Abstract
AbstractObservations 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.
Highlights
Observations on strain behaviour and direct demonstrations of natural selection establish that the scrapie agent and prions in general are able to evolve
PrPSc is virtually restricted to the central nervous system and is scant in the lymphoreticular tissues and cells of sheep with atypical scrapie [22, 28]
Classical scrapie was not observed in sheep over seven years old whereas 43% of cases of atypical scrapie were found in sheep over seven years old
Summary
Abstract: Observations on strain behaviour and direct demonstrations of natural selection establish that the scrapie agent and prions in general are able to evolve. It is conceivable that atypical non-contagious scrapie in sheep and goats can transform to classical contagious scrapie under particular circumstances. 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. The objective is a preliminary understanding of factors involved in the possible endogenous emergence of classical contagious scrapie from atypical non-contagious scrapie and the intent is to inform risk management. (4) accounts of the epidemiological aspects of atypical and Nor strain scrapie are analysed
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