Scrapie is a fatal, degenerative disease that affects the nervous systems of sheep and goats. It is one of several transmissible spongiform encephalopathies (TSEs), which are related to bovine spongiform encephalopathy (BSE or 'mad cow disease') and chronic wasting disease of deer. Like other spongiform encephalopathies, scrapie is caused by a prion. Scrapie has been known since the 18th century (1732) and does not appear to be transmissible to humans. 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 is infectious and transmissible among similar animals, so one of the most common ways to contain it (since it is incurable) is to quarantine and destroy those affected.
Contributions by SeanMyles, Joelmills, and Thegline.