Merino sheep breed's genetic resistance to scrapie: genetic structure and comparison of five eradication strategies.

Abstract

Three thousand one hundred and ninety-three Merino-breeding males were genotyped for Scrapie-resistance gene. They showed a high frequency of ARQ allele while VRQ allele frequency, linked to the highest susceptibility, was close to 0. Alleles linked to Scrapie-susceptibility frequencies have been studied in different Merino flocks. Most of the variability is intraflock, not interflock. In addition, there is an excess of heterozygotes due to crossbreeding or Wahlund effect. Five control and prevention strategies were studied: first genotyping males and females and eliminating VRQ carriers and ARQ/ARQ rams; second genotyping males and females and eliminating VRQ carriers; third genotyping males and eliminating VRQ carriers and ARQ/ARQ rams; fourth genotyping males and using only as breeders ARR/ARR, ARR/AHQ, AHQ/AHQ and ARQ/ARQ rams; fifth genotyping males and using only as breeders ARR/ARR, ARR/AHQ and AHQ/AHQ rams. They were simulated from allelic and genotypic frequencies to decide which animals had to be genotyped and which animals had to be eliminated due to their genotypes and risk levels. The third option, to genotype rams and to eliminate ARQ/ARQ and VRQ carriers, would be the best strategy to improve the resistance and would cause minimal cost and loss of animals.