Scrapie is a neurodegenerative disease, caused by a prion, that affects sheep, and less frequently, goats. Infected animals do not usually become ill for years; however, the clinical signs are progressive and invariably fatal once they develop. Scrapie can be transmitted between animals, either directly or via the environment, and infected premises are difficult to decontaminate. The presence of classical scrapie can result in trade sanctions, and many countries are conducting control or eradication programs. Breeding sheep for genetic resistance is an important tool in many of these programs; however, the understanding of resistance genes is still incomplete in goats.

As a result of increased surveillance, atypical (Nor98) scrapie prions have been detected in both sheep and goats. Atypical scrapie often occurs in sheep that are genetically resistant to classical scrapie. It has been reported in countries that do not have classical scrapie. Atypical/ Nor98 prions do not seem to be transmitted readily between animals in nature, and are rarely detected in more than one animal in a herd or flock. It is possible that they arise spontaneously in sheep, similarly to some genetic prion diseases in humans.