Different allelic efforts of the codons 136 and 171 of the prion protein gene in sheep with natural scrapie.
Three main polymorphisms have been described in the sheep prion protein (PrP) gene, at positions 136, 154 and 171. A strong association between susceptibility/resistance to natural scrapie and a dimorphism at codon 136 of the ovine PrP gene has been reported in several breeds, including Romanov. This dimorphism, however, is not found in all scrapie-affected breeds. The PrP genotypes of Lacaune sheep obtained from endemically affected flocks were compared with those of apparently healthy sheep. A third variant at codon 171 was also identified. The results were compared with those obtained in a single experimental Romanov flock inoculated orally 3 times with large amounts of third stage larvae of Teladorsagia circumcincta and Trichostrongylus columbriformis in which scrapie suddenly appeared and killed 80% of the sheep. Evidence was obtained to show that, even in different epizootological circumstances, the major genetic factor controlling the susceptibility/resistance to natural scrapie in sheep, is represented by codon 171 genotype of the PrP gene. It is also suggest that a modification of the allelic effects of codon 136 can occur in heavily infected animals.