Encephalitis is an acute infection of the brain characterized by fever, headache, and an altered state of consciousness, with or without seizures. Most cases of encephalitis are caused by viruses. They may also be limited to a single location (focal) or diffuse throughout the brain (generalized). Each year there are several thousand cases reported but, according to the National Institute of Neurological Disorders and Stroke, there are probably many more cases with minimal to mild symptoms that occur but are not documented.
Viral encephalitis may be caused by a variety of viruses including:
- Herpes simplex virus
- The rabies virus (from an animal bite)
- Arboviruses – those spread primarily by infected mosquitoes but also by a few ticks
Most people who are infected have mild to moderate symptoms. Only a very small percentage of people develop encephalitis.
In the United States, West Nile virus is the most common cause of arbovirus encephalitis. Other more rare U.S. arboviruses are distributed geographically. Throughout the world, different types of arbovirus-related encephalitis may predominate. Types of encephalitis caused by arboviruses include:
- West Nile encephalitis – about one in 5 people infected with West Nile virus will develop fever with other symptoms and less than 1% will develop severe illness, according to the Centers for Disease Control and Prevention (CDC).
- Western equine encephalitis – per the CDC, about 3% of those affected die; illness is more severe in infants than adults.
- Eastern equine encephalitis – though this is a rare illness with only a few cases in the U.S. each year, it tends to be one of the most severe; the CDC estimates that about a third of those with the infection will die and many survivors will suffer brain damage.
- LaCrosse encephalitis – found in the upper Midwest, mid-Atlantic, and southeastern states; most severe cases occur in children less than 16 years of age, according to the CDC.
- St. Louis encephalitis – a rare infection, most cases occur in eastern and central states in the U.S.; illness is most severe in the elderly.
Other arboviruses that may be seen in other parts of the world include:
- Japanese encephalitis – found naturally (endemic) in Asia and is associated with rural farming areas; a vaccine to prevent infection is available.
- Venezuelan equine encephalitis – very rare in the U.S.; has killed thousands of people in South American epidemics.
Viral encephalitis may also be seen as a secondary condition that occurs a few weeks after a viral illness.
Bacterial, fungal, and parasitic encephalitis are very rare. Bacterial meningoencephalitis may develop from the bacteria that cause meningitis. Tick-transmitted Lyme disease may cause bacterial encephalitis. Toxoplasma gondii, a parasite associated with cats, can cause parasitic encephalitis in some people with weakened immune systems.