Scrapie - Genetics

Like all animals, sheep receive one copy of each gene from their dam (ewe) and one copy from their sire (ram). A specific gene has been identified which regulates a sheep's response to scrapie. Current scientific literature indicates that the presence of certain combinations of amino acids at three specific locations (known as codons) on the scrapie gene influence a sheep's relative susceptibility to scrapie. The three locations are referred to by numbers 136, 154 and 171. The amino acids are referred to by letters: @136 V for valine or A for alanine, @ 154 H for histidine or R for arginine, @ 171 R for arginine and Q for glutamine. As sheep receives genetic instructions for one amino acid at each site from the ewe and one from the ram the genotype of an individual animal is listed as pairs of letters. The pairs of letter are either listed in numerical order: site 136, site154, site 171 e.g. AA/RR/RR or the specific site that is being referenced is listed with the pair of letters e.g. 171RR. The relative importance of the three locations varies with breed of sheep and the strain of scrapie. In North America 171 is of primary importance and 136 is considered to be of importance in certain situations. The possible amino acid combinations at these two locations and their impact on susceptibility to scrapie are listed below:

136/171

• Most Resistant - AA/RR, AA/QR, AV/QR
• Most Susceptible - AA/QQ, AV/QQ, VV/QQ

It is important to understand that scrapie genotyping is NOT disease testing. A 171QQ sheep does not automatically have scrapie, just as it is not a absolute guarantee that a 171RR sheep cannot get scrapie. The CFIA has NO intention to mandate the Canadian sheep flock to breed for scrapie resistance. Scrapie genotyping is a tool that can be used in an overall plan to manage the risk of scrapie on a particular premises. Whether or not a particular producer should use scrapie genotyping is a decision based on individual factors.

What can you do to minimize the risk of introducing scrapie into your flock?

The Canadian Sheep Breeder Association in partnership with government are implementing Voluntary Scrapie Flock Certification Programs for purebred sheep flocks. These programs allow a producer to chose between different approaches to managing the risk of scrapie on their individual premises. For information on these programs anywhere in Canada, please contact the Ontario Sheep Marketing Agency at (519) 836-0043.

Who might consider selective breeding for genetic resistance to scrapie?

• A producer who provides a large number of breeding ewes to other producers.
• A producer who purchases breeding ewes from multiple sources of unknown scrapie status.
• A producer who has a significant number of 171 RR breeding animals in their flock, thus breeding for resistance would be easy, achieved relatively quick and would not have a significant impact on other production traits.

Who might not consider selective breeding for genetic resistance to scrapie?

• A producer with a flock that has been closed for a long time and has no evidence of scrapie.
• A producer that has a breed of sheep or a flock with few or no animals with even a single 171 R gene.
• A producer that does not want to deviate from their breeding plan for selection of other production traits.

It is highly recommended that producer that chose not to selective breed for scrapie resistance either close their ewe flock or purchase ewes from flocks in a scrapie flock certification. program and commence scrapie testing in mature deadstock.