Genetic scrapie resistance in sheep?

At present, breeding for scrapie resistance means selecting for the prion protein (PrP) haplotype ARR and against the PrP haplotype VRQ. Currently, atypical scrapie cases vary in epidemiological aspects from classical scrapie cases and also show differences in respect to the well-established associations of PrP variants with scrapie susceptibility. German scrapie cases from 2002 to 2006 were analyzed regarding epidemiology and molecular genetics (PrP codons 136, 141, 154 and 171). Between classical and atypical scrapie, differences were observed concerning the geographical distribution of cases, the number of positive sheep per flock and the age of positive sheep at the time of death. Sheep with the PrP genotype ARQ/ARQ were most susceptible for classical scrapie, while the PrP haplotype VRQ played no significant role in classical and atypical scrapie cases. Sheep carrying the haplotype AHQ and/or the F allele at position 141 were most susceptible for atypical scrapie. Furthermore, sheep with the haplotype ARR – also in homozygous form – were more often affected by atypical scrapie than supposed before. After the current knowledge, selecting for the PrP haplotype ARR promises success with regard to the combat of classical scrapie, whereas a preferential suppression of the VRQ haplotype seems not to be necessary at least in Germany. In contrast, atypical scrapie appears not to be eliminable by selection for ARR.

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