Encephalitis

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Overview

Encephalitis is irritation and swelling (inflammation) of the brain. It often coexists with inflammation of the covering of the brain and spinal cord (meningitis) and most cases are caused by viral infection. Encephalitis ranges from mild to severe and may result in permanent neurological damage and death.

Types
Primary Encephalitis—This type results from viral infection of the brain and spinal cord. Primary encephalitis may occur in isolated cases (sporadic) or occur in many people at the same time in the same area (epidemic).

The most common type of sporadic infection is herpes simplex encephalitis, which is caused by the herpesvirus. This type carries a high risk for serious neurological damage and death and can occur in newborns if the virus is passed from the mother to the infant during birth.

Arthropod-borne viruses (transmitted through the bite of insects and ticks) may cause arboviral encephalitis. Mosquitoes are the most common agents of transmission and most cases occur during warmer weather, when the insects are more active. Arboviral encephalitis and rabies encephalitis (usually transmitted through the bite of an infected animal) may be sporadic or epidemic.

In the United States, the most common types of arboviral encephalitis are St. Louis, La Crosse, western equine, and eastern equine. Recent outbreaks of West Nile encephalitis (transmitted by mosquitoes, commonly infects birds) have occurred in eastern, southeastern, and midwestern regions of the United States, following bird migration.

Other types of arboviral encephalitis include the following:

• Japanese (widespread in Asia)
• Murray Valley (endemic in Australia)
• Powassan (transmitted by ticks; occurs in Canada and the northern United States)
• Tick-borne (occurs throughout Europe; vaccine available)
• Venezuelan equine (common in Central and South America)

Secondary Encephalitis—This type develops as a complication of a viral infection or reactivation of a latent virus. Viruses can become reactive when the immune system is suppressed by other conditions (e.g., malnutrition, stress, disease). Infections that may cause secondary encephalitis include influenza, chickenpox (varicella-zoster), measles (rubeola), mumps, and German measles (rubella).

Secondary encephalitis that develops as a result of a variola virus infection following smallpox vaccination or reactivation of another viral infection (called acute disseminated encephalitis) is often fatal.

Incidence and Prevalence
Incidence of encephalitis throughout the world is difficult to determine because the disease is often underreported. Approximately 150 to 3000 cases, most of which are mild, may occur each year in the United States. Herpesvirus accounts for most cases of encephalitis in the United States.

Arboviral encephalitis is more prevalent in warm climates and incidence varies considerably from area to area and from year to year. St. Louis encephalitis is the most prevalent type of arboviral encephalitis in the United States, and Japanese encephalitis is the most prevalent type in other parts of the world.

Encephalitis is more common in children and young adults.

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