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Study: Chronic Wasting Disease kills 19% of deer herd annually

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.

Wyoming Game and Fish Department created this map of the most recent hunt areas where Chronic Wasting Disease has been found in mule deer. The map does not include infected elk hunt areas, which don't cover as much territory as infected deer country. (Wyoming Game and Fish Department)

Wyoming Game and Fish Department created this map of the most recent hunt areas where Chronic Wasting Disease has been found in mule deer. The map does not include infected elk hunt areas, which don’t cover as much territory as infected deer country. (Wyoming Game and Fish Department)

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?

In this excursion to Kemmerer in 2014, DeVivo helps collect and process mule deer blood for migration and nutrition studies. Field work involves helping colleagues and the initial processing of samples, sometimes in far-flung motel rooms with pizza on the dinner menu. (Angus M. Thuermer Jr./WyoFile)

In this excursion to Kemmerer in 2014, DeVivo helps collect and process mule deer blood for migration and nutrition studies. Field work involves helping colleagues and the initial processing of samples, sometimes in far-flung motel rooms with pizza on the dinner menu. (Angus M. Thuermer Jr./WyoFile)

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.

Lucky, the 600 pound elk, has not caught CWD despite 13 years of exposure to the disease in a Game and Fish Department wildlife lab. Her rare genotype may make her resistant to the disease, possibly even immune. (Terry Kreeger/Wyoming Game and Fish Department)

Lucky, the 600 pound elk, has not caught CWD despite 13 years of exposure to the disease in a Game and Fish Department wildlife lab. Her rare genotype may make her resistant to the disease, possibly even immune. (Terry Kreeger/Wyoming Game and Fish Department)

“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.

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About the Author

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Angus M. Thuermer Jr. is the natural resources reporter for WyoFile. He is a veteran Wyoming reporter and editor with more than 35 years experience in Wyoming. Contact him at [email protected] or (307) 690-5586. Follow Angus on Twitter at @AngusThuermer

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7 Responses to Study: Chronic Wasting Disease kills 19% of deer herd annually

  1. Wyatt Wittkop March 3, 2016 at 3:04 pm #

    I don’t think the predator comment in the article was suggesting that carnivores do nothing to combat the spread or persistence of disease in the deer herd. It specifically said, “having more predators might not help limit CWD’s spread.” The key word there is more. It is apparent that carnivores have a strong presence in the area and are contributing greatly to the overall mortality of the deer. Mountain lions are the leading cause of mortality. The article acknowledges that mountain lions are selectively harvesting CWD positive deer, thereby informing the reader that they are likely having a beneficial effect. I would agree that, even given all the documents referenced in the comments, it is unclear whether MORE (additive) carnivores in the area would [further] limit the spread of disease. At some level of carnivore density, they may no longer have a positive effect on CWD prevelence, and who’s to say what that level is and whether or not the extent of benefits in the area have already been reached with the current level of predators?
    At what point in the procession of the disease are predators able to recognize decreased vigilance or increased susceptibility? Would they be more likely to kill healthy fawns than slightly CWD impaired does or bucks, thereby allowing for the persistence of the disease?
    Do any of the documents referenced in the comments actually address whether or not the introduction of MORE predators into that specific study area would further limit the disease?

    I don’t think the predator comment was nearly as far fetched as the comments would lead you to believe.

    Buffalo, Wyoming

  2. Louise nelson December 20, 2015 at 11:51 am #

    We also had this wasting problem on the or. Coast. The dear also had liver flukes. Our current local herd seams to have made some recovery since it first appeared. Today our local dear look very healthy. You might find some info from the or. Wildlife folks. To sad for words…. At first I thought it was from the timber industry spraying toxic chemicals on the Forrest, I still have my suspicions….. They continue to spray all over the country, add to that, farmers growing gmos & spraying round up, fracking operations everywhere loaded with methan & toxic chemicals in open pits & water systems…. The writing is on the wall but no one seams to be reading it…..

    Louise Nelson

    Lakeside, or. , Oregon

  3. Barb Rupers December 18, 2015 at 5:37 pm #

    Article in Bozeman about CWD with a map showing its recent advance in Wyoming.
    http://www.bozemandailychronicle.com/news/yellowstone_national_park/fatal-deer-and-elk-disease-moves-toward-yellowstone-park/article_6983a108-b344-5d05-97bb-77884cf19120.html

    Barb Rupers

    Sheridan, Oregon

  4. Damon Jensen December 16, 2015 at 1:57 pm #

    I would like to thank Wyofile and Mr. Thuermer for an excellent article. The disease scars me as in 2008 a moose tested positive for CWD near Bedford/West Wyoming. I think it is interesting that the article mentions that, “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.” Moreover, certain phenotypes such as LL elk and FF deer are more resistant to Chronic Wasting disease. Hunting may be a tool to shape a healthier herd. I am certainly glad that hunting licenses can help fund the Game and Fish as they try to address this important problem.

