Scrapie

The term scrapie encompasses two different pathological entities: classical and atypical scrapie. Both conditions are chronic, neurodegenerative, fatal diseases of sheep and goats and are caused by similar agents. However, they differ in clinical presentation, pathogenesis, pathology, biochemical properties and epidemiology. Classical scrapie is characterised by vacuolar changes in the central nervous system (CNS). It has been recognised as a clinical disorder for more than 250 years, and is classified as a transmissible spongiform encephalopathy (TSE), or prion disease, as defined by the accumulation of an abnormal form of a host cell membrane-bound glycoprotein (prion protein or PrP) referred to as PrPSc, in the CNS. Polymorphisms of the PrP gene are associated with susceptibility to scrapie. The more recently identified condition known as atypical scrapie has some clinical and pathological features similar to classical scrapie but is not considered to be transmitted in field situations. The epidemiology is consistent with a non-contagious condition that occurs sporadically. Classical scrapie may be transmitted from dam to offspring in the period from parturition to weaning, and in utero. It can also be transmitted horizontally to unrelated sheep or goats. The infectious material can persist for several years on pastures and in buildings. Fetal membranes are a source of infection, and milk from clinically affected animals can transmit disease. The incubation time between primary infection and clinical disease is usually longer than 1 year and may sometimes exceed the commercial lifespan of the animal. The majority of cases occur between 2 and 5 years of age. Clinical disease develops only if the agent enters the CNS. Atypical scrapie, where it presents clinically, is reported mostly in older animals, and occurs with a geographical distribution suggestive of a spontaneous disease, although it has been transmitted experimentally. There are no vaccines available. There is no evidence of a causal link between classical or atypical scrapie and human TSEs.