In the heartland of Wisconsin, deer are facing a formidable adversary: Chronic Wasting Disease (CWD), colloquially referred to as “zombie deer disease.”
As more deer fall prey to this wildlife pandemic and thousands of infected animals are consumed by Wisconsinites, concerns grow about the potential for interspecies transmission in the Badger State and beyond.
What is CWD?
CWD is a fatal disease that targets cervid populations — deer, elk, reindeer, sika deer and moose — according to the Centers for Disease Control and Prevention (CDC). CWD is part of a family of transmissible prion diseases, misfolded proteins that can cause abnormal folds in the brain.
“Prions are in all mammals naturally. But this is a misfolded prion, which eventually causes holes in the brain,” said Erin Larson, Wisconsin Department of Natural Resources’ Herd Health Specialist. “It's a very slow-progressing disease.”
After that extended incubation period, on average 18-24 months, most infected deer display symptoms of listlessness, weight loss, loss of awareness and excessive salivation, ultimately leading to death.
But infected deer can be contagious for a long time, spreading the infected prions through bodily fluids.
“The majority of our deer that test positive are going to look perfectly healthy if they are harvested and tested,” Larson said. “[There is] a long time where they could be potentially shedding those prions and infecting other deer but still look perfectly healthy.”
The biggest challenge with prions lies in their ability to stay infectious for extended periods in the environment. Studies indicate they can remain so for several years, and more recent studies suggest they may persist for over a decade, according to Dr. Cory Anderson, co-director of the University of Minnesota’s Center for Infectious Disease Research & Policy CWD Program.
“Deer and elk and other animals can move on their own, of course, but they do remain relatively localized. [The] bigger issue has been the human-assisted movement of these live animals,” he said. “[Humans] do a pretty good job of spreading the disease.”
By 2022, almost 11,000 CWD cases were found in Wisconsin’s deer population.
“In 20 years, we have unfortunately seen a geographic spread [of CWD cases], as well as a spread in the amount of positives in our southern part of Wisconsin,” Larson said.
What is the potential risk to humans and other species?
The answer is unclear. However, there have been no human cases of CWD, and studies have not shown “strong evidence” of possible transmission to humans, according to the CDC.
“I would not advocate for eating [infected animals],” said Anderson, whose master's project focused on the species barrier. “At this time, [scientists do not] have a good sense of what the risk is.”
Anderson explained that other prion diseases show a mixed bag of results. For example, scrapie, a prion disease that targets sheep and goats, has not been shown to cross the species barrier.
But Bovine Spongiform Encephalopathy, known as mad cow disease, created a prion-driven epidemic in the United Kingdom that peaked in January 1993 at almost 1,000 new cases per week, according to the CDC. To this date, 178 people in the UK have died of Variant Creutzfeldt-Jakob disease, caused by mad cow disease.
Studies with squirrel monkeys involved direct cerebral inoculation — an unnatural method of contamination — and feeding them contaminated meals, a more natural route. In both cases, the squirrel monkeys contracted CWD, according to Larson. There have also been studies revolving around macaques with some showing susceptibility.
And some research raised concerns about the potential for a strain that could cross the species barrier, she said.
Still, Larson urges caution in extrapolating those results to humans.
“Again, at this point, there is no evidence humans are able to get CWD,” she said. “And the human species barrier does seem strong with the current strains.”
Are Wisconsinites at risk?
The Wisconsin Department of Health Services, the CDC and the World Health Organization all recommend against consuming meat from deer that test positive for the disease, according to the Wisconsin Department of Natural Resources.
“We have testing locations in every single county. We're trying to make it convenient for folks if they want to get their deer tested,” Larson said. “They can bring it to one of those locations, and then we'll be able to get that test result back in about 10 days.”
According to Anderson, approximately 300,000 deer were harvested in the past year in Wisconsin, with only 6% of deer statewide tested for CWD. Of those tested, around 1,600, or 9%, were found positive.
“If you do back-of-the-envelope calculations, I think it's pretty safe to say that there's several thousand [infected] animals that are probably being consumed, unknowingly,” he said.
Once a deer tests positive for CWD, the Wisconsin Department of Human Services reaches out to the hunter as part of its long-term surveillance program.
But some hunters, upon learning of a positive result, still express intentions to consume the deer, and some join a long-term surveillance list, Anderson and Larson both said.
“I think we're just continually rolling the dice. We don't necessarily know what the risk is, many people say [that the risk is] low. But it's not zero,” Anderson said. “It's not necessarily comforting to know that's happening.”
What can Wisconsinites do to protect themselves?
Larson and Anderson both heavily emphasize the importance of continued hunting.
Aside from population management, Anderson stressed that hunting, along with CWD testing, serves as a key tool in limiting the potential spread of the disease.
“I grew up in southwest Wisconsin, and [I have] a family that hunts. Just this past year, we harvested our first deer that tested positive for CWD,” he said. “CWD doesn't have to impact your willingness or desire to hunt. [CWD testing] is one of those things where I think the long-term benefit really sort of warrants the buy-in.”
Larson said the Wisconsin Department of Natural Resources shares a similar goal.
“We have the resources to be able to get your deer tested if you'd like to,” she said. “Wisconsin has such a great historical deer hunting culture and we want that to be able to continue.”
Jasper Bernstein is the Associate News Editor for The Daily Cardinal. Follow him on Twitter at @jasperberns.