    Damon Jensen

    Afton, Wyoming

  5. Kent Nelson December 15, 2015 at 3:48 pm #

    Wyoming Wildlife Advocates would like to comment on one particular remark from the article:

    “Having more predators might not help limit CWD’s spread.”

    Wyoming Wildlife Advocates has examined the available research and discussed this issue at length. It is a topic that we follow closely.

    In general, I would note that the scientific literature does speak directly to the issue of large carnivore predation on ungulates with CWD. Some samples of the available scientific literature of the role predators play in a CWD environment follow:

    “The prolonged clinical course and type of clinical abnormalities associated with CWD make it the prototypic disease for selection by predators. Chronic wasting disease produces subtle changes in behavior and body condition that progress over weeks or months to overt signs of end-stage disease typified by loss of attentiveness or response to external stimuli, emaciation, and weakness.” (Williams and Young, 1980, 1992; Wild et al., 2002).

    “The results of selective predation were consistent and robust.” (Wild et al., 2011) and, “modest levels of selectivity might be expected to greatly diminish the persistence of CWD in a susceptible deer population” (Wild et al., 2011)

    “Simulation results suggested that selective predation could also dampen or eliminate the emergence of CWD in new locations, adding support to speculation that the absence of large predators presents an amplification risk factor for establishment of CWD.” (Samuel et al., 2003)

    “Early removal of infected individuals should markedly truncate CWD shedding and resultant opportunities for disease transmission.” (Wild et al., 2011)

    “The absence of predators may allow sick animals a longer period in which to spread CWD.” (Samuel, et al., 2003)

    “Like weak calves, limpets and toothless wonders, animals debilitated by CWD would quickly be culled by large carnivores – an elegant natural check on the spread of disease.” (Smith, 2012)

    “Thus far, control strategies relying on hunting or culling by humans to lower deer numbers and subsequently CWD prevalence have not yielded demonstrable effects.” (Conner et al., 2007).

    “What is most clear is a consistent and robust trend toward decreasing CWD prevalence in populations subject to predation, particularly selective predation, over a range of parameter estimates.” (Wild et al., 2011)

    “We consider the wolf, a large coursing predator, to be most effective in selective removal of deer vulnerable from CWD infection; however, opportunistic mountain lions (Krumm et al., 2009), and potentially coyote (Canis latrans) packs, would likely benefit from lack of vigilance by CWD-affected deer as well.” (Wild et al., 2011)

    “We suggest that predation, particularly wolf predation, may be a useful tool for management of CWD.” (Wild et al., 2011)

    Kent Nelson

    Wilson, Wyoming

  6. Lloyd Dorsey December 15, 2015 at 10:09 am #

    Margaret A. Wild, PhD, and three colleagues published “The Role of Predation in Disease Control: A Comparison of Selective and Nonselective Removal on Prion Disease Dynamics in Deer,” in Journal of Wildlife Diseases. (47(1), 2011, pp. 78-93). There are 78 reports and papers in Literature Cited at the end of this paper. Some snips from the paper: “We consider the wolf, a large coursing predator, to be most effective in selective removal of deer vulnerable from CWD infection; however, opportunistic mountain lions (Krumm et al. 2009), and potentially coyote (Canis latrans) packs, would likely benefit from lack of vigilance by CWD-affected deer as well. . . . We suggest that predation, particularly wolf predation, may be a useful tool for management of CWD.” (Wild et al. 2011:87)
    As indicated in Wild et al., and supporting studies, and CWD expert reports from Colorado to Canada, there is a significant body of science and professional opinion about predators selectively culling diseased prey and, thus, suppressing the effects of diseases in prey populations.

    Lloyd Dorsey

    Jackson, Wyoming

  7. Shane Moore December 15, 2015 at 6:57 am #

    The Wyoming Game and Fish Department has not been forthcoming to the public about the long term risks this disease presents to Wyoming wildlife. There is no mention of Melia Devivo’s study in the Draft CWD Management Plan that is currently being considered..

    Please ask the Game and Fish Department to make combating this disease a top priority.

    Shane Moore

    Jackson, Wyoming

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