POSTnote: The Geographic Expansion of Chronic Wasting Disease.

Overview

  • Chronic Wasting Disease (CWD), is a type of Prion disease, which effects members of the Cervidae family (deer).  
  • The disease effects the nervous system of the animal, and causes a spongy degeneration of the brain, resulting in abnormal behaviour, emaciation (wasting), loss of bodily functions, and ultimately death.
  • CWD is highly contagious, currently incurable, and is having huge effects in North America, and is showing signs of spreading to Europe (CDC, 2019).

Introduction
CWD is a contagious and lethal disease, which is affecting several species of Deer and Elk in Northern America, and some parts of Europe and Asia. The situation has been identified as an emerging threat to conservation and biodiversity (Sutherland, 2018). This POSTnote will explore the threat, its consequences, and potential solutions.

Background:
Prion diseases, or ‘transmissible spongiform encephalopathies (TSEs)’, are a family of neurodegenerative disorders that affect both humans and animals (Holman, 2010). The term ‘prion’ was coined to mean ‘proteinaceous infectious particle’ (Prusiner, 1982), but currently, prions are defined as: misfolded proteins that have the ability to spread by making other proteins misfold; the cause of this misfolding is currently under debate.
These malformed proteins result in a disintegration or degeneration of brain and nervous system tissue, into the characteristic ‘spongey’ formation.
In the specific case of CWD, the brain and spinal cord are the targets for infection. As the proteins in reproducing cells deform, this will eventually break down the structure of the brain, causing abnormal behaviour and loss of function, which invariably causes death, due to starvation or brain damage (Mysterud, 2019).
Instances of this disease were found in several keystone species, which are of great ecological and economical importance: Mule Deer, White-tailed Deer, Reindeer, Elk, and Moose (Mysterud, 2019).

Fig. 1 – As of March 2019, there were 270 counties in 24 states with reported CWD in free-ranging cervids (CDC, 2019).


The first documented case of CWD was in 1967, however, it took more than a decade for an official diagnosis to be given (Bunk, 2004), due to the complex nature of the disease (long incubation period, followed by rapid decline and death). Since then, CWD has been dubbed an ‘epidemic’ in America (Bunk, 2004), and its spread to Europe (Norway) and Asia (South Korea) has been concerning to scientists worldwide.
CWD is particularly challenging due to its long incubation period, which displays no symptoms, its high contagion rate, and its universal fatality. CWD is transmissible via either direct contact with an infected individual, or indirect environmental exposure (Hoover, 2017). Direct infection can occur during rutting (mating display fighting between males), from mother to calf (Nalls, 2013), during mating, or general interaction within herds (Hoover, 2017). Indirect exposure appears harder to control, as the prions can exist in the environment in the faeces, urine, salvia, antler velvet, contaminated pasture soils, and are taken into the animal via the mucosal membranes (nose, ears, mouth, cuts) (Hoover, 2017).


Potential Consequences:
The consequences of CWD will include population decline, inevitably leading to ecological impacts, such as effects on predation, overgrowth of previously grazed areas, and general biological instability. In addition to this, “cervids are associated with enormous cultural and economic value” (Mysterud, 2019) in many communities. Furthermore, the possibility of cross-contamination being explored by scientists (Haley, 2017), as it was feared that, as with other known TSE’s, this disease effect other animals, potentially including humans. In initial testing, it seems that prions may be able to convert the human PrPC protein (Barria, 2018), suggesting the possibility of cross-contamination. However, thus far these findings have only been documented in laboratory testing, and required in-vitro conditioning before it was able to cross the deer–human molecular barrier (Barria, 2018).
Although CWD is currently the last remaining unmanaged TSE, the discoveries during its study over the past several decades have greatly increased our knowledge of protein-misfolding disorders, which will ultimately benefit animal and human health in the long term (Haley, 2017).

Potential Solutions:
Currently, the most viable control method of CWD is the culling of animals presenting visible symptoms. In the USA, farmers are being forced to destroy whole herds if a single case of the disease is found, as the only way researchers can definitively diagnose is by studying the animal’s brain. Additionally, in states with high occurrence, such as Colorado, recreational hunters are being advised by wildlife wardens how to target high-risk individuals (Mysterud, 2019). Preventative measures are also possible, as Canada has closed its borders to the trade of U.S. deer and elk (Enserink, 2001). However, the eradication of visibly weak individuals will not eliminate CWD, as the dormancy phase of the disease is still infectious, and continues it’s silent spread (Enserink, 2001). This is the main issue that scientists and legislators face when tackling the spread of Chronic Wasting Disease, and its effects on biodiversity and ecology.

References

Barria, M., 2018. Susceptibility of Human Prion Protein to Conversion by Chronic Wasting Disease Prions. Emerging Infectious Diseases , 24(8), pp. 1482-1489.

Bunk, S., 2004. Chronic Wasting Disease – Prion Disease in the Wild. PLoS Biology , 2(4).

CDC, 2019. Centre for Disease Control and Prevention. [Online]
Available at: https://www.cdc.gov/prions/cwd/
[Accessed 19 03 2019].

Enserink, M., 2001. U.S. Gets Tough Against Chronic Wasting Disease. Science, 294(5544), pp. 978-979.

Haley, n., 2017. Evolution of Diagnostic Tests for Chronic Wasting Disease, a Naturally Occurring Prion Disease of Cervids. Pathogens, 6(3).

Holman, R., 2010. Human Prion Diseases in the United States. PLoS ONE, 5(1).

Hoover, C., 2017. Pathways of Prion Spread during Early Chronic Wasting Disease in Deer. Journal of Virology , 91(10).

Mysterud, A., 2019. A review of chronic wasting disease in North America with implications for Europe. European Journal for Wildlife Research, 65(26).

Nalls, A., 2013. Mother to Offspring Transmission of Chronic Wasting Disease in Muntjac Deer. PLoS One, 8(8).

Prusiner, S., 1982. Novel proteinaceous infectious particles cause scrapie. Science , 216(4542), pp. 136-144.

Sutherland, W., 2018. A 2018 Horizon Scan of Emerging Issues for Global Conservation and Biological Diversity. Trends in Ecology and Evolution, 33(1), pp. 47-58.

Published by amyandkatherine

We are two friends of 12 years, trying to start careers in journalism.

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