Scrapie is a fatal prion disease of sheep. Selective breeding for genetic resistance, particularly the ARR allele, has dramatically reduced incidence in flocks across Europe.
Scrapie causes a prolonged and distressing terminal illness. Affected sheep progressively lose coordination, develop intense pruritus causing severe self-trauma, and experience central nervous system deterioration over weeks to months before death. The welfare impact on affected individuals is substantial. Eradication through genetic selection is one of the most effective disease control strategies in livestock farming, preventing future welfare harm by removing susceptibility from the population. Maintaining high-resistance genotype frequency requires continued commitment from breeders even when disease prevalence is low, as relaxing selection pressure allows susceptible genotypes to return.