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.”