Tag Archives: scrapie

Chronic Wasting Disease

Last month Arkansas Game and Fish confirmed that a deer found dead near Ponca, Ar and an elk taken by a hunter were both found to have Chronic Wasting Disease (CWD). This ultimately fatal disease is not yet known to be transmissible to humans who consume the flesh of an infected animal. It is the same disease which showed up in Great Britain years ago, called Mad Cow Disease. In sheep, where the disease has been known for centuries, it is called Scrapie. In the remote highlands of New Guinea, warring tribesmen spread the same disease, known as Kuru, through ritual cannibalism. There is even a heritable form called Creutzfeldt-Jakob Disease (CGD.)

The assemblage of diseases are collectively known as transmissible spongiform encephalopathies (TSE.) What that means is that one may get it from eating the flesh of an infected animal and over time fatally damages the brain. The really unique thing about prion diseases is the nature of the infectious component of disease transmission.

With every other kind of transmissible disease from the common cold to the plague, from warts to ebola, transmission requires an organism such as a bacteria, or at least a virus. These agents replicate by well understood mechanisms involving DNA. Reproduction is necessary to create enough of the organism in a body to do it harm.

Stanley Prusiner won the Nobel Prize in Physiology or Medicine in 1997 for “his discovery of prions – a new biological principle of infection.”

As noted Scrapie has been known for centuries. Sheep farmers knew that the disease could be spread from one animal to another in a herd, although it may take years between infection and the presentation of symptoms in an infected animal. By the 1950s and with an awareness of the modern germ theory of disease, the cause of Scrapie was thought to be a viral. Because it took a long time for symptoms to present it was called a slow virus.

Prusiner began studying the disease in the ’70s. It was known that the disease could be spread from on organism to another via extracts of diseased brains tissue. What Prusiner discovered however was that when these extracts were treated with agents that destroyed DNA and RNA – nothing happened – the extracts were still infectious. This was previously unheard of. At first it was thought that a new kind of life (an organism) that replicated without DNA. Not so.

The infectious agent isn’t an organism, it is a simple molecule that already exists in all of us, quite possibly all multicellular organisms. Prions are proteins. Proteins all have unique three dimensional shapes. Prions are nothing more than a misshapen protein, except their misshapen form causes normally shaped protein to become similarly misshapen. The “bad” form catalyzes the change from the normal to the bad shape. Brain tissue riddled with the badly shaped protein take on a sponge-like appearance, with the loss of normal brain function.

The protein is found not only in brain tissue but virtually all nervous tissue. Consumption of any flesh of an infected animal, even an asymptomatic one carries a risk of contracting a lethal disease. Thorough cooking which normally destroys the usual infectious agents may not suffice to destroy the prion.