Genetic makeup has been determined to be a significant factor in a sheep’s susceptibility to infection with scrapie. At this time, a correlation between specific genetics and related scrapie susceptibility has not been determined for goats. Current experimental evidence indicates that there are different forms of the sheep sheep prion proteins. Some forms are highly susceptible to the structural transformation to the abnormal form associated with scrapie while others demonstrate resistance to this change. As in all mammals, sheep are diploid organisms, so that all cells contain two copies of each chromosome and thus two copies of the gene that codes for the prion protein. Genes are made up of codons. A codon is a stretch of DNA that determines which particular amino acid will be included at a particular location of a protein (in this case the prion protein). The prion protein is composed of 256 amino acids; therefore there are 256 codons determining these amino acids. In the literature concerning susceptibility to various strains of scrapie three codons are discussed: 171, 154 and 136. In North America (NA), 171 and 136 are given primary importance. The presence of arginine (R) at codon 171 of the prion protein confers resistance to the prion protein undergoing the structural change associated with NA scrapie. The presence of glutamine (Q) or histidine at codon 171 results in the prion protein being susceptible to the structural change associated with NA scrapie. Histidine at site 171 is considered to have the same susceptibility as Q and is therefore represented as Q in all further discussion. The coding for alanine (A) by codon 136 confers resistance to the prion protein undergoing the structural change associated with scrapie. The coding for valine (V) by codon 136 can produce susceptibility to structural change associated with scrapie. V at site 136 is linked with Q at site 171 such that R cannot be found at site 171 in combination with V at site 136.
The majority of cases of scrapie, worldwide have been in sheep homozygous for glutamine (QQ) at 171. There have been a small number of cases of scrapie reported in QR (171) sheep. In these cases, the amino acids coded for by codon 136 are examined and coding for valine appears to indicate greater susceptibility among the QR population. There are currently two theories as to when the QR (171) population becomes infected. One is that these sheep are susceptible only to certain strains of scrapie. The other theory is that when the prevalence of infection with the QQ (171) population of a flock builds to very high levels, there is a spillover of infection into the next most susceptible subpopulation within the flock. During 2003, a very small number of cases of scrapie were reported in RR (171) sheep in Europe. The profile of sheep’s prion genotype varies between breeds, between countries and between flocks.
Probability of a sheep with that particular genotype being susceptible to scrapie:
|Genotype (136, 171)
|Susceptibility to Scrapie
It is still not known definitively whether animals with less susceptible genotypes do not become infected with the scrapie agent or whether they are merely protected from developing the clinical signs of scrapie.
Recent science indicates that the genotype of the fetus influences the migration and accumulation of abnormal prion in the placenta of an infected ewe. A QQ (171) infected ewe carrying a QQ (171) fetus results in the accumulation of large quantities of abnormal prion, which is then shed during birth or abortion. Theoretically, in a fetus with a genotype QR (171) or RR (171), the abnormal prion does not accumulate to a significant degree in the placenta and associated fluids. This means that use of a RR (171) ram can prevent the shed of abnormal prion at lambing even from infected ewes. Heavy promotion of the use of RR (171) rams across Canada might be effective in minimizing the spread of scrapie, but it may result in a significant change in the genotypic profile of sheep in Canada. There might be impacts on secondary characteristics, such as carcass quality, or the unveiling of previously unidentified congenital problems. As in other countries, preliminary investigation of the demographics of the Canadian sheep population have clearly indicated that there is considerable genetic variation in the presence of resistance for scrapie between breeds and flocks. It may be impossible or impractical for certain sheep producers to select and breed for genetic resistance for scrapie.