Chronic Wasting Disease will cause a Wyoming deer herd to go virtually extinct in 41 years, a five-year study predicts.
The investigation, which relied on the capture of 143 deer, examined the dynamics in the Southern Converse County Mule Deer Herd that lives southwest of Douglas near Laramie Peak. There, a population that once numbered some 14,000 in the early 2000s dwindled to half that size in about a decade.
The Chronic Wasting Disease study is one of only three that have been conducted on wild deer, elk or moose herds, none of which have yet seen print. While wildlife managers have long suspected CWD as a principle agent in the ravaged Converse herd, the study puts numbers on the problem, calculating a 19 percent decline annually.
University of Wyoming doctoral student Melia DeVivo spent four years of fieldwork and another year crunching numbers before defending her PhD thesis on the herd. She calculated the herd would go extinct in 41 years, without taking into account genetic differences that make some deer more resistant to CWD, or accounting for deer migration into the area. Even when taking in those factors, the herd will decline dramatically, she said.
“I estimated that CWD was causing a 19 percent annual reduction in the population, which is pretty significant,” she said. “Potentially, in 41 years, it would be locally extinct.”
A malformed protein called a prion is widely believed to be the infectious agent in CWD, eroding the central nervous system and causing animals to waste away in a few years. While transmission to humans hasn’t been demonstrated in this variant of Mad-Cow Disease, experts recommend against eating meat from an infected animal. The human form of the disease family — transmissible spongiform encephalopathy — is called Creutzfeldt-Jakob Disease.
Managers are uncertain what will happen to wildlife as CWD — which infects deer, elk, and moose — spreads west across Wyoming.
There is no vaccine, no sure method to prevent the spread and no known cure for the invariably fatal CWD. One fear centers on 23 elk winter feedgrounds west of the Continental Divide where the specter of rapid transmission looms among wildlife that is artificially concentrated in the winter.
Essential to DeVivo’s study was the ability to test live deer for CWD when they were captured. The procedure took samples from animals’ tonsils before the deer were collared and released.
Infected adult mule deer had only a 32 percent annual survival rate. Uninfected deer survived at a rate of 76 percent annually.
Surprisingly, CWD-infected does birthed and raised fawns at the same rate as those that did not have CWD. “We actually found that CWD didn’t have an effect on pregnancy or recruitment,” DeVivo said.
The number-one cause of mortality was mountain lions, the study found.
“I was already on the lookout for that,” DeVivo said. “This has also been shown in CWD studies in Colorado. We did see that CWD-positive deer were more susceptible to mountain-lion predation.”
The second leading cause of death was CWD itself.
Infected deer were more likely to be killed by hunters than healthy deer, the study showed. But the sample size supporting that finding was small.
“The males that were harvested were all positive males,” DeVivo said. “That suggests CWD-positive deer are also more likely to be hunted.”
What to do with the findings?
Researchers are uncertain what to do with the information.
“We don’t have real good answers,” DeVivo said of potential recommendations. “This study wasn’t set up to answer management questions per se.
“There’s no vaccine treatment or good way to reduce transmission in free-ranging populations. We’re basically left with trying to study the disease.”
Game managers have implemented hunting regulations prohibiting transporting the brains and spines of animals from an infected area in hopes of stemming the spread of CWD. Wyoming Game and Fish Department has released a draft plan for updating its CWD strategy and is seeking comment. The one recommendation DeVivo would make for hunting already has been instituted.
“In my population, Game and Fish already went ahead and eliminated the doe-fawn season,” she said. “They [Converse deer] don’t need another cause of mortality. All I say is they should continue to not hunt does and fawns from that population.”
Healthy habitat, such as adequate winter range, remains important, she said. “Anything that’s good for overall deer health is going to be great for this population,” she said.
Having more predators might not help limit CWD’s spread.
“We don’t know a whole lot about how or if they could even regulate the disease,” DeVivo said. “We don’t know what their impact would be.”
