How Chronic Wasting Disease Kills Deer and Elk
By Dr. Nick Haley
Over the past few years, I’ve been very lucky to work closely with a number of deer and elk farming groups, and I feel like we’ve made great strides in improving how CWD is managed in farmed cervids now and into the future. I’ve been fortunate enough to meet a wide range of farmers from across North America, with various backgrounds and herd management styles, as well as a range of wildlife agencies and managers.
In that time and through those interactions, I’ve come across a few misconceptions I felt needed addressing, and I hope through some examples and analogies I can try to set the record straight on these misconceptions.
A brief discussion on how chronic wasting disease (CWD) is transmitted and how it progresses through the body would be a helpful introduction to this discussion, so let’s start there. CWD is caused by an infectious protein, unlike most anything we’ve come across in the past – viruses, bacteria, or fungi, which each have their own ways of causing disease in animals and people. This infectious protein (a “prion”) can be shed by deer in their saliva, feces, and to a much lower extent in urine. Shedding in bodily waste likely begins somewhere around the midway point of disease – typically about a year after infection. These bodily wastes can contaminate an environment where infectious deer or elk reside – and that contamination can persist for decades or more based on what we know from studies on scrapie and penned deer.
Because of that, CWD can be spread from animal to animal either directly through nose to nose contact during grooming, fighting, or feeding, as well as by grazing in an area where positive animals have been. This is the basis for many of the regulations regarding double fencing and property quarantines.
Following transmission, the infectious protein makes its way from the intestinal tract to the brain through two different routes – either via nerves to the spinal cord or through the lymph system to lymph nodes and eventually spilling over into the central nervous system. Once in the brain, it slowly accumulates through the conversion of normal proteins into additional infectious proteins, causing neurologic problems that are at first unnoticeable, though over time get worse. At the same time, the infectious protein is leaking from the brain into the circulation and reaching organs like the salivary glands and kidneys, completing the transmission cycle by allowing it to be shed again in bodily wastes.
The first, and most important misconception I’ve come across is that CWD doesn’t kill deer or elk – that they’ll die for some other reason just like any other deer or elk. As discussed above, CWD is a progressive neurologic disease, meaning it slowly but surely gets worse over time, very similar to Alzheimer’s disease and other causes of dementia in people. In fact, that’s an important reason that researchers have been working on CWD, because the work we do with deer and elk can be applied to diseases like Alzheimer’s disease, Parkinson’s disease, and Chronic Traumatic Encephalopathy (CTE) and improve human health.
Most of us are probably of an age where we’ve known someone with Alzheimer’s or other form of dementia, and those who have will recognize that while it’s not fatal on its own, it’s severely debilitating and results in a significant and progressive decline in quality of life – and not just for the person with the disease but their caretakers as well, with those who are afflicted eventually succumbing to something directly or indirectly related to their disease.
Imagine now, for a second, that a disease like Alzheimer’s was transmissible between people, and not only through direct contact, but also indirectly like the flu or common cold. While it may not be as contagious as the common cold, CWD certainly is every bit of a concern for deer and elk as human diseases like Alzheimer’s. The average reader here should agree that they’d have an easier time surviving if they were dropped somewhere in the wilderness than someone with dementia; in wild deer and elk, CWD can mean the difference between escaping a mountain lion, dodging a car, or eluding a hunter’s arrow.
A number of studies have shown this to be the case – deer with CWD are more likely to be found as lion kills, hit by a car, or harvested during hunting season. In the captive research animals that I’ve worked with that have CWD (animals that aren’t under those same pressures), the deer will steadily lose body weight and begin to develop subtle but progressive nervous system signs starting about halfway through the course of disease. This halfway point varies based on the dose of CWD an animal is exposed to and its genetic background, but it’s quite predictable. Despite losing weight, the deer typically have good or great appetites – eating their food, shavings off the floor, even rubber gloves, which could be a result of hormonal imbalances resulting from the effects of the disease on the central nervous system.
