Transmissible encephalopathies in animals.

Scrapie in sheep and goats is the best known of the transmissible encephalopathies of animals. The combination of maternal transmission of infection and long incubation periods effectively maintains the infection in flocks. A single sheep gene (Sip) controls both experimental and natural scrapie and the discovery of allelic markers could enable the use of sire selection in the control of the natural disease. Studies of experimental rodent scrapie show that neuroinvasion occurs by spread of infection from visceral lymphoreticular tissues along nerve fibers to mid-thoracic cord. The slowness of scrapie is due to restrictions on replication and cell-to-cell spread of infection affecting neuroinvasion and subsequent neuropathogenesis. Probably both stages in mice are controlled by Sinc gene, the murine equivalent of Sip. The glycoprotein PrP may be the normal product of Sinc gene. Posttranslationally modified PrP forms the disease specific "scrapie associated fibrils" and may also be a constituent of the infectious agent. Scrapie-like diseases have been reported in mink and several species of ruminants including cattle. All of them may be caused by the recycling of scrapie infected sheep material in animal feed. The human health implications are discussed.