Researchers are investigating whether predators might even be an agent in the spread of Chronic Wasting Disease. The malformed prion can survive in soil and can be transported in plants and feed.
“Are they [predators] somehow contributing to the spread?” DeVivo asked. “Maybe prions can pass through their system. While they key in on weak and sick, they can still kill [healthy] animals. I don’t think we know enough to start using predators as a management tool for this disease, basically.”
Genotypes make a difference
Researchers have found that deer with different genes react differently to CWD exposure. The key gene at a location known as codon 225 can have one of three combinations of alleles respectively named SS, SF or FF, DeVivo said. All three are represented in the Converse herd.
The majority of mule deer today are SS, and they get infected at a higher rate, the study showed. “They are 30 times more likely to be CWD-positive compared to deer that had SF or FF,” she said.
Only one of 29 mule deer of the SF genotype turned up CWD positive. FF deer are rare — only two of the 143 captured had that genotype. Neither was infected with CWD.
Although DeVivo’s mathematical model predicted the herd would go extinct in 41 years, without accounting for genetic variations, that won’t actually happen because new deer are expected to move into the herd’s home range. When accounting for genetic variation, the study predicts the genetic mix of a surviving population will change over time.
“What I found was within a 100-year period we do see a significant increase in those less-susceptible genotype — where the FF becomes the dominant genotype,” she said. “I still model a significant decline in the population.”
What scientists don’t know is whether the FF genotype is rare because it also carries a disadvantage. For example, suppose FF does were somehow unattractive to bucks or turned out to be bad mothers.
“That’s the debate right now,” she said. “Just because it’s rare does not mean [it is] some adverse genotype. Since there was no selective pressure on it ’til now, it’s just a rare genotype.”
Her study captured two FF does, but data on them spans only one year, and the small sample size precludes scientific conclusions.
“They were not positive [for CWD,] they both were pregnant and they both recruited fawns,” she said. “It seemed like they were just as fit as any other deer in the population.”
Does this research translate from deer to elk?
DeVivo’s research doesn’t translate easily to elk. “We know they [elk] are very susceptible,” she said. But wild elk in Wyoming have not been decimated in the same way as Converse deer.
“For some reason in free-ranging populations [of elk] we just don’t see the same thing we see in captivity,” DeVivo said. “I don’t know if anyone knows why that is.”
One modeling study published last year predicted severe declines among feedground elk in Sublette County, even after hunting seasons are modified in the face of infection. That conclusion took into account an apparent difference in susceptibility among elk of three different genotypes.
Like the deer, elk are classified into three genotypes when considering their sensitivity to CWD, said Brant Schumaker, assistant professor at the University of Wyoming Department of Veterinary Sciences. About 70 percent in a herd are MM genotype elk and most susceptible. Most of the rest are ML genotypes and live slightly longer. The only LL elk in a decade-long Wyoming experiment hasn’t become infected. LL elk make up approximately 2 percent of a normal population.
In a 10-year experiment Schumaker took part in, Wyoming Game and Fish captured a band of 39 elk in Jackson Hole in 2002, relocated them to a southeast Wyoming facility, and exposed them to CWD. In 10 years, all the other elk in the group contracted CWD from infected pens and withered away at the Tom Thorne/Beth Williams Wildlife Research Center at Sybille.
But not the lone LL elk. Her ear tag is No. 12, but researchers nicknamed her Lucky. She’s had a calf, and doesn’t look sick or test positive for CWD.
“She is, if anything, overweight,” Schumaker said. “My understanding is that she’s kind of a pill from the caretakers’ perspective. She stands up for herself.”
Is Lucky the 600 lb. elk immune to CWD? Will Lucky’s LL cousins become the mothers of all future Wyoming elk once CWD runs its course? Does the LL genotype also carry some disadvantage scientists don’t know about?
Even once Lucky dies and is autopsied, Schumaker said, researchers won’t know for sure whether she is immune. A new experiment would be necessary to determine that.