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Causes of Encephalitis

The viruses responsible for encephalitis are often common viruses. They include:

  • rubeola (measles)
  • varicella (chicken pox),
  • influenza,
  • rubella (german measles)
  • polioviruses
  • enteroviruses (tummy bugs)
  • herpes simplex (the cold sore virus)
  • Epstein barr (glandular fever)
  • mumps

The most common cause of life-threatening viral encephalitis is the herpes simplex virus. Herpes simplex viruses are highly contagious. Around 80% of the UK population carry the herpes simplex viruses, but for the majority of the time they lie dormant (inactive).

The herpes simplex infection is usually limited to sites around the mouth or the genitals, only very rarely does it spread to include the brain.

If you have severe immune deficiency, for example if you have HIV, you can develop encephalitis from many viruses and infections e.g. Toxoplasmosis, cytomegalovirus (CMV), which is a similar virus to herpes simplex. People with HIV can also get encephalitis from HIV itself.

Arbovirus encephalitis is caused by a group of viruses carried and passed on through bites from an insect group, known as arthropods, such as mosquitoes or ticks.

The insects pick up the viruses by biting small mammals, domestic animals and birds. This type of encephalitis is more common in the rest of the world and include the following:

  • West Nile encephalitis (or West Nile fever),
  • Japanese Encephalitis,
  • Tick Borne Encephalitis,
  • Murray Valley Encephalitis,
  • California encephalitis,
  • St Louis encephalitis,
  • Western equine encephalitis,
  • Eastern equine encephalitis,
  • or Colorado tick fever.

Very occasionally bacterial, fungal, parasitic infections cause encephalitis.

Other causes of encephalitis include listeriosis, brucellosis, whooping cough, rabies, allergic reaction to a vaccination and lead poisoning.

In over 50% of cases, the cause of infection cannot be identified.