Experimental subacute sclerosing panencephalitis in the hamster: ultrastructure of the chronic disease

Central nervous system (CNS) lesions were studied from weanling hamsters inoculated with the HBS strain of subacute sclerosing panencephalitis (SSPE) virus. The animals showed clincial signs of acute encephalitis between 8 and 18 days post-inoculation (PI), but all survivors were clinically recovered by day 21 PI. Nevertheless, 13 of 14 hamster brains examined by light and electron microscopy between days 21 and 59 PI had chronic lesions which contained morphologic evidence of persistent viral infection. The lesions developed preferentially in the subependymal areas of the lateral and fourth ventricles and involved degeneration of ependyma with subsequent damage to adjacent parenchyma. All CNS cell types were involved in degeneration. Viral inclusions occurred in both CNS parenchymal cells and in inflammatory cells. Giant cells were particularly common. No budding virus was seen in chronically infected animals, a finding in accord with previous studies. Demyelination was a common constituent of most lesions. It occurred in the presence of inflammatory cells and macrophages, and in later lesions, some remyelination was seen. It is suggested that the damage to myelin is a secondary phenomenon and is not a cellular immune reaction. The possible reasons underlying the latent nature of the virus and the similarities between this condition, canine distemper encephalomyelitis and human SSPE are discussed. It is concluded that the experimental chronic disease is a valid model for the study of human SSPE and may have usefulness in the understanding of other chronic CNS conditions of man, e.g. multiple sclerosis. Additional Key Words: Latent infection; Paramyxovirus; Slow Viruses; Demyelination; Inmmunologic defects; Multiple Sclerosis.