Genetic susceptibility to scrapie, a fatal disease of sheep and goats, is controlled by differences in one specific gene – the prion protein. The resulting scrapie genotype has been shown to be associated with an animal’s susceptibility to scrapie, with the ARR variant associated with resistance, while the ARQ, ARH, VRQ variants are associated with a reduced level of resistance.
Although a national scrapie programme was introduced in Ireland in 2002, the scheme ceased prior to the complete eradication of the most susceptible scrapie genotypes in Ireland. Since then, the frequency of the scrapie genotypes in the national population remains relatively unknown.
Scrapie genotypes are now readily available as part of the Sheep Ireland genotyping programme. Therefore a large dataset is now available to assess:
Scrapie genotypes were available on 16,416 animals originating from 252 lowland flocks born between the years 2004 and 2019 inclusive. The breeds represented included 733 Belclare, 333 Charollais, 739 Suffolk, 1,857 Texel, 191 Vendéen, as well as 12,563 animals from other breeds or crossbreds.
Performance data on a range of animal-specific events, including date of birth, lambing data, lamb liveweight records, as well as ewe performance metrics such as ewe liveweight, body condition score and litter size were also available from the national sheep database (www.sheep.ie).
The association between the 15 scrapie genotypes and animal performance was investigated.
All 15 of the possible scrapie genotypes were detected across the population, although the frequency differed by breed.
As detailed in Table 1, the frequency of Type 1 scrapie class was 70.85% across the entire population, but ranged from 44.70% in Texels to 85.93% in Suffolks (Table 1).
The most susceptible Type 5 scrapie was only detected in purebred Texels, Beclares and crossbreds (Table 1). The scrapie genotype of either the lamb or ewe did not associate with any of the lambing traits investigated including lambing difficulty score, lamb mortality and lamb birth weight.
With the exception of ultrasound muscle depth, the scrapie genotype did not associate with any of the lamb performance traits investigated including lamb liveweight pre-weaning to slaughter and any of the lamb carcase traits.
Lambs carrying the type 4 scrapie genotype (i.e ARR/VRQ) had 1.20mm, 1.38mm, and 1.47mm shallower ultrasound muscle depth relative to lambs of the less susceptible scrapie types of 1, 2, 3, respectively.
Nonetheless, a lamb’s scrapie genotype did not associate with the lamb’s eventual carcase conformation, the ultimate end goal of farmers.
Ewe litter size, body condition score or lameness did not differ by scrapie genotype of the ewe. For ewe mature liveweight, ARH/VRQ ewes were, on average, 3.79kg heavier than ARR/ARR genotype ewes.
Lamb dag score differed by the scrapie genotype of the ewe, although the differences were small.
Results from this study show that the all scrapie genotypes exist within the Irish sheep population albeit the frequency differs by breed. The scrapie genotype of either the lamb or the ewe, however, had a weak to no association with several animal performance metrics and, where associations were detected, the biological significance was small.
*This update was compiled by Noirin McHugh, Áine O’Brien and Donagh Berry, Teagasc, and Thierry Pabiou and Kevin McDermott, Sheep Ireland.