Senile dementia of the Alzheimer type: possibility of infectious etiology in genetically susceptible individuals.

The concept that SDAT is caused by an infectious agent acting in a genetically susceptible host was approached from a number of standpoints. The similarities between SDAT and the transmissible encephalopathies are discussed. One of the areas of similarity is the influence of genetic background on the development and expression of both conditions. Evidence is presented showing that genetics plays a role in many cases of SDAT and that there are known genetically controlled phenomena, the incidence of which is positively correlated with SDAT. For human encephalopathies the genetics of CJD and GSS are detailed. In experimental systems with scrapie, the influence of genetic control, operating through both host and agent, on the outcome of infection with scrapie is described. The events controlled include length of incubation period, type of lesions and their distribution and intensity. In the context of the human diseases, scrapie provides a model for the known human encephalopathies and for SDAT.