by Joanna Hay

Created on: June 27, 2009   Last Updated: June 28, 2009

Scrapies is a devastating disease that affects sheep and goats throughout the world. Although many details of this disease are still being debated by scientists and researchers, most agree that it is a prion disease much like mad cow disease or chronic wasting of deer. Scrapies has a slow onset but inevitably ends in the death of the animal. The contagious and fatal nature of the disease has led to eradication programs in America and throughout the world.

The causative agent of scrapies is a prion, or infectious protein. The original protein was a normal part of the genetic makeup of a sheep. At some point the protein in affected animals underwent a mutation into a protein called a PrP (prion protein). A protein is really just long sheets of amino acids strung together in a certain order and folded in a certain way. A PrP has the same amino acids but folds differently. The change in folding changes the shape of the protein, which in turns changes the way that it interacts with the other proteins and cells around it. The new interaction of the mutated PrP results in the clinical signs seen in scrapies disease.

Sheep that are infected with scrapies can shed PrP proteins in the fluids and placenta that is expelled after giving birth. This is the only known way that the virus is transmitted, although scientists believe that the prion may also be shed in other ways. Sheep normally eat their placentas and in a herd situation more than one sheep may 'help' a new mother by eating parts of her placenta. If prion is present in the placenta, it enters the intestines and quickly moves into the immune system. Consumed prions are capable of triggering normal proteins in the body to convert into PrPs, allowing the prion to multiply and spread through the host's immune system and into the nervous system.

Most of the clinical signs seen in affected sheep are the result of the actions of PrP on the nervous system. Although they may have been infected at any time in their life, the disease is usually only seen in animals between one and five years of age. At first, sheep suffering from scrapies show only mild behavioral changes. They may become nervous, restless, or aggressive. This often goes unnoticed in large herds. As the disease progresses, the animal will begin to scratch itself on fences and other rough surfaces. The intensity of the scratching slowly increases over the course of a few weeks and eventually becomes an obsessive behavior that results in wool loss and

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