Breeding lambs for long-term scrapie resistance

Breeding for scrapie resistance according to the current National Scrapie Plan (NSP) strategy does not affect lamb performance, but could compromise the national flock’s ability to withstand future scrapie strains.

Richard Moore of Roslin Institute told delegates at last week’s BSAS conference, York, that a study comparing PrP genotype of more than 3500 Charollais lambs with performance recording data from Signet, showed no significant link between performance and genotype.

“In particular, alterations to the PrP genotype frequencies [following the move from ARQ to ARR allele] in this breed will have negligible impacts on lamb performance.”

But Dr Moore did admit a move to a more limited genetic base, tilted in favour of one genotype could reduce the rate of improvement in flocks.

Meanwhile, similar studies in both Blackface and Swaledale flocks also reached similar conclusions, explained Rami Sawalha and Nicola Man, respectively.

Dr Sawahla though did find that Blackface lambs carrying the VRQ allele took 10 days longer to finish than lambs with other genotypes, although this wasn’t statistically significant.

While in Swaledales, Dr Man found lambs carrying the ARR allele were slower to reach slaughter weight, again this was statistically insignificant.

However, while this work failed to question the future of the NSP, the emerging threat of atypical scrapie posed questions about the validity of the current NSP model, said Matthew Baylis of Liverpool University.

“Nearly 300 cases of atypical scrapie have been found in 15 EU countries and the VRQ allele, currently being eliminated under the NSP, appears to confer resistance to this strain of the disease.”

Importantly, a large proportion of cases have the ARR/ARR genotype.

This genotype is resistant to classical scrapie and is most favoured by current breeding programmes, continued Prof Baylis.

“The question is, has each allele evolved to confer resistance to different scrapie strains that occurred in the past?

If so, a new long-term approach to control may be preferable, breeding out susceptibly in individual flocks while preserving genetic diversity in the national flock.”

But NSA chief executive Peter Morris urged caution.

“It’s important to retain the current strategy until a review of the NSP has been undertaken.

“DEFRA needs to decide, in the light of current information – particularly the fact that BSE still hasn’t been found in UK sheep, exactly what it wants the outcome of the NSP to be,” he told Farmers Weekly.

“At the moment there is no evidence for cross species transmission of atypical scrapie, so all we have is some sheep with a particular brain disease.

There are far bigger threats, both in terms of animal health and welfare and economic terms, than atypical scrapie.”

Questioned about the ability of back-up measures, such as the National Semen Archive to provide a safeguard against a situation where other scrapie strains could emerge to affect previously thought resistant genotypes, Prof Baylis said he believed any new strains, including atypical scrapie, could move through a flock quicker than susceptibility could be bred out of it.

“Should we end up with a national flock where one genotype dominates then we could find future scrapie problems are far greater than we have ever seen.

The NSP strategy is clearly effective against classical scrapie, but it might not be the best long-term strategy.”

For more from BSAS see page 48.