Chronic Wasting Disease in CervidsJoy [email protected] 2510Winter 2018Introduction    Chronic Wasting Disease is a fatal neurodegenerative disease that belongs to the family of prion diseases, also called transmissible spongiform encephalopathies (TSEs), which encompase the bovine disease BSE, the ovine disease scrapie, and the human equivalent, CJD. Chronic Wasting Disease (CWD) affects the wild and captive populations of cervids, including white-tailed deer, mule deer, moose, elk and reindeer. First identified in Wyoming and Colorado in the late 1960’s, CWD has continued to be recognized in numerous geographical locations including Canada, Norway and the Republic of Korea ((Bartz, Saunders & Bartelt, 2012., Government of Canada, 2017). This disease causes behavioural changes that spread through a population, leading to fatalities (White, et al. 2010). CWD is spread between members of the same species, crosses species barriers, and has the potential to be zoonotic in the appropriate environment. The growing number of fatalities and the potential for it to jump species arises a great concern, leading to developing research.               Causative Agent     Chronic Wasting Disease is caused by the abnormal folding of a small protein prion (PrPSc) located in the the membranes of neurons and other cells in the brain (Bartz, Saunders & Bartelt, 2012) . These cellular prions differentiate from their normal protein (PrPc)  as they are resistant to proteolysis and have a tendency to aggregate (Bartz, Saunders & Bartelt, 2012). This abnormal folding results in the death of neurons, and in turn, the death of the connected cells. It is able to replicate its abnormal structure (PrPSc) and spread across many cells, slowly damaging normal protein cells (PrPc) (Bartz, Saunders & Bartelt, 2012). Therefore, the prevalence of the disease is federally reportable in many areas, and under strict guidelines (Government of Canada, 2017). The prion is able to be spread in contaminated blood and feces, as well as other body fluids such as saliva and postpartum secretions (Nichols, et al. 2016).       CWD is a chronic, slow developing disease that has prolonged incubation periods. These incubation periods can be anywhere from 2-4 years in length, depending on sex, age, health, and transmission (Hannaoui et al. 2017).  During this incubation period, infected animals may not show clinical signs, but are able to secrete and excrete the prion to others and the environment (Cheng, et al. 2016). While it is not fully proven, research points to the idea that infection rates are seen higher in males of these species, and the prevalence of the infection increases with age, anywhere from 5-6 year olds (Potapov et al. 2016). However, just because the infection rates may be higher in males does not diminish the ability of infected males to pass on the prion to females of the population as well. As CWD is part of the TSE family, the potential of this prion being able to jump the species barrier to infect humans, not unlike BSE or scrapie, is a also a pressing concern. Clinic Signs        As CWD is a neurodegenerative disease, its clinical signs are associated with the brain and spinal cord, together known as the CNS. Infected individuals appear to be quite depressed, which worsens as the disease progresses (Government of Canada, 2017). Some of the more obvious signs of the disease are weight loss and lack of coordination that can be seen in individual and grouped animals (Bartz, Saunders & Bartelt, 2012., Government of Canada. 2017).  As Cervids rely on on their nimble ability to flee from danger, a decrease in coordination can be detrimental to their survival. The infected animals also show behaviour changes, such as decreased interaction with other animals, which is notable as individuals of these species rarely separate from the herd (White, et al. 2010). Another sign of CWD is the animals decreased responsiveness to stimulus from the surrounding environment, and producing repetitive behaviours that are not normal (White, et al. 2010).  