**Breeding for scrapie resistance in the U.S.
Selection for resistant prion protein genotypes should be a priority in breeds that have the highest incidence of scrapie in the U.S., i.e. Suffolks and, perhaps, other blackfaced breeds that have been "improved" by illicit crossing with the Suffolk. Breeders of other breeds that have a very high frequency of susceptible prion protein genes, and no or a very low incidence of scrapie, also may wish to increase the amount of selection emphasis placed on resistant genes in order to safeguard their breed against the effects of possible future exposure to the scrapie infectious agent. With the current strain(s) of scrapie present in the U.S., selection for the arginine (R) allele at codon 171 appears to be sufficient.
A flock where every ewe is of the susceptible genotype QQ can be converted to a flock of QR and RR resistant ewes fairly quickly by simply purchasing and using only RR rams. With a normal ewe replacement rate of 20% per year, over 2/3 of the ewe flock will be of the QR or RR genotypes after five years of RR ram use even with no selection of ewes or ewe lambs on prion protein genotype. Conversion of the flock to QR and RR ewes will be even faster if, in addition to using RR rams, replacement ewe lambs are preferentially selected from QR or RR dams and if QQ ewes are preferentially culled.
**Susceptibility versus scrapie-infected
It must be remembered that a susceptible genotype like QQ at codon 171 does not imply that the animal is scrapie-infected. Scrapie is not a genetic disease. Scrapie is not caused by a particular genotype. An infectious agent causes scrapie. In order to have scrapie, both an infectious agent and a susceptible genotype need to by present. For example, Australia and New Zealand are scrapie-free, but a DNA test is available commercially for prion protein genotyping. Generally, DNA is extracted from white blood cells from a sample of the sheep's blood collected in purple top (EDTA) blood tubes.