Many questions about CWD remain unanswered because of incomplete scientific information, and myriad policy issues yet remain unresolved. We summarize below what we know and don’t know about CWD by addressing 20 frequently asked questions (FAQs). Our goal is to aid scientists, wildlife professionals, policy makers, journalists, and hunters. We’ll update the FAQs as new information becomes available.
Chronic Wasting Disease Frequently Asked Questions (as of September 17, 2020):
- What is chronic wasting disease (CWD)?
- Why should people care about CWD?
- What infectious agent causes CWD?
- What are prions?
- What are the symptoms of CWD?
- How is CWD transmitted?
- How long can CWD prions persist in the environment?
- Where is CWD found?
- How is CWD spreading?
- Can CWD be treated?
- Are some cervids resistant to CWD?
- If CWD is always fatal, where are all of the dead cervids?
- What tests are being used to detect CWD?
- What live animal tests are available for CWD?
- What are the practical concerns about current CWD testing programs?
- Can CWD infect humans?
- What have studies shown regarding the CWD species barrier between cervids and humans?
- If CWD can infect humans, wouldn’t we have seen a case by now?
- How do we talk about the potential for CWD transmitting to people without scaring people from hunting cervids or eating venison?
- What can I do to reduce my exposure to CWD?
- What other resources are available?
What is chronic wasting disease (CWD)?
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).
Why should people care about CWD?
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.
What infectious agent causes CWD?
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.
What are prions?
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.
What are the symptoms of CWD?
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).
How is CWD transmitted?
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).
How long can CWD prions persist in the environment?
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.
Where is CWD found?
CWD was first documented in 1967 among captive mule deer at a Colorado research facility. Since that initial discovery, CWD has spread across North America and has also been documented in other continents. A notable increase in the geographic spread of CWD has taken place in the past two decades. In 2000, CWD was documented in five US states and one Canadian province; in 2010 it was identified in 17 states and two provinces; and in 2019, it was found in 26 states and three provinces. CWD has also been documented in South Korea, Finland, Norway, and Sweden. Despite best efforts, no agency effort has been capable of eliminating CWD after the disease establishes itself in the wild (Gillin et al., 2018). Preventive efforts are essential for avoiding further spread of CWD and sustaining healthy cervid populations.
How is CWD spreading?
Despite the long-standing thought that CWD originated in the American West and has since spread from those original cases, new findings suggest that the disease may also be sporadically emerging across other locations in North America and Europe in the form of multiple strains. Recent cases in Norway and Sweden and experimental evidence that the cervid PrPC may be more capable of folding into new prion strains support this hypothesis (Benestad et al., 2016)(Bichell, 2019)(Pirisinu et al., 2018)(Meyerett-Reid et al., 2017). For example, experts have found that cases of CWD in Texas appear to have unique pathogenic features compared to cases of CWD from areas like Colorado, indicating that they may be distinct infections (Osterholm et al., 2019)(Zaktansky, 2019). Additional research will help clarify the likelihood and impact of sporadic CWD emergence.
It is known, however, that a variety of factors have contributed to the continued geographic spread of CWD. The movement of live animals has been identified as a key risk factor for continued CWD transmission (CWD Alliance, 2019)(Gillin & Mawdsley, 2018)(Miller & Williams, 2004). The natural movement of infected cervids is a challenge for the management of CWD and prevention of disease establishment in localized populations. Human-involved movement of infected live cervids has also been shown to contribute to the geographic spread of CWD (Gillin & Mawdsley, 2018)(Zabel & Ortega, 2017).
The movement of infected live cervids between game farms has been identified as the cause of CWD introduction to South Korea (Kim et al., 2005)(Sohn et al., 2002). Movement of infected live cervids has also been implicated in the spread of CWD among North American deer and elk farms, with 10 states (Iowa, Michigan, Minnesota, Missouri, Montana, New York, Ohio, Oklahoma, Pennsylvania, and South Dakota) and three provinces (Alberta, Quebec, and Saskatchewan) detecting their first CWD cases in captive cervids. Additionally, circumstantial evidence suggests that CWD in captive facilities may have led to infections among wild cervids via fence-line contact and/or following the escape of infected captive cervids in Saskatchewan, Nebraska, South Dakota, and Wisconsin (Gillin & Mawdsley, 2018). Until a highly sensitive antemortem test (one conducted on live animals) is available, the movement of live cervids by citizens and agencies is a risk for continued transmission of CWD in captive and wild cervid populations.