These infected animals also show increased salivation and excessive urination, resulting in increased water consumption (Bartz, Saunders & Bartelt, 2012., Government of Canada, 2017). While all of these clinical signs can be expressive of many diseases, a diagnosis is needed to determine if the cause is CWD. Transmission    As previously stated, CWD is a prion disease that is shed in fluids and feces of infected cervids and carriers. As these prions are shed, other animals pick them up, which contributes to the horizontal transmission of the disease process (Cheng, et al. 2016) . The prions are excreted from the animal at any stage of infection, ranging from preclinical to when they are deceased (Cheng, et al. 2016). These prions are also able to be spread and picked up by direct or indirect contact, and are believed to be able to jump the inter-species barrier (Bartz, Saunders & Bartelt, 2012). It is also believed that the soil acts as a mediator in the spread of these prions into the environment (Bartz, Saunders & Bartelt, 2012., Mejía-Salazar, et al. 2018). This is because the prions are able to bind to molecules in the soil, and still retain the ability to replicate (Bartz, Saunders & Bartelt, 2012).      Vertical transmission may also be possible with CWD. The prion is able to be transferred from the dam to the offspring of the animal through their saliva, and through the mother’s milk (Nalls et al. 2013). Studies have shown that the reason for the high infection rate in cervids from CWD may be due to in utero transmission of the prion from mother to offspring (Nalls, et al. 2013). Because many of these infected animals are in wild populations, it is hard for researchers to track the disease transmission.    The likelihood of a zoonotic potential for CWD has been determined as low, as there is not enough definitive research to back the claim (Bartz, Saunders & Bartelt, 2012). However, because CWD is a prion disease, and is able to cross the inter-species barrier and be carried by pre-clinical scavengers (such as coyotes and raccoons), the possibility human infection is probable. Therefore, numerous governments are implicating strict laws and protocols that ensure no possible prion infected animals enter the food chain (Government of Canada, 2017). By doing this, governments are attempting to decrease the chance of the disease infecting humans through consumption. Diagnosis    The disease is diagnosed by routine testing of hunting animals. This allows research to be gained, and allows us to know if the rest of a population could have or be carriers of the prion. Currently, surveillance and diagnosis depend on hunter’s compliance to have heads analyzed after harvest to have the brain tested (Cheng, et al. 2016). The obex of the cervids brains are examined by immunohistochemistry (IHC), at the level of the dorsal motor nucleus of the vagus nerve to determine if the animal is CWD positive or not, and to determine the amount of damage caused by the prion (Nichols, et al. 2016). Another way of detecting the prions are through the lymphoid tissues, as it is believed that the prion replicates its way to the lymph nodes (Nichols, et al. 2016, Race et al. 2007). These tests analyze IHC of palatine tonsils or rectal lymph nodes. The fluids that the prions are shed in, such as blood, saliva, urine and cerebrospinal fluid contain minute amounts of prions that leave these tests unsuccessful in determining whether the cervid was infected (Nichols, et al. 2016).A newer test has been developed by researchers called a “Real-time Quaking Induced Conversion Assay (RT-QulC), which enables the detection of CWD in fluids and feces (Cheng, et al. 2016). This study shows that using the RT-QulC method allows for in the field testing that supports surveillance of potentially infected populations, while also giving early diagnosis of the prion in feces of cervids. As CWD is a fatal disease, many of the diagnosis’ are made post mortem. However, there are ways to test both captive and wild cervids pre mortem.  First, an animal is determined to be potentially infected by witnessing visible clinical signs. Then, using a biopsy of lymphoid tissue, researchers are able to detect the presence of the prion (Cheng, et al. 2016). While accurate, this process is stressful in the fact that the cervids, both wild and farmed, have to be captured to obtain a sample (Cheng, et al. 2016). Therefore, this method is generally exclusive to research based diagnosis.  