Scrapie Resistance

Scrapie is a progressive degenerative disease of the central nervous system that is fatal to sheep and goats. It is one of the ‘prion’ diseases, also called transmissible spongiform encephalopathies (TSEs). These diseases have long incubation periods during which an abnormal protease-resistant isoform of a prion protein accumulates in the central nervous system.

The main mode of transmission of scrapie is from mother to offspring immediately after birth and to other susceptible newborns exposed to the birth fluids and tissues of an infected animal. There is no cure and no treatment for scrapie.

While Australia is free of scrapie, it is present in several European Union member states, including the United Kingdom, as well as Canada, the United States, Iceland, India, Japan and Brazil. Israel has also reported outbreaks of scrapie, with the most recent being in early 2007.

Testing for scrapie is not a priority for most Australian sheep breeders, however, it is of vital importance to breeders using imported genetics or breeders seeking to gain access to international markets for export.

Genetic Basis of Scrapie Resistance
Genetic susceptibility to scrapie appears to be influenced by variation in the prion gene (PRNP). In sheep, variation in the PRNP gene has been identified at a number of codons, but polymorphism at some codons is rare and only three codons (136, 154 and 171) have a reported linkage with the incidence of scrapie.

The association between ovine PRNP polymorphism at codons 136, 154 and 171 and scrapie susceptibility makes it possible to identify whether sheep are resistant or susceptible to scrapie by genotyping the PRNP gene.

Based on variation at these three codons, a number of different scrapie-related alleles in sheep have been identified.

Testing for scrapie assesses the combination of genetic markers that contribute together to provide resistance to the disease. The results of the test are reported as a type from scrapie resistant through to scrapie susceptible.