Scrapie is a fatal brain disease affecting sheep and goats. Scrapie is a transmissible spongiform encephalopathy. Clinical symptoms include irritation, and changes in behaviour and changes in posture. In the beginning, changes are mild and include slight behavioural changes and an increase in chewing movements may occur. Ataxia and neurological signs then develop, and affected sheep struggle to keep up with the flock. Some sheep scratch excessively, and patches of wool loss and lesions can be seen on the skin. Scratching sheep over the rump area may lead to a nibbling reflex, which is characteristic for the condition. Signs of a chronic systemic disease appear later, with weight loss, anorexia, lethargy, and death. These clinical signs can be confused with other sheep diseases. It is not known to pose a risk to human health.

Post mortem examination is important for the diagnosis of scrapie. Histology of tissues shows accumulation of prions in the central nervous system, and immunohistochemical staining and ELISA can also be used to demonstrate the protein.

There is a well-established association between sheep prion protein (PrP) genotype and the risk of death from scrapie. Certain genotypes are clearly associated with susceptibility to the disease and others to resistance. The test checks for the following alleles: ARQ, VRQ, ARH and AHQ. The greatest risk are genotypes: ARQ/VRQ, ARH/VRQ and VRQ/VRQ . The next greatest risk are sheep with the genotype ARQ/ARQ. There are no reported scrapie in sheep with the genotype ARR/ARR and so this is considered to be very low risk. The AHQ allele conferred resistance and the risk of scrapie in AHQ/VRQ sheep was very low, although there was a higher and moderate risk for the AHQ/AHQ genotype. Scrapie risk varied with age: for VRQ/VRQ and ARH/VRQ the risk peaked at 2 years of age; that for ARQ/VRQ peaked at 3 years.