The historical aspects of spongiform encephalopathies, Creutzfeldt-Jakob disease (CJD) and kuru of man, as well as scrapie and transmissible mink encephalopathy, are outlined. Transmissions of these diseases to animal hosts are presented, with emphasis on CJD transmissions to guinea pigs, hamsters, and mice. The relationship of CJD to scrapie with reference to the pathological findings is discussed. In CJD the incubation period is cut in half in guinea pigs and hamsters in the second passage. The spongiform changes occurring in the neuropil are reviewed. These changes are related to the type of inoculum, e.g., there is more vacuolization after inoculation with brian, and less after inoculation with spleen. Spongiform changes are also dependent upon the route of inoculation; these are more severe in intracerebral inoculation compared to intraperitoneal inoculation. Viremia is present. Maternal transmission and lateral transmission are absent. No virus-like particles are detected, and no other organisms are visible by electron microscopy. Isolations of the causative agent and strains of the agent in spongiform encephalopathies remain elusive. The hypotheses concerning the nature of the agent are critically reviewed. Novel data on the production of tumors derived from CJD brains are presented. Tissue culture cells arising from such brains become permanent lines and are similar to neoplastic lines. When such CJD lines are injected subcutaneously into nude mice, malignant neoplasms are formed. No evidence of an infectious etiology in Alzheimer's disease exists. Reported similarities between this disease and CJD are reviewed. Animal models of CJD are useful for the investigation of dementias.

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