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Encephalitis is inflammation of the brain. Myelitis refers to inflammation of the spinal cord. When both the brain and the spinal cord are involved, the condition is called encephalomyelitis.

Infections and other disorders affecting the brain and spinal cord can activate the immune system, which leads to inflammation. This inflammation can produce a wide range of symptoms and, in extreme cases, cause brain damage, stroke, or even death.

Individuals with encephalitis often show mild flu-like symptoms. In more severe cases, people may experience:

  • Problems with speech or hearing
  • Double vision
  • Hallucinations
  • Personality changes
  • Loss of consciousness
  • Loss of sensation in some parts of the body
  • Muscle weakness
  • Partial paralysis in the arms and legs
  • Impaired judgment
  • Seizures
  • Memory loss

Important signs of encephalitis to watch for in an infant include fever, lethargy, not waking for feedings, vomiting, body stiffness, unexplained/unusual irritability, and a full or bulging fontanel (the soft spot on the top of the head).

Who is more likely to get encephalitis?

Anyone—from infants to older adults—can get encephalitis. People with weakened immune systems, including those persons with HIV or those taking immunosuppressant drugs, are at increased risk.

Some forms of encephalitis are contagious and can be spread through contact with:

  • Saliva
  • Nasal discharge
  • Feces
  • Respiratory and throat secretions (often spread through kissing, coughing, or sharing drinking glasses, eating utensils, or such personal items as toothbrushes, lipstick, or cigarettes)

For example, people sharing a household, at a day care center, or in a classroom with an infected person can become infected. College students living in dormitories—in particular, college freshmen—have a higher risk of contracting meningococcal meningitis than college students overall.

Because the disease can occur suddenly and progress rapidly, anyone who is suspected of having encephalitis should immediately contact a doctor or go to the hospital.

Causes of encephalitis. Infectious causes of encephalitis include bacteria, viruses, fungi, and parasites. For some individuals, environmental exposure (such as a parasite), recent travel, or an immunocompromised state (such as HIV, diabetes, steroids, chemotherapy treatment) are important risk factors. There are also non-infectious causes such as autoimmune/rheumatological diseases and certain medications.

Up to 60 percent of cases of encephalitis remain undiagnosed. Several thousand cases are reported each year, but many more may occur since the symptoms may be mild to non-existent in most individuals.

Most diagnosed cases in the U.S. are caused by herpes simplex virus types 1 and 2, arboviruses (such as the West Nile Virus), which are transmitted from infected animals to humans through the bite of an infected tick, mosquito, or other blood-sucking insect, or enteroviruses. Lyme disease, a bacterial infection spread by tick bite, occasionally causes meningitis, and very rarely encephalitis. Rabies virus, which is transmitted by bites of rabid animals, is an extremely rare cause of human encephalitis.

Herpes simplex encephalitis (HSE) is responsible for about 10 percent of all encephalitis cases, with a frequency of about two cases per million people per year. More than half of untreated cases are fatal. About 30 percent of cases result from the initial infection with the herpes simplex virus; the majority of cases are caused by reactivation of an earlier infection. Most people acquire herpes simplex virus type 1 (the cause of cold sores or fever blisters) in childhood.

HSE due to herpes simplex virus type 1 can affect any age group but is most often seen in persons under age 20 or over age 40. This rapidly progressing disease is the single most important cause of fatal sporadic encephalitis in the country. Symptoms can include headache and fever for up to five days, followed by personality and behavioral changes, seizures, hallucinations, and altered levels of consciousness. Brain damage in adults and in children beyond the first month of life is usually seen in the frontal lobes (leading to behavioral and personality changes) and temporal lobes (leading to memory and speech problems) and can be severe.

Type 2 virus (genital herpes) is most often transmitted through sexual contact. Many people do not know they are infected and may not have active genital lesions. An infected mother can transmit the disease to her child at birth, through contact with genital secretions. In newborns, symptoms such as lethargy, irritability, tremors, seizures, and poor feeding generally develop between four and 11 days after delivery.

