Encephalitis is most often caused by a viral infection, and many types of viruses may cause it. Exposure to viruses can occur through insect bites, food or drink contamination, inhalation of respiratory droplets from an infected person, or 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, poliovirus, and echovirus. Other causes include herpes simplex infection, varicella (chickenpox or shingles), measles, mumps, rubella, adenovirus, rabies, West Nile virus, and extremely rarely, vaccinations.

Once the virus has entered the bloodstream, it may localize in the brain, causing 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 cause destruction of nerve cells, bleeding within the brain (intracerebral hemorrhage), and brain damage.

Encephalitis is uncommon. It affects approximately 1,500 people per year in the U.S. The elderly and infants are more vulnerable and may have a more severe course of the disease.

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

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

• A lumbar puncture test and cerebrospinal fluid (CSF) examination may show clear fluid, high pressure, high white blood cell count and protein levels -- indications of inflammation. Blood may be present in the CSF.
• Sometimes the virus can be detected in CSF, blood, or urine through a laboratory test called viral culture. However, this test is cumbersome and rarely useful. In some cases, viral PCR (polymerase chain reaction), a test able to detect very tiny amounts of viral DNA) may identify the virus. Health care providers also rely on serology tests (serologies detect some proteins called antibodies, which are produced in response to an specific virus) to provide evidence of viral infection.
• An EEG (a test of the electrical activity of the brain) may provide indirect clues for the diagnosis of encephalitis. Some EEG wave patterns may suggest a seizure disorder, or point to a specific virus as cause of the infection. Certain EEG wave patterns can suggest encephalitis due to herpes, for instance.
• A brain MRI, which provides high-quality pictures of the brain, or a CAT scan of the head may be used to determine internal bleeding or focal areas of brain inflammation.

The goals of treatment are to provide supportive care and relieve symptoms. Antiviral medications may be prescribed for herpes encephalitis or other severe viral infections. Most of the time, however, no specific antiviral drugs are available to combat the infection.

Antibiotics may be prescribed when the infection is caused by some organisms, such as certain bacteria. Anti-seizure medications (such a phenytoin) are used to suppress seizures. On rare occasions, potent anti-inflamatory drugs called steroids (such as dexamethasone) are used to reduce brain swelling.

Sedatives may be needed to treat irritability or restlessness. Other medications, like acetaminophen, may be used for fever and headache.

Supportive care (rest, nutrition, fluids) allows the body to fight the infection. Reorientation and emotional support of confused or delirious persons may be helpful.

If brain function is severely affected, interventions like physical therapy and speech therapy may be necessary after the acute illness is controlled.

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

Animal vaccination is important to prevent encephalitis caused by rabies virus. Vaccination is available to prevent a form of viral encephalitis that often affects people living in dorms or in the military.