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Study Tells The CWD Story: Large-Scale Research Project Illustrates Impacts In Wisconsin
Caitlyn Nalley
Caitlyn Nalley is a communications specialist in the DNR’s Office of Applied Science.
For many Wisconsinites, fall is synonymous with deer hunting. This not only means time spent in the field pursuing Wisconsin’s most iconic wild animal, but also brings chronic wasting disease front and center for avid deer hunters and conservationists alike.
For those unfamiliar with CWD, it is a disease that can affect animals in the cervid family such as deer, elk, moose and reindeer. It is caused by infectious, misfolded proteins known as prions that progressively accumulate throughout the body, including in the brain, eventually leading to cognitive decline and death.
Because CWD is always fatal, a big concern is its potential impact on deer populations. This is what prompted the DNR to launch the Southwest Wisconsin CWD, Deer and Predator Study in 2017.
Led by DNR deer research scientist Daniel Storm, Ph.D., the Southwest Study sought to provide the most comprehensive understanding yet of how CWD is impacting Wisconsin’s deer population.
The sheer scale of the project was massive, with over 1,100 captured and collared deer and more than 100 collared deer predators (bobcats and coyotes).
“We worked in the part of Wisconsin where CWD has been around the longest and where CWD prevalence is highest,” Storm said. “This ensured that we’d collar a good sample size of CWD-infected deer.”
Key Takeaways
For years, scientists closely monitored the lives of these collared animals, performed necropsies (animal autopsies) when they died and thoroughly analyzed the data. Their hard work and the robust study design resulted in two main takeaways.
First, for both females and males, the probability of a CWD-infected deer surviving to the next year was substantially lower than that of an uninfected deer surviving. For infected females, the survival rate was 41%, compared to 83% for uninfected females, and for infected males it was 17%, compared to 69%.
Scientists used this finding, in conjunction with data on deer recruitment (deer that survive to around 6 months of age are considered “recruited” into the population), to model how population growth might change depending on the prevalence of CWD.
The model predicted that when about 29% of adult female deer are CWD-infected, the local deer population can be expected to decline.
Data Details
So, CWD spells trouble for white-tailed deer in Wisconsin, but how do we know that CWD specifically is what’s causing these deer to die? To answer that, let’s look at the life of deer #7202, one of the 1,133 deer included in the Southwest Study.
On Feb. 27, 2017, the research team captured #7202, a 4-year-old doe in Grant County, and fit her with a
GPS tracking collar. During this initial collaring, the team also performed a quick biopsy by taking a small amount of rectal tissue and later testing it for CWD.
The test revealed that #7202 was CWD-positive. Despite this, she did not appear sick at capture. In fact, she appeared to be in great physical condition and weighed a whopping 170 pounds, considerably heavier than the weight of an average adult doe (about 145 pounds).
Just three months after her initial capture, on May 29, 2017, scientists received an alert from #7202’s collar indicating she had died. The team rushed out and, after locating her at the base of a hill she had fallen down, brought her body to UW-Madison for a necropsy.
#7202 now weighed an alarming 106 pounds, but there was minor predator scavenging, so she probably weighed around 110 pounds at death. In the time since her initial capture, she had lost roughly 35% of her body weight, including all body fat and some muscle mass.
The necropsy also revealed that #7202 had contracted pneumonia and developed holes in her brain, known as spongiform encephalopathy, typical of endstage CWD.
“CWD-infected deer produce more saliva and have trouble swallowing, so saliva or food can go down the wrong pipe, which can introduce bacteria to the lungs and cause infection,” Storm explained.
“It’s also important to note that this deer starved during a time of plentiful food, so we’re confident that the starvation was driven by CWD, not food availability.”
Sadly, #7202 was also found to be carrying a mummified fetus, meaning she had been healthy enough to get pregnant about seven months before her death.
End Results
Along with the extreme and rapid weight loss, the case of deer #7202 illustrates just how brief and severe the end stage of CWD can be compared to the disease’s entire infection period, which is believed to be between 18 and 24 months. However, it’s important to note that infected deer are still spreading the disease well before they might appear sick.
#7202 is one of many deer from the Southwest Study that demonstrates the end result of CWD infection. Although each deer has its own unique circumstances, they all consistently tell the same story: CWD is directly reducing deer survival.
So what does this mean for Wisconsin’s white-tailed deer?
The bottom line is that where CWD prevalence is high, Wisconsinites can expect a smaller, sicker deer herd. Fortunately, however, Wisconsin does have an otherwise highly productive deer population, with uninfected deer boasting relatively high survival rates.
This means we’ll always have deer — they won’t go extinct. And while it can understandably feel a bit doom and gloom to talk about CWD, it’s important to acknowledge the threat it poses, recognize opportunities and focus on what we can do.
How You Can Help
Aside from prevention, thorough sampling to detect new introductions as early as possible and active steps to reduce disease transmission are our best tools to combat CWD, especially in areas of the state where the disease is not yet highly prevalent.
If you’re a hunter, the best thing you can do to help is to keep hunting. It’s a powerful population management tool because, when the hunter harvest is high enough, it can help keep local deer populations at reasonable levels and thus help reduce CWD transmission rates.
If you have opportunities to harvest more deer than you normally do, consider it! Just be sure to follow all listed baiting and feeding bans, dispose of any carcasses properly and, if you harvest a deer, get it tested for CWD.
Testing is a valuable source of information that allows scientists and decision makers to better understand how CWD is spreading throughout the state, especially in the identified priority sampling areas. View the most recent sampling map online.
If you do harvest a deer, keep in mind that although there have not yet been any reported cases of CWD transferring to humans, the U.S. Centers for Disease Control and Prevention advises against consuming meat from deer testing positive for the disease.
If you don’t hunt, review the deer feeding regulations in your county, dnr.wi.gov/topic/hunt/bait. Even if feeding is allowed where you live, consider avoiding feeding deer in your backyard. While well-intentioned, feeding sites can cause deer that might not otherwise interact to come together and potentially transmit CWD and other diseases.
Finally, anyone can help by learning more about CWD, asking questions and attending future public forums on the subject.
The primary results of the Southwest Study have been an important step in gaining a better understanding of CWD in Wisconsin. The unprecedented dataset the project has provided will continue to inform additional analyses well into the future.
As we look toward that future, we hope that by sharing the science of the Southwest Study today, we motivate more hunters, landowners and wildlife lovers to help combat the spread of CWD in our state.
Learn More
To see additional case examples of deer from the Southwest Wisconsin CWD, Deer and Predator Study, check out the study’s Field Notes newsletter. (Please note that in an effort to best explain the science behind the Southwest Study results, this newsletter contains images of dead deer that may be upsetting to some readers.)
More information on CWD as well as guidelines and best practices can be found on the DNR website.
