Research, Scientific Breakthroughs, & Caregiver Tips
What Causes Encephalitis?
Very often, encephalitis is a result of a viral infection that directly affects the brain, causing inflammation and swelling. These types of the disease are called primary encephalitis, and they are often transmitted to humans by the bite of a tick, insect, or another animal that is infected with the virus.
Types of viral encephalitis include:
- Herpes simplex encephalitis (HSE) is caused by the herpes simplex virus types 1 and 2, and the brain inflammation may result from reactivation of a virus that was already present and inactive in the body. Herpes simplex virus type 1 causes cold sores and is commonly acquired in childhood. The type 2 virus is usually spread through sexual contact. HSE is rare but is potentially very serious or even fatal.
- Mosquito-born viral encephalitis types include Eastern equine encephalitis, Western equine encephalitis, West Nile encephalitis, La Crosse encephalitis, and St. Louis encephalitis. These types usually cause flu-like symptoms within days or weeks after the mosquito bite.
- Tick-borne viral encephalitis includes Powassan encephalitis, which produces symptoms within 7-10 days after the bite of an infected tick.
- Rabies virus is transmitted through a bite or scratch from an infected animal, and symptoms rapidly progress to severe encephalitis. Cases of rabies are rare in the United States, but if left untreated, the disease is almost always fatal.
- Bacterial or protozoan infections such as toxoplasmosis or malaria may also be a cause.
Sometimes encephalitis is caused by the body’s immune system as it tries to fight off another infection. In this case, the immune system mistakes proteins in nerve cells with those in a virus, causing the immune system to attack the healthy nerve cells. This type of encephalitis is called secondary encephalitis, post-infection encephalitis, or autoimmune encephalitis.
Infections that may trigger autoimmune encephalitis include:
- Enterovirus infections
- Epstein-Barr virus
- Hepatitis A or B
- Human immunodeficiency virus (HIV)
- Childhood diseases such as measles, mumps, and rubella
Another form of autoimmune encephalitis occurs when the immune system attacks nerve-cell proteins called N-methyl-d-aspartate (NMDA) receptors. Called anti-NMDA receptor encephalitis, this type of the disease sometimes follows cases of herpes simplex encephalitis
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