Can Large Carnivores Control CWD?

We are fortunate that teams of disease specialists and managers are working together to learn as much as they can about all the ways CWD is spreading to more deer and more areas. The same abundant deer and elk populations that are affected have also allowed the recovery of large carnivores in many locations, so perhaps it can be expected that we find ourselves talking about these issues together.

Although there is no evidence carnivores can contract this disease of the deer family, there recently has been a lot of very bold and definitive assertions they are controlling it in deer and elk. In this case, “control” could mean anything from slightly reducing the rate of increase, holding it stable, or decreasing the prevalence rate (percent of the population testing positive). Carnivores preying on or scavenging CWD-positive deer or elk could potentially either reduce the amount of infectious material available in the environment or spread that infectious material around the environment through their feces. Of course, both can also be true at the same time.

The message that wild carnivores will reduce or manage CWD for us is an increasingly popular narrative. The loudest sources of this idea are predictable segments of the public who are always looking for reasons to make large carnivores out to be saviors of the environment.

Those on the other, equally predictable end of the spectrum who see large carnivores more as satans of the environment would like you to think about carnivores spreading CWD and making it worse. Scientists have investigated this topic, but the messaging has been mixed and confusing.

Let’s look at what science can tell us about carnivores cleansing, or accelerating, CWD so we can craft—or temper—our messages accordingly.

Increasingly, we hear large carnivore protectionists promoting fanciful stories about why their favorite animal is restoring ecosystems, curbing climate change, changing the course of rivers, cleaning up CWD, or otherwise saving the world in general.

Wolves, in particular, have a very effective and relentless volunteer public relations team. There is no need to justify the restoration of large carnivores, we saved their prey and as conservationists we have an obligation to finish the job while recognizing some places they formerly existed are no longer suitable or appropriate. Besides the constant carnivore cheerleaders, some legitimate studies have been conducted to look into this topic of mountain lions and wolves controlling CWD.

Research in Colorado found mountain lion-killed adult mule deer were more likely to be infected by CWD than hunter-killed deer, and infected deer were more likely to be killed by mountain lions than uninfected herdmates. This could be from infected deer being less alert or being less able to get away when attacked. Carnivores killing CWD-infected deer and elk at a higher rate than healthy animals is the kind of information that underlies statements about them cleansing our herds of CWD. Some selectivity of CWD-positive prey animals doesn’t automatically translate to the control or management of CWD at the population level.

If you build a simple population model where diseased animals are killed at a higher rate than healthy animals, it would be unusual if the model didn’t predict that carnivores will control the disease. However, there are many complex ecological forces that can dilute and counterbalance any cleansing effect of carnivores selecting CWD-positive animals out of the population. Researchers in the Greater Yellowstone Ecosystem built a population model, not to predict CWD trends, but to learn more about the relationships between predation and CWD. They summarize their study by saying the model suggests cougar and wolf predation may decrease CWD outbreaks and delay the rise in prevalence.

The focus of this research was not to document carnivores controlling well-established CWD outbreaks, but on whether it is theoretically possible and, if so, under what conditions. The model aggressively increased how intensively the infected animals were removed from the population in later stages of CWD infection compared to healthy animals.

As infection progressed, CWD-positive deer and elk were preyed upon at an exponentially higher rate, assuming that is what happens in the wild. Researchers reported that the model does indicate carnivores could control CWD under the right conditions; however, those theoretical conditions don’t currently exist in their Greater Yellowstone study area. Their results show that predation and disease relationships would have to be different than they currently are before we would see carnivores significantly changing CWD disease dynamics in a population.

In the Greater Yellowstone Ecosystem, wolves and mountain lions tend to select fawns, calves, and old-age individuals, but the model indicates that to control CWD, predation would have to focus more on adults—especially on infected adults—than on young animals.

This paper is often cited as scientific evidence that carnivores can control CWD, but the researchers programmed the model to conduct many different scenarios describing which animals are preyed upon the most, when during the course of the infection cycle they are killed, and when the animals are transmitting the disease to other deer or elk. Some of these scenarios resulted in various levels of control of CWD and still maintained viable deer and elk populations, and some scenarios did not control CWD effectively but still reduced deer and elk populations dramatically.

In some cases where CWD was maintained at a very low level in prey populations, the overall abundance of deer and elk was cut by more than half. As with all modeling exercises, they made reasonable assumptions and reported under what conditions predators may be most effective in controlling CWD, but they did not confirm that they were. It seems that reporters looking for a flashy headline and carnivore protectionists are only promoting the scenarios where it looks like CWD control is possible as if that were the scenario happening in the wild.

Researchers reported that to eradicate (not just control) CWD, predator densities would have to be some of the highest ever recorded in North America, they would have to prey upon at least 10 infected adult deer or elk for each healthy one, and that doesn’t consider continued infection from all the prions in the environment.

In model scenarios with those unrealistic conditions where CWD was mostly eradicated, deer and elk populations were cut in half in 10 to 15 years, and there were no deer and elk left after 30 years. Obviously, predation is not the solution to eradicate CWD from our herds.

For carnivores to clean up CWD and reduce the amount of infectious material in the environment, they would have to neutralize the infectious prions during digestion and not just spread it around in their feces. As it turns out, carnivores are very good at digesting protein (including prions) because that’s the way their digestive systems work.

In a recent study, researchers fed two captive mountain lions a meal of CWD-infected mule deer tissue and then collected and tested their feces for a week after. They only found infectious prions in their first feces after the meal three days later and estimated that the lions passed less than 4% of the CWD prions they ate. Some of these same researchers later reported variability in CWD detection in coyote scats with this same method, which is why in science we like to see a sample size of more than two mountain lions fed one meal.

