Encephalitis is an inflammation (swelling accompanied by heat) of the brain tissue. Encephalitis can be a very serious, sometimes fatal, condition. However, with treatment, adults can recover within a couple of weeks, but children and older adults may have permanent issues, such as seizures, memory loss, personality changes or brain damage.

Encephalitis usually starts with fever and a severe headache and continues with one or more of the following symptoms:

  • Abnormal sensitivity to light
  • Coma
  • Drowsiness
  • Lack of energy, sluggishness
  • Memory loss
  • Nausea and vomiting
  • Personality change
  • Restlessness, confused speech and hallucinations (delirium)
  • Seizures or tremors
  • Stiff neck and back
  • Trouble learning and understanding


Viruses that cause cold sores and genital herpes (herpes simplex virus), mumps, measles, chickenpox, Epstein-Barr, influenza and German measles (rubella) can cause encephalitis. These common viruses usually cause less serious illnesses, but on rare occasions they can get into the brain and cause encephalitis. Among these types of viral encephalitis, herpes simplex encephalitis is the most common in the United States. Another group of viruses (arboviruses) are mainly the result of mosquito and tick bites. Most people who are bitten by infected mosquitoes or ticks do not develop any symptoms. Only a very small number of people who exhibit symptoms actually develop encephalitis.

Infection with the rabies virus can cause encephalitis and is almost always fatal if not treated before symptoms develop. People who have weakened immune systems, especially those with AIDS (acquired immunodeficiency syndrome), are at increased risk for developing encephalitis from infections.


The most important test in diagnosing encephalitis is an analysis of the fluid in the spine. Samples are taken during a spinal tap, a procedure in which a needle is inserted in the lower back between the bones of the spine. The spinal fluid is checked for infection, such as increased white blood cells and increased protein. Blood tests, and sometime a brain biopsy, can also be done to help identify specific causes of encephalitis.

A magnetic resonance imaging (MRI) scan may be used to provide a cross-section picture of the brain. If encephalitis is present, the scan may reveal bleeding, inflammation or other changes in certain areas of the brain, depending on the type of encephalitis present.

An electroencephalogram (EEG) can also help confirm encephalitis. In this procedure, a computer records the brain's electrical patterns. An EEG on a patient with encephalitis may show an abnormal electrical activity in the brain.


When encephalitis is caused by the herpes simplex virus or chickenpox (varicella-zoster virus), the condition can be treated with intravenous (IV) acyclovir, an antiviral drug. Because early treatment can reduce the chances of dying from encephalitis, it is very important to start treatment with acyclovir as soon as encephalitis is suspected, even if the exact cause of the illness is unknown.

Viruses carried by mosquitoes and ticks that cause encephalitis cannot be destroyed with drugs and will not respond to acyclovir. Instead, medications may be used to help alleviate symptoms.

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