An Interplay of Genetics, the Environment, and Human Health

Recently labeled as the “zombie deer disease” by prominent media outlets including the New York Times, Chronic Wasting Disease has emerged as a relevant threat to the environment and cervid populations. Over 30 states and three continents now report the presence of infected cervids as the disease spreads uncontrollably. As a transmissible spongiform encephalopathy, Chronic Wasting Disease harms cervids when prions accumulate in their lymphoid tissue and central nervous system. Prions, pathogenic agents that trigger the abnormal folding of certain proteins, are particularly harmful to cervids as they impair brain functioning and are lethal (25). The durability of prions makes them particularly dangerous as they continue to contaminate environments and are exposed to new geographical cervid populations. 

A unique interplay between genetics and the environment plays a role in a cervid’s susceptibility to CWD. Upon contact with prions in their environment, some cervids with certain PRNP gene polymorphisms are more likely to become infected (7). In particular, a study by Yu-Ran Lee et al. found that, out of 23 SNPs of the PRNP gene, deer with a particular SNP, S100G, were far more susceptible to be infected by prions (7). 

It has been proven that typical components of the environment, including soil, plants, and water, can easily be contaminated by prions from infected carcasses or diseased cervids (10). Therefore, the contamination of natural components of the environment proves to be quite dangerous for cervid populations. Soil can serve as a vector for horizontal transmission of the disease. As the infected carcasses and diseased individuals shed into the soil, prions are absorbed by the soil. Certain substances in soil, such as montmorillonite, are particularly dangerous, as they form a strong bond with CWD prions. Soil and surface waters contaminated with any body fluids of infected cervids have also been shown to be possible vectors for the transmission of CWD prions (13). Additionally, plants are susceptible hosts of the disease because they absorb prions from their roots and consequently spread them to above-ground tissues (11). With the confirmed presence of prions throughout the environment, deer casualty rates only increase and CWD only expands across geographical regions. 

Human health is faced with clear risk if the lack of regulatory measures persist. There is confirmed knowledge that prion diseases are able to spread to humans. For instance, prions from bovine spongiform encephalopathy, also known as “mad cow disease,” spread to humans and caused significant brain damage with 232 people dying worldwide (16). 

With this historical perspective, the negligence of the zoonotic potential of Chronic Wasting Disease is frightening. Currently, only 5.06% of deer samples are voluntarily tested for Chronic Wasting Disease in Wisconsin, the state with one of the highest CWD rates (26). This statistic is alarming, as research has shown that human exposure to CWD is likely to occur as a result of oral exposure to infected meat byproducts (15). The continued expansion and escalation of Chronic Wasting Disease in cervid populations as they continue to contaminate surrounding environments poses a significant risk because the species barrier has been crossed before for CWD. For instance, squirrel monkeys and mice have been infected with CWD in laboratories (18, 19).  Additionally, new CWD strains have been found. Future CWD strains could have zoonotic potential. 

While there are no cases of CWD in humans that have been documented, the prevalence of prions in the environment and the genetic susceptibility of cervids make it a matter worth addressing (21). As seen with previous prion diseases, human health is at risk if CWD strains continue to evolve and more components of the environment become vectors of horizontal transmission. William Werkheiser, science advisor to the Secretary of the Department of the Interior, stated that “there is no known treatment or cure for CWD. Therefore, prevention of the disease and limiting its spread is essential.” While current understanding and efforts towards CWD prevention are evolving with the establishment of the RT-QuIC assay which detects CWD and with the testing of vaccines for prion diseases, human health is put at risk as with the lack of an incentive for testing (23, 24). If CWD were to be transmitted to a human, it is plausible that this would severely impact brain functioning. While it is difficult to control the interplay of the deer’s PRNP gene and the contamination of the environment, the most feasible step towards protecting human health is promoting CWD testing. Currently, all U.S. states offer free testing for hunters. However, the incentive for voluntary testing still remains. Organizations and labs such as MNPRO and CWD Today seek to address this problem by spreading awareness and furthering the understanding of CWD.