At least 26 U.S. states,1 three Canadian provinces and countries including South Korea, Finland, Sweden and Norway have been affected by chronic wasting disease (CWD), a contagious neurological disease that affects deer, elk, reindeer and moose.
Experts are racing to understand and contain this deadly condition before it wipes out entire herds or, worse, spreads to humans — a possibility that’s been raised by a number of studies. CWD is part of the transmissible spongiform encephalopathies (TSEs) disease family — the most notable member of which is bovine spongiform encephalopathy (BSE), also known as mad cow disease, a condition that affects cattle.
A human version of mad cow disease, known as variant Creutzfeldt-Jakob disease (vCJD), also exists and made headlines when it was discovered that it can be caused by eating beef contaminated with brain, spinal cord or other central nervous system tissue from infected cattle.2
Given the rising prevalence of CWD, experts are now asking whether the disease poses a risk to humans, especially since it has elements that make it fit for a horror movie. Epidemiologist Michael Osterholm said, speaking to Minnesota lawmakers, “This is kind of a worst-case nightmare … If Stephen King could write an infectious disease novel, he’d write it about prions.”3
What Are Prions, the Cause of CWD?
CWD is thought to be caused by prions, which are quite different from “ordinary” pathogens of the bacterial, viral or fungal nature. Colorado State University’s Prion Research Center calls prions “unprecedented infectious agents,”4 in part because they do not have a nucleic acid genome, such as DNA.
While viruses and bacteria need DNA or RNA to replicate, prions can do so even in its absence. Further, as noted by the U.S. Geological Survey (USGS), “[P]rions exhibit an extraordinary resistance to common treatments used to stop other infectious agents, such as ultraviolet and ionizing radiation, exposure to chemical disinfectants, and heat treatments.”5
Prions can be transferred not only via direct transmission but also indirectly, via exposure to contaminated materials in the environment. The infectious agents in CWD persist in the environment, which is why deer and elk raised in captivity (or concentrated via artificial feeding) have an increased likelihood of transmitting the disease among them.
In fact, even plants may harbor infectious prions, as research shows they can bind to plant roots and leaves. In one study, hamsters were infected by eating prion-contaminated plants, and the infectious prions remained on the plants for several weeks. Plants can also uptake prions from contaminated soil,6 and prions are believed to persist in the environment for decades.
It’s possible then, though not proven, that CWD could even spread via agricultural crops, as wild deer defecate in the fields and the feces contaminate the crops or the infectious proteins are taken up by plants, such as wheat.
Further, it’s common practice for manure from concentrated animal feeding operations (CAFOs) to be spread over agricultural land, where it often runs off into waterways. If CWD mutates into a form that can infect cattle, this could have major implications for its spread.
To be clear, not all prions are problematic. We all have these proteins in our bodies. It’s when they become distorted, or misfolded, that they begin to damage brain cells, in a way similar to Alzheimer’s and Parkinson’s diseases.
What Is CWD?
CWD causes progressive, neurological degeneration that leads to physiological and behavioral changes, including increased drinking and urination, weight loss, lowering of the head, listlessness, drooling, stumbling and death. Animals may be infected for a long period and show no symptoms, only to suddenly succumb to the disease.
“Through time [CWD] degrades, essentially, their brain tissue,” ecologist Heather Swanson told High Country News. “That seems to happen pretty rapidly. To our eyes, they look fairly healthy, and within a number of weeks they reach that point — and then they’re gone.”7
Research by Swanson and colleagues found that mountain lions preyed on CWD-infected deer nearly four times more than noninfected deer,8 perhaps because they could sense that the deer were unwell — even though they appeared otherwise normal.
Despite the intense predation, “remarkably high infection rates sustained,” with about one-fourth of the deer sampled in the study infected.9 CWD was first identified in 1967 and is on the rise with new and ongoing outbreaks.
In Iowa County, Wisconsin, for instance, the proportion of adult white-tailed deer infected more than doubled over a six-year period, and as of 2016 approximately 40% to 50% of males and 20% to 30% of females were infected.10 Overall, in the U.S., the U.S. Centers for Disease Control and Prevention stated:11
“Nationwide, the overall occurrence of CWD in free-ranging deer and elk is relatively low. However, in several locations where the disease is established, infection rates may exceed 10 percent (1 in 10), and localized infection rates of more than 25 percent (1 in 4) have been reported.
The infection rates among some captive deer can be much higher, with a rate of 79% (nearly 4 in 5) reported from at least one captive herd.”
Subsidizing the Source
CWD was first identified in captive deer in Colorado and wasn’t found in wild deer until 1981.12 The transport of captive deer is thought to have contributed to the spread of CWD throughout the U.S. Today, deer farms persist across the U.S., including in Wisconsin, one of the states hardest hit by CWD.
The state has 380 deer or elk farms, 23 of which have tested positive for CWD. Fourteen of the facilities have been depopulated as a result,13 but some are allowed to stay open, despite the known presence of the disease. What’s more, when a deer farm tests positive for CWD and is depopulated, the business owner receives a subsidy or bailout from the government.
It may seem strange that a business such as a captive deer farm, which promotes the spread of CWD by raising animals in close quarters, would receive government subsidies. It occurs because captive deer are considered livestock and as a result are covered under the Condemnation of Diseased Animals statute, which was enacted in 1977.
