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Encephalitis is irritation and swelling (inflammation) of the brain, usually due to infections.

See also: Meningitis

Encephalitis is most often caused by a viral infection. Many types of viruses may cause it. Exposure to viruses can occur through:

• Breathing in respiratory droplets from an infected person
• Contaminated food or drink
• Insect bites
• Skin contact

In rural areas, arboviruses -- carried by mosquitoes or ticks, or accidentally ingested -- are the most common cause.

In urban areas, enteroviruses are most common, including:

• Coxsackievirus
• Echovirus
• Poliovirus

Other viruses that can cause encephalitis include:

• Adenovirus
• Cytomegalovirus
• Eastern Equine Encephalitis Virus
• Epstein-Barr virus
• Herpes simplex infection
• Measles
• Mumps
• Rabies
• Rubella
• Varicella (chickenpox or shingles)
• West Nile virus

AIDS patients and others at high-risk can develop encephalitis due to parasites such as:

• Certain roundworms
• Cysticercosis
• Toxoplasmosis

Although most forms of encephalitis are caused by viruses, the condition may also be caused by bacterial diseases, such as:

• Lyme disease
• Syphilis
• Tuberculosis

Extremely rarely, an allergic reaction to vaccinations can cause encephalitis. Autoimmune disease and the effects of cancer can also cause encephalitis.

Encephalitis is uncommon. The elderly and infants are more vulnerable and may have a more severe case of the disease.

When the virus enters the bloodstream, it may cause inflammation of brain tissue and surrounding membranes. White blood cells invade the brain tissue as they try to fight off the infection. The brain tissue swells (cerebral edema), which may destroy nerve cells, cause bleeding in the brain (intracerebral hemorrhage), and brain damage.

Symptoms include:

• Clumsiness, unsteady gait
• Confusion, disorientation
• Drowsiness
• Fever
• Headache
• Irritability or poor temper control
• Light-sensitivity
• Stiff neck and back (occasionally)
• Vomiting

• Loss of consciousness, poor responsiveness, stupor, coma
• Muscle weakness or paralysis
• Seizures
• Sudden change in mental functions:
  • Diminished interest in daily activities
  • "Flat" mood, lack of mood, or mood inappropriate for the situation
  • Impaired judgment
  • Inflexibility, extreme self-centeredness, indecisiveness, or withdrawal from social interaction
  • Memory loss (amnesia), impaired short-term or long-term memory

An examination may show:

• Abnormal reflexes
• Increased intracranial pressure
• Mental confusion
• Mouth ulcers
• Muscle weakness
• Neck stiffness
• Signs in other organs, such as the liver and lungs
• Skin rash
• Speech problems

Tests may include:

• Brain MRI
• CT scan of the head
• Culture of cerebrospinal fluid (CSF), blood, or urine (however, this test is rarely useful)
• Electroencephalogram (EEG)
• Lumbar puncture and CSF examination
• Tests that detect antibodies to a virus (serology tests)
• Test that detects tiny amounts of virus DNA (polymerase chain reaction -- PCR)

The outcome varies. Some cases are mild and short, and the person fully recovers. Other cases are severe, and permanent impairment or death is possible.

The acute phase normally lasts for 1 - 2 weeks. Fever and symptoms gradually or suddenly disappear. Some people may take several months to fully recover.

Permanent brain damage may occur in severe cases of encephalitis. It can affect:

• Hearing
• Memory
• Muscle control
• Sensation
• Speech
• Vision

Go to the emergency room or call the local emergency number (such as 911) if you have:

• Sudden fever
• Other symptoms of encephalitis

Controlling mosquitoes (a mosquito bite can transmit some viruses) may reduce the chance of some infections that can lead to encephalitis.

• Apply an insect repellant containing the chemical, DEET when you go outside (but never use DEET products on infants younger than 2 months).
• Remove any sources of standing water (such as old tires, cans, gutters, and wading pools).
• Wear long-sleeved shirts and pants when outside, particularly at dusk.

Vaccinate animals to prevent encephalitis caused by the rabies virus.

Human vaccinations that are available include:

• A vaccination to prevent a form of viral encephalitis that often affects people living in dorms or in the military
• Measles
• Herpes zoster

Bleck TP. Arthropod-borne viruses affecting the central nervous system. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 406.

Modlin JF. Enterovirus infections. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 402.

Nath A. Berger JR. Acute viral encephalitis. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 439.

Whitley RJ. Herpes simplex virus infections. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 397.