Infection: Encephalitis

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Infection: Encephalitis

Causes and Symptoms
The Future
For more information


Encephalitis is a severe infection or inflammation of the brain itself. It should be distinguished from meningitis, which is an inflammation of the protective membranes that cover the brain and line the spinal cord. However, the two conditions can exist together. Encephalitis is usually caused by viruses, but can also result from infections caused by bacteria, fungi, or parasites.


Encephalitis is an illness that can range from mild cases with flu-like symptoms to life-threatening, severe cases that can kill patients or leave them with permanent problems. The severity of the illness varies according to the patient's age, the organism that is causing the encephalitis, and the condition of the person's immune system. Mild viral encephalitis may have no symptoms at all, or a brief period of headaches, tiredness, and irritability.

Severe encephalitis often begins with a prodrome, or a period of warning symptoms. After the prodrome, victims of the disease may become feverish, vomit, become drowsy, find that their eyes are sensitive to light, complain of a stiff neck, or feel unsteady on their feet when they try to walk. They should see their doctor at once. If they lose their memory, have speech problems, are paralyzed, or go into a coma, they need emergency medical treatment and should be taken to the hospital as soon as possible.


The exact number of cases of encephalitis in the United States in an average year is difficult to estimate because mild cases may not be diagnosed, and the reporting of severe cases is not standardized. Between 1,500 and 3,000 cases of viral encephalitis are reported to the Centers for Disease Control and Prevention (CDC) each year, but most doctors think that the actual number of cases is higher. According to the CDC, it costs about $150 million each year to control the mosquitoes that spread some types of viral encephalitis.

Some statistics for specific types of viral encephalitis are as follows:

  • Japanese encephalitis: 30,000–50,000 cases each year in Asia; about one case per year in Americans traveling in Asia.
  • Eastern equine encephalitis, found in the eastern United States: five cases per year on average.
  • La Crosse encephalitis, found in California and the Great Lakes region: seventy cases per year on average.
  • West Nile virus: about 1,200 cases per year in the United States.
  • Herpes simplex encephalitis, caused by the virus that causes cold sores: about one person in every 250,000 per year in the United States.
  • Encephalitis following measles: about one case per 1,000 infected persons.
  • Encephalitis following chickenpox: one case in every 2,000 infected persons.
  • Encephalitis following rabies: one to three cases in the United States each year.

Causes and Symptoms

Most cases of encephalitis in the United States are caused by viruses:

  • Herpes simplex virus (HSV) 1 and 2. This is the virus responsible for cold sores and genital herpes.
  • Poliovirus.
  • Arboviruses. The arboviruses are a family of viruses transmitted to humans by infected mosquitoes, ticks, or other blood-sucking insects. These types of encephalitis include West Nile virus, Japanese encephalitis, equine encephalitis, La Crosse encephalitis, and St. Louis encephalitis.
  • The rabies virus.

Toxoplasmosis, a disease caused by a parasite, and Lyme disease, a bacterial infection, can cause encephalitis.

Symptoms of encephalitis in infants may include:

  • Vomiting
  • Body stiffness
  • Crying that increases when the baby is picked up or held
  • A fullness or bulge in the soft spot at the top of the baby's head

In addition to the symptoms of encephalitis in adults discussed earlier, people with severe infections may also have:

  • Visual hallucinations
  • Seeing double
  • Personality changes
  • Loss of sensation in some parts of the body
  • Impaired judgment
  • Muscle weakness


The patient's history of recent travels, exposure to animals or infected persons, or exposure to the herpes simplex virus or arboviruses are important clues in diagnosis. If the patient is unconscious or cannot speak, family members may be asked for information about his or her recent activities.

When to See the Doctor

A person who has been exposed to organisms that can cause encephalitis combined with symptoms of the illness should see their doctor quickly if:

  • The person has cold sores or sores around the genitals following close contact with another person.
  • He or she has recently had measles or chickenpox, or been exposed to someone with those diseases.
  • The person knows that he or she has recently been bitten by a tick.
  • He or she has been bitten by a bat, raccoon, dog, or coyote.
  • The person has been in wooded, damp, or marshy areas where mosquitoes are likely to breed.
  • He or she has traveled to foreign countries where various types of encephalitis are common, particularly eastern Asia.

The specific diagnostic tests that may be done depend partly on the type of disease organism that the doctor suspects is causing the encephalitis. Some common diagnostic tests include:

  • Blood tests. Blood tests can be used to diagnose West Nile virus
  • A neurological examination. This examination allows the doctor to evaluate the patient's memory and other mental functions, sight and hearing, reflexes, gait and movement, mood, level of consciousness, and other functions of the nervous system that may be affected by encephalitis.
  • Spinal tap. In a spinal tap, a small sample of spinal fluid is withdrawn through a needle inserted into the lower back. It can be tested for evidence of a bacterial or fungal infection. In some cases, a spinal tap can also be used to diagnose HSV encephalitis.
  • Electroencephalogram (EEG). An EEG is a test that measures patterns of electrical activity in the brain. HSV encephalitis produces a telltale abnormal EEG pattern.
  • Imaging studies of the brain. Computed tomography (CT) scans and magnetic resonance imaging (MRI) are the types of imaging studies most frequently used to diagnose encephalitis.
  • Brain biopsy. In a brain biopsy, a surgeon removes a small sample of brain tissue to analyze it for the presence of HSV. This test is rarely done except when brain imaging or other tests for HSV do not provide definite results.