Four common forms of mosquito-transmitted viral encephalitis are seen in the United States:

  1. Equine encephalitis affects horses and humans
    • Eastern equine encephalitis also infects birds that live in freshwater swamps of the eastern U.S. seaboard and along the Gulf Coast. In humans, symptoms are seen within four days or up to two weeks following transmission and include sudden fever, general flu-like muscle pains, and headache of increasing severity, followed by coma and death in severe cases. About half of infected individuals die from the disorder. Fewer than 10 human cases are seen annually in the United States.
    • Western equine encephalitis is seen in farming areas in the western and central plains states. Symptoms begin within five and up to 10 days following infection. Children, particularly those under 12 months of age, are affected more severely than adults and may have permanent neurologic damage. Death occurs in about three percent of cases.
    • Venezuelan equine encephalitis is very rare in the U.S. Children are at greatest risk of developing severe complications, while adults generally develop flu-like symptoms. Epidemics in Central and South America have killed thousands of people and left others with permanent, severe neurologic damage.
  2. LaCrosse encephalitis occurs in the upper midwestern states (Illinois, Wisconsin, Indiana, Ohio, Minnesota, and Iowa) but has been reported in the southeastern and mid-Atlantic regions of the country. Most cases are seen in children under the age of 16. Symptoms such as vomiting, headache, fever, and lethargy appear up to 10 days following infection. Severe complications include seizures, coma, and permanent neurologic damage. About 100 cases of LaCrosse encephalitis are reported each year.
  3. St. Louis encephalitis is most prevalent in temperate regions of the U.S. but can occur throughout most of the country. The disease is generally milder in children than in adults, with elderly adults at highest risk of severe disease or death. Symptoms typically appear within seven days and up to 10 following infection and include headache and fever. In more severe cases, confusion and disorientation, tremors, convulsions (especially in the very young), and coma may occur.
  4. West Nile encephalitis is usually transmitted by a bite from an infected mosquito but can also occur after transplantation of an infected organ or transfusions of infected blood or blood products. Symptoms are flu-like and include fever, headache, and joint pain. Some individuals may develop a skin rash and swollen lymph glands, while others may not show any symptoms. At highest risk are older adults and people with weakened immune systems.

Outside the U.S., Japanese encephalitis is one of the most common causes of encephalitis worldwide. It is widespread in Asia and transmitted by a mosquito. A vaccine is available so travelers to at-risk areas should discuss this with their healthcare provider.

Powassan encephalitis is rare but is the only well-documented tick-borne arbovirus in the U.S. and Canada. Symptoms are noticed seven to 10 days following the bite (most people do not notice tick bites) and may include headache, fever, nausea, confusion, partial paralysis, coma, and seizures.

It is also possible to develop encephalitis that has non-infectious or autoimmune causes. Some cases of encephalitis are caused by an autoimmune disorder that may in some instances be triggered by an infection (“post infectious”) or by a cancer—even one that is microscopic and cannot be found (so-called paraneoplastic neurological syndromes). NMDA-Receptor encephalitis is a type of autoantibody-mediated encephalitis and is being increasingly recognized; it was the most documented form of non-bacterial meningitis reported in the long-term study and follow-up of participants in the California Encephalitis project. Treatment involves immunosuppression and/or tumor removal if such a cause is found.

How is encephalitis diagnosed and treated?

Diagnosing encephalitis. Following a physical exam and medical history to review activities of the past several days or weeks (such as recent exposure to insects, ticks or animals, any contact with ill persons, or recent travel; preexisting medical conditions and medications), the doctor may order various diagnostic tests to confirm the presence of infection or inflammation. Early diagnosis is vital, as symptoms can appear suddenly and escalate to brain damage, hearing and/or speech loss, blindness, or even death.