The movement of infected carcasses has also played a role in CWD spread. Infected cervids that are harvested by hunters have CWD prions in various tissues in their body, although the highest concentrations appear to be in the central nervous system and lymphoid tissues (Angers et al., 2006)(Hoover et al., 2017)(Kramm et al., 2017). Despite regulations that limit the movement of harvested carcasses or identified high-risk parts in some states, provinces, and territories, the lack of a uniform policy or awareness of existing policies makes it probable that cervids infected with CWD are being harvested and transported to areas without current detection of the disease. If these carcasses are being improperly disposed of and susceptible cervids can have contact with them, they could contribute to the continued spread of CWD. Being aware of state, provincial, and territorial regulations and options for carcass transport and disposal is vital for preventing further CWD spread.
Baiting and feeding can also contribute to the spread of CWD. Promoting artificial congregation via baiting and feeding can introduce infected and susceptible animals. Even if infected animals don’t come into direct contact with susceptible animals through baiting and feeding, they can shed prions in their saliva, urine, and feces that can later infect animals that are feeding in the same spot. A comprehensive summary of the scientific studies looking at the role of baiting and feeding on CWD was developed by the CWD Alliance (Van Deelen, 2003). Additional information from the Association of Fish & Wildlife Agencies regarding this topic is also available (Gillin & Mawdsley, 2018). Regulations on baiting and feeding vary by region. Visiting an appropriate wildlife agency website can provide more specific information on whether baiting or feeding is allowed in a specific area.
Evidence indicates that CWD prions can be consumed and moved by predators and scavengers feeding on infected carcasses, with the prion remaining infectious after it passes through that animal (Fischer et al., 2013)(Nichols et al., 2015). Experimental studies have also shown that prions can be taken up by certain plants, including alfalfa, corn, tomatoes, and wheat, and remain infectious (Johnson, 2013)(Pritzkow et al., 2015). However, further research on these topics and their true role in non-experimental CWD transmission is needed. Additionally, other potential risk factors that warrant further research include the sale and use of cervid urine products and reproductive tissues (Gillin & Mawdsley, 2018). Evaluating these factors and their role in the spread of CWD is important for guiding future management efforts.
Following existing state, provincial, and territorial regulations regarding the movement of live cervids, transportation and disposal of carcasses, and baiting or feeding can help prevent the spread of CWD. In addition, implementing uniform policies and regulations based on the best practices for CWD management would help prevent confusion caused by the variation in regulations between different states, provinces, and territories.
Can CWD be treated?
Currently, there is no available vaccine that can effectively prevent CWD infection in cervids. Similarly, there is no successful treatment for infected cervids available at this time. These characteristics of CWD emphasize the importance of preventing disease spread whenever possible.
Are some cervids resistant to CWD?
Certain genotypes that provide some level of resistance to CWD infection have been identified in various cervid species, including mule deer, white-tailed deer, and elk. The resistant polymorphisms, which vary between species, make an animal less susceptible to infection and delay the onset of clinical symptoms when CWD infection does occur. Cervids with resistant genotypes are not immune to infection and the disease is still invariably fatal in these animals.
Concerns have been raised regarding the consequences of elongated incubation periods observed among infected animals with resistant genotypes. Research indicates that infected cervids with some resistance to CWD have the same probability of shedding prions in their saliva as infected cervids with a more susceptible genetic makeup. This finding suggests that infected cervids with resistance to CWD are just as infectious as non-resistant infected cervids and may shed more prions into the environment over the course of their lifetime due to the longer incubation periods (Davenport et al., 2018). Prolonging the length of time that infected cervids excrete CWD prions would likely further complicate efforts to control further disease transmission.
Overall, the prevalence of cervids with CWD resistant genotypes is low, indicating that it may be associated with traits that are less desirable for fitness or long-term survival. For example, white-tailed deer that were selectively bred in captivity to have a CWD resistant genotype were found to behave abnormally (Robinson & Samuel, 2018). Continued research on selective breeding and genetic resistance to CWD is warranted, but it does not yet appear to be a viable solution to prevent or mitigate the threat of CWD.
If CWD is always fatal, where are all of the dead cervids?
Infected cervids appear healthy for a vast majority of their infection, only showing clinical symptoms for a brief period of time before death occurs. Cervids with clinically evident CWD eventually become emaciated and can die from starvation if they avoid other causes of death. Ongoing studies have found that CWD infected cervids have died from starvation despite an abundance of available row crops available for consumption (WI DNR, 2019). Additionally, some infected cervids that die directly from CWD show evidence of aspiration pneumonia, which may be caused by preceding symptoms like difficulty swallowing and excessive salivation (CWD Alliance).
