Recently scientists have been exploring the idea that we can fight chronic wasting disease in wild deer by releasing captive-bred, “CWD resistant” deer. Some in the deer farming industry endorsed the idea, and legislators in Oklahoma even authorized a program to begin breeding and releasing deer. Most scientists, however, are urging everyone to pump the brakes.

On July 15, the Theodore Roosevelt Conservation Partnership invited two CWD experts to give a live webinar on the topic, titled “Breeding to Battle CWD: Can Wildlife Evolve Their Way Out of Disease?” To help shed more light on this important topic, this article is a summary of the most important take-home messages for deer hunters that came out of the two presentations. You can also watch the full presentations here.

Part 1: Dr. Debbie McKenzie

The first speaker was Dr. Debbie McKenzie, an emeritus professor at the University of Alberta’s Centre for Prions and Protein Folding Diseases. Her research focus for the past 35 years has been on prion diseases, specifically CWD.

“Resistance” Needs More Evidence

This entire discussion originated with one study that suggested certain genetic strains of deer – known as the 96SS genotype – could be “resistant” to CWD. That study is a statistical model that suggests 96SS deer test positive for CWD less often than other deer. The study did not challenge live deer with CWD infection. Both speakers suggested they’d welcome the use of Genomically Estimated Breeding Value (GEBV) if it can slow the spread of CWD among captive herds, but they say real-world evidence is lacking so far.

“Though I think there’s some utility with GEBV, I really think we need some challenge experiments so we can demonstrate that these statistical analyses do point to increased resistance,” said Dr. McKenzie.

“Resistant” Deer Still Get CWD But Live Longer

Meanwhile, studies of actual deer, according to Dr. McKenzie, show that 96SS deer are not “resistant” but experience a longer incubation period before dying. “96SS and other polymorphisms are linked to slower disease progression, but they are not resistance genes,” she said. “If those animals are exposed, they will get CWD. It’s just going to take longer from the time they are infected until they are clinically sick.”

Living Longer With CWD Means Greater Spread

Longer incubation periods with CWD are counterproductive, because even CWD-infected 96SS deer are still shedding prions into the environment and sharing them with other deer. “Shedding is still occurring throughout those longer incubation periods,” said Dr. McKenzie. “If you have double the incubation period, but you’ve only made a 10% reduction in the prion shedding, you’re actually going to end up with more environmental contamination.”

New Strains Might Make “Resistant Deer” Irrelevant

New strains of CWD have already evolved, and more are coming. According to Dr. McKenzie, selecting for 96SS genetics in deer could theoretically lead to reduced susceptibility to some CWD strains. “On the other hand,” she said, “it might mean we’re selecting for susceptibility to a different strain.”

Part 2: Dr. Sonja Christensen

The second talk by Dr. Sonja Christensen took the science into the real world to look at the likelihood of using genetically resistant deer – assuming they are found to be a real thing – to fight CWD in wild deer populations. Dr. Christensen is an assistant professor at Michigan State University who specializes in wildlife disease ecology, population health, and wildlife management.

Wild Deer Are a Different Issue Than Captive Deer

Genetic resistance to CWD could have usefulness in captive deer, where people control which deer do the breeding. “However, I am focused solely on wild deer,” said Dr. Christensen. “Wild populations are inherently messy.” 

We Can’t Manage Genetics in Wild Deer

Hunters have mistakenly believed for years that they can change the genetics of future deer through selective harvest, but repeated scientific experiments have failed to show that it’s possible to make a measurable difference – even under very intensive manipulation of deer harvest. The reasons why also explain why releasing a few “CWD resistant” deer in an area would be unlikely to have any impact on wild deer genetics.

Dr. Christensen listed several of the factors that would complicate any kind of genetic fight against CWD in wild deer. For example, there are over 30 subspecies of whitetails with genetic variations across regions. We lack the ability to control which deer do the breeding. Any genetically modified deer released into the wild might never reproduce due to being killed by predators, other diseases, and other mortality sources beyond our control.

Existing Native Deer Prevent Success

Stocking genetically modified deer could be more successful only if native deer with higher susceptibility to CWD are first removed. Dr. Christensen pointed out that fighting scrapie, a prion disease of sheep that is similar to CWD, required massive culling of sheep to increase resistance, and this was with captive livestock. Obviously, in the wild it would be nearly impossible to identify which deer are more susceptible or remove enough of them to make a difference. 

It Would Take a Very, Very Long Time

“In the best case scenario, assuming everything’s working perfectly and we can control all of these factors in a very messy, free-ranging herd, it’s still going to take years and years and years and years to really see this change and pick up on that signal,” said Dr. Christensen.

Why Not Try It Anyway?

Despite all the evidence that stocking “resistant” deer would have no impact on CWD in wild deer populations, Dr. Christensen played devil’s advocate and asked, “Why not try it anyway?” She answered by highlighting the extreme costs and the unintended consequences. 

“Captive deer genetics are not like wild deer genetics,” she said. “There might be other traits that are unknowingly entering a population when you release captive deer. Something else might be expressed. Because of the long incubation period, we could unknowingly release CWD-positive deer into an area that doesn’t have the disease. We could be increasing susceptibility to other diseases.”

“The outcome has a lot of uncertainly and cost for wildlife agencies, and that’s a risk to our natural resources that the wildlife agencies are managing in the public trust,” said Dr. Christensen. “Ultimately, that is undermining the North American Model of Wildlife Management and how wildlife management works.”

Other Comments About Release of Captive-Bred Deer to Fight CWD

• The CWD Research Consortium, a group of independent researchers from diverse disciplines and institutions currently working on CWD, prepared a document to provide science-based information on the use of selective breeding and release of captive deer for CWD management.
• The Association of Fish & Wildlife Agencies passed a resolution opposing “the release of any captive cervids into the wild to influence free-ranging cervid population genetics for the purpose of controlling or managing CWD, based on the current best scientific information, and encourages its members in their own jurisdictions to promote and implement the best scientific management practices for CWD.”
• The United States Animal Health Association issued a resolution calling on the U.S. Department of Agriculture to conduct a controlled experiment to test the validity of CWD resistant genetics.