Chronic wasting disease is a chronic weight loss disease that till 2016 has only been found in the deer family. It is a transmissible spongiform encephalopathy (TSE) of mule deer, elk (wapiti), moose or white-tailed deer. 
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bull elk herd

Image: “Bull elk herd at the crack of dawn between Noris and Canyon village.” by Anayst – Own work. License: CC BY-SA 3.0


Overview

Chronic Wasting Disease first came into notice and was identified in 1967 in Colorado, USA, and it was found in free and captive animals. Ultimately, due to muscle wasting and weight loss it leads to death.

Reports are being made about the disease occurrence in the human population, but there is no definitive data to prove that. One epidemiological study states that in the areas where the disease has been reported, the hunters should avoid eating deer or moose as a precautionary measure.

Clinical Signs and Symptoms of CWD

The disease usually affects adult cattle, but the lowest age at which it has been identified is 17 months. The earliest signs that come into notice are difficulty in movement and walking.

Other signs and symptoms include decreased alertness, agitation, decreased contact with other animals, tremors, nervousness, continuous teeth grinding and increased milk consumption and urination. Increased salivation and urination are the major cause of spread of the disease to other animals.

Causative Agent of CWD

The agent responsible for chronic wasting disease and other transmissible spongiform encephalopathies—and mad cow disease—is a prion protein, which converts a normal structural protein into an abnormal structural prion protein (PrPres or PrPd).

The abnormal prion protein first affects the central nervous system (CNS) and then spreads peripherally affecting the muscles and peripheral nervous system (PNS). The build-up of protein in the brain causes neurodegeneration.

Diagnosis of Chronic Wasting Disease

Many animals die during early illness due to aspiration pneumonia and poor health, so it becomes difficult to diagnose them. Definitive diagnosis can only be made by examining the post mortem specimens’ brain and spinal cord.

On microscopic examinations, the lesions resemble those of the other prion diseases. The prion proteins can be visualized by using immune-histochemical stains in the brain, lymph and other tissues. The gold standard for diagnosis is obex with positive IHC findings.

Diagnosis can also be made by bioassay of fluids from the suspected animal and incubating them in mice. However, this method is considered to be unethical, and that is why it is not used in practice.

Epidemiology of Chronic Wasting Disease

The origin of the disease is still unknown, but some researchers suggest that the prion protein is excreted in urine. It, then, contaminates the soil, and animals feeding on that grass can acquire the disease.

Animals born in captivity and in the wild are equally affected by the disease. Transmission can occur in animals who feed and drink from the same source as the prion protein is secreted in the saliva as well, so animals sharing the same water source are more prone to develop the disease. Maternal transmission can also occur.

Transmission Pathways

Direct transmission pathway

Chronic wasting disease is transmitted via body tissues and fluids directly. Research done on rocky mountain elk showed that the infected animals transferred the disease into 80% of the offspring transplacentally.

Experimental transmission

Research has shown that Black-tailed deer and European red deer are naturally susceptible to the disease as compared to other populations of deer.

Indirect/environmental transmission

Environmental transmission occurs directly from the body tissues as well as from the contaminated environment. Prion proteins can survive for many years in the soil, and they remain infective for long periods of time. The chronic wasting disease prion adheres tightly to the soil, and this soil can serve as a major source of disease transmission.

Transmission to humans

There is no evidence of transmission to humans till date. The disease cannot be transmitted by the infected animal.

Research

Research is being done to determine:

  1. How to dispose of the animals who died from the disease.
  2. How the infected animals can be diagnosed at an early stage of the disease.
  3. Better ways to clean the soil carrying the prion protein.
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