Encephalitis: Viral - Causes

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Varicella-Zoster Virus. The varicella-zoster virus is responsible for both chickenpox (when the virus is called varicella) and shingles (when it is referred to as herpes zoster). Chickenpox 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 chickenpox (varicella-zoster virus).]

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Epstein-Barr Virus. Epstein-Barr virus is the cause of infectious mononucleosis, which is most common in children and young adults. Symptoms of the disease are severe fatigue, headache, sore throat, and fever. In 1% of cases, neurological complications occur about 1 - 3 weeks after the onset of the infection. If encephalitis develops, it is almost always mild with full recovery.

Cytomegalovirus Encephalitis. Cytomegalovirus is also very common and usually mild. In immunocompromised patients, such those with AIDS, it can be dangerous, with severe complications including encephalitis.

Arboviruses

Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks. Most of the time, the viral infections initially develop in birds. Insects that feed on the infected blood from a diseased bird (which functions as the reservoir of infection) carry the virus, and transmit it when they bite a susceptible host (such as an animal or a human). Insects, such as mosquitoes, that play a role in the disease-transmission process are referred to as vectors.

Arboviruses multiply in blood-sucking vectors, nearly always mosquitoes. There is no evidence that these infections can be transmitted casually from one infected person or animal directly to another uninfected person without passing through a mosquito (or tick) first. (However, a small number of West Nile virus cases have occurred through blood transfusions, organ transplantation, and possibly breastfeeding.) Only about 10% of people who are infected by an arbovirus develop encephalitis and only about 1% of those infected have symptoms.