A fatal brain infection caused by malformed proteins called prions – Chronic Wasting Disease (CWD)— is ripping through deer, elk, and moose populations across the US – but  studies have shown that mountain lions may play a role in slowing the spread.

The incurable disease was first discovered in Colorado in 1967 in captive mule deer and now infects cervids across most of the US, two Canadian provinces, Norway, and South Korea. Highly contagious, CWD is spread directly from animal to animal or via vegetation laced with contaminated urine, faeces, or saliva.

As scientists scramble to try to stem the outbreaks, one of their most promising findings involves mountain lions targeting infected mule deer, a phenomenon that may reduce the concentration of the prion in the wild.  

Lions prey on infected deer

“If predation, or some other removal process, was preferentially focused on infected animals…and removed infected animals at a relatively high rate and relatively early in the disease course, then chronic wasting disease might be suppressed,” Dr Michael W. Miller, a senior wildlife veterinarian with Colorado Parks and Wildlife (CPW) told The Ecologist. 

In 2009, Miller co-authored a study published in Biology Letters that fitted nine mountain lions with GPS collars in Colorado’s Front Range and tracked them over a three-year period.

The study concluded that adult mule deer preyed upon by lions were more likely to have CWD than deer shot by hunters, “suggesting that mountain lions were selecting for infected individuals when they targeted adult deer.” 

“The subtle behaviour changes in prion-infected deer may be better signals of vulnerability than body condition, and these cues may occur well before body condition noticeably declines,” according to the study.

In other words, the keystone predator may be more effective at culling sick deer than hunters who rely on more obvious signs of emaciation that occur in later stages of the disease.

What’s more, the lions consumed over 85 percent of carcasses, including brains, thereby removing a significant amount of contamination from the environment. 

High infection rates

Miller was also lead author of a 2008 study published in PLoS One that found prion infection in Front Range mule deer herds led to a fourfold increase in predation by lions.

However, the study also pointed out the “remarkably high infection rates”—up to one-quarter—of local deer, indicating that lions are far from a silver bullet.

Just the same, based on the solid data showing the predator’s preference for CWD-infected deer, it behooves one to ask whether an increase in predator population might make a significant dent in preventing future outbreaks.

Paradoxically, instead of working to boost the number of lions in Colorado, CPW is doing the opposite. 

Increasing mule deer numbers

Together, two of the agency’s predator control plans, the Piceance Basin Predator Management Plan and Upper Arkansas River Predator Management Plan, would eliminate between 15 and 45 mountain lions (and 30 to 75 black bear) over a period of three years on Colorado’s Western Slope and over half of the mountain lion population in a 2,370-square-mile area in south-central Colorado.

The killing would be carried out through the use of cage traps and foot snares, tracking by hounds, and dispatching via firearms.

CPW’s intention with the Piceance Basin plan is to “evaluate the extent to which predation is limiting deer population size,” citing local newborn fawn survival rates at less than 40 percent, which the agency blames “largely” on predators. 

While CPW’s eventual goal is to increase mule deer numbers in the state from the current 450,000 to 560,000, the agency admits that killing predators isn’t the “solution” to getting there.

Impact of habitat fragmentation

Michelle Lute, a wildlife coexistence campaigner for WildEarth Guardians—who travels up and down the Rocky Mountain front from her office in Missoula, Montana—says lions are wrongly scapegoated for the decline in deer numbers.

Instead, she thinks more attention should be paid to how deer are being “impacted by oil and gas and habitat fragmentation, and degradation that is associated with that.”

Over the last couple of decades, Colorado has seen a significant amount of fracking for natural gas, and the Piceance Basin is no exception.  

In 2006, CPW (formerly Colorado Division of Wildlife) arranged for a land exchange with Shell Oil in the area, and noted a decline in deer numbers due, in part, to “changes in human use primarily related to energy development.” 

Fossil fuels extraction created an “extensive” road network in the Basin, which the agency said has “most likely been responsible for reducing deer populations in the area.”

Forecasting more oil and gas development, CPW “anticipated increased impacts to deer and elk in the area.”

Avoiding political backlash

Peer-reviewed studies, such as one published in 2017 in Global Change Biology, backs up these claims.

Mule deer and energy development—Long-term trends of habituation and abundance by Hall Sawyer et. al., found that “following fifteen years of natural gas development in western Wyoming, mule deer did not habituate to disturbance and continued to avoid energy infrastructure.”

In fact, the study found “no evidence” of mule deer living any closer than an average one kilometre from gas wells. 

“You aren’t going to be able to have these historic mule deer numbers unless you start doing something very serious in terms of habitat,” says Stuart Wilcox, Denver-based staff attorney for WildEarth Guardians.

“CPW just indicated at every stop that they’re not really willing to take a strong stand on that.” 

“It’s not really a study,” he says of the Piceance Basin plan. “They’re covering it in the guise of science to try and temper the political backlash against it.”

A major part of this backlash involves three ongoing lawsuits launched by the organisation, along with Center for Biological Diversity and the Humane Society, in an attempt to get CPW to abandon the hunt. 

Mismanaging the issue

Wilcox says that increasing deer numbers in the Basin without expanding suitable habitat will result in “overbrowsing” of the landscape, which can cause more “sickly” animals prone to CWD.

“You’re going to see CWD infections and deaths go up in this area as the result of CPW mismanagement of this issue,” he says. “And that’s sad because it could be—if not avoided—mitigated.”

Furthermore, CPW’s vision of management doesn’t just mean less lions, it also may influence the behaviour of those left alive, says Lute, whose Ph.D. focused on conflicts between humans and carnivores. 

Lute says typical predator populations keep prey species in check—which can limit CWD—but hunting them “affects behaviour and that affects interspecies interaction and that can affect how predators hunt their prey, when they hunt their prey, and where they hunt their prey.”

Leave well alone

So what’s to be done about Chronic Wasting Disease in the state? 

“I think the solution is a simple one,” says Lute. “It’s that we don’t really need nearly as much wildlife management as we’re used to thinking we do.”

Lute believes the first and most important step for dealing with CWD is to address habitat degradation and fragmentation from the oil and gas and other industries.

“Other than that, mountain lion and mule deer have been together a long time before we started mucking up the system, so they’re fine on their own,” she says.

But if habitat continues to dwindle, what does that mean for the future of deer, elk, and moose in Colorado? 

Uncertain future for cervids

A February 2018 report from CPW found that infection in Colorado deer and elk herds appears to be rising, shrinking the lifespan of the animals and striking fawns at a younger age.

It cautions that, if rates get too high, “CWD can affect a herd’s ability to sustain itself.” 

What these disturbing trends mean for the future of cervid populations in Colorado—or anywhere else the disease has taken hold—and the vital role herds play in balancing a given ecosystem, remains to be seen. 

This Author

Josh Schlossberg is a freelance journalist.