Adult mice given two or three intraperitoneal inoculations with avirulent Semliki Forest Virus showed typical lesions of a viral encephalitis similar to those caused by a virulent strain of the virus. Demyelination also was seen in the medulla and in the folia of the cerebellar white matter. Neuronophagia was seen only in mice that had had three successive infections. The repeated inoculations of avirulent virus exacerbates the encephalitis of a single inoculation and causes demyelination. The mice did not have neurological clinical signs except for a short-lived weakness of the hind legs. No central nervous system lesions were seen by the 7th and 8th week after the initial infection and all mice recovered.
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