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What is Scrapie?
Scrapie is a slowly progressive, fatal disease of the nervous system of sheep. It is characterized by compulsive rubbing or scraping against fixed objects and by uncoordinated movements. It has been transmitted experimentally to sheep, goats, and various rodents by injecting tissue, especially brain tissue, from an infected animal. Passage from animal to animal can be continued indefinitely. The clinical disease may appear months or even years after injection of infected tissue. Since the 1960's two fatal degenerative diseases of the human nervous system, kuru of the primitive Fore people of New Guinea and the rare Jakob-Creutzfeldt syndrome, have been found to be similar to scrapie. Thus scrapie has a scientific importance greater than the relatively small economic loss that it causes farmers.
There has been much speculation on the nature of the causal agent of scrapie. Progress in the identification of the agent has been slow, because it can only be detected indirectly by its ability to produce disease. Until the late 1960's it was thought that the transmissible agent of scrapie was a virus because the agent passes filters that retain all particles larger than a virus and because it apparently multiplies when passed through animals. But the scrapie agent has not been seen under the electron miscroscope; does not stimulate the production of antibodies, a characteristic of viruses; shows marked resistance to boiling and to various chemicals, including betapropiolactone, that destroy viruses; and is highly resistant to doses of ultraviolet light that are lethal to viruses. These findings indicate that the scrapie agent does not contain nucleic acid, the genetic material of viruses. The size of the scrapie agent (about one half that of the smallest known virus) plus its tendency to stick to larger particles have added to the difficulties of purifying and characterizing the agent.