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Study contemplates predators’ role in reducing chronic wasting disease


With its diversity of predators, could Yellowstone National Park be a disease-free island in a surrounding sea of chronic wasting disease?

That’s speculation based on a recent study by Ellen Brandell who, with other scientists, built a model to analyze the role predators play in removing sick animals from the environment.

“The Yellowstone Ecosystem is an exciting area to study this because there is a rich predator community and CWD has just started to infect elk and deer,” Blandell wrote in a blog posted on the Animal Ecology in Focus website.

Dan MacNulty, from Utah State University’s Department of Wildland Resources and Ecology Center, has studied wolves in Yellowstone for years. He collaborated with Blandell on her study and said, “There is a growing body of scientific research, including our recent study, that predicts that Yellowstone National Park will become an island mostly free of CWD in a sea of CWD infection thanks to its diverse and abundant community of large carnivores.”

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Could wolves be a way to control chronic wasting disease’s spread? A recent study contemplates the possibility in Yellowstone National Park.

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Doug Smith, lead wolf biologist in Yellowstone National Park, collaborated on the study as well. He said the modeling is missing one key piece of information: How effectively wolves and cougars will prey on CWD-infected elk and deer.

If the predators identify a sick animal early in its infection, the effect will be much greater, he said.

Another question is how many infected animals will predators kill? And will the kill rate of sick animals be higher than that for healthy ones?

Despite the weak points in some knowledge, Smith said the model shows that predators could help slow CWD’s spread by removing sick animals from the landscape.

“I would say this is one of the benefits of having an intact predator community,” he said.

By collecting brain stem material from elk and deer that predators have killed and analyzing it for CWD, Yellowstone researchers are attempting to find out infection and kill rates in the park. Smith and his colleagues also capture elk every winter and tests them for the disease.

Smith is also hopeful that with time, evidence may be collected to support the CWD theory.

“The utility of models like this is a deeper understanding of complex ecological relationships, but models require simplifying certain processes as well and should be interpreted with some caution,” Brandell wrote.

Other collaborators on Brandell’s modeling included: Paul C. Cross, Will Rogers, Nathan L. Galloway, Daniel R. Stahler, John Treanor and Peter J. Hudson.

Unlike wolves who chase prey before bringing it down, cougars often attack from ambush. Whether they can target weaker animals, possibly those sick with CWD, seems uncertain.


Scientists already know predators will often kill sick, old, young and weak prey that are easier to take down. So maybe mountain lions and wolves could play a role in removing elk, deer or moose that have chronic wasting disease, Brandell’s study hypothesizes.

A 2010 study in Colorado’s Front Range “showed mule deer killed by mountain lions were more likely to be infected with CWD than mule deer killed by hunters,” Colorado State University reported. A 2008 study, however, found that “such selective predation by mountain lions…did not limit CWD transmission in deer populations with high infection rates.”

The theory was that because lions ambush their prey, instead of chasing like wolves, cougars may be “less likely to detect sick animals compared to wolves.”

Reducing the number of animals, with or without CWD, is one tool state wildlife agencies employ to lessen the disease’s spread. This is based on the fact that CWD is more easily dispersed if animals are in close contact. It also recognizes that hunters can remove sick animals from the population, sometimes before they show symptoms of infection.

Seeing predators as another tool to keep disease prevalence low is an argument wolf advocates have long been making to the Montana Fish and Wildlife Commission. With that in mind, groups like Wolves of the Rockies have been seeking to lower wolf hunting and trapping quotas in the state, so far to no avail.

The Wyoming Game and Fish Department’s website shows the rate of chronic wasting disease infection across the state.

In Yellowstone, wolves primarily dine on elk, accounting for around 96% of their diet in winter and 85% in summer, according to the National Park Service. Mountain lions rely on elk for about 55% of their diet, with about 45% coming from mule deer, the Yellowstone Cougar Project found.

Chronic wasting disease tends to infect adult male deer more often than females. Elk seem somewhat resistant to the disease, although Montana detected its first infected elk in 2019 and a Wyoming elk shot in Grand Teton National Park tested positive for CWD in 2020.

Montana’s first detected case of CWD in wild deer occurred in 2017 in south-central Montana’s Carbon County. In Wyoming the disease is working its way north and west since it was first identified in 1985.

The disease, which causes damage to the infected animal’s brain, is always fatal. The abnormal proteins that cause CWD, called prions, are spread from an infected animal’s bodily fluids or feces. Once in the environment, the prions can survive for years, making it difficult to eradicate.

So far, 27 states and two Canadian provinces have detected CWD infected animals. The sickness has also been found in South Korea and Norway.

Wolves rely on elk for a large part of their diet in Yellowstone National Park. Elk that are sick with chronic wasting diseased may be more likely to be killed, raising the question about whether the park may become a place where CWD infection rates are lower.

Brandell’s study comes out in the wake of the October publication of a U.S. Geological Survey study documenting the benefit of scavengers — from ravens and crows to coyotes and golden eagles — as landscape sanitizers. The study placed disease-free cattle fetuses in different types of habitat. Cameras were set up to film what animals arrived to feed and how long it was before the fetuses were eaten.

The goal was to mimic when elk, which can carry brucellosis, abort their fetus. The birthing material is believed to be one of the main ways brucellosis is spread.

Wolf hunt aftermath

Wolves have become a red-hot issue in Montana this winter following changes to hunting regulations that allowed more killing of animals wandering out of Yellowstone National Park.

“On the one hand, that’s the way it goes,” said Doug Smith, the park’s lead wolf biologist. “It’s not illegal to shoot a collared wolf. But that’s how we get our data.”

In all, a total of 25 wolves known to researchers were killed by hunters, 20% of the park’s wolf population – now estimated at 89 animals. Yellowstone contains one of the few protected populations of wolves in North America.

Smith said he will be watching to see if the loss of wolves impacts how many breed and how many pups are born.

“One thing I’m worried about is the hunt could stimulate reproduction because it broke up the wolves’ social structure,” he said. “This is well known in coyotes.

“When you have a stable pack, reproduction is controlled by the dominant wolves,” he added.

If wolf numbers grow, Smith said wolf hunters may see the reproduction rate as proof that killing a lot of wolves won’t affect their populations.

“The goal of the National Park Service is to preserve behavior and social structure as well,” he said. “That’s just as important to us. That’s a very hard point to communicate, but it’s a very important point.”

A 2006 analysis by a University of Montana researcher estimated wolf watchers in Yellowstone have around a $35 million economic impact to the park and surrounding communities. As those dollars circulate in the economy, the figure jumps to a total of $70 million annually.

In 2006, visitation to Yellowstone was about 2.8 million. Last year, a record 4.8 million people toured the park.

CWD legislation

Congress has made $10 million available to state wildlife agencies for chronic wasting disease management. This is an increase of $3 million from the previous year and double the funding made available in 2020.

Other states are using funds to increase the availability of carcass disposal and testing sites or develop educational materials. The Tennessee Wildlife Resources Agency is researching the potential use of dogs to detect the disease in live animals.

If the Senate approves the CWD Research and Management Act, which passed the House of Representatives by an overwhelming margin in late 2021, $35 million would be authorized annually for cooperative agreements with states and tribes, as well as an additional $35 million to support research into the disease.

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Montana Untamed editor for the Billings Gazette.


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