Studies were conducted to determine whether accumulation of the scrapie agent protein Sp33–37 in brain correlated with the appearance of the scrapie agent or with pathology. The concentrations of the scrapie agent and Sp33–37 were measured in purified fraction P isolated from hamster brains at weekly intervals after inoculation. The scrapie agent concentration in fraction P was approximately 10 LD/g brain 1 day post-inoculation and increased to 10LD/g at day 77. Sp33–37 was first detected in P at day 21, when the agent titre was 10 LD/g. Sp33–37 concentration increased in concert with the scrapie agent concentration, although the apparent rate of increase was somewhat lower for the protein than for the agent. The histopathological evidence of disease, consisting of mild vacuolation and gliosis, was first seen at 35 days, but was not conspicuous until 49 to 56 days post-inoculation. Vacuolation and gliosis increased until termination of the experiment at day 77. Amyloid plaques were first detected at 56 days and were widespread at day 77. Clinical disease was first seen in these animals at day 66, with an average onset at day 71. Control animals inoculated with buffer alone showed some mild gliosis, but were otherwise normal. The fact that Sp33–37 purified with the scrapie agent isolated from brain 14 days prior to detectable (light microscopic) pathology supports the theory that Sp33–37 is the major structural component of the scrapie agent and not solely a product of the pathology.


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