CWD: FAQ

CWD is a progressive neurodegenerative disease that is a member of a family of diseases known as transmissible spongiform encephalopathies (TSEs). TSEs are caused by prions, which are pathogenic agents made up entirely of protein, and are always fatal. Other examples of TSEs include scrapie in sheep, Creutzfeldt-Jakob disease (CJD) in humans, and bovine spongiform encephalopathy (BSE) in cattle. CWD is a TSE that affects members of the cervid family, including mule deer, black-tailed deer, white-tailed deer, Rocky Mountain elk, sika deer, moose, and wild reindeer (Spickler, 2016)(CDC, 2019).

CWD is an important cervid health issue and possible health issue for humans and animal species other than cervids for multiple reasons—which is why science-based efforts to help limit further CWD transmission are crucial. First, studies have shown that CWD can reduce cervid populations over time. Population-level impacts among elk, mule deer, and white-tailed deer have been modeled and observed in localized areas with high CWD prevalence (DeVivo et al., 2017)(Edmunds et al., 2016)(Miller et al., 2008)(Monello et al., 2014)(Williams et al., 2014). Preliminary results from an ongoing study in southwestern Wisconsin also show that CWD significantly decreases the survival of white-tailed deer (WI DNR, 2019). While controlling the spread of CWD is challenging and expensive, these results indicate that efforts to limit transmission of CWD are necessary for the long-term well-being of cervids.

The serious consequences that would result from CWD transmission to other species, including humans or livestock, represent another important aspect of CWD and its continued spread. Some TSEs, such as BSE and kuru, have severely affected human health and required immediate interventions to prevent further harm (Osterholm et al., 2019). It is not yet known whether CWD will transmit to humans or animals such as livestock. However, taking proactive steps to limit the transmission of CWD among cervids and reduce the potential for human exposure to CWD prions is important to prevent adverse outcomes.

Another concern about CWD is its potential to stifle hunter participation. Hunting is important for wildlife conservation and plays a critical role in limiting the transmission of CWD. Surveys gauging hunter perceptions about CWD have indicated that hunting participation would decrease if the prevalence of CWD reached certain levels in the cervid population (Needham et al., 2004)(Quartuch, 2019)(Vaske & Lyon, 2010). Decreasing hunter participation would further complicate efforts to control CWD, lead to economic losses, and challenge the model of conservation in North America, where state wildlife agencies are largely funded by hunters, many of whom hunt deer and elk (US Fish & Wildlife Service, 2016). The ongoing spread of CWD could limit funds that state agencies use to conserve all wildlife.

CWD could also result in more widespread economic loss. Experimental studies have provided evidence that CWD prions can bind to or be taken up into plants, including alfalfa, corn, tomatoes, and wheat, and can remain infectious (Johnson, 2013)(Pritzkow et al., 2015). These findings could impinge on trade between countries with CWD and those without the disease. Norway recently placed restrictions on hay and straw exports from North America, requiring a certified veterinarian to confirm that the hay and/or straw is from an area where CWD has not been detected (Richards, 2019)(Schuler, 2019). More research is needed to assess the risk and realistic implications of this in the wild. The impact of these potential consequences highlights the critical need for management strategies that will help limit CWD transmission.

Substantial evidence indicates that prions cause CWD (Schuler, 2019). Alternative theories, such as the recent suggestion that Spiroplasma bacteria are the causative agent, are not supported by scientific evidence. Krysten Schuler, PhD, wildlife disease ecologist at Cornell University’s Wildlife Health Lab and a CWD expert, has summarized the evidence that prions serve as the causative agent for CWD.

Prion proteins are formed under the instruction of the PRNP gene. Normal, non-disease-causing cellular prion proteins (PrPC) are expressed on the membranes of different cells in many species, including mammals, amphibians, birds, and reptiles (Riek et al., 1996). The exact biological function(s) of PrPC have not been identified at this time.

Prion diseases occur when a misfolded prion protein is introduced into a susceptible host. This introduction can occur sporadically, genetically, or following exposure to prion-contaminated material (Weissmann et al., 2002). Following introduction, the abnormally folded prion proteins, termed PrPSc, can directly interact with PrPC. Infection occurs when PrPSc successfully binds to PrPC and causes the normal protein to also become misfolded (Prusiner, 1998).

Once infected, PrPSc acts as a template and a chain reaction of PrPC misfolding takes place. The resulting cascade effect generally takes place over long periods of time, as disease-causing prions continue to accumulate. High concentrations of PrPC are found throughout the central nervous system and the continued binding of PrPSc eventually leads to the formation of amyloid plaques. Neurodegeneration ultimately becomes clinically evident.

