Gene tests point way to national scrapie control
By Sue Rider
SCRAPIE can be controlled in the national flock and it need not take too long.
Speaking to the National Sheep Associations eastern region sheep night at Newmarket last week (p48), Mike Dawson, of the Central Veterinary Laboratory, said that genetic testing for some breeds, for example Swaledales, Suffolks and Shetlands, was already under way.
"We can isolate the gene responsible for scrapie resistance or susceptibility and can use that knowledge to eliminate disease in an infected flock," said Mr Dawson. "If we could develop a co-ordinated strategy to target influential breeds in the industry, there is a real prospect of our controlling disease in the national flock."
He said a blood test would determine scrapie resistance or susceptibility. "We can now offer the test to many breeds because we have a fairly solid base of information of how alleles (each gene comprises two alleles) behave in each breed," he said.
For example, there were 256 positions on the scrapie gene, each coding for an amino acid. Three positions on the gene influenced susceptibility to scrapie – known as codon numbers 136, 154, 171.
Codon 171 could be R (arginine) or Q (glutamine). Some breeds, Suffolk and Texel, had a third variant, H. Given that individual sheep inherit one allele from each parent, the 171 results could be R/R, R/Q, Q/Q R/H and QH. Evidence suggested sheep with 171Q/Q, and possibly 171Q/H (especially in Suffolk and Texels), were at greatest risk of developing scrapie; 171Q/Rs or (171R/H) had a low risk.
"Over 95% of scrapie cases in any breed have QQ at 171; the remainder have the Q/R or Q/H result," said Mr Dawson. "No cases in any breed in Europe or North America have had R/R at 171. This type of ram or ewe is associated with resistance and its progeny will have a low risk of contracting scrapie – under 5%."
In breeds where the only variation is at 171, most Suffolks, the strategy to increase genetic resistance was straightforward.
"Avoid using 171 Q/Q rams and where possible use 171 R/R rams in preference to 171 Q/R rams," he advised. *
Commercial testing is through the Scottish Agricultural College (0131-445 5544) or the CVL, Weybridge (01932-357570). Current cost of £70 a test will be cut to £40 from April.