CSF pleocytosis Measles encephalitis is also less common since the advent of widespread live immunization The onset usually occurs during the prodromal period or after the rash has appeared Ataxia is the most common neurologic abnormality, and sequelae occur in up to 30% of cases Varicella encephalitis occurs to days after the onset of the rash; severe infections are uncommon, except in the immunosuppressed host HSV is a relatively common cause of sporadic encephalitis Disease in neonates is usually caused by perinatal transmission of HSV type In previously healthy older children and adults, encephalitis more often results from infection with HSV type and may be a complication of acute primary infection or reactivated latent infection Early recognition of herpes encephalitis can be difficult, but is important because specific antiviral therapy reduces the substantial morbidity and mortality of this disease TABLE 97.7 AGENTS OF VIRAL ENCEPHALITIS Arboviruses Eastern equine encephalitis Western equine encephalitis St Louis encephalitis Japanese encephalitis California (LaCrosse) encephalitis West Nile Herpesviruses Herpes simplex Varicella zoster Epstein–Barr Cytomegalovirus Mumps Measles Enteroviruses Rabies The arthropod-borne encephalitides including St Louis, Western equine, Eastern equine, and California encephalitis occur in sporadic and epidemic forms, often in late summer or early fall, and tend to cluster in localized geographic areas Sequelae may be severe and mortality high, especially in Eastern equine