Encephalitis is an inflammation (swelling accompanied by heat) of the brain tissue. Encephalitis can be a very serious, sometimes fatal, condition. People often recover fully within a couple of weeks, but others, especially children and older people, may have permanent problems, such as seizures, memory loss, personality changes or brain damage. A person's chances of dying from encephalitis or having permanent problems after recovery depend on the cause of the encephalitis, its severity, the person's age and how quickly the illness develops. Depending on which virus caused the encephalitis, the death rate ranges from 1 to 80% in the United States.

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. In the case of herpes simplex encephalitis, doctors look for traces of the herpes simplex virus. Blood tests can be used 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 on paper. An EEG on a patient with encephalitis may show an abnormal electrical activity in the brain.

A brain biopsy is sometimes used to help determine the cause of encephalitis, especially if herpes simplex encephalitis is suspected and the patient does not appear to be improving after treatment with acyclovir (an antiviral drug used to treat herpes). A brain biopsy involves taking a small sample of brain tissue and examining it for evidence of infection by the virus. Brain biopsies are not used very often because tests on spinal fluid usually can accurately diagnose encephalitis caused by the herpes simplex virus.

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. If encephalitis caused by the herpes simplex virus is treated promptly, the death rate of 70 to 80% drops to less than 30%.

Viruses carried by mosquitoes and ticks that cause encephalitis cannot be destroyed with drugs and will not respond to acyclovir. Instead, doctors treat the symptoms with medications to keep the patient comfortable and allow the body to heal itself.