Scrapie
The name scrapie is derived from one of the clinical signs of the condition, wherein affected animals will compulsively scrape off their fleeces against rocks, trees, or fences. The disease apparently causes an itching sensation in the animals. Other clinical signs include excessive lip smacking, altered gaits, and convulsive collapse.
Scrapie
The cause of scrapie, as with other transmissible spongiform encephalopathies, is unknown and is a matter of debate; the agent is very difficult to destroy with heat, radiation and disinfectants, does not evoke any detectible immune response, and has a long incubation period of between 18 months and 5 years. The agent is thought to be much smaller than the smallest currently known virus and the prevailing theory is that it is caused by a misfolded protein called a prion. Prions multiply by causing normally folded proteins of the same type to 
take on their abnormal shape, which then go on to do the same, in a kind of chain reaction. These abnormal proteins are gradually accumulated in the body, especially in nerve cells, which subsequently die.