Treatment of mild encephalitis usually includes bed rest at home, drinking lots of fluids, and taking over-the-counter pain relievers to bring down fever and treat headaches. If the patient gets worse suddenly, or if the infection is severe from the start, emergency treatment in a hospital is necessary.

Specific forms of treatment for encephalitis in the hospital may include:

  • Acyclovir for HSV encephalitis. Acyclovir is an antiviral drug that is given intravenously when the doctor suspects HSV encephalitis. It is given for a period of two to three weeks.
  • If the encephalitis is caused by a bacterium, the doctor may prescribe antibiotics. Antibiotics cannot be used to treat encephalitis caused by viruses, however.
  • Patients are monitored for signs of swelling of the brain, and they are given intravenous fluids to prevent dehydration.
  • Patients who are having seizures are given anticonvulsants (antiseizure drugs).
  • Sedatives may be given to patients who are restless or having trouble sleeping.
  • Patients who are partially paralyzed or having trouble breathing are put on a respirator.

Patients who have had severe encephalitis may suffer permanent damage if they survive. They may need physical therapy, speech therapy, or psychological counseling after the acute illness has passed.


The prognosis for recovery from encephalitis depends on the virus or other disease agent that is causing the infection, the patient's age, and his or her general state of health. In general, infants and the elderly are at the greatest risk of death from encephalitis.

People with mild cases of encephalitis usually recover without long-term complications.

The prognoses for specific types of encephalitis are as follows:

  • Rabies encephalitis: 100 percent mortality.
  • Japanese encephalitis: 60 percent mortality, usually within the first week of illness.
  • Untreated HSV encephalitis: 50–75 percent of patients die within eighteen months. The fatality rate drops to 20 percent in patients treated with acyclovir. Forty percent of survivors have long-range learning disabilities, epilepsy, movement disorders, memory loss, and psychiatric problems.
  • St. Louis encephalitis: 30 percent mortality.
  • West Nile virus: 10 percent mortality.


Japanese encephalitis is the only encephalitis for which there is an effective vaccine for humans. People planning a trip to Japan or eastern Asia may want to consider being vaccinated before departure. There was a vaccine for horses against West Nile virus but not one for humans as of late 2008.

Other preventive measures to lower one's risk of encephalitis include:

  • Draining water from bird baths, wading pools, and other outdoor water containers when not in use.
  • Using insect repellent when outdoors; applying it to exposed skin and spraying clothing with repellent.
  • Wearing long-sleeved shirts and long pants on hikes or other outdoor activities when the weather permits.
  • Keeping window and door screens in good repair so that mosquitoes cannot get inside the house.
  • Covering baby strollers or carriers with mosquito netting when taking the baby for a walk outdoors.
  • Having pet dogs and cats vaccinated against rabies, and avoiding bats, raccoons, and other wild animals that are acting aggressive.
  • Avoiding sexual contact with people infected by genital herpes.
  • Avoiding kissing or sharing drinking glasses with people with cold sores.
  • Having children vaccinated against diseases such as measles and mumps.
  • Seeking prompt medical treatment for any high fever associated with an infection.

The Future

Researchers are working on developing vaccines against West Nile virus and other mosquito-borne viral infections. As of 2008, the National Institutes of Health was testing a recently developed vaccine against equine encephalitis, and a new drug related to acyclovir as a treatment for HSV encephalitis.

SEE ALSO Chickenpox; Cold sore; Genital herpes; Lyme disease; Measles; Meningitis; Rabies; Toxoplasmosis; West Nile virus infection


Arboviruses : A family of viruses spread by blood-sucking insects.

Gait : A person's manner of walking.

Meningitis : Inflammation of the membranes that cover the brain and line the brain and spinal cord.

Prodrome : A period before the acute phase of a disease when the patient has some characteristic warning symptoms.

For more information


Bloom, Ona, and Jennifer Morgan. Encephalitis. Philadelphia: Chelsea House Publishers, 2006.


Centers for Disease Control and Prevention (CDC). Infectious Disease Information: Encephalitis. Available online at (updated October 2, 2007; accessed July 16, 2008).

Journal of the American Medical Association Patient Page. Viral Encephalitis. Available online at (posted July 27, 2005; accessed July 16, 2008).

National Institute of Neurological Disorders and Stroke (NINDS). Meningitis and Encephalitis Fact Sheet. Available online at (updated December 11, 2007; accessed July 16, 2008).

TeensHealth. Encephalitis. Available online at (updated November 2007; accessed July 16, 2008).

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Infection: Encephalitis

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