Causes and Risk Factors for Encephalitis

Encephalitis is caused by several types of viral infections. Herpesvirus is the most common cause and encephalitis can also result from infection following smallpox vaccination and reactivation of viral infection such as influenza, chickenpox, measles, mumps, and rabies (usually in undeveloped countries).

Arthropod-borne viruses, which are usually transmitted by mosquitoes, cause arboviral encephalitis. People who live in warm, moist climates are at higher risk for this type.

Encephalitis Signs and Symptoms

Primary symptoms of encephalitis include sudden fever, stiff neck, malaise, sensitivity to light (photosensitivity), and headache. Infants may develop bulging of the soft spots (fontanels) of the skull.

Other early symptoms include the following:

  • Abnormal sleep patterns
  • Behavioral changes (e.g., lethargy, confusion)
  • Exhaustion
  • Nausea
  • Muscle stiffness
  • Sore throat
  • Upper respiratory tract infection (coughing, sneezing, congestion)

Neurological complications that may be permanent or improve as the infection runs its course include the following:

  • Altered mental state (e.g., disorientation, personality changes)
  • Convulsions
  • Drooping eyelids (ptosis), double vision (diplopia), crossed eyes (strabismus)
  • Hyperactive deep tendon reflexes
  • Increased intracranial pressure
  • Loss of consciousness
  • Mental retardation
  • Motor dysfunction
  • Partial paralysis (paresis) of the extremities
  • Projectile vomiting
  • Pupil irregularities
  • Restlessness
  • Seizures
  • Tremor

Most people infected with an arthropod-borne virus do not develop encephalitis. Infection is usually does not produce symptoms (called asymptomatic) or causes flu-like symptoms such as fever, headache, and malaise.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 24 Sep 2002

Last Modified: 14 Sep 2015