If large carnivores deactivate most CWD prions they consume, that would reduce at least some infectious material in the environment, but is it enough? Throughout the course of CWD infection, a deer or elk releases thousands of infectious doses of CWD prions in its life. CWD-infected animals release infectious prions through saliva, feces, urine, antler velvet, and birthing fluids, then they die and release even more.

These prions can last a very long time and accumulate by binding to the soil and minerals and may even be taken up into plants. This means there are abundant potential points of infection all around in the environment in CWD-endemic areas, and that is too complex to include in a model of CWD disease dynamics and predation. New infections from environmental sources could not be included in the Greater Yellowstone model, and that complicates the idea that most infection is occurring from animals in their last few months when the model ramps up exponential predation rates on those animals.

Disproportionately killing and consuming infected animals in the last few months of their life when symptoms start to show up is a good thing. However, the crux of this discussion is: what scientific evidence do we have to say that large carnivores are controlling CWD at a population or game management unit level—or statewide for that matter?

Despite arguments that carnivores “could” control CWD, evidence that they actually do has been elusive in the real world. In 2008, Colorado researchers reported that in their study on Table Mesa, CWD-infected mule deer were nearly four times more likely to be killed by mountain lions, but that selective predation was not controlling CWD infection. They concluded: “Remarkably high infection rates sustained in the face of intense predation show that even seemingly complete ecosystems may offer little resistance to the spread and persistence of contagious prion diseases.”

Some of those same researchers returned to that study area in 2018-19 and found that even though the deer population was far from healthy, deer abundance, CWD prevalence rate, and mountain lion predation were still about the same. CWD prevalence rate only changed from 29% to 34% of the deer population in 13 years despite mountain lions killing 4.5 infected deer for every one uninfected deer, which led them to conclude that “some combination of predation by mountain lions and perhaps subtle genetic shifting in the mule deer host or unidentified environmental factors may have contributed to the net absence of measurable change, at least on the surface.” Another interpretation might be that mountain lions continue to be unable to meaningfully change the percentage of the population that is infected by CWD.

Other research in Colorado provides strong support for the use of hunters to control CWD in wild, free-ranging mule deer populations. When hunter harvest was high enough and sustained, CWD was kept lower. From 2002 through 2018, those areas with the lowest deer harvest levels had the highest increases in infection rate. Those areas with stable or increasing harvests had stable CWD infection trends. In light of this, it is curious that carnivore protectionists are not standing in line to speak at commission meetings, asking for more deer and elk hunting opportunities.

Animals in the later stages of CWD infection may be targeted over healthy animals and may find it harder to escape predators.

Wild carnivores kill and feed on members of the deer family and then leave their feces around containing infectious prions. CWD-positive prions have been detected in the feces of crows, bobcats, mountain lions, coyotes, wolves, black bear, raccoon, and eagles. That is a lot of potential prion spreaders and has sparked concern that predators and scavengers may not be cleaning up CWD, but instead just spreading it around the landscape—especially those with large territories.

As a carnivore species, humans have contributed to the spread of CWD more than any other through live animal translocation, captive and wild animal concentration, and the transportation and dumping of hunter-harvested deer species. Deer and elk are spreading CWD in the environment every day. The question is to what degree are carnivores involved, and is their role significant enough to have any effect on the local transmission or geographic spread of CWD in the wild? Just like the idea that carnivores can control CWD, we don’t want to get ahead of the science and make unsupported statements.

Coyotes normally roam about five to 20 square miles annually, but transient coyotes may cover more than 100 square miles. Mountain lions typically have home ranges of 50 to 250 square miles, so that is a lot of ground that could be the recipient of a prion-laden turd. However, remember from the CWD meal fed to two captive mountain lions, researchers were only able to detect less than 4% of the prions originally fed to them in their CWD burger. Despite this small sample size, we can probably assume meat eaters are pretty good at digesting proteins, including infectious prion proteins, with their stomach acid and digestive enzymes.

Importantly, studies also showed coyotes shed infectious prions for only three days after the meal and for mountain lions, only the first feces after a meal contained infectious prions. Since they both likely stay close to a food source for a few days, they probably don’t spread them far from the source. Considering human movements of live and dead deer and elk, as well as massive big game migrations of hundreds of miles in the West, how important to overall CWD disease dynamics are the prions remaining in predator feces?

In 2024, the National Academies of Sciences, Engineering, and Medicine assembled a panel of leading experts to report on the state of our knowledge of CWD. On the topic of carnivores spreading CWD, they noted, “…there is currently no evidence of such….”

Deer like this CWD-positive buck continue to look healthy until their final months. Photo by M. W. Miller.

Advocacy should be based on science, but our science should not be based on advocacy. Beyond the evidence that infected deer are relatively vulnerable to predation and that most of the consumed prions are deactivated in the digestive track of carnivores, questions remain about whether, and to what extent, predation can slow, stabilize, or reduce CWD in a wildland setting.

Whether carnivores are controlling or spreading CWD are not mutually exclusive scenarios—they both can be true, and they both can be happening at levels that are inconsequential to population-level trends in CWD disease dynamics.

Carnivores are neither destructive monsters nor saviors of the environment; they are simply meat eaters trying to make a living. We should refrain from crafting exaggerated stories about both ends of that spectrum. Our focus should always be on what science, not advocates, tell us.

The continued interest in CWD and predator research assures that more people will be looking into this topic in the years to come. We do not know all we will know in a few years as we continue to learn more about how carnivores might influence the spread and prevalence of CWD. In the meantime, it is important that we curb the temptation to only discuss large carnivores with a saint or sinner narrative.