Observations on strain behaviour and direct demonstrations of natural selection establish that the scrapie agent and prions in general are able to evolve. Accordingly, it is conceivable that atypical non-contagious scrapie in sheep and goats can transform to classical contagious scrapie under particular circumstances. In consequence, atypical scrapie can be regarded as a latent hazard that warrants comprehensive risk assessment and biosecurity preparedness planning. Evidence for this proposition comes from differences in the expression of atypical and classical scrapie that may make scrapie contagious, historical records of scrapie in Western Europe, and contemporary accounts of the epidemiology of atypical scrapie. Biosecurity preparedness can be based on current knowledge of pathophysiology and epidemiology and can be built around a three-stage model for the endogenous emergence of a propagating epidemic of scrapie. The first stage concerns the occurrence of atypical scrapie. The second stage concerns the acquisition of communicability in prion populations provided by atypical scrapie and the third stage concerns circumstances allowing disease transmission and the initiation of a propagating epidemic. The range of component causes envisaged for possible outbreaks of endogenous classical scrapie is broad. However, exposure of sheep and goats to cyanobacterial toxins qualifies for special attention.
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