By Marcelo Jorge PhD, Southeastern Cooperative Wildlife Disease Study, University of Georgia Warnell School of Forestry and Natural Resources
You may love to watch these graceful animals frolic, you may despise them for destroying your garden, or you may even look forward to the next hunting season and stocking your freezer. Whatever your relationship to deer, we have all been witness to their massive population explosion, and ORLT landowners often seek advice about managing deer on their property. Chronic wasting disease, while not yet documented in Georgia, is on the state’s doorstep and may have vast repercussions on deer populations. ORLT asked Marcelo Jorge, from UGA’s Warnell School of Forestry and Natural Resources, to help inform our readers about this critical issue.
Chronic Wasting Disease: Isn’t that a “Zombie Deer” Disease?
Chronic wasting disease (CWD) is unlike most wildlife diseases because it is not caused by bacteria, fungi, parasites, or viruses. CWD is a prion disease. Prions are misfolded proteins that cause other proteins to misfold, which subsequently negatively impacts the health of infected individuals. Broadly, prion diseases are called transmissible spongiform encephalopathies (TSEs). Other TSEs you may have heard about include “mad cow disease”, or bovine spongiform encephalopathy, in cattle and Creutzfeldt-Jakob disease, a human-borne prion disease. Breaking down the words in transmissible spongiform encephalopathies, we can see that these diseases can be spread between individuals (transmissible), that they create holes in the brain making it look like a sponge (spongiform), and that these are brain diseases leading to neurological symptoms similar to dementia (encephalopathy). CWD was first detected in a research facility in Colorado in 1967, but the origin of the disease in that facility remains uncertain. CWD may have jumped from sheep in the facility with scrapie (another TSE) to captive deer. Alternatively, CWD could have already existed in wild populations around the captive deer facility. Since its discovery, CWD has been detected throughout North America (35 US states and 5 Canadian provinces) as well as Finland, Norway, Sweden, and South Korea (in a captive facility). As a “deer” disease CWD is not limited to white-tailed deer. Mule deer, elk, red deer, moose, fallow deer, sika deer, and Reeves’s muntjac are species known to be susceptible.
How does CWD impact infected individuals?
A deer can pick up these prions through direct contact with other deer and potentially through ingesting free prions in the contaminated environment (i.e., soil, feed, plants, water). Research has demonstrated that environmental transmission is possible, but how often it occurs in nature is unknown. Once inside the body, these prions begin to replicate during an incubation period when no apparent symptoms can be seen to the naked eye. After approximately three months, infected deer can begin to shed these prions via bodily fluids as more prions continue to replicate in the body. Importantly, this shedding can occur while they are still asymptomatic. An infected deer often appears healthy between this point and when the terminal stage of the disease occurs 1.5-3 years post-infection. Deer with terminal stage disease exhibit symptoms such as weight loss, lack of coordination, excessive salivation, behavioral changes, drooping ears, and lethargy begin to manifest as a result of deteriorating brain function. The disease ultimately culminates in premature death killing 100% of the individuals it infects.
Fortunately, the slow progression of the disease allows infected individuals the opportunity to reproduce. Female white-tailed deer can produce 1-3 fawns each year depending on their nutrition and health, and research indicates no significant difference in annual fawn production and survival between CWD positive and negative females. Although there is some evidence of prions occurring in the fetal tissues of offspring, we still don’t know the probability of vertical transmission (mother to fawn transmission) and how much of an impact it may have. CWD positive deer can still add to the population but with a reduced lifetime reproductive potential compared to a healthy doe.
These game camera images of two individuals demonstrate differences in body condition. Either of them could potentially be infected with CWD, as body condition is not always an indicator.
The million-dollar question: how do we solve this problem?
Preventative measures for CWD are currently a hot research topic. Vaccines, the use of copper supplements, and genetics are a few of the current areas of research; however, there is likely no silver bullet solution. Thus, management should be the goal. We must focus on how to slow the spread of the disease and how to manage and maintain infected populations. Although CWD has not yet been detected within Georgia, it has been detected in Florida roughly 30 miles from the Georgia-Florida state line. Therefore, it’s important for Georgia to prepare in case CWD moves across the state line.
One way to reduce the spread of the disease is by decreasing the density of deer herds. This isn’t a new idea but one that has been tried and tested for many transmissible diseases. The underlying concept for population density reduction is to minimize the probability of interaction between CWD-negative and CWD-positive individuals. This idea is functionally the same as the lockdown and social distancing protocols instituted globally during the COVID-19 pandemic. Fewer infected people coming into physical contact with each other decreased the chances of a susceptible person contracting the disease. In terms of CWD management, we can’t possibly know every deer’s infection status, so we lower the density of the entire herd. Many deer populations are already overpopulated, especially in suburban areas, so decreasing some deer herd densities would also align with good herd management practices. In areas where deer densities are already low, the goal would be to maintain that existing herd density and weather the storm.
