Abnormal prion protein in genetically resistant sheep from a scrapie-infected flock

The central molecular event in transmissible spongiform encephalopathies, such as scrapie in sheep, is the accumulation in tissues of an abnormal isoform of the cellular prion protein. A previous investigation of 26 sheep showed that the accumulation of PrP^res in brain correlated more with the prnp genotype than with the severity of the clinical disease. Here, the ability of a sandwich ELISA to detect PrP^res distribution in the brain was demonstrated. Immunohistochemistry also strongly supported the hypothesis that the dorsal motor nucleus of the vagus nerve is the possible entry site in the brain for the scrapie agent. Remarkably, three asymptomatic (or possibly asymptomatic for scrapie) sheep carrying an allele known to be associated with clinical scrapie resistance (ARR), which were negative for the detection of PrP^res by Western blotting and immunohistochemistry, were positive for the presence of PrP^res by ELISA, raising the possibility of carriers resistant to the disease and possibly contributing to the persistence of scrapie in certain flocks.