Encephalitis: Viral - Causes

Herpes Viruses

The herpesvirus group includes a number of common infections, including herpes simplex, varicella-zoster (the cause of chicken pox and shingles), cytomegalovirus, herpesvirus 6, and Epstein-Barr (EB) virus (the cause of mononucleosis). About 2,100 people are hospitalized each year from herpes-associated encephalitis. These viruses share certain features, including the capacity to cause an infection and then to go into hiding. They can lie dormant for periods of time as short as months or as long as a lifetime. In a few cases, when the viruses reactivate, they cause encephalitis. In fact, some evidence suggests that varicella-zoster, cytomegalovirus, and Epstein-Barr (EB) virus may be more common causes of encephalitis than previously thought. In most cases, however, encephalitis from these viruses occurs in people with impaired immune systems, such as people with HIV or organ transplant patients.

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Herpes Simplex Virus. Herpes simplex virus (HSV) is the most common cause of encephalitis in developed countries and is responsible for about 10 - 20% of all adult cases of viral encephalitis. There are two distinct types of the herpes simplex virus: HSV-1 (which is commonly associated with oral herpes) and HSV-2 (which usually causes genital herpes, although HSV-1 can also cause this form). HSV-2 causes 70-90% of encephalitis cases in neonatal infants; the virus is transmitted through the mother's genital secretions. Although HSV-1 is the primary culprit in most adult cases of herpes encephalitis, HSV-2 may also cause a small number of these cases.

Herpes simplex encephalitis is the only effectively treatable form of encephalitis but treatment (typically intravenous acyclovir), must be administered within the first few days of symptom onset. If left untreated, the mortalit rate for patients with HSV-1 is approximately 70%; if treated, the mortality rate declines to 30%. The mortality rate for neonatal HSV-2 encephalitis ranges from 15-57%. [For more information, see In-Depth Report #52: Herpes Simplex.]

Varicella-Zoster Virus. The varicella-zoster virus is responsible for both chicken pox (when the virus is called varicella) and shingles (when it is referred to as herpes zoster). Chicken pox is the initial infection, after which the virus remains dormant, often for a lifetime. If it erupts, usually years later, is does so in the form of shingles. Encephalitis caused by varicella can occur in both children and adults and be very serious. If it occurs as a result of herpes zoster in adults, the brain inflammation tends to be mild except in immunocompromised patients. In such cases, symptoms can appear weeks to months after an attack of shingles and resemble those of a stroke. Fortunately, encephalitis is rare with both varicella and zoster. [For more information, see In-Depth Report #82: Shingles and Chicken Pox (Varicella-Zoster Virus).]