TONSILLAR BIOPSY AND PRPSC DETECTION IN THE PRECLINICAL DIAGNOSIS OF SCRAPIE

Preliminary findings have indicated that in naturally infected sheep, fully susceptible to scrapie (VRQ-homozygous), PrPSc can be detected i n the tonsils approximately one year before the expected onset of clin ical disease, whereas no immunostaining can be detected in animals wit h a semi-resistant genotype. This paper describes the technique for ta king tonsillar biopsies from sheep and gives the results of the comple ted experiment. In another experiment PrPSc was detected even earlier in comparable VRQ-homozygous sheep barn and raised in different surrou ndings. At three-and-a-half months of age no PrPSc could be detected i n three homozygous susceptible sheep (VRQ/VRQ), but PrPSc was detected at four months in one similar sheep. At eight months of age all seven sampled VRQ/VRQ sheep showed positive immunostaining in the biopsies, but none of the biopsies from three VRQ/ARQ heterozygotes showed any immunostaining; they were positive when sampled at 14 to 15 months of age. Biopsies from VRQ/ARR sheep were negative throughout this period. On the basis of the established or expected incubation period, PrPSc could thus be detected in the tonsils of live susceptible animals at b etween one-third and a half of the incubation period, more than one-an d-a-half years before clinical signs normally appear in both these gen otypes.