Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/16/2004
Publication Date: 2/1/2005
Citation: Alexander, B.M., Stobart, R.H., Russell, W.C., Orourke, K.I., Lewis, G.S., Logan, J.R., Duncan, J.R., Moss, G.E. 2005. The incidence of genotypes at codon 171 of the prion protein gene (prnp) in five breeds of sheep and production traits of ewes associated with those genotypes. Journal of Animal Science. 83(2):455-459. Interpretive Summary: Scrapie is a fatal disease of sheep, occuring throughout the United States. The disease is the subject of an intensive federal/state/industry eradication program that includes diagnosis of infected animals and removal of genetically susceptible sheep from flocks with evidence of the disease. Selection of scrapie resistant sheep in flocks with potential exposure to scrapie will be cost effective for producers if the resistant allele is not linked to deleterious production triats or if the resistant allele is very rare within a breed. In this study, the scrapie susceptibility alleles were determined for a group of sheep from five major breeds and western white-faced commercial ewes, and the relationship between lamb production and scrapie susceptibility genotype was determined. The scrapie resistant allele was found in all five breeds, although the allelic frequency varied among the groups. In this population, the scrapie susceptibility genotype did not influence ultimate lamb production in any of the breeds investigated. Based on these findings, no deleterious effect to selecting for scrapie resistance was identified.
Technical Abstract: Scrapie is a member of the transmissible spongiform encephalopathies, a family of fatal disorders characterized by accumulation of abnormally folded prion proteins in the central nervous system. Scrapie is the subject of an intensive eradication effort in the US, which includes selective removal of genetically susceptible animals from a flock following diagnosis of scrapie in a breeding ewe. Genetic susceptibility in sheep is associated with particular alleles of the gene encoding the prion protein precursor. In the US, most cases of scrapie are diagnosed in sheep homozygous for the allele encoding glutamine (Q) at codon 171; sheep homozygous or heterozygous for the alternative allele encoding arginine (R) are rarely affected. The risk of scrapie can be reduced by selective breeding for 171QR and 171RR sheep. However, the economic advantage of this approach is reduced if the 171R allele is very rare in a breed or if the production traits of sheep with the resistant allele are affected. In this study, several flocks of sheep representing five major breeds and a group of western white-faced ewes were examined for the frequency of the resistant allele, and the association of scrapie susceptibility genotype with particular production traits was examined. The resistant allele was found in all breeds. Suffolk ewes with the susceptible genotype gave birth to more lambs than did resistant ewes, although they weaned similar total lamb weights as did the resistant ewes. Production traits of Columbia, Hampshire, Rambouillet, and commercial ewes in the study flock did not differ with scrapie susceptibility genotype. Under the husbandry conditions in these flocks, the scrapie susceptibility status of the ewes did not influence lamb production and should therefore not be a hindrance to selection for resistant breeding stock.