Scrapie is a naturally occuring disease affecting sheep. It causes debilitating degeneration fo the central nervous system after two to three years of incubation in the host. With no immunity produced to counter the disease and a high rate of transmission, particularly amongst parents and their offspring, Scrapie is both deadly and incurable.
Though little is known about the Scrapie causing protein, as it shows very few of the characteristics of viruses and is relatively unique in its structure, it has been linked to the presence of certain prions in the host body. These prions are proteins naturally produced in a number of cells, though primarily in the cells of the nervous systems of many animals. Variations of the prion protein producing gene (PrP) are known to have a notable effect on the susceptibility and control of the disease within the host organism (Hartwig, 2000). As a result of this strong genetic basis, selective breeding has been used substantially to isolate and reduce the presence of the disease among herds, though this has presented its own difficulties.
Recent studies have shown a strong association between the PrP genotype and coat colour genotypes amongst Badgerfaced Welsh Mountain and Shetland sheep (Sawalha, Bell, Brotherstone, White, Wilson, Villanueva, 2009). With regards to the PrP gene, the allele ARR appears to be the most resistant, while the VRQ allele appears to be the most susceptible. Studies have shown that a both Welsh Mountain and Shetland sheep showed a high correlation between the presence of the ARR allele and the torddu coat colour (light dorsum with dark abdomen). In addition, Shetland sheep also showed a high correlation between the presence of the ARR allele and non-spotted coats.
As a result of this link between coat colour and the PrP gene, the assumption that selective breeding targeting the disease will have an effect on coat colour is supported. As a large proportion of the sheep livestock is aimed at the wool industry, this presents an issue. As sheep coat colour is generally required to be specific based on industry preferences, such selective breeding preferences do not necessarily support the selective breeding in place to manage Scrapie disease. Put simply, there is a strong need to find a way to manage the links between these genes individually, and understanding the link is the first step.
Peter Staff
23 March, 2009
Main Article: Sawalha, Bell, Brotherstone, White, Wilson, Villanueva, 2009, Scrapie-resistant sheep show certain coat colour characteristics, Cambridge University Press Genetics Research
Link: http://journals.cambridge.org/action/displayFulltext?type=6&fid=4105844&jid=GRH&volumeId=91&issueId=01&aid=4105840&fulltextType=RA&fileId=S0016672308009968
Resources: Hartwig et al, 2000, Scrapie of Sheep, Iowa State University
Link: http://www.extension.iastate.edu/Publications/PM829X9.pdf
Image: Holes in the brain of Scrabie infected sheep
Link: http://www.jyi.org/volumes/volume11/issue5/features/iyer.html