However, CWD is a neurodegenerative disease and infected cervids seem to be more prone to other causes of mortality, including vehicular collisions and predation, compared to healthy cervids (Haley & Hoover, 2015). Additionally, cervids infected with CWD appear to be more susceptible to hunter harvest due to potential behavioral changes (Edmunds et al., 2016). The role of predators, scavengers, and natural decomposition make observations of intact dead cervids a relatively rare occurrence regardless of disease status. In combination with the limited number of infected cervids that die directly from CWD, these factors contribute to the infrequent observation of cervids that suffer acute deaths from CWD in the wild.
There are documented observations of cervids infected with CWD courtesy of the CWD Alliance, the Wisconsin Department of Natural Resources, the Wyoming Game and Fish Department, and the University of Wyoming. Additionally, there is the story of a hunter witnessing a CWD-infected cervid’s death directly from the disease, courtesy of the Quality Deer Management Association.
What tests are being used to detect CWD?
Currently, two validated diagnostic assays are available to determine if a harvested cervid is infected with CWD prions. Enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry (IHC) are post-mortem tests that detect the presence of abnormal prion proteins in the obex area of the brain stem or the retropharyngeal lymph nodes. ELISA testing has not been validated in elk or moose, so samples collected from these animals are confirmed via IHC. Valid ELISA and IHC testing for CWD can be conducted only in certified laboratories, so results can take days to weeks.
Although ELISA and IHC are not validated food safety tests and therefore cannot guarantee that an animal is completely free of CWD prions, they are the best available options to reduce human exposure. Continuing to use ELISA and IHC is essential until improved, validated assays are available.
What live animal tests are available for CWD?
Various antemortem, or live animal tests, are currently being evaluated. Multiple groups have considered the use of IHC on tissues collected from live cervids, including tonsillar, medial retropharyngeal lymph node (MRPLN), and rectoanal mucosa-associated lymphoid tissue (RAMALT). Completed study results indicate that these tests can be useful tools in certain settings. The US Department of Agriculture (USDA) recently revised its CWD Voluntary Herd Certification program standards for captive cervid facilities and now considers antemortem IHC analysis of RAMALT and/or MRPLN tissue as an official CWD test in specific situations (USDA APHIS, 2019). For example, antemortem IHC is only an official CWD test on white-tailed deer in CWD-exposed herds and epidemiologically linked herds. In other words, this test can be used as a tool only in response to detections of CWD among captive cervid facilities. Additional criteria, such as knowing the genotype at codon 96, must also be met for antemortem IHC testing to be classified as an official CWD test for USDA’s CWD Voluntary Herd Certification program. At this time, the Canadian Food Inspection Agency (CFIA) does not list an official antemortem diagnostic test for CWD, although it does provide more information on RAMALT testing and its strengths and weaknesses (CFIA, 2019).
Antemortem tests have considerable limitations that restrict their use. The collection of tissues from live animals can be invasive, time-consuming, and expensive (Kramm et al., 2017). The reliability of antemortem IHC testing on tissues also depends on factors like genotypic differences, the stage of infection, and the number of diagnostic follicles present in the biopsy (Keane et al., 2009)(Monello et al., 2013)(Thomsen et al., 2012)(Wolfe et al., 2007). The combination of these factors limits the effectiveness of broadly using antemortem IHC tests for CWD.
Other potential antemortem tests like protein misfolding cyclic amplification (PMCA) and real-time quaking-induced conversion (RT-QuIC) are also being evaluated with various tissues. Again, results show that these tests can be beneficial in certain situations, but test sensitivity levels are significantly lower than the existing validated post-mortem tests (Haley et al., 2016)(Haley et al., 2018)(Kramm et al., 2017). More detailed information on antemortem tests is available courtesy of the Association of Fish & Wildlife Agencies (AFWA) Technical Report on Best Management Practices for Prevention, Surveillance, and Management of Chronic Wasting Disease (Gillin & Mawdsley, 2018). Further research is needed for the development of a rapid, sensitive, and validated CWD assay.
What are the practical concerns about current CWD testing programs?
State and provincial wildlife agencies primarily determine policies for CWD testing. Testing is voluntary for hunters in some states and provinces and mandatory in others. And many have mandatory testing policies, but not in all CWD-affected areas. Hunters are often left to decide for themselves whether to test their harvested cervids.
Despite the variation in approaches, CWD testing is critical for wildlife agencies to determine the prevalence of CWD in wild cervid populations and for informing hunters if their harvested cervid is infected. Therefore, efforts must be taken to encourage hunter participation in voluntary CWD testing efforts, particularly in areas where cervids are known to be infected.
