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Encephalitis (cont.)

What Are Other Less Frequent Causes of Encephalitis?

  • Eastern equine encephalitis (EEE): According to the CDC, there have been confirmed cases in the United States of EEE since 1964 with a rate of about 0-21 diagnosed infections per year (average about six per year). This virus is found along the East and Gulf Coasts. The virus causes severe disease in horses, puppies, and birds such as pheasants, quails, and ostriches. In humans, flu-like symptoms develop four to 10 days after the bite of an infected mosquito. Usually, human illnesses are preceded by those in horses. EEE can cause death in 50%-75% of human infections; 90% of infected people have mild to severe impairments. Those who recover may suffer severe permanent brain damage such as mental retardation, seizures, paralysis, and behavior abnormalities.
  • Western equine encephalitis (WEE): This virus was isolated from the brain of a horse with encephalitis in California in 1930. The worst epidemic was in Canada and the western U.S. when over 300,000 horses and mules were diagnosed, along with over 3,300 humans in 1941. Since 1964, there have been at least 639 confirmed cases, but currently only a few per year are reported. However, it remains a cause of encephalitis in the western part of the United States and Canada. In 1994, there were two confirmed and several suspected cases of WEE reported in Wyoming. In 1997, 35 strains of WEE virus were isolated from mosquitoes collected in Scotts Bluff County, Nebraska. The WEE virus cycles between certain types of birds (small, mostly songbirds) and Culex tarsalis mosquitoes, a species associated with irrigated agriculture and stream drainage. The virus has also been found in several other mammals. Horses and humans become sick through bites by infected mosquitoes. Infants are particularly affected and can have permanent problems such as seizure disorders and developmental delay as a result of the infection. A vaccine is not available for humans. WEE is becoming more frequently encountered in the U.S.
  • Venezuelan equine encephalitis (VEE): This virus is found in Central and South America and is a rare cause of encephalitis in the southwestern part of the United States. It is an important cause of encephalitis in horses and humans in South America. From 1969-1971, an outbreak from South America to Texas killed over 200,000 horses. In 1995, there were an estimated 90,000 human infections with VEE in Columbia and Venezuela. The virus cycles between forest-dwelling rodents and mosquito vectors, especially the species Culex. VEE infection in humans is much less severe than that of WEE and EEE. While adults tend to develop a flu-like illness, children tend to develop overt encephalitis. Deaths are rare in humans but are common in horses. There is an effective vaccine for horses but none for humans.
  • Japanese encephalitis: This virus is responsible for 50,000 cases and 15,000 deaths per year. Most of China, Southeast Asia, and the Indian subcontinent are affected. The geographic distribution is expanding. Rarely, cases may appear in United States civilians and military personnel traveling to and living in Asia. Children and young adults are mostly affected. Older adults are affected when there are epidemics in new locations. The virus cycles between domestic pigs, wild birds, and Culex tritaeniorhynchus mosquitoes, which breed in rice fields. The disease is not transmitted through human contact, pigs, or birds. Only the mosquitoes can transmit the disease during feedings.
  • Zika virus: This virus is spread by the bite of an infected Aedes species mosquito; the virus can be passed from a pregnant woman to her fetus, and this has been associated with certain birth defects, including microcephaly, Guillain-Barré disease, and disseminated encephalomyelitis. A major outbreak of this disease began in 2015 and is ongoing in Brazil and has spread to other countries. Zika typically is spread via the mosquito vector, but transmission through sexual intercourse has also been documented.

The following is a short summary of the viruses that cause the majority of encephalitis infections, although they may also cause other diseases.

DiseaseGeographic LocationVector/ HostsComment
Herpes encephalitisUnited States/the worldHuman-to-human
Prompt treatment with acyclovir increases survival to 90%
West Nile encephalitisAfrica, West Asia, Middle East, United StatesMosquito/mostly birdsMajority are mild cases. Less than 1% of those infected will become severely ill. Full recovery is expected. A vaccine for humans is not commercially available.
Eastern equine
East Coast (from
Massachusetts to Florida),
Gulf Coast
Mosquito/birdsOften occurs in horses. High mortality rate (50%-75%); frequent outcomes (seizures, slight paralysis), especially in children
Western equine
Western United States and
Mosquito/birdsOften occurs in horses.
Particularly affects infants
Venezuelan equine
Western HemisphereMosquito/rodentsRare in United States; low mortality rate, rare after-effects
La Crosse encephalitisThroughout the United States, especially in midwestern & southeastern regionsMosquito/ chipmunks,
Most common cause of
encephalitis in children younger than 16 years of age
St. Louis encephalitisMidwestern & mid-Atlantic
United States
Mosquito/birdsMostly affects adults
Japanese encephalitisTemperate Asia, southern and southeastern AsiaMosquito/birds and pigsVaccine available for ages 17 and older. See
Prevention section.
High morbidity/mortality rates
Zika virusSouth America, Asia, Pacific Islands, Central AmericaMosquitoesBirth defects including microcephaly, neurologic damage

A special cause of viral encephalitis is HIV. This virus is mainly known for its damage to the human immune system. However, as HIV disease progresses, some individuals develop encephalitis symptoms termed AIDS dementia complex. It results in cognitive disorders (There is memory loss, abstract thinking and verbal fluency decline, and motor control may be markedly decreased.). Other causes of encephalitis are as follows but will not be further discussed in detail in this article; the reader is referred to the links provided:

  • Bacteria, such as N. meningitidis, and those that cause Lyme disease, syphilis, tuberculosis, and occasionally other bacteria such as Mycoplasma spp. have been implicated in a few individuals.
  • Fungi such as Candida, Mucormycosis, Cryptococcus, and others
  • Rabies virus
  • Parasites such as Toxoplasma (often seen in HIV-infected patients) or the parasite Naegleria
  • Allergies to vaccinations
  • Autoimmune disease such as Rasmussen's encephalitis
  • Cancers involving the brain tissue
  • Prion caused encephalitis (rare) such as bovine spongiform encephalitis or mad cow disease
  • Myalgic encephalitis or chronic fatigue syndrome (no defined cause)
  • Chemical encephalitis such as that seen with alcohol (Wernicke-Korsakoff syndrome) due to a decline in liver functions and ultimately, affecting the brain tissue, or by drug use

In recent years, researchers have begun more intensive studies of some types of encephalitis. In 2012, the CDC began a multicenter study of epidemic myalgic encephalomyelitis (or chronic fatigue syndrome or CFS) that is ongoing to better understand this problem.

Another cause of encephalitis under study is encephalitis believed to be caused by an autoantibody attack on subunits of brain N-methyl-d-aspartate (NMDA) glutamate receptors. The autoantibodies are termed anti-NMDA receptor antibodies, and the disease is termed NMDA receptor encephalitis, first identified in 2007. The disease is found mainly in young women (over 80%) and has been associated with ovarian teratomas (germ cell tumors). Some investigators think it may have been seen previously as an outbreak of encephalitis of unknown cause termed (epidemic) encephalitis lethargica that occurred worldwide between 1918 and 1928. Recent research suggests that the anti-NMDA receptor symptoms (seizures, unresponsiveness, motor-control problems, and others) caused by this autoimmune disease can be treated with immunotherapy to reduce or halt symptoms in some patients. Some individuals believe this disease is related to autism but currently no convincing evidence has shown such a relationship.

Medically Reviewed by a Doctor on 8/5/2016

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