Chronic Wasting Disease and its Impacts on Human Dimensions

by Natalie Heyward

Photo by Joyce McCown on Unsplash

Chronic Wasting Disease (CWD) is a Transmissible Spongiform Encephalopathy (TSE) that causes neurologic disease in species like white-tailed deer, mule deer, and other members of the Cervidae family. Chronic wasting disease is caused by misfolded proteins called prions that are extremely hard to disinfect from surfaces and are present in the environment for long periods, resulting in high pathogenicity. Bleach is the only known effective method of decontaminating infected surfaces. The effects of CWD prions on humans are largely unknown, and due to the slow progression of the disease, food safety is a big concern for hunters in areas of high CWD prevalence. There is a lack of up-to-date information available to hunters. Local natural resource departments should be regularly updating hunters in order to promote testing of carcasses. However, testing methods are time-consuming, making it difficult to inform hunters of an infection until they have already been exposed. Additionally, hunters who are better informed of CWD presence tend to hunt less, leading to a decrease in hunting-related taxes and fees that could be used towards disease management1. If not managed, Chronic Wasting Disease may cause population-level impacts on mule deer and white-tailed deer, but methods of genetic protection against disease severity are being explored.

prion, wasting, cervids, hunting

Introduction

Chronic Wasting Disease (CWD) is a neurological disease that leads to lethargy and incapacitation in cervids. It was first discovered in 1967 in captive mule deer at a research facility in Colorado and is now present in 26 states in the USA, two provinces in Canada, and is also found in Finland, South Korea, Norway, and Sweden2 (Figure 1). The infectious agents of CWD are formed through the misfolding of naturally occurring proteins into their infectious isoform, the prion3. These prions are not processed by the body properly and accumulate in the cerebral tissue, over time leading to neurodegenerative disease and ultimately death3. This type of disease is classified as a Transmissible Spongiform Encephalopathy (TSE), and similar ones like mad cow disease in humans and bovine, and scrapie in sheep and goats have historically been detrimental to human health and the food industry2. Chronic Wasting Disease is currently the only TSE found in wild animals that also has the potential to harm domestic populations of cervids4. Infected individuals are characterized by lethargy and listlessness and are often found wandering aimlessly on the brink of starvation. Disease severity progresses as prions build up in the cerebral tissues, but clinical signs take several months to a couple of years to manifest. The lack of obvious indicators of illness makes it difficult to identify individuals with acute CWD infections. Additionally, quick ELISA tests and other blood-based tests are not typically sensitive enough to detect low concentrations of prions in the blood, making post-mortem evaluation of central nervous system tissue the best method of detection, despite being time consuming and resource intensive 4. The difficulties associated with the detection and treatment of CWD make it a natural concern for human health, particularly in areas where cervids make up a large portion of the food source and culture. The impacts of CWD on wild cervid populations are also not well understood, but evidence shows some effect on rutting behaviors and reproductive success5. This literature review summarizes recent findings on CWD transmission, the impact of transmission routes on human exposure within food systems, and the effects of CWD expansion on wild cervid populations, specifically white-tailed deer in the United States.

Figure 1. USGS map of the distribution of CWD in North America as of 2021 (Osterholm et al., 2019). Link here.

Discussion

Disease transmitted through direct contact is density-dependent and spreads faster in close proximity. This means that CWD probably spreads faster in areas of higher density, like captive deer farms, than in wild populations of cervids. However, the environmental persistence of prions makes culling a relatively ineffective tool for management1,2. Prions are resistant to many disinfectant methods, such as autoclaving, ethanol, and formalin, and can remain infectious in the environment for years3,5. Chronic Wasting Disease is primarily transmitted through nose-to-nose contact, through bodily fluids, or picked up from the environment, especially in highly trafficked areas2. Other methods of transmission, through arthropod vectors or sexual intercourse, have been investigated but determined insignificant in the epidemiology4. Because there is no known cure, the only way to combat CWD is through elimination and prevention within the host population. Effective preventative measures include significant monitoring of the population and have historically meant quarantining or culling infected individuals in captive situations2. However, it is evident that eliminating the infectious agent from the environment is likely impossible, so developing some biological protection within cervids is crucial.

