RESISTANCE
RACE FOR INBUILT SCRAPIE
RESISTANCE
Scrapie genotyping is a technological advance the sheep industry can no longer afford to ignore. But how should it be used for the best advantage of the industry? Sue Rider reports
THE race to breed scrapie resistance into UK sheep flocks is now on.
Mike Dawson, of MAFFs Central Veterinary Laboratory and who, with his colleague Linda Hoinville, is studying scrapie in several flocks in England and Wales, reports a significant upturn in demand for genetic testing.
Increased interest is both from individual breeders keen to have rams tested ahead of this summers sales, and breed societies, especially the Swaledale, Shetland, and Cheviots. Demand is driven by premiums likely to be available for high indexing rams with a scrapie resistant tag, says Mr Dawson.
"More and more producers recognise the value of a resistant animal and are keen to take blood samples, mainly from rams, to send off for the gene test. Demand is such that well be busy this summer, but its encouraging that we can now do something positive about controlling scrapie incidence in the national flock."
The scrapie gene test fingerprints the gene which encodes for PrP, the protein involved in the development of scrapie. Fingerprinting identifies those forms of the gene linked to scrapie susceptibility and those linked to its resistance.
Commercial testing is available through the CVL or the SAC, but the concept and early technology was pioneered at the Institute of Animal Healths neuropathogenesis unit in Edinburgh, explains Mr Dawson.
IAH work has identified 256 positions, or codons, on the PrP gene. Each codes for an amino acid – the building blocks of the PrP protein.
The PrP gene, like all genes in mammals, is composed of two strands, or alleles, one from each parent. At least five variations of PrP alleles have been identified in sheep, but not all have been found in every breed studied so far.
Differences between the five alleles are due to variations in the amino acids specified at codons 136, 154 and 171 (see box).
As mentioned previously, a gene comprises a pair of alleles, and the options for pairing are determined by the alleles carried by ewe and ram.
Pairing can be of like or unlike alleles, says Mr Dawson. For example, VRQ could pair with another VRQ, giving the genotype V/V at 136 – R/R at 154 – Q/Q at 171, or VRQ could pair with ARR, giving V/A at 136 – R/R at 154 – Q/R at 171.
Indicates susceptibility
The VRQ allele indicates scrapie susceptibility in many breeds, such as the Swaledale, Shetland, Welsh Mountain, Cheviot, Charollais, Texel and Bleu du Maine.
In Suffolks, the VRQ allele is very rare: in this breed scrapie susceptibility is linked to the ARQ allele. As for AHQ, there is evidence that it provides some scrapie resistance in breeds where scrapie is linked to VRQ.
ARH is common in Texels, but its significance is not clear: It may have a neutral effect on disease susceptibility. ARR seems to be associated with scrapie resistance in all breeds studied to date, says Mr Dawson. But whether this will apply to all breeds of sheep and all strains of scrapie remains to be seen.
"Strategies for developing scrapie resistance require negative selection for those alleles linked to scrapie susceptibility and positive selection for those linked to resistance," he adds. "Selection will also have to take into account impact on other breed traits, but it may be that development of scrapie resistance should be a priority."
The common alleles in the Suffolk are ARQ and ARR: There is no variation at 136 and 154 so testing is only needed for 171. Mr Dawson and his team are currently developing a one codon test for breeds with variation at 171 only to enable cheaper testing.
"In Suffolks, increasing the genetic resistance to scrapie is straightforward. Avoid using 171Q/Q rams and where possible use 171R/R rams in preference to 171Q/Rs.
"By using 171 R/R rams year-on-year the level of resistance will increase. Progeny of such rams will at least be 171Q/R and have a significant degree of protection against clinical disease caused by all known strains of scrapie," says Mr Dawson.
Mike Dawson and colleague Saira Hamid analyse the results of blood tests – which are in increasing demand – to determine scrapie genotype.
Scrapie genotypes
136154171
VRQ
ARQ
AHQ
ARH
ARR
V = valine, A = alanine, R = arginine, Q = glutamine and * = histidine.