Health Information



Encephalitis is inflammation of the brain. The inflammation may involve the whole brain, or just parts of the brain. Encephalitis may just occur in individuals (sporadic) or may affect many people in a particular area (epidemic).
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Encephalitis is most often caused by a viral infection. In the United States, the most common cause of sporadic encephalitis is the herpes simplex virus (HSV). Epidemic causes of encephalitis are usually mosquito or tick-borne viruses.
The most common viruses that cause encephalitis include:
Not all encephalitis is caused by a virus. Some may be due to an overreaction of the immune system.

Risk Factors

Factors that may increase your chance of encephalitis include:
  • Living, working, or playing in an area where mosquito-borne viruses are common
  • Not being immunized against diseases, such as:
    • Measles
    • Mumps
    • Chickenpox
    • Polio
    • Rotavirus
  • A suppressed immune system caused by certain medications, or health conditions, such as HIV infection
  • Having cancer—sometimes immune system overactivity may be the first sign of cancer
Newborns of mothers who have genital herpes simplex are at risk for herpes simplex encephalitis


The symptoms may range from mild to severe. Severe symptoms can include permanent neurological damage. Encephalitis can also lead to death.
Milder symptoms include:
  • Fever
  • Weakness, severe fatigue
  • Headache
  • Sensitivity to light
  • Stiff neck and back
  • Vomiting
  • Muscle aches
  • Rash
  • Yawning
More severe symptoms may include:
  • Changes in consciousness
  • Personality changes
  • Confusion
  • Irritability
  • Seizures
  • Partial or complete paralysis
  • Progressive drowsiness
  • Trouble walking
  • Trouble speaking
  • Trouble swallowing


The doctor will ask about your symptoms and medical history. A physical exam will be done.
Tests may include:
  • Blood tests
  • Lumbar puncture—to evaluate cerebrospinal fluid that surrounds the spinal cord and brain
  • CT scan and/or MRI scan—to evaluate internal structures of the head
  • Electroencephalogram (EEG)— to look for abnormal electrical activity in the brain
  • Brain biopsy—removal of a small sample of brain tissue to test for signs of infection


Treatment is mostly supportive. It may include:
  • Antiviral drugs to shorten the duration of the illness
  • Steroids to reduce brain inflammation
  • Diuretics to decrease elevated intracranial pressure
  • Intubation with hyperventilation to decrease elevated intracranial pressure, and to maintain respiration and ventilation
  • Anticonvulsants to prevent and/or treat seizures


To help reduce your chance of getting encephalitis, take these steps:
  • Make sure that you and your children are vaccinated against preventable viral illnesses
  • Protect yourself from mosquito bites
    • Fix window screens
    • Drain standing water around your home
    • Wear long clothes after dark
    • Use repellent when you are outside
    • Use proper mosquito netting at night. Look for netting treated with insecticide.


Encephalitis Information Resource

National Institute of Neurological Disorders and Stroke


Canadian Neurological Sciences Federation

Health Canada


Herpes simplex encephalitis. EBSCO DynaMed website. Available at: Updated February 21, 2013. Accessed May 30, 2014.

California encephalitis. EBSCO DynaMed website. Available at: Updated February 21, 2013. Accessed May 30, 2014.

Eastern equine encephalitis. EBSCO DynaMed website. Available at: Updated September 19, 2013. Accessed May 30, 2014.

NINDS meningitis and encephalitis information page. National Institute of Neurologic Disorders and Stroke website. Available at: Updated April 16, 2014. Accessed May 1, 2014.

West Nile virus infection. EBSCO DynaMed website. Available at: Updated August 9, 2013. Accessed May 30, 2014.

10/1/2013 DynaMed Systematic Literature Surveillance Reimer LJ, Thomsen EK, Tisch DJ, et al. Insecticidal bed nets and filariasis transmission in Papua New Guinea. N Eng J Med. 2013 Aug 22; 369(8):745-53.

Revision Information

  • Reviewer: Rimas Lukas, MD
  • Review Date: 05/2014 -
  • Update Date: 00/53/2014 -