The cause of encephalitis is often unknown, but the most commonly diagnosed cause is a viral infection. Known causes of encephalitis may include:

  • Viruses
  • Bacteria
  • Fungi
  • Parasites

An infection may result in one of two conditions affecting the brain:

  • Primary encephalitis occurs when a virus or other infectious agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive (latent) after a previous illness.
  • Secondary (post-infectious) encephalitis is a faulty immune system reaction in response to an infection elsewhere in the body. This likely occurs when disease-fighting proteins enlisted to fight an infection mistakenly attack molecules in the brain. Secondary encephalitis often occurs two to three weeks after the initial infection. Rarely, secondary encephalitis occurs as a complication of vaccination against a viral infection.

Common viral causes

Common causes of encephalitis include:

  • Herpes simplex virus. There are two types of herpes simplex virus (HSV). HSV type 1 (HSV-1) more commonly causes cold sores or fever blisters around your mouth, and HSV type 2 (HSV-2) more commonly causes genital herpes. While HSV-1 encephalitis is rare, it has the potential to cause significant brain damage or death.
  • Other herpes viruses. Other herpes viruses that may cause encephalitis include the Epstein-Barr virus, which commonly causes infectious mononucleosis, and the varicella-zoster virus, which commonly causes chickenpox and shingles.
  • Enteroviruses. These viruses include the poliovirus and the coxsackievirus, which usually causes an illness with flu-like symptoms, eye inflammation and abdominal pain.
  • Mosquito-borne viruses. Arboviruses, or arthropod-borne viruses, are transmitted by mosquitoes or other blood-sucking insects. Mosquito-borne viruses can cause infections that include West Nile, La Crosse, St. Louis, western equine and eastern equine encephalitis. Mosquitoes transfer the virus from a nonhuman host — such as a bird, chipmunk or horse — to humans. Symptoms of an infection may appear within a few days to a couple of weeks after exposure to an arbovirus.
  • Tick-borne viruses. In the United States, the only known tick-transmitted virus that causes encephalitis is Powassan virus. Symptoms usually appear about a week after exposure to the virus.
  • Rabies virus. Infection with the rabies virus, which is usually transmitted by a bite from an infected animal, causes a rapid progression to encephalitis once symptoms begin. The disease causes death if the infection isn't treated before it reaches the spinal cord and brain.
  • Childhood infections. Common childhood infections — such as measles (rubeola), mumps, and German measles (rubella) — used to be fairly common causes of secondary encephalitis. These causes are now rare because of the availability of vaccinations for these diseases.
May. 05, 2011