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2015-08-13

CWD - What we just don't know!

By Lauren Langford, MD

Courtesy photo
A 2-year-old white tail buck, similar to this one, in a Medina County breeding facility has recently been confirmed positive for Chronic Wasting Disease (CWD). This discovery might deal Texas' hunting industry a huge blow.


By Lauren Langford, MD
Special to the Courier

(Editor's note: This summer a breeding buck in Medina County tested positive for Chronic Wasting Disease. For more on this development, see the article "Cervid CWD detected in Medina County on page 2A.)

Chronic Wasting Disease (CWD) of cervids, which are members of the deer family, including not only white tails, but also non-natives like sika, fallow, red, axis and mule deer, is part of an incurable disease spectrum called Transmissible Spongiform Encephalopathy (TSE).
Although CWD shares features with other TSEs, such as bovine spongiform encephalopathy (BSE) in cattle and scrapie in sheep, it is a distinct disease known at this time to only affect members of the deer - cervid - family. CWD was first recognized as a "wasting syndrome" in a research facility in Colorado in 1967. CWD has only been found in captive and wild cervids in North America and in captive cervids in South Korea.
TSEs, including chronic wasting disease, are caused by a deadly protein called a prion (PREE-on). Prion disease is unstoppable. Prions defy all attempts at sterilization and inactivation. Victims can spread the disease even further via their bodily fluids and tissue. Victims often are contagious long before they appear sick.
For example, their saliva, urine, blood and feces spread the pathogen further every day. Prions linger in the environment and on objects infinitely. They migrate, mutate, multiply and kill with unparalleled efficiency. Because of these dynamics, Canada has declared CWD unstoppable. Unlike viruses or bacteria, prions are not alive; therefore, prions cannot be killed.
No one knows for certain how deer become infected with CWD. Is it through deer-to-deer contact, by consuming bodily wastes dropped or sprayed onto edible plants or by eating grains or leaves from plants whose roots take up prions from contaminated soil? Effective methods to inactivate prions in the soil are currently lacking, and the effects of natural degradation mechanisms on prion infectivity are largely unknown. Recent evidence shows that even grass can take up prions and, presumably, could be a carrier of the infectious agent.
For years, prions were thought to be confined to mostly brain and spinal cord, but we now know that prions can be detected in lymph nodes, spleen, salivary glands and even muscle. Processing facilities that cut and grind wildlife carcasses for hunters - before CWD tests are complete - are contaminated forever. Every animal processed after an infected carcass could become infected.
Pickup trucks and trailers that transport infected animals are contaminated. The humans who handle the animal are contaminated. The prion pathways created by one hunting trip can explode exponentially within hours.
Regarding possible transmission to humans, almost every formal statement about CWD simply says: "There is no strong evidence CWD can infect humans, but the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) recommend not eating venison from CWD-positive deer or elk."
How do we know if the animal is infected? How do we know if CWD prions can jump species to humans?
In 1995, a 19-year-old boy in Great Britain was the first human to die from a TSE that officials had said could not be transmitted from cows to humans - Mad Cow Disease or bovine spongiform encephalopathy, first diagnosed in British cattle in 1986. Variant Creutzfeldt-Jakob disease (vCJD) became known as the fatal neurodegenerative disorder in humans originating from exposure to bovine spongiform encephalopathy prions.
In contrast to traditional CJD, vCJD affects younger individuals, 28 years opposed to 68 years, and has a longer duration of illness, 14 months as opposed to 4.5 months. From 1996 to 2011, 175 cases of variant CJD, the human form of BSE, have been documented. The prions appear to become more aggressive with a species jump. Unfortunately, the small number of documented vCJD cases has created a false sense of security about the danger of the disease.
Claiming that there is no known risk associated with handling or consuming wildlife with CWD is absurd - just look at the guidance issued regarding mad cow disease before vCJD appeared.
Prion infections are associated with long and clinically silent incubations. Prions can inhabit the body of a person or animal for up to 50 years before presenting symptoms. During this time, there is a chance a carrier could pass on the infection to others, for example through blood transfusion or even through surgical and medical instruments as prions can easily attach onto metal surfaces.
The number of asymptomatic individuals with vCJD prion infection is unknown, posing risk to others via blood transfusion, blood products, organ or tissue grafts, and contaminated medical instruments. Could not CWD also be such a threat?
One of the reasons that the transmissible spongiform encephalopathies are such a dreaded disease group and have caused such disruption and expense to health services is the lack of knowledge of who is and who is not a carrier of the infection. For humans, there are preliminary studies for a blood test for diagnosis of vCJD in symptomatic individuals, which could allow development of large-scale screening tests for asymptomatic prion infection.
For suspected prion disease in animals, a portion of the brain or lymph nodes found in the throat of the animal must be tested to confirm if the animal in question is infected with CWD. Unfortunately, there is no live-animal test to detect CWD.
Although there is no scientific evidence or confirmed cases of companion animals, such as dogs, contracting a form of TSE, there have been confirmed cases of Feline Spongiform Encephalopathy (FSE) in approximately 100 cats in Europe. To date, there have been no confirmed cases of FSE in the United States.