One state that relies on voluntary CWD testing is Wisconsin. Wisconsin’s Department of Natural Resources (DNR) has established surveillance areas around detections of CWD among wild and captive cervid populations (WI DNR, 2019). However, despite the Wisconsin DNR’s offering of CWD testing to hunters in surveillance areas at no cost, only 5% of Wisconsin’s 336,464 deer harvested in 2018 were tested (WI DNR, 2019). Additionally, only 4,925 (21%) of the 23,441 deer harvested in four Wisconsin counties (Dane, Iowa, Richland, and Sauk) where CWD is most established were tested in 2018, with 894 (18%) testing positive. Thus, more than 18,500 deer harvested in these four Wisconsin counties were not tested for CWD, suggesting that the venison from more than 3,000 CWD-positive animals was consumed, given the percent of positive tests. We therefore need to identify and eliminate factors that deter hunters from having their harvested cervids tested, as well as promote CWD testing participation among hunters, who are key to any effective CWD testing program.
The turnaround time for results from current CWD tests can be seen as burdensome to hunters who are considering having their harvested animal tested. Hunters may have to travel far to deliver their samples, depending on the availability of collection stations or drop-off points in a particular state or province. Once submitted samples are collected, valid ELISA and IHC testing for CWD can be conducted only in federally approved laboratories that are part of the National Animal Health Laboratory Network, so results often take days to weeks. A survey among hunters in Alberta found that two thirds of respondents who submitted samples for CWD testing in 2018 had to wait at least a month to receive their results (Adamowicz, 2019). This delay is a major deterrent to an effective testing program, since hunters might choose to process or consume the meat from harvested cervid in the interim. It is especially concerning for indigenous communities relying on harvested cervids for sustenance (Allen, 2019).
Expanding the availability of sample collection stations or drop-off points, particularly in rural areas, will likely encourage more hunters to participate in CWD testing. Additionally, creating more laboratories approved for CWD testing will reduce bottlenecks caused by an annual wave of samples being collected and analyzed by a limited number of facilities. This will generate faster turnaround times for test results, allowing agencies to respond faster to a positive test and hunter to know more quickly if their meat is contaminated.
While ELISA and IHC are the current best available options to reduce human exposure to CWD, policymakers need to place more emphasis on developing and validating highly sensitive and rapid alternatives. “Point of kill” tests that are able to be used by hunters in the field will limit human exposure to CWD prions in the animal, particularly in endemic areas. Such tests will also improve wildlife agency response times. While alternative diagnostics are being researched and developed, more needs to be done to ensure current tests are readily available at a low cost.
CWD management and surveillance is expensive and can quickly deplete state and provincial wildlife agencies’ funds, leaving little left to invest in other services, such as increasing the number of sample collection stations or drop-off points. Adequately addressing CWD challenges requires further federal investment. With sufficient federal support, these agencies can conduct critical CWD surveillance, enhance hunter education, and better develop and implement comprehensive CWD management plans. Additionally, investment in CWD research would improve diagnostics, ultimately helping prevent human exposure to CWD prions.
Can CWD infect humans?
The World Health Organization (WHO), US Centers for Disease Control and Prevention (CDC), Health Canada, and numerous other public health agencies recommend against eating meat from CWD-positive cervids. There has not been clear and compelling evidence of CWD transmission to humans. At this time, studies evaluating the CWD species barrier have not been capable of providing any certainty as to whether or not interspecies transmission will happen. Available data suggest that the risk of transmission to humans may be low, but it is not zero (Rinella, 2017). However, additional emerging factors must be considered when evaluating this species barrier.
Since the initial documentation of CWD in 1967, the geographic spread of the disease and its prevalence in areas where it has been detected have grown, meaning more people are likely being exposed to CWD prions. Surveillance data show that most of this geographic spread has taken place recently. In 2000, CWD was documented in 5 US states and 1 Canadian province; in 2010 it was identified in 17 states and 2 provinces; and in 2019, it was found in 26 states and 3 provinces. While exposure to CWD-positive animals certainly occurred in the past, it’s now becoming much more common. The Alliance for Public Wildlife estimates that 7,000 to 15,000 CWD-infected animals are consumed annually, a number that may increase by 20% each year (Geist et al., 2017).
Prion diseases in humans are also characterized by notoriously long incubation periods. Adequate levels of exposure may be taking place, but clinical infection may not be recognized for years or decades. In the case of BSE, so-called "mad cow" disease was first documented among cattle in 1986. The first case of vCJD in humans was not identified until 1996; one decade later. Additionally, while clinical cases of vCJD have remained relatively low despite the widespread exposure of humans to BSE-contaminated beef, researchers found, after observing abnormal prion protein deposits in appendix samples, that up to 1 in 2,000 UK residents could have subclinical vCJD infections (Gill et al., 2013)(Ironside et al., 2006). Multiple factors such as an individual’s age and genotype could determine whether any of these potential subclinical infections will result in clinical vCJD cases (Diack et al., 2017).
