Breeding Lambs for Long-term Scrapie Resistance
*From the UK, as printed in the St Croix Newsletter Vol 4(3) 2007
This article takes issue with the current National Scrapie Plan (NSP) of breeding toward AARR genotype at codon 171 to improve scrapie resistance even though studies have proven that this strategy does not affect lamb performance, but could compromise the national flock's ability to withstand future scrapie strains.
One study by Richard Moore of Roslin Institute, compared genotypes of 3500 Charollais lambs with performance recording data and found no significant link between performance and genotype when moving from ARQ to ARR. But Dr. Moore did admit a move to a more limited genetic base, tilted in favor of one genotype could reduce rate of improvement in flocks. Similar studies with Blackface and Swaledale flocks also reached similar conclusions. Dr. Rami Sawalha did find that Blackface lambs carrying the VRQ took ten days longer to finish than lambs with other genotypes. Dr. Nicola Man found that lambs (Swaledale) carrying the ARR were slower to reach slaughter weight. Neither study was statistically significant, however.
Dr. Matthew Baylis of Liverpool University has accumulated data of some 300 cases of atypical scrapie found in 15 EU countries and the VRQ genotype currently being eliminated under the current NSP model were resistant to these strains where a large portion of the ARR/ARR genotype contracted these atypical strains of scrapie (italics added). This ARR/ARR genotype is resistant to classical scrapie and is favored by most current breeding programs.
"The question is, has each allele evolved to confer resistance to different scrapie strains that occurred in the past?" says Prof. Baylis. "If so, a new long term approach to control may be preferable, breeding out susceptibility in individual flocks while preserving genetic diversity in the national flock."
NSA chief executive Peter Morris urged caution. "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."
Prof. Baylis, when questioned about 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, 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 far greater than we have ever seen. The NSP strategy is clearly effective against classical scrapie, but might not be the best long-term strategy."