brain, spinal cord, and nerve disorders
Encephalitis
Types of Encephalitis
Infections that can directly lead to encephalitis can occur in epidemics or occasionally as isolated cases (sporadically). In the United States, the most common types of epidemic encephalitis are caused by one of the following: Overview of Enterovirus Infections (which more often cause meningitis) Overview of Arbovirus, Arenavirus, and Filovirus Infections None Arboviruses are viruses transmitted to people through the bites of arthropods, usually mosquitoes, fleas, or ticks. (Arbovirus is short for arthropod-borne virus.) The viruses are transmitted to arthropods when arthropods bite infected animals or people. Many species of domestic animals and birds carry these viruses. Epidemics occur in people only periodically—when the population of mosquitoes or infected animals increases. Epidemics tend to occur when arthropods are biting—for mosquitoes and ticks, usually during warm weather. Infection spreads from arthropod to person, not from person to person. Many arboviruses can cause encephalitis. The different types of encephalitis that result are usually named for the place the virus was discovered or the animal species that typically carries it. In the United States, mosquitoes spread several types of encephalitis, including the following: La Crosse encephalitisis caused by the La Crosse virus (also called California virus). It is most common in the Midwest but can occur anywhere in the country. This encephalitis accounts for most cases in children. Many cases are mild and undiagnosed. Fewer than 1% of infected people die from it. Eastern equine encephalitisoccurs predominantly in the eastern United States. A few cases have occurred in the Great Lakes region. Eastern equine encephalitis affects mainly young children and people older than 55. In children younger than 1 year, it can cause severe symptoms and permanent nerve or brain damage. Over half of infected people die. West Nile encephalitis,once present only in Europe and Africa, first appeared in the New York City area in 1999. It has spread throughout the United States. Several species of birds can be infected with the virus when they are bitten by an infected mosquito. This encephalitis affects mainly older people. This virus also causes a milder infection called West Nile fever, which is much more common. West Nile encephalitis develops in fewer than 1% of people who develop West Nile fever. About 9% of people with West Nile encephalitis die. However, those who have only West Nile fever usually recover fully. St. Louis encephalitisoccurs mostly in urban areas of central and southeastern states of the United States but also in western states. Infection is more common in the summer and is more likely to affect the brain in older people. Epidemics once occurred about every 10 years but are now rare. Western equine encephalitiscan occur throughout the United States but, for unknown reasons, has largely disappeared since 1988. It can affect all age groups but is more severe and more likely to affect the brain in children younger than 1 year. A few types of encephalitis are spread by ticks. They include Tick-borne encephalitisoccurs in Northern Asia, Russia, and Europe. The infection usually causes a mild flu-like illness that clears up within a few days, but some people, usually those age 50 or older, develop more severe symptoms. Because many cases occur in Europe and Russia, a vaccine is available there. Powassan virus infectionoccurs mainly in Canada and in the Great Lakes region and northeastern United States. Powassan virus has also caused cases of encephalitis in Russia. The virus is similar to the one that causes tick-borne encephalitis in Europe. Powassan virus infection usually causes mild or no symptoms. However, the infection can also cause severe encephalitis with headache, vomiting, seizures, loss of coordination, speech problems, or coma. About 10% of people with severe encephalitis die. Powassan virus is spread by the deer tick, which also transmits Lyme disease. In Lyme disease, the tick must be attached for 24 to 48 hours to spread disease. In contrast, Powassan virus infection may be transmitted if an infected tick is attached for as little as 15 minutes. The vaccine that is effective against tick-borne encephalitis in Europe and Russia is not effective against the Powassan virus. Colorado tick feveroccurs in areas of western United States and Canada that are 4,000 to 10,000 feet above sea level. Colorado tick fever causes a flu-like illness. Occasionally, people with Colorado tick fever develop meningitis or encephalitis. Colorado tick fever rarely causes death. It is rarely transmitted by blood transfusion. Several viruses that cause encephalitis were once present in only a few parts of the world but now are spreading, probably because travel has increased. These viruses include Chikungunya virus Japanese encephalitis virus Venezuelan equine encephalitis Zika Virus Infection None All are spread by mosquitoes. TheChikungunya viruswas first identified in Africa but has spread to Southeast Asia, India, China, some parts of Europe, the Caribbean, and Central, South, and North America. Most people with chikungunya disease feel better within a week. However, chikungunya disease can lead to severe encephalitis and even death, especially in infants and people over age 65. TheJapanese encephalitis virusis a common cause of encephalitis in Asia and the western Pacific. In the United States, Japanese encephalitis occurs only in travelers who acquired the virus in areas of the world where the virus is common. Venezuelan equine encephalitisoccurs mainly in parts in South and Central America. The Venezuelan equine encephalitis virus caused an epidemic of encephalitis in Texas in 1971 but now rarely causes encephalitis in the United States. It occurs mainly in travelers returning from areas where the virus is common. was first identified in the Zika Forest of Uganda, then spread to the South Pacific islands, then South America, Central America, the Caribbean, Mexico, and Florida. Zika infection may cause fever, joint and muscle aches, headache, and a red, bumpy rash. Having Zika virus infection during pregnancy can cause Microcephaly and severe brain damage in the baby. In the United States, the most common cause of sporadic viral encephalitis is Herpes Simplex Virus (HSV) Infections type 1, the same virus that causes cold sores. This encephalitis occurs at any time of the year and is often fatal if not treated. Rabies is a significant cause of encephalitis in developing countries and still causes a few cases of encephalitis in the United States. Human Immunodeficiency Virus (HIV) Infection (HIV) causes a slowly developing brain infection, resulting in HIV-associated encephalopathy (also called Encephalitis can result from reactivation of a virus, including Herpes simplex virus type 1 Varicella zoster virus (the virus that causes Chickenpox ) JC virus (which causes a usually fatal disorder called Progressive Multifocal Leukoencephalopathy (PML) —common among people who have AIDS or other conditions that impair the immune system) The virus that causes measles (which, if reactivated, leads to a usually fatal disorder called Subacute Sclerosing Panencephalitis (SSPE) years after measles occurs) Reactivation can occur long after people have the infection. A reactivated infection can severely damage the brain. After people have certain viral infections or get certain vaccines, the body’s immune system sometimes attacks the layers of tissue that wrap around nerve fibers (called the Overview of Demyelinating Disorders ) in the brain and spinal cord—an An autoimmune encephalitis may also develop if the immune system produces antibodies that attack proteins on the surface of nerve cells calledN-methyl-d-aspartate (NMDA) receptors. The resulting encephalitis is called anti-NMDA receptor encephalitis. Some evidence suggests that anti-NMDA receptor encephalitis is a more common type of encephalitis than was previously thought. It occasionally develops after encephalitis due to the herpes simplex virus, even when that encephalitis was successfully treated. Rarely, people with COVID-19 develop what appears to be encephalitis. (COVID-19 is caused by the newly identified severe acute respiratory syndrome coronavirus, or SARS-CoV2.) This encephalitis may be caused by the virus that enters and affects the brain. But it may also be an autoimmune or a partly autoimmune reaction. 