Treatment and Prevention    As Chronic Wasting Disease is a fatal disease, there is no treatment that can be done to cure it. Therefore, steps are being put in place to control the outbreaks and potential spread through many areas. To help stop the spread of the shed prions, the carcasses of the animals should not be consumed by any other animals or humans, as there is the potential they could be infected off of it (Bartz, Saunders & Bartelt, 2012., Arduegno, M.P., Lawrence, D.M. 2013). Infected animals should be culled, and surrounding populations (both farmed and wild) should be under surveillance for appearance of clinical signs (Bartz, Saunders & Bartelt, 2012).      Just as hunters are required to submit heads of animals in selected area for testing, they benefit the surveillance programs by providing data (Alberta Government, 2016). These surveillance programs aid research in many areas, including understanding how transmission occurs, monitoring whether genetic resistance is beginning to occur, develop tools in understanding the epidemic, and analyzing surrounding species for their role in the transmission of this disease (USGS, 2016). Conclusion    Chronic Wasting Disease is a growing concern in many populations of wild and farmed cervids in countries, including North America. As this prion disease has no therapeutic treatment available, research is being conducted that attempts to determine prevention tactics and control the spread. Surveillance programs are being implicated to aid in tracking the progression of CWD, and to aid in the understanding of how this disease works. Despite the efforts of governments and researchers, no definitive treatments or prevention have been developed to slow the spread of the disease. Because so much remains unknown about the disease, the thought of a zoonosis is a looming concern that fuels researchers and puts a potential safety concern onto humans around these areas. The continuation of these surveillance programs and research are imperative in being able to fully understand, and hopefully one day treat and prevent, Chronic Wasting Disease. Literature Cited:Alberta Government. 2016. Chronic Wasting Disease and Deer Management. 2017 Alberta Guide to Hunting Regulations. Retrieved from: Arduengo, M. P., Lawrence, D. M. (2013). Chronic Wasting Disease (CWD). Magill’s Medical Guide (Online Edition) Cheng, Y., Hannaoui, S., John, T. R., Dudas, S., Czub, S., & Gilch, S. (2016). Early and Non-Invasive Detection of Chronic Wasting Disease Prions in Elk Feces by Real-Time Quaking Induced Conversion. PLoS ONE. 11(11), 1. Doi: 10.1371/journal.pone.0166187Government of Canada. (2017). Chronic Wasting Disease (CWD)- Fact Sheet. Canadian Food Inspection Agency. Retrieved from: Hannaoui, S., Schaltzl, H. M., & Gilch, S. (2017). Chronic Wasting Disease: Emerging Prions and their Potential Risk. Plos Pathogens, 13(11), 1-5. Doi: 10.1371/journal.ppat.1006619Mejía-Salazar, M. F., Waldner, C. L., Hwang, Y. T., & Bollinger, T. K. (2018). Use of Environmental Sites by Mule Deer: A Proxy for Relative Risk of Chronic Wasting Disease Exposure and Transmission. Ecosphere, 9(1), p1-18. Nalls, A. V., McNulty, E., Powers, J., Seelig, D.M., Hoover,C., Haley, N.J., & … Mathiason, C.K. (2013). Mother to offspring transmission of chronic wasting disease in reeves’ muntgac deer. Plos One, 8(8), e71844. Doi: 10.1371/journal.pone.0071844 Nichols, T. A., Spraker, T. R., Gidlewski, T., Powers, J. G., Telling, G. C., VerCauteren, K. C., & Zabel, M. D. (2012). Detection of prion protein in the cerebrospinal fluid of elk (Cervus canadensis nelsoni) with chronic wasting disease using protein misfolding cyclic amplification. Journal of veterinary diagnostic investigation,(4). p764-749. edsagr.US201400077638Potapov, A., Merrill, E., Pybus, M., & Lewis, M. A. (2016). Chronic Wasting Disease: Transmission mechanisms and the possibility of harvest management. Plos ONE, 11(30, page 1-20. Doi: 10.1371/journal.pone.0151039 Race, B. L., Meade-White, K. D., Ward, A., Jewell, J., Miller, M. W., Williams, E. S., & … Race, R. E. (2007). Levels of abnormal prion protein in deer and elk with chronic wasting disease. Emerging Infectious Diseases, (6), 824.  USGS. (2016). Chronic Wasting Disease (CWD). National Wildlife Health Center. Retrieved from:   White, S.N., Spraker, T. R., Reynolds, J. O., & O’Rourke, K. I. (2010). Association analysis of PRNP genere region with chronic wasting disease in Rocky Mountain elk. BMC Research Notes, 3314-320. Doi: 10.1186/1756-0500-3-314


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