Prions are an infectious agent that lack any genetic material. PrPC are able to be quickly broken down by proteases. However, conversion to PrPSc causes biochemical property changes in the protein, often making them extremely resistant in the environment for long periods of time (Hawkins et al., 2015). PrPSc cannot be broken down by proteases. Additionally, PrPSc are resistant to disinfectants such as formalin and alcohol (Spickler, 2016). They can also withstand high levels of heat and radiation.

While CWD has been detected in clinically normal fawns as young as 5 to 6 months old, symptoms of the disease have exclusively been found among adult cervids because of the disease’s long incubation period, which is the time between initial infection and clinical symptoms (Fischer, 2019). The average incubation period of CWD is thought to be 18 to 24 months, although longer periods have been occasionally reported. The maximum incubation period for CWD is currently unknown and might vary between species, but experimental studies have indicated that it can exceed 34 months (Fischer, 2019)(Moreno & Telling, 2018).

Clinical signs of CWD infection are typically evident for only a few weeks or months before the animal dies, although some infected cervids may live for over a year with clinical symptoms (Spickler, 2016). Symptoms of CWD include drastic weight loss, altered gait, confusion, excessive salivation and urination, grinding of teeth, slumped head, drooping ears, and a lack of fear of people (CDC, 2019)(Spickler, 2016). CWD may also result in reproductive losses, with some evidence suggesting that some infected cervid species are prone to stillbirths and offspring death soon after birth, but further research is needed (Spickler, 2016).

CWD is most likely transmitted horizontally (i.e., animal-to-animal contact) through infectious bodily fluids such as saliva, urine, and feces (Haley et al., 2011)(Henderson et al., 2015)(Plummer et al., 2017). Excreted substances can even contain CWD prions during the preclinical phase of the disease, with the concentration of excreted prions appearing to increase as the disease progresses (Davenport et al., 2017)(Henderson et al., 2015). CWD prions have also been found in the blood and antler velvet of infected cervids (Kramm et al., 2017)(Mathiason et al., 2006). Although CWD prions have been detected in the blood of infected cervids, ticks do not appear to play a role in transmission of the disease (Shikiya et al., 2020). Additionally, CWD prions have been detected in the reproductive tissue and semen of infected deer, although it remains unclear whether transmission of CWD can occur via sexual contact between cervids (Kramm et al., 2019). Carcasses of animals infected with CWD have high amounts of prions present and can remain infectious for extended periods (Miller et al., 2004)(Saunders et al., 2012)(Zabel & Ortega, 2017).

Infection likely takes place following oral and/or intranasal exposure to CWD prions (Nichols, 2013). Susceptible animals are infected following direct contact with a CWD-positive animal or via indirect environmental exposure to CWD prions (Zabel & Ortega, 2017). Evidence also suggests that vertical transmission (i.e., parent to offspring) can occur, although its impact on the ecology of CWD is not entirely understood (Hoover et al., 2017)(Selariu et al., 2015).  

It is not currently known how long CWD prions persist in the environment, but they have been shown to remain infectious in the environment for at least 2 years (Miller et al., 2004). However, scrapie, which is a similar prion that infects goats and sheep, has been shown to remain infectious in the environment for at least 16 years (Georgsson et al., 2006). In a separate study, researchers used power washing, chlorine treatment, and sodium hypochlorite to decontaminate a facility known to be infected with scrapie, and they still found that sheep were infected following reintroduction (Hawkins et al., 2015). It is reasonable to believe that the environmental persistence and resilience of CWD prions is comparable to scrapie.

The length of time that prions can remain infectious in the environment appears to be influenced by various environmental factors. The interaction of prions with soil has been studied, with results dependent on soil type. CWD prions that bind to a mineral in clay known as montmorillonite appear to be very stable and more infectious than unbound prions (Johnson et al., 2007)(Smith et al., 2011). Similarly, prions that bind to kaolinite and quartz may also be more infectious (Johnson et al., 2006). However, humic acid, which is commonly found in soil organic matter, has been shown to reduce the infectivity of CWD (Kuznetsova et al., 2018). The complexity of various soil types in different geographic regions makes it difficult to extrapolate information at this time. Other factors like weather and prion strain further complicate the ability to determine how long prions can remain infectious in the environment (Smith et al., 2011)(Yuan et al., 2015). Continued research on the environmental persistence of CWD is needed to guide public policy and management strategies.