WHAT IS SCRAPIE?
In comparison to the major problems, scrapie is not a significant problem for the majority of sheep flocks. Media interest raised the profile of scrapie, although the disease incidence has been in gradual decline. Swaledale breeders have for many years been working to reduce scrapie susceptibility within the breed.
Scrapie has been recognised in British flocks since 1732. It is one of a group of diseases called Transmissible Spongiform Encephalopathies. These diseases result in 'holes' developing in the brain tissue (hence the name 'spongiform' -looks like a sponge).
It is an infectious disease, although the causative agent and the way it spreads are not understood.
Clinical signs of scrapie:
- Skin irritation - Scratching of the flanks
- Rubbing/ scratching the head
- Grinding teeth
- Nibbling of feet/legs
- Behavioural changes- Lag behind flock
- Increased nervousness
- Depression/ vacant stare
- Eventually leads to incoordination, weight loss, recumbency and death.
For clinical disease to develop, two elements are required:
1. The presence of the scrapie agent in the environment.
2. A genetically susceptible sheep.
agent is believed to be present in foetal fluids and removal of
affected animals from the flock has, until recently, been the only
available control measure.
In recent years it has become possible to genetically manipulate the flock by blood sampling and selective breeding; the aim being to increase scrapie resistance of the flock as a whole.
This is obviously desirable in a flock which is clinically affected by scrapie. It may also be politically desirable given the public perception of scrapie and its relation to other spongiform encephalopathies. The dilemma is whether it is advisable to base selective breeding on a laboratory test rather than the physical characteristics of the sheep.
Genotyping is the technique which identifies an individual's genes - usually done by blood testing. Scrapie susceptibility or resistance is determined in the sheep by two genes (one from each parent). One area of the 'scrapie gene' is composed of three amino acids, the combination of which varies between individuals. Some combinations offer more protection against scrapie than others. The amino acids involved are as follows:
- A - alanine
- H - histidine
- Q - glutamine
- R - arginine
- V - valine
It has been
established that combinations involving valine provide the least
protection against scrapie and therefore steps have been taken to
eliminate this gene type.
The most resistant genotype is ARR/ARR ie ARR from both parents. Naturally occurring scrapie has not been observed in animals of this genotype.