An assessment of genetical methods in the control of scrapie

Scrapie is caused by a virus-like agent which is transmitted to sheep by both maternal and horizontal routes of infection. The disease can be controlled by selective culling of bloodline relatives of scrapie cases. However, this depends on accurate breeding records which are often not available. Two major studies, with Cheviot and with Herdwick sheep, have shown that ‘susceptibility’ to experimental infection with one source of agent is controlled mainly by a single gene and that the dominant allele confers ‘susceptibility’. There is evidence that scrapie agent may still replicate in some ‘resistant’ sheep, but only after a long delay. For this reason, ‘resistance’ is best regarded in terms of an extended incubation period. Also, ‘resistance’ to one strain of agent does not mean ‘resistance’ to all strains. A ‘resistant’ flock of Swaledale sheep is being developed by experimentally infecting all animals and breeding from the survivors. There have been extensive outbreaks of natural scrapie in the ‘susceptible’ flocks of Cheviot and Herdwick sheep. Investigations of these outbreaks might be able to show whether or not sheep selected for ‘resistance’ could be used to limit the spread of infection in flocks with a serious scrapie problem.