Studying prions has historically been tricky for a variety of reasons, one of which is the limited capacity for decontamination within research facilities. This naturally raises concerns for the safety of butchers, hunters, and anyone else who handles cervid meat. One study determined that stainless steel items can be disinfected fully when soaked in 40% sodium hypochlorite (bleach) for at least five minutes6. However, tools must first be cleaned thoroughly of any tissue particulates because bleach does not penetrate tissue well and therefore cannot sterilize contaminated organic matter5. Contaminated organic matter should be properly disposed of under local government guidelines. This is an extremely important finding because it assures that prions can be purged from materials in a relatively cheap and easy way.

Currently, there is no strong evidence showing that humans can develop a TSE from CWD prions. Previous studies show that prion diseases with zoonotic potential have been able to jump across significant phylogenetic barriers, such as how mad cow disease can be transmitted from cows to humans7. Deer are as phylogenetically distinct from humans as cows are, meaning that there is no assured species barrier protecting humans from CWD7. One study showed that some non-human primates can develop disease after intracerebral inoculation of prions or oral inoculation, albeit after a much longer incubation period8. The generally long incubation period for this disease (2-4 years) makes analyzing its potential effects on humans difficult7. Despite these ambiguous findings, the World Health Organization still encourages the removal of any potential prion-infected meat from the human food chain1. Hunters are encouraged to test carcasses before consuming the meat in areas where CWD is prevalent. 

The uncertainty around the zoonotic capability of Chronic Wasting Disease creates a danger for some populations that rely heavily on hunting and consuming wild cervid meat for their livelihood such as the Indigenous peoples of Western Canada. The Canadian government protects Indigenous people’s right to freely hunt on reservations, resulting in little oversight and few animals getting submitted for CWD surveillance1. However, greater surveillance is occurring due to increased awareness of the disease and its potential danger to people. One survey conducted in Alberta found that 73% of respondents would be willing or interested in monitoring for CWD in white-tailed deer, mule deer, and moose1. Respondents also reported that they are confident in selecting healthy animals for harvest, which may be true except that CWD is largely asymptomatic until late-stage infection. Contrastingly, there is evidence that CWD-positive deer are overrepresented in hunter-harvested surveys, indicating that early infection can alter behavior to make individuals more susceptible to harvest6. For example, bucks infected with CWD are less involved in the rut and less aware of the hunting season, making them more likely to be singled out by a hunter5. These behavioral changes complicate a hunter’s ability to avoid harvesting CWD-infected individuals. Providing rapid testing of CWD in carcasses combined with greater information on disease prevalence could help protect people from exposure.

Hunters and Indigenous peoples that rely on cervid meat are an important component to managing CWD. The amount of information that reached hunters decreased dramatically from 2008 to 2018 while there was a drastic increase in CWD cases during that same period1. With the addition of an educational component to surveys, a large majority of the respondents expressed their support for monitoring and managing CWD, which is an important first step in government intervention for disease management1. Statistics show that an increase in education leads to a decrease in harvest rates. In Wisconsin, after information about CWD was shared with the public there was a 19% decrease in non-resident hunting and subsequently a $6 million decline in revenue for the state1. Hunting licensing in the United States provides a significant amount of funding for wildlife managers. If people do not feel safe consuming the meat they harvest, harvest rates will decrease. Infection does not appear to alter fecundity or doe behavior during the rut, but it can make them more likely to deteriorate from pregnancy if they have a late stage infection5. This would impact the birth rate of heavily infected populations and subsequently reduce recreational and food opportunities for people. If white-tailed deer populations begin to decline from CWD, harvest rates would need to decrease to compensate for the additional mortality. White-tailed deer are widespread, but these principles can be applied to other, less common cervids that may suffer more dramatic population impacts or species in need of conservation management.