Furthermore, classical BSE prions responsible for the epidemic appear to remain highly localized in the central nervous system, with prions primarily found in the brain, spinal cord, and retina of infected cattle (Michigan Medicine, 2018)(USDA FSIS, 2013). While CWD prions are also found in the central nervous system, their distribution in an infected host’s body is more widespread than BSE's. CWD prions are found in numerous extraneural tissues, including the blood, skeletal muscle, and lymphoid organs (Angers et al., 2006)(Benestad & Telling, 2018)(Osterholm et al., 2019). Therefore, people consuming the skeletal muscle of infected cervids would be directly consuming CWD prions. On the other hand, exposure to BSE mostly occurred following the contamination of meat with infectious tissue such as the brain or spinal cord, which were sometimes mixed in with the meat during processing (Chen et al., 2013)(Chen & Wang, 2014)(Michigan Medicine, 2018). This difference in tissues is important, as Béringue and colleagues discovered that prions in extraneural tissues appear more capable of overcoming transmission barriers compared with prions in neural tissues (Béringue et al., 2012)(Osterholm et al., 2019). Consequently, CWD prions found in commonly consumed cervid tissue such as venison could have a higher disease potential than prions in the central nervous system.
The origination of novel CWD strains raises concern that this barrier can be overcome. Animal studies show that CWD prions can adapt following serial passage (i.e., transmission from one animal to another, then on to another, etc), resulting in new strains. Recent research has also provided evidence that CWD prions from the same infectious source can conform into unique strains, depending on the species that becomes infected (Bian et al., 2019). Prion strains possess slight differences in their conformation and transmission properties, and different strains can be identified through both their biological and biochemical features in the host. For example, various strains appear to have noticeably different incubation periods or can cause unique clinical symptoms (Morales, 2017)(Zabel & Reid, 2015). Additionally, recent studies have provided evidence that emerging CWD strains can have broader host ranges and higher zoonotic potential (Duque Velasquez et al., 2015)(Herbst et al., 2017). This is a concerning conclusion that needs to be considered during evaluation of the CWD species barrier.
It is not yet known at this time whether or not CWD will cause human infections. Continued research, including observational, in vitro (outside of a living animal), and in vivo (in a living animal) studies, is necessary to assess the risk that CWD might pose to human health. Additionally, the maintenance of a robust human prion disease surveillance system is essential for identifying and investigating human prion disease cases. Meanwhile, people should use precaution and take steps to avoid exposure to CWD prions whenever possible.
What have studies shown regarding the CWD species barrier between cervids and humans?
Multiple epidemiologic, in vitro (i.e. testing done outside of a living animal), and in vivo (i.e. testing using an animal model) studies have attempted to assess the integrity of the CWD species barrier between cervids and humans. Epidemiologic studies have attempted to determine any occurrence of human prion disease resulting from exposure to CWD prions. Currently, no cases of human prion disease have been linked to human exposure to CWD prions (Waddell et al., 2017). However, the characteristics of prion diseases complicate epidemiologic studies, as populations of exposed and/or infected individuals tend to be small (fewer than several thousand people per year during the first 20 to 30 years of the presence of CWD in cervids). Incubation periods of 10 years or more between exposure and illness onset and potentially low attack rates of several percent create further challenges for finding a statistically significant increase of a CWD-like illness in exposed humans. Despite these challenges, maintaining comprehensive human prion disease surveillance systems and performing ongoing epidemiologic studies are critical initiatives for determining if an association exists between exposure to CWD prions and the development of human prion-related disease.
In vitro species barrier studies expose human PrPC to CWD prions outside a living organism and measure whether or not the human PrPC are converted. While most in vitro studies found CWD prions capable of converting human PrPC, the efficiency of conversion was low or relied on protein modifications (Waddell et al., 2017). Recent results suggest that the efficiency of prion conversion varies depending on the cervid species that is infected and genetic variations in the prion protein between the infected cervids and exposed humans (Barria et al., 2018)(Morales, 2018). With time, more genetic variations in prions from cervids have been identified and as such may represent a greater likelihood for the increased efficacy of prion conversion. This is an important consideration in interpreting the CWD species barrier today or in the future compared with results of studies conducted in the past. Although in vitro studies cannot account for all of the complexities presented by prion interactions that occur in a live exposed animal, they remain important, as they provide timely insights into the CWD species barrier between cervids and humans and other animal species.
In vivo studies determine if CWD infection occurs in live animals following exposure to CWD prions. Such studies have often involved using either transgenic mice or non-human primates as a model. Genetic sequence similarity between non-human primates and humans make them an adequate model for prion disease experiments focusing on the possibility of interspecies transmission. The transgenic mouse model also provides a unique opportunity for studying the species barrier between humans and cervids, as the mice can be genetically modified to express human prion proteins, with their original prion protein gene inactivated (Waddell et al., 2017).
