Abstract
History. Scrapie, a slow progressive fatal infection affecting the central nervous system (CNS), occurs naturally in sheep and goats (1) and was first described in the scientific literature of the last century by Besnoit and Morel (2). Due to a number of common features five other degenerative neurological diseases of the CNS have been grouped together with scrapie. Two of the others are animal diseases, transmissible mink encephalopathy (TME) (3) and wasting disease of mule deer and elk (4, 5), and the other three are human diseases, kuru, Creutzfeldt-Jakob disease (CJD), (6) and Gerstmann-Sträussler syndrome (7, 8).
In 1899 Besnoit (9) attempted to transmit scrapie by inoculating a ewe with brain tissue from a sheep with scrapie. Later Cuillé and Chelle (10) transmitted scrapie from one sheep to another, demonstrating the filterable nature and viruslike properties of the agent. The infectious nature of the scrapie agent was also shown in England during the 1930s by a major vaccine accident in which formalized tissue of louping-ill virus vaccine caused a large number of scrapie cases in 18,000 sheep in a flock formerly unaffected by scrapie.
The agent appears to replicate at a very slow rate with apparently no eclipse phase and requires many months to reach high titers (11). The characteristic features of the disease are both structural and functional. Pathological changes are restricted to the CNS without involvement of any other organs, and the disease is fatal. The late Björn Sigurdsson (12), because of the long incubation period, listed Rida as an example of slow infections and proposed the concept of slow or latent virus infection. Chandler successfully transmitted the disease in the early 1960s to mice (13) and then serially in both mice and rats. This facilitated laboratory investigations and confirmed the transmissible nature of scrapie.
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