Chronic wasting disease (CWD) is an infection found in captive (farmed) and free-ranging (wild) members of the deer family (hereafter “cervids”) [1]. North American species known to be affected include white-tailed deer (Odocoileus virginianus), mule deer (Odocoileus hemionus), Rocky Mountain elk (Cervus elaphus), and moose (Alces alces) [2,3]. First observed in 1967 in Colorado and Wyoming as an affliction of mule deer [3–5], CWD has since (as of December 2019) spread across North America – into 26 US states and 3 Canadian provinces – as well as overseas, to South Korea, Norway, Sweden, and Finland [6–8].
In 1980, CWD was recognized as a type of transmissible spongiform encephalopathy (TSE) [4]; other TSEs include scrapie in sheep, bovine spongiform encephalopathy (or “mad cow disease”) in cattle, and Creutzfeldt-Jakob disease in humans [1,9,10]. CWD is the only TSE known to affect both captive and wild animals [1,3,11]. TSEs are caused by misfolded versions of naturally-occurring proteins known as prions [12]. Before becoming malignant, prions exist throughout the mammalian body (concentrated most highly in the central nervous system [8]) in benevolent conformations (PrPC) that are encoded by mammalian DNA [10,13–15] and contribute to cellular functionality [8]. However, under certain circumstances PrPC can undergo a conformational change into PrPSc, an infectious isoform of the protein that is able to propagate by inducing further conversion of PrPC into PrPSc [1,10]. After a sufficient amount of PrPSc accumulates in a brain, neurodegeneration starts to occur [10]. And because TSEs’ causative agents are proteins (as opposed to bacteria or viruses), a host’s body does not recognize them as pathogens, so immune responses are not triggered by prion diseases [16,17]. Epidemics are self-sustaining [11,18], and CWD can therefore reach incredible levels of prevalence in a population: in one facility’s population of captive deer (mule and black-tailed), CWD led to the death of 80% of individuals in just six years [4].
Prions exhibit remarkable resistance to traditional attempts at decontamination; these include ultraviolet and ionizing radiation, chemical disinfectants, and high levels of heat [8–10]. This affords prions an incredible persistence once they’re released into an environment [19]. One demonstration of this was seen on a farm that had previously housed scrapie-infected sheep; the farm was still contaminated with infectious prions *sixteen years* after the last infected animals had been removed [20].
CWD is passed via direct or indirect contact (i.e., exposure to an environment contaminated with infectious saliva, urine, feces, etc.) [8,9,19,21]. Following an animal’s initial exposure to prions, an extended incubation period occurs in which no outward symptoms present themselves [4,22]. After typically >1.5 years [17], the disease enters its clinical stage, and thus begins the animal’s progressive deterioration: first is a deviation from normal behavior, followed by loss of coordination and physiological changes [4,22]. This symptomatic stage can last from a handful of days to an entire year, but its average duration is a few weeks to 3-4 months [21,22]. Ultimately, the single inexorable conclusion to chronic wasting disease is death [4,22]. There are no records of an individual – of any species – recovering from this disease.
The movement of cervids – whether by choice or by human orchestration – is one of the primary ways in which CWD gets established in new areas [7,17]. Attempts to control and eradicate the disease from wild populations of cervids have been largely unsuccessful [7], so current management strategies rely on the implementation of methods aimed at interrupting or hindering CWD’s movement into new environments [2,22,23]. Examples of specific strategies being put to use (or considered for use) are attempts at reducing the abundance of prions in the environment (e.g., carcass disposal, depopulation of infected farms), decreasing population densities of deer (e.g., culling, extended hunting seasons), and preventing cervid movement in certain areas (e.g., fencing, natural barriers) [24]. Despite these tactics and others, CWD is continuing to spread across the continent, and there is a definitive need to take steps to curtail it.
Since its detection more than fifty years ago, CWD has brought varying levels of concern to land managers, scientists, and stakeholders [8,22,25,26]. (In 2001, it was even declared a “State of Emergency” by US Secretary of Agriculture Anne Veneman [27].) This disease poses numerous threats – to animals, to the environment (e.g., ecosystem function), and to humans (at the scale of both the individual and society) – but neither the magnitude of these threats nor the details of what they might entail are fully understood. There is a pressing need for scientists and policymakers to identify and enact strategies aimed at controlling this disease, as notions of eradication have already been ruled out [23].
Some of the clearest ramifications of a CWD epidemic involve its impact to wild cervids. In one study, a population of white-tailed deer in Wyoming declined by 10.4% annually between 2003 and 2010 [28]. In another Wyoming study, a population of mule deer decreased by 21% annually between 2010 and 2014, and the researchers singled out CWD as “a significant contributor” to that decline [2]. It is accepted that CWD leads to reductions in cervid populations.
A widespread CWD outbreak amongst cervids would also have ramifications for American society: big game hunting in the US contributes about $170 billion every year to the national economy [8] – but Needham et al. (2014) [29] found that if 50% of a given state’s deer or elk were infected with CWD, about half of hunters would elect to stop hunting in that state. Such a drop in hunting would lead to the loss of revenue (in the forms of travel expenses, equipment purchases, license sales, etc.) of the local communities that benefit from – and sometimes depend on – expenditures by hunters. Additionally, conservation and wildlife management efforts are often funded to some degree by money brought into an area by visiting hunters [29]. Therefore, an epidemic of CWD would affect human livelihoods.
A final consideration regarding CWD is one of particularly grave concern: the question of human susceptibility. To date, there have been no reports of a non-cervid naturally contracting CWD; however, infections of such non-cervids as livestock, rodents, carnivores, and non-human primates (squirrel monkeys) have contracted CWD in laboratory settings [7,8]. A nontrivial amount of evidence suggests that the cervid-human “species barrier” (the natural restriction on a disease’s ability to jump species [17]) is strong when it comes to CWD [23,30,31]. Nevertheless, it’s prudent to remain cautious, as misplaced trust in this instance could prove devastating. Luckily, people are more apt to take CWD’s zoonotic potential more seriously due to the history of mad cow disease [32,33], and the CDC advises against the handling and consumption of CWD-infected animals and their body parts [34].
Although much has been learned about chronic wasting disease since it was first observed, there remains much to be discovered. CWD poses a threat to both captive and free-ranging cervid populations [9], and any long-term impacts it could have on ecosystems are poorly understood [21,23]. Despite (or maybe because of) this, it is important that quick action be taken. Moreover, due to infectious prions’ persistence after being shed, they will, in the early (~50) years of the epidemic, accumulate into sizeable pools of environmental contaminants, and as a consequence, any attempts at disease management will prove increasingly ineffective as time passes [35]. But getting a handle on CWD will be hugely challenging even under the best of circumstances, as laid out by Williams et al. (2002) [22]: “Long incubation periods, subtle early clinical signs, absence of a practical antemortem diagnostic test, extremely resistant infectious agent, possible environmental contamination, and incomplete understanding of transmission all constrain options for controlling or eradicating CWD.” It is imperative that work on CWD is not put off; we must work now to develop a deeper understanding of this disease, its implications, and any ways we might mitigate the harm it is to inflict on cervids, the environment, and human society.
References
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