together.”

Imagine if the pieces of this particular puzzle had never been scattered by CWD. If the predators that keep herds healthy by weeding out the sick had never been removed, Baldes said, this terrible disease, with its unknown origins, might never have had the chance to spread.

That’s partly why tribal nations like the Eastern Shoshone traditionally didn’t allow predator hunting on the reservation — though there is currently a hunting season for lions — and why tribal leaders continue to oppose hunting grizzly bears and wolves. It’s also why Baldes and others are working so hard to restore the buffalo. Only a complete ecological system can ensure that all species have a chance against threats like disease and even climate change. 

“We can’t continue to think we have an answer when we don’t look at it holistically,” Baldes said. 

A PAIR OF BLUETICK HOUNDS ran across lion tracks in the snow as the sun cracked the horizon on a late January morning, north of where Binfet sorted through the elk calf carcass. The dogs followed the big cat for miles over sandstone boulders, up hills and down ridgelines, across a road and a bridge and then yet more hills.

Their human handlers watched the dogs’ progress on GPS trackers. The crew of Wyoming state biologists wanted to get a collar on the lion so they could add its movements — and its dining habits — to the study. 

Barking and howling erupted in the distance. Minutes later, the handlers hiked up a ridge and found the source: The dogs had treed the lion on a thumb of rock about halfway down a 400-foot precipice, hanging above a frozen river. 

The lion had careened down a narrow, steep couloir etched into the side of the cliff, then skirted an impossibly small ledge to reach a lone ponderosa growing from a crevice in the rock. The dogs followed. There all three remained for almost an hour. Then, in a flash of fur and claws, the lion bailed from the tree and streaked back along the ledge with the dogs in pursuit. One grabbed the lion’s neck in its mouth. The lion spun around, planted her teeth in the hound’s face, and the two began to roll. 

The crew at the top of the cliff — now including a search and rescue team the biologists had called to help the dogs — gasped and watched, helpless.

Somehow the animals separated and made it to the bottom of a gully, where the lion flew into another ponderosa and the dogs once again waited below. 

The lion’s clear blue eyes remained fixed on the humans at the top of the rim, unblinking. 

It was late afternoon now, and the sun was sinking. The rescuers decided to rappel down the cliff, fit the dogs in cloth slings and winch them back up. The lion, the biologists decided, would not be sedated and collared after all; it was too close to dark. She would go free, untraceable. 

Minutes later, the dogs safely back with their human companions, she fled. 

In late March, small latches in the remaining lion collars unclicked, and the collars dropped to the ground. The lions will once again fade into shadows and tracks, rarely seen by humans — another piece in the very large puzzle of a disease that won’t stop moving.  

Christine Peterson lives in Laramie, Wyoming, and has covered science, the environment and outdoor recreation in Wyoming for more than a decade. Her work has appeared in National Geographic, Outdoor Life and the Casper Star-Tribune, among others. We welcome reader letters. Email High Country News at [email protected] or submit a letter to the editor. See our letters to the editor policy.

Help us create more stories like this.This coverage was supported by contributors to High Country News.


Hi there, reader.

We hope you found value in the article, above.

Before you go, I’d like to ask you for a few dollars of support.

For more than 50 years, High Country News has been funded by readers just like you: citizens of the world who are curious and passionate about the West.

I’m guessing you are the kind of person who stays abreast of news and takes time to keep informed about issues that matter to you. As such, I’m sure you’re well aware that journalism around the globe is in real trouble.

You can help our team of independent writers, editors, illustrators and photojournalists stay focused on the important stories with your tax-deductible donation today. Even $4 – or the cost of a small latte – makes a difference to us.


We hope you’ll return to this website again, and read as much as your tough or tender heart can consume. And you’ll feel better knowing you’ve invested in fair-minded, in-depth journalism.

We need you now, more than ever.

Onward, with courage and resolve!
Greg Hanscom
Executive Director/Publisher