Natural Transmission and Genetic Control of Susceptibility of Sheep to Scrapie

Abstract

Scrapie has often been the subject of controversy and speculation both by farmers and by scientists. Not the least strange of numerous etiologic ideas was that put forward by Roche—Lubin in 1848 who suggested that scrapie was caused by sexual excess, sunstroke after shearing, or lightning. The recent suggestion that kuru, a human scrapie-like disease, was the result of a volcanic eruption (Schoental 1990) is therefore part of a long tradition. The lightning analogy is not a bad one, however. One of the most common remarks made by farmers having to deal with a scrapie outbreak is that it appeared suddenly and apparently from nowhere; reports like this are still being made from, for instance, Sweden (Elvander et al. 1988), Cyprus (Toumazos 1988) and Italy (Mechelli and Mantovani 1988). Another common belief is that the disease was “brought in” by a recently purchased ram. Such anecdotal evidence is difficult to substantiate, especially as scrapie has such stigma attached to it that true incidence figures have been difficult to obtain. It is probably a much more common disease than would seem from official diagnosis figures (Chatelain et al. 1983; N. Hunter 1991). In addition, the lack of a preclinical dignostic test means that animals incubating scrapie cannot be identified until they develop symptoms. The number of carrier-status animals, if indeed they exist at all, is completely unknown, despite their obvious importance in any hypothesis of natural transmission mechanisms.