Studies using humanized transgenic mice as a model for potential CWD infection have so far suggested that a robust CWD species barrier may exist between cervids and humans (Barria et al., 2018)(Kong et al., 2005)(Race et al., 2019)(Sandberg et al., 2010)(Tamguney et al., 2006)(Wilson et al., 2012). While results from these studies are reassuring, the continued evolution of prions following their adaptation to hosts and emergence of novel strains suggests that this apparent species barrier cannot be considered absolute (Hannaoui et al., 2017)(Herbst et al., 2017)(Osterholm et al., 2019). A recent study provided evidence that a novel strain of CWD, known as H95+, was capable of infecting wild type mice that were previously thought to be resistant to CWD infection (Herbst et al., 2017). The study concluded that the zoonotic potential of CWD could increase over time as new strains emerge.
Several studies using nonhuman primates have provided conflicting results. Squirrel monkeys appear to be highly susceptible to CWD infection following either oral or intracerebral exposure (Marsh et al., 2005)(Race et al., 2014). Cynomolgus macaques have also been used as an animal model because of their genetic similarity to humans. Before 2017, evidence of prion infection among cynomolgus macaques exposed orally or intracerebrally to CWD prions had not been documented (Race et al., 2009)(Race et al., 2014)(Race et al., 2018). However, recent research involving four cynomolgus macaques supports the potential for infection (Czub et al., 2017)(Bichell, 2019). Two monkeys showing signs of prion infection had a CWD-contaminated steel wire implanted in their brain, while the other two ate skeletal muscle tissue, approximately equivalent to a human eating one 7-ounce steak per month, from asymptomatic, CWD-positive deer. These latter possible infections are most concerning, as that route of exposure is the most realistic for humans. Although the study is ongoing and has not been subjected to peer review, results reported to date cause concern over robustness of the CWD species barrier.
Major limitations in both in vitro and in vivo studies make the assessment of risk to human health challenging. In vitro studies can provide timely insight into the species barrier, but they cannot represent the complexities that exist within a living organism. Additionally, variations in methodology complicate comparisons of results (Waddell et al., 2017). In vivo studies of prion diseases are expensive and lengthy owing to the ecology of prion diseases and their prolonged incubation. While in vivo studies can address factors such as the influence of exposure routes, prion interactions taking place in animals might not equate to those in humans. This difference is important in prion studies, where one polymorphic variation in the host PRNP gene can play a role in susceptibility to infection (Lloyd et al., 2013). While in vitro and in vivo species barrier studies are important, they have not provided definitive answers regarding whether CWD will infect humans or other animals, such as bovines.
If CWD can infect humans, wouldn’t we have seen a case by now?
CWD was initially described in captive mule deer in 1967 at a Colorado research facility. The disease was subsequently detected for the first time among free-ranging cervid populations in the early 1980s (Gillin & Mawdsley, 2018). Since that time, CWD has continued to spread geographically. In 2000, CWD was documented in 5 US states and 1 Canadian province, in 2010, it was identified in 17 states and 2 provinces, and in 2018, it was found in 26 states and 3 provinces (Osterholm et al., 2019). CWD has also been documented in South Korea, Finland, Norway, and Sweden.
Despite the continued geographic spread, the overall occurrence of CWD in North American cervids remains relatively low (CDC, 2019). However, the prevalence of CWD continues to grow amongst cervid populations in endemic areas (Osterholm et al., 2019) (Saunders et al., 2012). For example, in 2010, about 17% of adult male deer harvested in the north central area of Wisconsin’s Iowa County were CWD-positive (WI DNR, 2019). Data from 2018 show that more than 50% of adult male deer in this same area are now infected with CWD. The trend of rising CWD prevalence among cervid populations in endemic areas is paralleled by increasing human exposure (e.g., consumption of venison, field-dressing carcasses) to CWD prions (Osterholm et al., 2019). This differs from the minimal levels of human exposure to CWD in the 20 to 30 years following its detection in free-ranging cervids during the 1980s, owing to initially low CWD prevalence among cervid populations at that time (Geist et al., 2017).
Currently, the Alliance for Public Wildlife estimates that 7,000 to 15,000 CWD-infected animals are consumed annually, a number that may increase by 20% each year (Geist et al., 2017). This rise in human exposure to CWD prions is a recent phenomenon that should be considered when evaluating the zoonotic potential of CWD. Given that human exposure to CWD prions has greatly increased in just the past 10 years—and, based on other TSEs like vCJD, the incubation in humans may be 10 years or longer—it is not possible to determine if CWD prion infections in humans have occurred more recently but have not yet resulted in clinical disease.
