Neurovirulence of in vivo-passed wild measles virus and that of the cell-associated SSPE virus were compared by intracerebral inoculation into monkeys. The wild measles virus was found to lack neurovirulence without producing neurological signs or significant histological changes in the brains, whereas the virus was confirmed to preserve the properties characteristic of the wild virus. In contrast, inoculation of the SSPE virus induced rapid onset of neurological signs with mild but definite histological changes including degeneration of nerve cells. The fact that SSPE virus exhibited neurovirulence in monkeys indicated importance of the current assay system for neurovirulence of measles vaccine by intracerebral inoculation into monkeys.
