Doubts cast on scrapie resistance aims
A LEADING independent vet consultant has questioned the validity of the national scrapie plans objective to breed sheep which are resistant to the disease and possibly BSE.
Although breeding for resistance to scrapie means sheep with a resistant genotype will not show signs of scrapie, they could still be carriers, warns Dr Andrews. "This means they could transmit it to other sheep without our knowing."
National scrapie plan co-ordinator Colin Penny admits that DEFRA does not know whether resistant sheep can carry the scrapie-causing agent. "Government BSEadvisory committee, SEAC has recommended research into this which is on-going," he says.
New Zealand flocks are known to be scrapie free, but it would be interesting to find out whether its sheep are genetically resistant or susceptible to the disease, says Dr Andrews.
"It seems probable there are no disease-causing agents present in New Zealand meaning susceptible flocks remain free from the disease or, alternatively, that sheep are genetically resistant to it.
"In the UKs case, it may make sense to have a mixture of scrapie control measures. Current NSP policy ultimately means sheep with susceptible genotypes will be culled, but there must be other additional ways."
Unlike diseases such as maedi visna, there is currently no blood test for scrapie. But brains from all culls from susceptible flocks could be checked for scrapie and over time, where there was no sign of disease, these flocks could be declared free, he suggests.
"Using this alternative testing procedure could prevent the unnecessary slaughter of sheep with susceptible genotypes. In the long term, it may even be counterproductive to breed scrapie resistant sheep as it could make it more difficult to tell whether scrapie is present in the national sheep population or not."
As most sheep enter the food chain between 12-18 months old, checking brains from culls would make scrapie detection too slow and expensive, says Mr Penny. However, the scrapie agent has never been found in sheep under 18 months old, regardless of their genotype, he adds.
Nevertheless, when it was thought BSE may have passed to sheep, SEAC recommended only lambs with the most scrapie resistant ARR ARR genotype and under 12 months old should enter the food chain, says Mr Penny. *