Spontaneous Paramyxoviral Encephalitis in Nonhuman Primates (Macaca mulatta and M. nemestrina)

Two female rhesus macaques (Macaca mulatta) and one female pigtailed macaque (M. nemestrina) developed acute neurological signs including ataxia, muscular incoordination, and seizures. Light microscopy showed Cowdry Type A intranuclear inclusion bodies in astrocytes and neurons of the cerebral cortex and Purkinje's cells of the cerebellum. Many cells also had intracytoplasmic inclusion bodies. Electron microscopy identified paramyxoviral nucleocapsids in the intranuclear and intracytoplasmic inclusions in two monkeys. We believe these monkeys had measles encephalitis because measles virus is the only paramyxovirus known to cause inclusion bodies in macaques and because the lesions in these monkeys resembled measles encephalitis in man.