It is now understood that, as CWD continues spreading in cervids, the prion is evolving and novel strains are emerging (Moreno & Telling, 2018). Studies reveal that different CWD strains, which are presumed to originate via structural differences in the prion, have unique potentials for interspecies transmission (Duque Velasquez et al., 2015)(Herbst et al., 2017)(Morales, 2017). Novel CWD strains are formed when the prion adapts to a new species and/or it infects a cervid with genetic variations in its prion protein (Bian et al., 2019)(Hannaoui et al., 2017). Therefore, as CWD continues to persist on the landscape and more animals are exposed to the prions, the origination of new strains becomes more common. This is especially concerning for CWD, which appears to adapt to a host more readily than BSE (Davenport et al., 2015). Additionally, studies have provided evidence that CWD prions in the neural tissue of an infected cervid can be distinct from CWD prions in the extraneural tissue (Beringue et al., 2012) (Davenport et al., 2018). These same data suggest that prions in the extraneural tissue may have a higher zoonotic potential compared with prions in the neural tissue (Beringue et al., 2012) (Osterholm et al., 2019).
While it is reassuring that no cases of clinical human prion infection have been linked to human exposure to CWD, the apparent CWD species barrier between cervids and humans should be viewed as dynamic. Human exposure to CWD is increasing as the disease expands its range and prevalence grows in endemic areas. Prion diseases are also characterized by notoriously long incubation periods. If CWD is capable of infecting humans, cases may not arise for years to decades following initial exposure. Additionally, evidence suggests that current CWD prions may be increasingly different from past CWD prions. The growing spread and emergence of unique CWD strains may indicate that the zoonotic potential of CWD may be increasing.
It is not yet known whether CWD will cause human infections. Continued research, including observational, in vitro, and in vivo studies, is necessary to assess the risk that CWD might pose to human health. Additionally, maintaining robust surveillance for disease in people and other animal species is essential. Meanwhile, humans should use precaution and take steps to avoid exposure to CWD prions whenever possible.
How do we talk about the potential for CWD transmitting to people without scaring people from hunting cervids or eating venison?
The bedrock mission of public health is to prevent disease and promote health. When solid, evidence-based science is available, people's health can be protected effectively through apparent, preventive action. In the face of inadequate evidence-based science but some data supporting the potential for increased risk, the underlying premise of public health is to base statements and actions on the precautionary principle. Precaution has been at the heart of public health protection for centuries, and the precautionary principle is related to acting under uncertainty. Public health professionals must keep the public out of harm’s way when data suggest that severe outcomes are possible but are not robust enough to allow for indisputable conclusive action. This approach requires public health professionals to consider both the strength and biologic plausibility of the scientific evidence versus the potential harm that would occur if the risk is not mitigated.
In December 2014, Peter Sandman, PhD, and Jody Lanard, MD, two of the world’s experts on risk communication, authored an article on crisis communication lessons from the 2014 Ebola outbreak in West Africa. The article focused on four main crisis communication errors that public health officials and government leaders “committed” during the outbreak: (1) over-reassurance, (2) overconfidence and even absolutism instead of acknowledging uncertainties about Ebola science, (3) misdiagnosing the public as panicking, and (4) ridiculing the public’s Ebola fears instead of accepting and guiding them. These same lessons hold true for CWD risk communication today.
Although no human infections from CWD have been documented, current data support that the risk is not zero and the risk may be increasing with time (Osterholm et al., 2019) (see Can CWD infect humans? and What have studies shown regarding the CWD species barrier between cervids and humans?). This statement does not give us any reason for reassurance of the public, including cervid hunters. In fact, the past century has provided compelling evidence that effective public health interventions are needed to prevent the transmission of prion-related TSEs between animals and humans and between humans. A historical perspective of TSEs such as CJD, kuru, BSE, and vCJD was recently published (Osterholm et al., 2019). As CWD continues to spread geographically and the prevalence increases among cervid populations in endemic areas, human exposure to CWD prions increases. The current data and unknowns surrounding CWD warrant the undertaking of precautionary measures, whenever possible, to help limit human exposure to CWD while also allowing hunters to continue taking part in the traditions they enjoy and to support hunting as a critical population management tool. This, in turn, reduces the risk of cervid-to-cervid CWD transmission.
