North American Wensleydale Sheep Association

SCRAPIE RESISTANCE

The Wensleydale has the highest genetic resistance to scrapie of all recognized sheep breeds in the United Kingdom, with PrP tests revealing a 92% codon 171 R/R genotype in the breed. The commercial significance of this is that the buyer of a Wensleydale ram has a 92% chance that the ram will be of this genotype and that on using it as a crossing sire, none of the crossbred progeny will be of the genotype susceptible to scrapie.

Wensleydales All NAWSA rams must test RR

Based on the fact that the Wensleydale breed in the U.K. is reported to test 92% RR at Codon 171, our membership strongly feels that this attribute be perpetuated in the North American Wensleydale. At a time when other breed associations are struggling to develop a more scrapie-resistant population, we feel that we have the rare opportunity to establish our breed from the onset with highly resistant individuals, based on current industry information. As a result, we have designated that all rams recognized in the NAWSA registry, whether imported as semen or naturally bred, must test RR at Codon 171.

We realize there will be rams ineligible for registration as they may test QR at Codon 171, but these rams may still be utilized in spinners flocks, in the NCWGA registry (if colored), or for other uses without having NAWSA paperwork. Until all of the facts are known about scrapie, carrier status, resistance and its link to genetics, we currently feel that it is important to maintain this position.

Please read the Upgrading Guidelines carefully before considering breeding for Wensleydale sheep.

The following has been reprinted from an article in The Wensleydale World, a publication of the Wensleydale Longwool Sheep Breeders Association, U.K.

SCRAPIE:
Prospects for its Control have never been better

By Michael Dawson, Central Veterinary Laboratory, England

Michael Dawson graduated from the Royal Veterinary College, London, in 1974 and worked in mixed veterinary practice in Leicestershire and Essex before moving to the Central Veterinary Laboratory.

Scrapie is a disease that has affected British sheep for over 200 years. The signs – typically in 2-4 year old sheep, of subtle behavior changes in the early states, body and head rubbing, nibbling at legs, loss of condition, clumsy gait and advancing paralysis – are linked to progressive and irreversible degeneration of the brain and spinal cord. Death is inevitable, usually within weeks of signs being seen. Sheep farmers tend to avoid talking about scrapie. There is a stigma associated with it, which is not matched by any other sheep disease.

It remains one of the major challenges to veterinary science. What causes it? Is it a genetic disease or an infection? Is it a combination of the two? For many years opinion has been divided on the subject, and the debate has, at times, been acrimonious.

Without a lab test that can be used to identify infection in live animals, it has proved difficult to control the disease within a flock. Experimental work suggests that the disease can be transmitted through placental membranes, and within affected flocks it has appeared that cases occurred in maternal lines. This led to attempts to control scrapie by rigorous recording and culling of such affected lines. While there was moderate success in some flocks, the policy has never proved to be completely effective. In Iceland, affected flocks are slaughtered out and re-stocked after two years from areas where scrapie is not recognized. Although some farms have remained free of disease, breakdowns have occurred on others.

Wensleydales The scrapie (PrP) gene
So, what is understood about the genetic influence on scrapie? Significant progress has been made in the last few years. Until recently, it has been usual to talk about the genetics of scrapie in terms of the scrapie incubation period (Sip) gene that was discovered following studies of experimental scrapie in Cheviot sheep. Experiments with scrapie in the Herdwick and Swaledale breeds also pointed towards involvement of the Sip gene. Although not definitely proven, it is now widely accepted that Sip is the gene that codes for a protein called PrP which is found on the surface of nerve cells, and on cells I other tissue, and is closely involved in the development of scrapie. In other words, the Sip gene is probably the PrP gene.

With the new tools of molecular biology, it is now possible to fingerprint the PrP gene and identify those forms of the gene that are linked to scrapie susceptibility and those that are linked to resistance. Nora Hunter and her colleagues at the Institute for Animal Health’s Neuropathogenesis Unit in Edinburgh, Scotland have been largely responsible for pioneering this work in Britain.
There are 256 positions, or codons, on the PrP gene, each coding for an amino acid, e.g. arginine, glutamine, alanine, etc. Amino acids are the building blocks from which proteins are made. Depending on the breed, current evidence indicates that up to three positions on the gene, out of the 256, can influence an animal’s susceptibility to scrapie: these are codon numbers 136, 154 and 171.

Codon 171 determines overall susceptibility
Variation at 171 seems to be common to all breeds, the possibilities being R (arginine) and Q (glutamine). Some breeds, e.g. Suffolk and Texel have a third variant H (histadine). In most Suffolks, 171 is the only site on the PrP gene at which variation seems to exist. Bearing in mind that individual sheep inherit half their genes from each parent, the 171 results can be RR, RQ, QQ, RH, etc. More than 95% of scrapie cases studied in a wide variety of breeds in Europe and North American have the QQ result at 171; the remainders have the QR or QH result. In almost 300 cases from a range of breeds studied in our laboratory over the last two years, all but one were QQ at 171. The odd one was QR. With the exception of a single Suffolk case reported in Japan, no scrapie cases have genotyped RR at 171. These results demonstrate that sheep with 171QQ (and possibly 171QH) are at the greatest risk of developing scrapie. 171QRs (or 171QHs) have a low risk and 171RRs are the most protected.

Wensleydales Possible breeding strategies to control scrapie
For those breeds where the only variation is at 171 (e.g. most Suffolks), and is therefore at risk mainly from the group C scrapie strains, the strategy to increase genetic resistance to scrapie is straightforward. Avoid using 171QQ rams, and where possible, use 171RR rams in preference to 171QR rams. Availability of rams will be determined not only by the frequency of the 171RR genotype within the breed, practical considerations of selection for desirable breed traits will also have to be considered. All first generation progeny of 171RR rams will have a significant degree of protection against scrapie, being at least 171QR, a genotype that is only rarely found in scrapie cases. By using RR rams year on year, the level of resistance will progressively increase.

What about the carrier status?
Is it known if sheep that are resistant to disease are still capable of acting as silent carriers, and if so, what are the consequences? Experiments to answer the first question are underway but results are a few years off. Even if transmission occurs, it would be of little consequence in respect to scrapie if the overall genetic background prevented disease from surfacing. It seems reasonable to argue that creating an increasingly resistant population of sheep should reduce opportunities for scrapie agent to spread.

Genetic testing is already underway in some breeds, e.g. Swaledales, Shetlands and Suffolks. By further strategic targeting of genetic testing at those breeds which have the greatest influence on the industry, there is a real prospect of controlling disease in the national flock and it shouldn’t take too long.