With CWD finally confirmed in Yellowstone National Park, Predators Could be Yellowstone's Salvation

“[The buck] was healthy when we caught it. Over the summer, GPS data shows at least one other deer in that area where we found him [dead],” Mong said. “But in Yellowstone, Grand Teton; a majority of our deer live in those areas. I wouldn’t be surprised if we do find some others, just based on the 10-15 percent [infection] rate that we have for CWD in this herd unit and the number of animals moving from the Cody area to the park for the summer. I think it’s inevitable that there are other individuals that probably have CWD that use the park. It’s just that this one had a collar and alerted us when it did die.”

Mong informed Yellowstone National Park, which sent out a press release on Nov. 14 announcing that the first known case of chronic wasting disease had been found in Yellowstone. It was a sad revelation for America’s first national park, where many visitors come specifically to see and photograph its abundant wildlife.

It’s uncertain how the park will respond, beyond a stated push to increase monitoring of deer, elk and moose, and testing carcasses for CWD like Mong did with his deer. Yellowstone published a Chronic Wasting Disease Surveillance Plan in 2021 but removed it from its website this past week. When Mountain Journal requested a copy of the plan, Public Affairs Officer Morgan Warthin said in an email that they’d removed it because they’re revising it to include additional actions the park will take to manage the disease and will share it when it’s complete sometime in 2024.

The news attracted significant attention on social media, where some expressed despair while others said they were disappointed but not surprised.

Dan Vermillion, former Montana Fish, Wildlife and Parks commissioner, was in the latter camp, saying it was “only a matter of time.” During the dozen years he was on the commission, Vermillion watched CWD spread and worsen throughout Wyoming and over the Canadian border in Alberta and Saskatchewan. He wasn’t surprised with Montana’s first positive test in 2017, and he isn’t now with the Yellowstone case.

What does surprise him is that Wyoming insists on maintaining the elk feedgrounds south of Yellowstone and Grand Teton national parks. CWD spreads more quickly where elk or deer congregate, and elk gather on the feedgrounds in high densities during winter. Some of those elk migrate to other locations, so they could carry the disease with them.

Grand Teton National Park had its first CWD case in 2018 when a car struck and killed a buck mule deer that tested positive. According to a 2018 Wyoming Game and Fish release, biologists said it wasn’t a surprise based on finding infected mule deer to the south in Star Valley and Pinedale in 2017. Some muleys that summer in Grand Teton National Park and near some elk feedgrounds spend winters near Dubois and Cody, so migration aids the spread.

Vermillion hopes the attention on the Yellowstone deer will prompt Wyoming to reconsider the feedgrounds.

“When I was on the commission, we had really contentious meetings with the Wyoming Fish and Game commission and their director on this very issue,” Vermillion said. “We sent them a pretty stern letter asking them to stop feeding elk. They took great umbrage, but I think we were right. And I still do … The fact we see CWD pretty much all around the [Greater Yellowstone Ecosystem] shows you this is a situation where the toothpaste may out of the tube, and all we can do is hope to figure out how to slow the spread, not eliminate it.”

Washington State University pathology professor Margaret A. Wild agrees that Wyoming should reconsider their policy on feeding elk now that it’s certain that animals in Yellowstone have been exposed. However, it’s unlikely the Yellowstone deer was the first case; it just happened to be collared so its illness was identified. Wild should know. Not only does she study emerging infectious diseases like CWD, but she was the chief wildlife veterinarian for the National Park Service until 2018.

However, Wild has hope for Yellowstone because, of all the national parks, it may have the one strength that could help its deer, elk and moose populations endure the disease: a healthy dose of predators.

“They’ve got wolves, they’ve got mountain lions. These predators can detect animals that are sick long before people can,” Wild said. “Some modeling we’ve done in the past, we showed that removal [of CWD-infected cervids] by hunters may help some. But what really helps is selective removal by predators. Assuming that predators can detect and remove animals earlier in the disease course, they can reduce the amount of time a deer or elk is transmitting the disease to other animals either directly or by putting feces, urine or saliva into the environment that their counterparts could encounter. So, a complete predator guild like Yellowstone has is one of the best things I can think of for chronic wasting disease management.”

Other parks aren’t so lucky. The USGS has found that CWD was the leading cause of death in the elk population in Wind Cave National Park south of Rapid City, South Dakota, with infection rates up to 24 percent. So, scientists are evaluating the effectiveness of reducing elk density on chronic wasting-disease mortality. But without predators around, that requires that people either haze or harvest elk.

Paul Cross, USGS research biologist at the Northern Rocky Mountain Science Center, and his team have done some of the computer modeling that shows predators can help in two ways. First, they keep densities of elk and deer down and they devour carcasses, so animals have fewer opportunities to come into contact. Second, they may be able to cull the sick individuals out of the herd early so they have less of a chance to spread the infection.

Their modeling shows that works. But they’ve yet to demonstrate the second hypothesis in real populations, partly because they don’t yet know when infected deer or elk become contagious. If deer or elk experience symptoms before they are contagious, predators could be the key to keeping infection rates down. Otherwise, predators play a smaller role.

“What we learned on a personal level from Covid-19 is it’s hard to control a disease when transmission occurs before symptoms appear,” Cross said. “So, this is a challenging question that will take us a long time to empirically answer. But national parks serve an important role in providing places to observe and study how ecosystems function. In this case, Yellowstone, Grand Teton—they’re critical in understanding how predators might serve to keep the herd healthy. We have a hard time learning that in other places.”

In the meantime, Mong said it will be up to the park and state biologists like him to remove animals showing obvious signs of CWD.

“Hopefully we can use this as an educational tool and get people on board for managing chronic wasting disease,” Mong said.