Encephalitis is an acute inflammation (swelling) of the brain, usually caused by a viral infection or the wrong attack on brain tissue by the body’s immune system. Encephalitis can also be caused by problems with the immune system, as well as, less commonly, bacterial or fungal infections. Rarely, bacteria, fungi, or parasites can also cause encephalitis.
Some parasitic or protozoan infections, such as toxoplasmosis, malaria, or primary amebic meningoencephalitis, can also cause encephalitis in people with weakened immune systems. In most cases, infection with one of several viruses does not cause encephalitis. Bacterial infections and noninfectious inflammations can also cause encephalitis.
The cause of most viral encephalitis is unknown, but the most common identifiable cause of viral encephalitis is herpes simplex infection. The most common causes of viral encephalitis are herpes simplex virus types 1 and 2, varicella zoster virus, and enteroviruses that cause gastrointestinal illness. Most diagnosed cases of encephalitis in the United States are caused by herpes simplex virus types 1 and 2, arboviruses (such as West Nile virus) that are transmitted from infected animals to humans through the bites of ticks, mosquitoes, or other infected blood-sucking insects, or enteroviruses.
Herpes simplex encephalitis (HSE) The herpes simplex virus affects the temporal lobes, the area of the brain responsible for memory and speech. This form of encephalitis usually affects the temporal lobe, the part of the brain responsible for memory and speech. Encephalitis occurs when a virus or other agent directly attacks the brain.
Autoimmune encephalitis occurs when a person’s antibodies or immune cells attack the brain. Instead of attacking only the cells that cause the primary infection, the immune system mistakenly attacks healthy brain cells as well. Encephalitis can be caused by infections or autoimmune conditions, in which the body’s immune responses attack the brain.
Encephalitis is an inflammation of active brain tissue caused by an infection or an autoimmune reaction. Encephalitis can affect one area of the brain or several different areas. Encephalitis is inflammation of the brain due to an infection such as a virus or bacterium, drugs, or a malfunction of the immune system.
The most serious potential complication of viral encephalitis is permanent brain damage. When a direct viral infection of the brain or spinal cord occurs, it is called primary encephalitis. Primary or infectious encephalitis can occur when fungi, viruses, or bacteria infect the brain. Secondary encephalitis is an infection that starts elsewhere in the body and spreads to the brain.
It is caused by an infection that enters the brain (infectious encephalitis) or by the immune system mistakenly attacking the brain (post-infectious or autoimmune encephalitis). Some types of autoimmune encephalitis, such as acute disseminated encephalomyelitis (ADEM), are caused by infection, in which case the term postinfectious encephalitis is used. Some cases of encephalitis are caused by an autoimmune disease, which in some cases may be due to infection (“post-infectious”) or cancer, even microscopic and undetectable (so-called paraneoplastic neurological syndromes).
Herpes encephalitis is dangerous and can lead to severe brain damage and death. HSV type 1 encephalitis is rare but can lead to significant brain damage or death.
When encephalitis strikes, it can be very severe, causing personality changes, seizures, weakness, and other symptoms, depending on which part of the brain is affected. Encephalitis can also affect the function of the brain (cognition), leading to confusion and behavioral changes.
In children under 1 year of age, Eastern Equine Encephalitis can cause severe symptoms and permanent nerve or brain damage. Western equine encephalitis can affect all age groups, but is more severe and more likely to affect the brain in children younger than 1 year of age. These viruses can cause infections such as West Nile, La Crosse, St. Louis, Western equine encephalitis, and Eastern equine encephalitis.
West Nile encephalitis also causes a milder infection called West Nile fever, which is much more common. Several species of birds can become infected with West Nile encephalitis when bitten by an infected mosquito. Several viruses that can cause encephalitis are spread by insect bites (such as mosquito or tick bites).
Additional possible viral causes are arbovirus flavivirus (St. Louis encephalitis, West Nile virus), bunyavirus (La Crosse strain), arenavirus (lymphocytic choriomeningitis virus), reovirus (Colorado tick virus), and genipavirus infections. Other viruses that are less likely to cause encephalitis include mumps virus, Epstein-Barr virus (glandular fever), ecovirus, coxsackie virus, human immunodeficiency virus (HIV), and cytomegalovirus (CMV). Infectious causes of meningitis and encephalitis include bacteria, viruses, fungi, and parasites. Lyme disease, a bacterial infection transmitted by tick bites, sometimes causes meningitis and, very rarely, encephalitis.
Rabies virus infection is usually spread through the bite of an infected animal and can rapidly progress to encephalitis after symptoms appear. The infection usually causes a mild flu-like illness that resolves within a few days, but some people (usually 50 or older) experience more severe symptoms. Powassan virus infection can also cause severe encephalitis, including headache, vomiting, seizures, loss of coordination, speech problems, or coma. In children, damage to parts of the brain that did not occur at onset may occur at a later age, long after the onset of encephalitis.
Several types of bacteria first cause upper respiratory tract infections and then travel to the brain through the bloodstream. Viral encephalitis may be caused by bacterial infection, such as bacterial meningitis, or may be a complication of current syphilis infection (secondary encephalitis).