Public reassurance should be given only when it is completely justified by data. Residents of the United Kingdom were incorrectly reassured by government officials that eating British beef was safe in the midst of its ongoing BSE epidemic. A good example of this approach was captured in a 1990 BBC interview with the UK agriculture minister and the chief medical officer. More than 200 cases of vCJD have now been identified and linked to consumption of BSE-contaminated beef. Detection of human cases despite the government’s assurance that British beef was safe led to public distrust and petitions for criminal charges against involved officials (Blake, 2011). While prion science has greatly advanced since then, much remains unknown regarding CWD and the potential for human infection. Factors such as the ability for CWD to adapt to hosts and emerge as novel strains obscure past interpretations of the CWD species barrier between cervids and humans. Therefore, avoiding reassurances is responsible and necessary while the data remain inconclusive.
Some leaders in government and professional organizations have said that discussing the potential for transmission of CWD to humans scares people—so we shouldn’t say more than we actually know. Others argue that CWD could be transmissible to humans, but right now there’s no evidence of that, so why voice that possibility? Such statements represent absolutism instead of acknowledging uncertainties. There is no evidence that hunters or other members of the general public are panicking over the CWD situation. They are generally seeking understandable interpretations of current prion science and the implications of this information on the safe consumption of venison, as well as specific steps they can take to reduce or eliminate the risk of CWD with venison consumption.
Hunting is an essential component of wildlife conservation and plays a critical role in limiting CWD transmission. Fortunately, hunters and other can take steps to limit human exposure to CWD. Controlling the spread of CWD begins with implementing science-based management strategies by appropriate wildlife and agricultural agencies. AFWA recently published a document featuring the best CWD management practices. The application and enforcement of these practices will help limit CWD transmission among cervids and maintain healthy herds. In addition, implementation of these practices buys time for necessary CWD research and development of improved diagnostic tests. The development of a validated, rapid, and reliable test for CWD that can be conducted in the field could encourage more frequent hunter testing. Tools such as improved diagnostics, therapeutics, and vaccines require adequate funding. Therefore, further investment in CWD research and management is critical for reducing CWD spread between animals and limiting human exposure.
Hunting in areas known to have CWD can still occur while allowing for safe consumption of the venison. Taking precautionary steps, which are featured in the response below, can help limit an individual’s exposure to CWD prions and prevent further transmission of CWD within cervids. This can help protect both public and wildlife health. In turn, the hunting tradition can be enjoyed for generations to come.
What can I do to reduce my exposure to CWD?
Given recent studies suggesting there may be a risk of CWD prion transmission to humans, it is prudent to adopt a precautionary approach to human consumption of prion-infected venison. The United States Geological Survey (USGS) maintains an updated map showing the distribution of CWD in North America (USGS, 2019). Maps of CWD in Norway and Sweden are also available (NINA, 2019)(SVA, 2019). As with other hunting regulations, strategies for CWD management vary between states and provinces. Visiting an appropriate wildlife agency website can provide more specific information on CWD and regulations if you are hunting in an area known to have CWD.
Relevant wildlife agency websites can provide information on whether or not CWD testing is recommended or mandatory in the area. They can also provide information about the process for getting an animal tested.
The CDC has the following recommendations when hunting in an area with CWD:
- Do not shoot, handle or eat meat from cervids that look sick or are acting strangely or are found dead (road-kill).
- When field-dressing a cervid:
- Wear latex or rubber gloves when dressing the animal or handling the meat.
- Minimize how much you handle the organs of the animal, particularly the brain or spinal cord tissues.
- Do not use household knives or other kitchen utensils for field dressing.
- Check state wildlife and public health guidance to see whether testing of animals is recommended or required. Recommendations vary by state, but information about testing is available from many state wildlife agencies.
- Strongly consider having the cervid tested for CWD before you eat the meat.
- If you have your cervid commercially processed, consider asking that your animal be processed individually to avoid mixing meat from multiple animals.
- If your animal tests positive for CWD, do not eat meat from that animal (CDC, 2019).
A study published by the National Institutes of Health provided evidence that treating contaminated equipment with sodium hypochlorite after field dressing and butchering a harvested cervid will inactivate the CWD prions. The study found that soaking steel equipment used for processing, such as knives and saws, for a minimum of five minutes in a 40% dilution of household bleach containing 6% sodium hypochlorite inactivated CWD prions present on the equipment’s surface. However, this treatment strategy was unable to inactivate CWD prions in solid tissues, so equipment should be adequately cleaned of any remaining tissues prior to treatment.
Health professionals with remaining questions about CWD can visit the CWD section of the CDC website for more information. Other helpful resources for health professionals include The Center for Food Security and Public Health at Iowa State University’s CWD factsheet and the Montana Department of Public Health and Human Services list of FAQ for healthcare providers.
Additionally, in an effort to combat CWD misinformation, the Theodore Roosevelt Conservation Partnership (TRCP) wrote an article that featured three CWD experts responding to skeptics.