Ultrastructural links between scrapie and Alzheimer's disease

The discovery of abnormal fibrillar structures, scrapie-associated fibrils (SAF), in fractions with high infectivity from scrapie-infected brains has led to the proposal that SAF are a form of the infectious agent. On the basis of this proposal and on the congophilia shared by SAF and amyloid, it has been speculated elsewhere that the amyloid in Alzheimer's disease is infectious. This speculation is not supported by available evidence and therefore a conventional origin for the amyloid in Alzheimer's disease is favoured--that it originates by partial degradation of a host protein, as occurs in all other forms of amyloidosis characterised so far.