The incidence of the various types of viral encephalitis varies from year to year, depending on whether sporadic or epidemic outbreaks occur. In relative terms, the incidence is about one-tenth that of bacterial meningitis. Arboviruses account for 10 percent of cases during times of sporadic, isolated cases but can account for up to 50 percent of cases during epidemic outbreaks. The four most common arboviral encephalitides in the United States are the California encephalitis serogroup, St. Louis equine encephalitis, western equine encephalitis, and eastern equine encephalitis. California encephalitis is reported most often, with 76 cases reported in 1994. It is seen in the midwestern and eastern United States exclusively, despite its name. Ninety percent of reported cases occur in children. St. Louis encephalitis is seen throughout the United States in periodic outbreaks, with the yearly incidence ranging from 20 reported cases to 2000. It is seen primarily in young children and the elderly and has a 20 percent mortality rate in the latter. Western equine encephalitis prevails in the western United States and Canada. Disease is most severe in the very young, causing seizures in 90 percent of affected infants and permanent neurologic deficits in 50 percent. Eastern equine encephalitis is prevalent in the Atlantic and Gulf coast regions and accounts for the most severe cases of arboviral encephalitis. It tends to occur in sporadic epidemics and has a mortality rate of nearly 70 percent.

Herpes simplex virus type 1 (HSV-1) and herpes simplex virus type 2 (HSV-2) encephalitis occurs in 1 in 250,000 to 1 in 500,000 cases annually in the United States. HSV-1 is typically seen in older children and adults as reactivation disease, while HSV-2 is seen in neonates as a result of perinatal transmission and generally leads to devastating neurologic outcomes.

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