Scrapie - A Sheep Breeder's View

Dr Sandy Sutherland 09/10/02

New Channelkirk, Oxton, Lauder, Berwickshire, Scotland

History

Scrapie is a particularly unpleasant disease of sheep. It does not appear much before the animal is two to three years old and the first sign is usually a loss of condition followed by the development of neurological symptoms – involuntary movements and an intense desire to itch. This leads to the appearance of bare patches in the fleece and provides the origin of the name. There follows a fairly rapid decline with complete loss of appetite, weakness and inevitable death.

In the two hundred or so years that the disease has been recognised it has always been a disease limited to sheep alone with no evidence that it could cross species barriers to man – a fact that provided some false comfort in the early days of the BSE outbreak.

Flock owners have long since realised that there was a considerable inherited element to the disease. This made closed flocks, both where male and female replacements are selected from within the flock, particularly susceptible to outbreaks of the disease. There have been cases where very high levels of the disease have developed leading to the need to cull the entire flock.

Very early on in my career as a Suffolk sheep breeder an experienced breeder told me the way to get rid of the disease was to cull the female line, mother, daughters and granddaughters, of the infected sheep. When one of the first sheep I bought developed the disease I followed this advice and have never seen a case since.

The Upsurge in Interest

The BSE outbreak in cattle, with the subsequent appearance of new variant CJD (vCJD) in the human population led to an enormous upsurge in interest in prion diseases. This combined with the huge advances in genetics to give scrapie research a much higher priority so that the understanding of the disease is now pretty well complete.

In the mid nineties discussions among breeders in my breed, the Suffolk, were around the implications for the sheep industry of the appearance of vCJD. Even then the writing was on the wall that, despite the continuing lack of evidence that scrapie transmitted to humans, the public and politicians were not willing to live with the slightest risk and it was up to us to put our house in order.

The development of a specific, albeit expensive, genetic test about this time enabled us to start selecting stock rams that had resistance to the disease. The success of this self funded programme has led to the present situation where more than 90% of the Suffolk breed are resistant with the prospect of 100% resistance in the next couple of years.

Unfortunately the rest of the sheep industry has lagged behind and the introduction of the government funded National Scrapie Plan has revealed a much less satisfactory situation in other breeds.

The Genetics

The disease is caused by a specific protein in the sheep’s brain converting to an abnormal form over a period of time. The susceptibility of the animal to this process is determined by variations in a specific gene called the prion protein gene. Each gene is made up 256 amino acids and variations in the amino acids at positions 136, 154 and 171 determine the resistance or otherwise to scrapie. Each gene is of course made up of pairings from each parent so there can be single or double variations at each position.

The Suffolk breed is perhaps the simplest as variations occur only at position 171. The result is that there are effectively three possibilities. Resistance inherited from each parent, giving total resistance to the disease, resistance inherited from only one parent, which seems to confer almost total resistance, and susceptibility from both parents. This provides the group in which almost all cases of scrapie are found in the Suffolk breed.

By using a ram that is doubly resistant at position 171 it is possible to confer almost total resistance to the disease in its offspring irrespective of the genetic make up of its mother.

In other breeds the situation is made more complicated by susceptibility being dictated by variations at more than one site on the gene. As a result there are 15 possible genotypes determining susceptibility in sheep.

The National Scrapie Plan

This scheme is a government funded voluntary scheme, with the aim of reducing and eventually eliminating scrapie from the national flock.

The benefits are seen as twofold. To improve the image of British sheep meat with consumers at home and abroad and to prevent the as yet theoretical possibility of BSE appearing in the sheep flock. (Resistance to scrapie seems to confer resistance to BSE when attempts have been made to infect sheep in laboratory experiments.) In addition there are obvious animal welfare benefits.

The Plan is currently concentrating on preventing rams with double susceptibility at two or all three sites from being used for breeding with the intention that eventually double susceptibility at one site alone will also be included. There has recently been an extension to the deadline for the latter move in certain breeds as breeders are concerned that important breed characteristics will be lost if there is a very tight schedule imposed.

The opportunity exists to eliminate this disease from the national flock and it is encouraging to see that most breed societies have signed up to the plan. Already resistant rams suitable for commercial breeding are commanding a substantial premium and while this is not yet apparent at a commercial level it is only a matter of time before buyers of prime lambs will insist on proof of resistance in the sire of their purchases, leading to increased rewards for those who have acted promptly.

Editor’s Notes on Dr Sandy Sutherland's article

Readers may wish to use links within this website to the following:

What is a prion?

Predicted Future Incidence of vCJD in the UK Population

Risk of vCJD From Blood Transfusion

Scrapie Statistics for Great Britain (and for England, Scotland and Wales separately)

National Scrapie Plan (NSP)

National Scrapie Plan: Rule Changes Proposed (Veterinary Record, September 28 2002, pp. 369-370)

The Editor’s trip to Kelso Ram Sales

Dr Sandy Sutherland's flock and farm holding were featured in the Scottish Farmer the week prior to the Kelso Ram Sales

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