Because of restrictions placed on animal used in research, deer in studies that I’ve been a part of in the past are typically euthanized when they’ve lost about 25% of their body weight and are showing a number of very obvious neurologic problems like aggression, inability to stand, or difficulty swallowing – symptoms mirrored by aging Alzheimer’s patients. The key differences are that CWD is transmissible, as mentioned above, and that the disease affects animals in the prime of their life – typically between 2-6 years of age, depending on when they were exposed and their genetic background.
The second misconception I’ve come across is that CWD won’t affect wild deer and elk population numbers. Many of us in states where CWD has been found can appreciate this observance – that deer populations in southern Wisconsin, northern Illinois, or elk populations in Colorado and Wyoming are, at worst, consistent or in some cases increasing. Those with a background in wildlife management or animal sciences might be able to see why that might be, but let’s take a closer look.
Most of us know that when left unchecked, wild populations tend to increase to a point of being destructive to their habitat and easing disease transmission, for example, which would eventually lead to animals dying off. It’s the very reason that hunting is important – it keeps the population at the appropriate balance with what the environment can sustain. Wildlife agencies have a fairly good understanding about what this healthy population size should be, and they use it to determine how many antelope tags to give out in central Wyoming or how many doe tags to give out in southern New York. By removing deer and elk through hunting, the remaining animals have better reproductive rates and fawn/calf recruitment with enough food and other resources to survive and thrive.
Diseases work very much like hunters in this sense – in that deer and elk dying from CWD, directly or indirectly, results in more resources being available for fawn and calf recruitment and, at the level of disease (prevalence) in the population in some places, we see that as a stable or in some cases ironically increasing population size. CWD is somewhat more subtle than diseases like EHD, which kill animals quickly, because an infected animal with CWD can still live long enough to reproduce potentially through another 1-2 seasons after becoming infected.
However, when the prevalence of a disease reaches a certain point, just like with hunting pressure, the population can and will suffer. We don’t necessarily know what that prevalence is with CWD, but we do know that we can control the number of licenses given out, while the spread of disease that contributes to animal death, when there’s no treatment or vaccine available, is nearly out of our control. The worry wildlife agencies have is that the level of disease in an area will increase to the point where the populations will eventually start to suffer, and that’s really all we can expect from a disease that spreads between animals both through direct contact and environmental contamination.
There’s no reason not to think, without the steady development of resistance, that the prevalence of any disease that behaves like CWD would continue to increase slowly over time. Wildlife agencies try to counteract the spread of CWD by increasing tag numbers and harvest objectives, which would hopefully decrease population size to a point where transmission slows – it’s really the only tool they have to control disease without a vaccine or treatment. Apart from that, their only real hope is resistance building up in the herd, and that takes a lot of time in wild animals.
It’s this last aspect that is at the center of our latest project with NADeFA and NAEBA. We know a little about CWD resistance in deer and elk, but we don’t know the whole story. One problem with trying to understand resistance in free-ranging animals is they’re able to move around freely, and it’s difficult to identify who’s related to who so we can get a better handle on which genetic lines may be contributing to resistance. On the other hand, we have no control over who’s doing the breeding in wild populations. In a closed population like a deer or elk ranch, we can develop a better idea of genetic relationships and which groups of animals get CWD and which don’t. Typically we lose all of this information when a herd is placed under quarantine and depopulated.
It is our hope that by actively managing a herd where CWD is present we can identify resistant lines of animals quickly, and use them as breeding animals to improve overall herd resistance to CWD. This, I think, will help correct a misconception commonly held by wildlife agencies and regulators – that CWD is a farmed deer and elk problem and nothing good comes from deer and elk farming.
I think, apart from helping to sustain a livelihood and grow an agricultural industry, the practice of deer and elk farming can help improve the genetics of deer and elk herds – and not just in terms of antler size which has its own benefits – but in terms of disease resistance in the case of CWD. I’ve given this very statement to a wildlife management director who was opposed to deer and elk farming: “someday in the future, a deer or elk farm may be the only place you can find an animal that won’t get CWD, in which case you’d be more than happy to have them back to help support the wild populations.”
Deer and elk farming has a long history in North America, and in states like Iowa the practice has helped restore populations that were almost eliminated by over-hunting and habitat destruction. That’s enough reason for me to believe that with the collective help of the deer and elk farming industry today, we can make a difference with diseases like CWD.