NR AWBT
AU Baylis,M.
TI The epidemiology of scrapie
QU International Conference - Prion 2006: Strategies, advances and trends towards protection of society - 3.10.-6.10.2006, Torino, Italy, Lingotto Conference Centre - Oral sessions ORAL-03
PT Konferenz-Vortrag
AB
Whether a sheep can acquire scrapie infection or, following infection, proceed to clinical disease, is affected by different alleles of the sheep PrP gene. An aspect of scrapie genetics that has attracted relatively little attention is the different patterns across breeds. First, the five major PrP alleles are present at very different frequencies within different breeds. The origins of these differences are unknown, but may reflect the different breed origins (phylogeny or founder effects), genetic drift following reproductive isolation or, perhaps, are a response to different scrapie histories.
There is also variation between breeds in the genotypes that succumb to scrapie, with an unknown biological basis. They may reflect a variety of strains of scrapie, with different genotype-tropisms, circulating in different breeds; alternatively, they may indicate certain innate differences between breeds in their responses to scrapie infection.
In this talk I present the results of an analysis of several UK-derived datasets on scrapie in different breeds of sheep and their underlying genotype frequencies. The purpose is not to provide explanations, but to identify the patterns that need explaining and measure their robustness.
Breed differences notwithstanding, the strong genetic control of sheep in susceptibility to classical scrapie has led to the possibility of its control by selective breeding. A two pronged-approach is now being applied across the EU and elsewhere: (1) culling of all sheep or all but the most genetically resistant sheep, within scrapie-affected flocks; (2) culling of the most susceptible sheep within unaffected flocks of high genetic merit.
A genetic approach to control within scrapie-affected flocks is both appropriate and effective. The application of large-scale selective breeding to unaffected flocks may make them resistant to becoming newly infected by classical scrapie. However, this latter approach will also reduce the overall variability of the ovine PrP gene in our sheep populations and it is worth asking, therefore, why this variability arose in the first place and why it has persisted until now. One possibility is that the gene has evolved under 'negative frequency dependent selection'; ie, resistant genotypes are resistant to only some scrapie strains and as one 'resistant' genotype becomes more common, so it becomes the target of a novel strain. 'Atypical' scrapie may be one such novel strain, targeting the alleles that confer resistance to classical scrapie.
If this hypothesis is correct, current selective breeding programmes may provide a short-to-medium, but not a long-term solution for scrapie control in sheep. Accordingly, the preservation of variability at the ovine PrP gene may be worth increased consideration.
AD Veterinary Clinical Science, University of Liverpool, Leahurst, Neston, Cheshire, UK. E-mail: matthew.baylis@liverpool.ac.uk
SP englisch
PO Italien