Extraneural competition between different scrapie agents leading to loss of infectivity

MANY strains of scrapie agent have been isolated which differ in their biological properties, such as incubation period and type of brain lesion 1 . We have shown previously that competition occurred between different scrapie agents intracerebrally injected into mice 2 , indicated by an increase in the incubation period of the lethal scrapie agent when a different scrapie agent had also been injected to impede its pathogenesis. We did not establish whether the increased incubation resulted from replication of the lethal agent being hindered throughout or at particular stages of pathogenesis, or whether the increased incubation period resulted from some loss of effective titre.