USGS CWD Distribution Map. Lightly shaded counties are counties where CWD has been detected in wild populations, dark shaded counties are where CWD was detected in wild populations prior to 2000. Yellow circles indicate a captive deer facility that has been since depopulated after a CWD detection and red circles indicate a CWD-positive captive deer facility. Source - .https://www.usgs.gov/media/images/distribution-chronic-wasting-disease-north-america-0
How do managers reduce herd densities?
Hunters who harvest deer play a critical role in herd management. Hunters directly remove infected individuals and simultaneously decrease population density. Furthermore, they also provide wildlife agencies with invaluable data on demographic information such as sex, age classes, and fertility rates along with disease prevalence information, which helps managers monitor herd health and disease progression. If you’re not already a hunter but want to help combat CWD, consider taking up hunting as a hobby. Not only does it build comradery with your family and friends, but hunting is also a great way to gain access to lean, organic, free-range protein.
If you are a hunter and are worried about CWD, get your deer tested as soon as possible. Many state agencies provide free or low-cost testing for hunters. While there are no confirmed cases of human illness associated with the consumption of CWD-positive venison, the Center for Disease Contro (CDC) advises against consuming meat infected with CWD. Because these proteins become misfolded, they form a strong structure that is difficult to break, meaning that they are resistant to degradation and can last in the environment for 5,10, or even 15 years. Researchers have tested a slew of tool decontamination methods, including boiling, dry heat, formaldehyde, radiation, microwave, UV light and autoclaving (249.9 °F for 15 minutes), with little success. Ironically, the sterilization technique most commonly used is household bleach. If you ever harvested a CWD-positive deer, scrub your knives to remove blood and tissue, and then soak your knives and tools in bleach for 5-10 minutes. It should be noted that although this is very effective for metals and plastics, absorbent materials like wood may only have the surface disinfected while the interior could remain largely unaffected.
If deer hunting isn’t something you care to take part in, you can still help by reporting oddly behaving or sickly-looking deer. If you observe a deer that looks emaciated with its ribcage and hip bones poking out, consider calling your state wildlife agency to investigate. However, deer can also become emaciated as a result of non-CWD related factors such as fawn-rearing, a nutritionally demanding time or season, or in overpopulated areas lacking adequate resources. In my experience, the first signs of CWD don’t become apparent from their physical condition but rather from their behavior. For example, deer are typically most active around sunrise and sunset, and generally find a secluded area to bed down and rest during the day. However, cognitive decline in CWD-positive deer could result in an individual feeding in a field by itself during the middle of the day. While I was working on the Arkansas CWD deer study, every so often I would find an odd deer foraging by itself in a field near the road during the day. Within a week I might see that same deer three times in the same location at similar times always by itself. Every single time I would call the biologist the third time I would see it. To date, every deer I called in came back CWD-positive despite otherwise appearing healthy. Additionally, CWD-positive deer have been known to bed down against people's houses and they generally lack a fear of humans. It should be noted that in non-hunted urban and suburban populations it might be more difficult to identify diseased deer based on behavior. In these areas, deer often become acclimated to human presence and may not have a fear of humans.
One final (and more controversial) means of managing the spread of CWD is to minimize the practice of baiting and feeding deer. Because the saliva of infected deer contains the prions, setting up corn piles may be leading more deer to become infected. This issue is controversial because in many places like Georgia, there is a culture around baiting for deer and many people see it as a way to improve hunting success. Harvest is one of our best tools to slow the spread of CWD, so we must weigh the pros and cons of baiting and feeding. If baiting allows us to reduce the deer herd density more effectively, that’s great, but that benefit might be a complete wash or worse if baiting is also significantly increasing transmission rates between deer. As previously mentioned, research suggests that environmental transmission is possible, but we don’t yet know how much of an impact it has under natural conditions. Given the potential for transmission at feed or bait sites, providing forage through habitat management is a superior alternative to feeding/baiting in terms of disease transmission, forage quality, and the number of species benefitted. Habitat management practices such as wildlife plantings, the use of prescribed fire, and/or the creation of forest canopy openings can effectively provide quality deer forage while also improving habitat conditions for a multitude of other wildlife species. Food plots planted with desirable deer forage (e.g., soybean, corn, clover, winter wheat, rye, brassicas, etc.) also allow for great hunting opportunities with a much lower risk of disease spread. If you are adamant about feeding or baiting, I would suggest that you use a feeder that will cast the feed out spreading it across a localized area instead of piling it up in one concentrated spot. Regardless, baiting and feeding aggregate deer into a relatively small area. Although you might not have thought about your bird feeder as a problem, it too can congregate deer around a point source of food. You don’t need to remove the bird feeder entirely. Instead, I suggest raising it to keep it out of reach from deer and periodically removing any bird seed that has fallen on the ground.
Conclusion
Chronic wasting disease is a challenging disease to cope with, and global research and management efforts are underway to develop and implement ways to manage the disease. The public also plays a part in slowing down the spread of the disease, and I encourage you to join the fight against CWD, either by continuing to hunt, getting started on your hunting journey, calling in any sick or odd deer, not baiting or doing so more carefully, or cleaning up around your bird feeders. The task at hand is a big one but, with a concerted effort from us all, we can weather the storm and make sure generations to come can hunt, watch, and fall in love with deer and the natural world.