What causes encephalitis?
The inflammation caused by encephalitis can be the result of an infection that invades brain tissue (infectious encephalitis), or by the immune system attacking the brain in error (post-infectious or autoimmune encephalitis).
Infectious encephalitis (also called primary encephalitis) is most often caused by a virus that directly infects the brain. The infection may be concentrated in one area or widespread. Symptoms develop within days to weeks. Some of the most common encephalitis-causing viruses include herpes simplex types 1 and 2, Epstein-Barr, varicella-zoster, enteroviruses, mosquito-borne viruses (West Nile, Japanese, La Crosse, St. Louis, Powassan, Zika, Chikungunya, and both Western and Eastern equine), and tick-borne viruses. Rabies virus infection causes rapid onset of encephalitis, although rabies is a rare cause in the United States (US).
While any virus can potentially cause encephalitis, not everyone who has a viral infection will develop encephalitis. Although rare, bacteria, fungi or parasites can cause encephalitis. Noninfectious inflammatory conditions can also cause encephalitis. The exact cause of an individual case of encephalitis is often unknown.
Autoimmune encephalitis (also called secondary, or post-infection encephalitis) is the result of a faulty immune-system reaction to an infection in another part of the body. Instead of attacking only the cells causing the infection, the immune system also attacks healthy brain cells. These infections can result from a reactivation of a virus that became inactive after a previous illness. The reaction occurs (and symptoms develop) two to three weeks after the initial infection. Common childhood infections (measles, mumps, chickenpox, German measles) used to be fairly common causes of secondary encephalitis. Those cases are now rare in the US because of the availability of vaccinations for these diseases.
Autoimmune encephalitis can also be caused by specific antibodies in blood that attack the brain. Antibodies are proteins that identify and remove foreign viruses and bacteria. Why these antibodies are produced by patients with autoimmune encephalitis is not known. In some cases, they can be caused by a tumor.
Who’s most at risk for encephalitis?
Although anyone can develop encephalitis, these factors can increase your risk: