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Symptoms and Signs of Encephalitis

Doctor's Notes on Encephalitis

Encephalitis is inflammation of the brain. Encephalitis is not the same as meningitis, which is inflammation of the membranes covering the brain. Both of the diseases may be present at the same time, and both conditions share many of the same symptoms so they may be difficult to distinguish. Causes of encephalitis include viruses, bacteria, parasites, chemicals, and even autoimmune reactions.

Symptoms of encephalitis may last for two to three weeks, are flu-like, and may include fever, fatigue, muscle weakness, rhythmic muscle contractions, muscle pain, sore throat, stiff neck and back, loss of appetite, nausea, vomiting, headache, confusion, irritability, unsteady gait, weakness, problems with coordination, drowsiness, and visual sensitivity to light. Symptoms of encephalitis in infants may also include poor feeding, irritability, vomiting, bulging fontanel, and body stiffness. Symptoms of severe cases of encephalitis may include seizures, muscle weakness, paralysis, memory loss, impaired judgment, delirium and/or hallucinations, disorientation, poor responsiveness, or altered level of consciousness

Medical Author: John P. Cunha, DO, FACOEP
Medically Reviewed on 3/11/2019

Encephalitis Symptoms

The signs and symptoms of encephalitis are the same for adults and children. Infants may have poor feeding, irritability, vomiting, bulging fontanel, and body stiffness; such symptoms in an infant always constitute a medical emergency.

  • Signs and symptoms may last for two to three weeks, are flu-like, and can include one or more of the following which in some individuals may become progressively worse and continue over time:
    • Fever
    • Fatigue, muscle weakness, rhythmic muscle contractions, muscle pain
    • Sore throat
    • Stiff neck and back
    • Loss of appetite
    • Vomiting and nausea
    • Headache
    • Confusion
    • Irritability
    • Unsteady gait, weakness
    • Problems with coordination
    • Drowsiness
    • Visual sensitivity to light
  • More severe cases may involve these signs and symptoms:
    • Seizures
    • Muscle weakness
    • Paralysis
    • Memory loss
    • Sudden impaired judgment
    • Delirium and/or hallucinations
    • Disorientation
    • Poor responsiveness or altered level of consciousness

Encephalitis Causes

  • Herpes simplex (HSV): This type of virus causes cold sores of the mouth and lesions of the genitals. HSV is transmitted directly through human contact. Newborns can also get the virus by passing through an infected birth canal. Once inside the body, the virus travels through nerve fibers and can cause an infection of the brain. The virus may also undergo a period of latency in which it is inactive. At a later time, emotional or physical stress can reactivate the virus to cause an infection of the brain. It causes the most subacute (between acute and chronic) and chronic (lasting three or more months) encephalitis infections in humans.
  • Arbovirus: Hosts are animals such as birds, pigs, chipmunks, and squirrels that carry the virus. Mosquitoes (known as vectors, or ways of transmitting the virus) feed on these animals and become infected. The virus grows and cycles between the hosts and the vectors. Humans become infected through mosquito bites. Once inside the body, the virus replicates and travels in the bloodstream. If there is a large enough amount of the virus, the brain can become infected. The majority of cases occur between June and September when the mosquitoes are most active. In warmer climates, the disease can occur year-round.
    • West Nile virus (WNV): This virus was first isolated from an adult woman with a fever in the West Nile District of Uganda in 1937. The nature of the virus was studied in Egypt in the 1950s. In 1957, as a result of an outbreak in Israel in the elderly, the WNV became recognized as a cause for severe inflammation of the spinal cord and brain in humans. In the early 1960s, it was first noted that horses were becoming ill in Egypt and France. This virus then emerged in North America in 1999, with encephalitis reported in humans and horses.
      • The virus cycles between the Culex mosquito and hosts such as birds, horses, cats, bats, chipmunks, skunks, squirrels, and domestic rabbits. The mosquito feeds on the infected hosts, carries the virus in its salivary glands, and then passes it on to humans or other animals during a blood meal. It usually takes three to 15 days from the time of infection to the onset of disease symptoms. In the U.S., crows became a major host, but the disease would kill the crows; as a result, the number of infections dropped dramatically as the crow population died off.
      • West Nile encephalitis is not transmitted from person to person (such as through touching or kissing or from a health-care worker caring for a sick person) nor can it be passed from animal to human. Blood transfusions are the exception; the virus may be passed among people by blood transfusions if the donor is infected.
      • The chance that people will become severely ill from a mosquito bite is extremely small. According to the Centers for Disease Control and Prevention (CDC), even in areas where mosquitoes are reported to carry the virus, much fewer than 1% of them are infected. Furthermore, less than 1% of the people who get bitten and become infected will become severely ill. Therefore, the majority of cases are mild, and people can fully recover. For example, the CDC reported 690 people infected in 2011, with a total of 43 deaths.
      • Prognosis is usually guarded in the extremes of age (infants, young children, and the elderly). Death rates as a result of West Nile encephalitis range from 3%-15% and are highest among the elderly. At the present time, there is no documented evidence to suggest that a pregnancy is at risk due to WNV infection. It is assumed that if a person contracts WNV, he or she will develop a natural immunity that is lifelong. However, it may wane in later years. Currently, there is no commercially available vaccine for humans.
      • A relative of West Nile virus, Powassan virus, was discovered in 1958, but its vector is the blacklegged (deer) tick. It is very rare; only about 60 individuals have been diagnosed since 1958. There is no vaccine available. It has a 10% death rate.
  • La Crosse encephalitis: The first case occurred in La Crosse, Wisconsin, in 1963. Since then, the largest number of cases has been identified in woodland areas of the midwestern and mid-Atlantic United States. This virus is the most common cause of mosquito-borne encephalitis in children younger than 16 years of age. Each year, about 75 cases are reported to the Centers for Disease Control and Prevention (CDC). The virus cycles between the daytime-biting treehole mosquito (Aedes triseriatus) and hosts such as chipmunks and squirrels. Some investigators consider the cause of California encephalitis to be the La Crosse virus. The La Crosse encephalitis virus can cause adverse effects on IQ and school performance. About 80-100 people are diagnosed each year in the U.S., and 1% of people with this infection may die.
  • St. Louis encephalitis: Since 1964, an average of about 102 people is reported infected per year. Outbreaks can occur throughout most of the United States, although large urban epidemics have occurred in the midwestern and southeastern regions. The last major epidemic of St. Louis encephalitis occurred in the Midwest from 1974-1977. There were 2,500 cases in 35 states reported to the CDC. Additionally, there were 20 reported cases in New Orleans in 1999. The virus cycles between birds and the Culex mosquitoes breeding in stagnant water. It grows in both the mosquito and the bird but does not make either one sick. Only the infected mosquito can transmit the disease to humans during the blood meal. The virus cannot be transmitted from person to person through kissing or touching nor can it be transmitted from the infected bird. The disease tends to affect mostly adults and is generally milder in children.
  • 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.

Viral Infection Types, Treatment, and Prevention Slideshow

Viral Infection Types, Treatment, and Prevention Slideshow

Viruses are small particles of genetic material (either DNA or RNA) that are surrounded by a protein coat. Some viruses also have a fatty "envelope" covering. They are incapable of reproducing on their own. Viruses depend on the organisms they infect (hosts) for their very survival. Viruses get a bad rap, but they also perform many important functions for humans, plants, animals, and the environment. For example, some viruses protect the host against other infections. Viruses also participate in the process of evolution by transferring genes among different species. In biomedical research, scientists use viruses to insert new genes into cells.

When most people hear the word "virus," they think of disease-causing (pathogenic) viruses such as the common cold, influenza, chickenpox, human immunodeficiency virus (HIV), and others. Viruses can affect many areas in the body, including the reproductive, respiratory, and gastrointestinal systems. They can also affect the liver, brain, and skin. Research reveals that that viruses are implicated in many cancers as well.


Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.