The environmentally persistent nature of prions limits management options, although there has been success in the past through biological protection. Scrapie has been largely eliminated from the international farmed sheep population as a result of selective breeding of sheep9. In cervids, certain genotypes in the host’s prion gene make individuals more or less susceptible to developing CWD9. Selective breeding in white-tailed deer and other cervids could create a collective resistance against CWD. The prion gene in cervids is PRNP. This gene has several variations that make the host more or less susceptible to developing CWD. As a result, mendelian genetics can be used to select for combinations of less susceptible variations in individuals9. In one study, deer with these less susceptible PRNP genes were released into a controlled area of CWD prevalence in the environment and allowed to integrate genes into an existing captive herd9. After several generations, testing showed that CWD was present in only the individuals with highly susceptible PRNP gene variations, indicating that less susceptible PRNP variations did protect individuals from infection within the study constraints9. Mule deer and elk also have gene polymorphisms that reduce disease prevalence. This resistance is promising although more research is needed before applying this method of disease management.

Conclusion

Chronic wasting disease, a transmissible spongiform encephalopathy in the wild cervid population of North America and select European countries, was first discovered in the late 1960s and is now present in over half of the United States. This disease should be considered endemic due to its environmental persistence and resistance to many sterilization techniques. Additionally, CWD is a terminal illness that ultimately has little known effect on the population of wild cervids in the long term. It is important to consider human health concerns when managing CWD and recognize the need for testing of harvested animals to keep CWD prions out of the human food chain as much as possible. Education is a critical component in increasing testing rates, especially in places where there is not much information provided, like Canadian Indigenous reservations. Chronic Wasting Disease will probably not be eradicated, so traditional methods of management, like culling or mass vaccination, are impractical. A promising management method to conserve cervid populations is to create biological protection through selective breeding for individuals with prion-resistant alleles. 

References

  1. Parlee, B., Ahkimnachie, K., Cunningham, H., Jordan, M., and Goddard, E. “It’s important to know about this”—Risk communication and the impacts of chronic wasting disease on indigenous food systems in Western Canada. Environmental Science and Policy. 123, 190-201. (2021)
  2. Osterholm, M.T., et al. Chronic Wasting Disease in Cervids: Implications for prion transmission to humans and other animal species. mBio. 10. https://doi.org/10.1128/mBio.01091-19 (2019)
  3. Arifin., M. I., et al. Large-scale prion protein genotyping in Canadian caribou populations and potential impact on chronic wasting disease susceptibility. Molecular Ecology. 29: 3830-3840. DOI: 10.1111/mec.15602 (2020)
  4. Kramm, C., et al. In Vitro detection of Chronic Wasting Disease (CWD) prions in semen and reproductive tissues of white-tailed deer bucks (Odocoileus virginianus). PLoS ONE 14. https://doi.org/10.1371/journal.pone.0226560 (2019)
  5. Edmunds, D.R., et al. Chronic Wasting Disease drives population decline of white-tailed deer. PLoS ONE 11. DOI:10.1371/journal.pone.0161127 (2016)
  6. Williams, K., Hughson, A.G., Chesebro, B., and Race, B. Inactivation of chronic wasting disease prions using sodium hypochlorite. PLoS ONE. 14. https://doi.org/10.1371/journal.pone.0223659 (2019)
  7. Zink, R.M. Genetic and evolutionary consideration of the Chronic Wasting Disease—Human species barrier. Infection, Genetics and Evolution. 84, 104484. https://doi.org/10.1016/j.meegid.2020.104484. (2020)
  8. Race, B., et al. Susceptibilities of nonhuman primates to Chronic Wasting Disease. Emerging Infectious Diseases. 15, 1366-1376. (2009)
  9. Haley, N., Donner, R., Merrett, K., Miller, M., and Senior, K. Selective breeding for disease-resistant PRNP variants to manage Chronic Wasting Disease in farmed. (2001)

Acknowledgements: Thank you to Seth Alex McWhorter for help in the editing of this article.

Citation Style: Nature

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