Diagnostic tests include:

  • A neurological examination involves a series of physical examination tests designed to assess motor and sensory function, nerve function, hearing and speech, vision, coordination and balance, mental status, and changes in mood or behavior.
  • Laboratory screening of blood, urine, and body secretions can help detect and identify brain and/or spinal cord infection and determine the presence of antibodies and foreign proteins. Such tests can also rule out metabolic conditions that may have similar symptoms.
  • Analysis of the cerebrospinal fluid that surrounds and protects the brain and spinal cord can detect infections in the brain and/or spinal cord, acute and chronic inflammation, and other diseases. A small amount of cerebrospinal fluid is removed by a special needle that is inserted into the lower back and the fluid is tested to detect the presence of bacteria, blood, and viruses. The testing can also measure glucose levels (a low glucose level can be seen in bacterial or fungal meningitis) and white blood cells (elevated white blood cell counts are a sign of inflammation), as well as protein and antibody levels.

Brain imaging can reveal signs of brain inflammation, internal bleeding or hemorrhage, or other brain abnormalities. Two imaging procedures are routinely used to diagnose meningitis.

  1. Computed tomography (CT)
  2. Magnetic resonance imaging (MRI)

Additionally, electroencephalography (EEG) can identify abnormal brain waves by monitoring electrical activity in the brain.

Treating encephalitis. People who are suspected of having encephalitis should receive immediate, aggressive medical treatment. The disease can progress quickly and has the potential to cause severe, irreversible neurological damage.

Antiviral drugs used to treat viral encephalitis include acyclovir and ganciclovir. For most encephalitis-causing viruses, no specific treatment is available.

Autoimmune causes of encephalitis are treated with additional immunosuppressant drugs and screening for underlying tumors when appropriate. Acute disseminated encephalomyelitis, a non-infectious inflammatory brain disease mostly seen in children, is treated with steroids.

Anticonvulsants may be prescribed to stop or prevent seizures. Corticosteroids can reduce brain swelling. Affected individuals with breathing difficulties may require artificial respiration.

Once the acute illness is under control, comprehensive rehabilitation should include cognitive rehabilitation and physical, speech, and occupational therapy.

Prevention. People should avoid sharing food, utensils, glasses, and other objects with someone who may be exposed to or have the infection. People should wash their hands often with soap and rinse under running water.

To lessen the risk of being bitten by an infected mosquito or other arthropod, people should limit outdoor activities at night, wear long-sleeved clothing when outdoors, use insect repellents that are most effective for that particular region of the country, and rid lawn and outdoor areas of free-standing pools of water (where mosquitoes like to breed). Repellants should not be overapplied, particularly on young children and especially infants, as chemicals like DEET may be absorbed through the skin.

The outlook for individuals with encephalitis generally depends on the particular infectious agent involved, the severity of the illness, and how quickly treatment is given. In most cases, people with very mild encephalitis can make a full recovery, although the process may be slow.

In more serious cases, the disease can cause hearing and/or speech loss, blindness, permanent brain and nerve damage, behavioral changes, cognitive disabilities, lack of muscle control, seizures, and memory loss. These individuals may need long-term therapy, medication, and supportive care.

How can I or my loved one help improve care for people with encephalitis?

Consider participating in a clinical trial so clinicians and scientists can learn more about encephalitis and related disorders. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.

For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with encephalitis at

Where can I find more information about encephalitis?

Information may be available from the following resources:

Centers for Disease Control and Prevention (CDC)
Phone: 800-232-4636

HHV-6 Foundation
Phone: 888-530-6726


National Institute of Allergy and Infectious Disease (NIAID)
Phone: 301-496-5717 or 866-284-4107

Content source: Accessed June 22, 2023.

The information in this document is for general educational purposes only. It is not intended to substitute for personalized professional advice. Although the information was obtained from sources believed to be reliable, MedLink, its representatives, and the providers of the information do not guarantee its accuracy and disclaim responsibility for adverse consequences resulting from its use. For further information, consult a physician and the organization referred to herein.

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