Scrapie-free Merino and Poll Dorset Sheep from Australia and New Zealand Have Normal Frequencies of Scrapie-susceptible PrP Genotypes

As natural scrapie occurs only in sheep of specific PrP genotypes, one proposed aetiology was that scrapie is simply a genetic disease. However, Cheviot and Suffolk sheep of scrapie-susceptible genotypes are found in Australia and New Zealand, both generally accepted to be scrapie-free countries. A study of more common Australia and New Zealand sheep breeds (Merinos and Poll Dorsets) was carried out in order to obtain more generally applicable estimates of Australia and New Zealand sheep PrP genotype frequencies. We have confirmed that animals of highly susceptible PrP genotypes are found in Australia and New Zealand. Interestingly, the Poll Dorset sheep, although born in New Zealand, were brought to the UK as young adult animals and subsequently remained free of clinical scrapie despite 21% of the sheep having scrapie-susceptible genotypes. These results have implications for the genetic control of occurrence of the equivalent human diseases.