In Wisconsin alone, deer farmers have received more than $330,000 in compensation from state and federal officials after their animals were killed over CWD fears.14 The money to bail out the diseased farms comes either from tax revenues paid by Wisconsin residents or, if there’s not enough available, from federal funds also generated by taxpayers.
In May 2018, Wisconsin Gov. Scott Walker announced new rules for deer farmers operating in counties affected by CWD, effective via an emergency order. Both captive deer breeding and hunting facilities were supposed to be required to install additional barriers around their facilities, while hunters would be required to cut hunted deer into quarters and leave the spinal cord, where the disease may concentrate, behind.15
Deer farmers in the state opposed the new measures, with some saying the requirements would put them out of business. The rule was amended, giving deer farmers a year to comply, but it expired in February 2019 — before it ever took effect.16 Further, in October 2018, lawmakers rejected the emergency rule’s limit on hunters moving deer carcasses from CWD-affected counties.
Meanwhile, the Department of Agriculture, Trade and Consumer Protection has allowed some deer farms with CWD-positive animals to continue operating, including Wilderness Game Farm Inc., which has had 84 known CWD cases and still sells hunts that cost up to $9,000.17 As prion disease continue to rise in animals, Alzheimer’s continues to rise in humans — a connection that deserves a much closer look.18
The Alzheimer’s Connection
The Prion Research Center regards other diseases that involve misfolding proteins, including Alzheimer’s, Parkinson’s, Lou Gehrig’s and Huntington’s diseases, as prion diseases, much like CWD.19 And there is reason to believe, according to a review in Medical Hypotheses, that Alzheimer’s may develop similarly to mad cow disease and other spongiform encephalopathies.
“In fact, Creutzfeldt-Jakob and Alzheimer’s often coexist and at this point are thought to differ merely by time-dependent physical changes,” the researcher stated. “A recent study links up to 13% of all “Alzheimer’s” victims as really having Creutzfeldt-Jakob disease … All of this brings up the unthinkable: that Alzheimer’s, Cruetzfeldt-Jackob, and Mad Cow Disease might just be caused by eating the meat or dairy in consumer products or feed.”20
Another concern is antler velvet from elk, which is sometimes taken in supplement form. Prions have been detected in elk antler velvet, which suggests it plays a role in disease transmission among elk and “humans who consume antler velvet as a nutritional supplement are at risk for exposure to prions.”21
There is also growing suspicion that Alzheimer’s may have an infectious component capable of human-to-human transmission. More than 200 people are known to have developed CJD as the result of receiving growth hormone contaminated with prions that came from human cadavers.
When eight of them later were autopsied, four of them had buildups in the brain characteristic of early Alzheimer’s disease, with the researchers suggesting misfolded amyloid beta protein, a hallmark of Alzheimer’s, may be transmittable similar to other prions.22
Further, because prions aren’t killed by ordinary sterilization methods, it’s possible they could be transmitted during common medical procedures, including dental work and colonoscopies. Meanwhile, prions may be spread via contaminated feces, saliva, blood or urine, as well as via contact with contaminated soil, food or water.
Can CWD Be Transmitted to Humans?
The burning question is whether or not CWD can be transmitted to humans, and while hard data aren’t yet available, there are some concerning signs that the answer could be yes. In a study on macaques, monkeys that share genetic similarities with humans, the animals developed CWD after eating CWD-infected meat or brain tissue.23
Some of the meat came from deer that had CWD but showed no symptoms of the disease, yet was still able to spread the infection to monkeys.24 Studies are currently underway to determine if people in contact with CWD-infected animals or meat are at increased risk of prion diseases, but results won’t be available for some time.
The World Health Organization recommends products that could be contaminated with CWD or any related disease should be kept out of the human food chain.25
Some experts, including Osterholm, who serves as director of the University of Minnesota’s Center for Infectious Disease Research and Prevention, tracked the ability of BSE to be transmitted to humans decades ago, however, and have already sounded the alarm. Many believed that mad cow disease couldn’t infect people — until it did. Osterholm believes the same fate will become of CWD:26
“It is my best professional judgment based on my public health experience and the risk of BSE transmission to humans in the 1980s and 1990s and my extensive review and evaluation of laboratory research studies … that it is probable that human cases of CWD associated with the consumption of contaminated meat will be documented in the years ahead. It is possible that number of human cases will be substantial and will not be isolated events.”
If You Eat Venison, Be Sure It’s Not Infected
If you’re a hunter or consume meat from elk or deer, you should ensure the meat is tested for CWD before it’s consumed. While some states require testing of deer from high-CWD areas, others do not, and some areas offer free testing while in others it must be done at the hunter’s expense.
If you’ve obtained a deer from a captive farm, which isn’t recommended, it’s especially important to have the animal tested, as the farms are high-risk zones for CWD. It’s important to understand that you cannot judge whether an animal has CWD by symptoms alone. It can be years before symptoms develop, and a healthy-looking animal may still be infected and capable of transmitting the disease.
In addition, the CDC recommends hunters use caution when handling a deer in the field, including wearing latex or rubber gloves when handling the meat and minimizing contact with the organs, especially the brain and spinal cord.
If CWD disease has already mutated into a form that could infect humans, symptoms may not be seen for years or decades, and they may appear similar to other prion diseases like vCJD, making the diseases virtually indistinguishable.