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What is encephalitis?

When infections or underlying medical conditions affecting the brain and spinal cord activate the immune system, this can lead to inflammation. Inflammation itself can cause a wide range of symptoms and, in extreme cases, can lead to brain damage, stroke, or even death.

Inflammation of the brain is called encephalitis.

Symptoms of encephalitis

People with encephalitis often show mild flu-like symptoms such as fever, fatigue, headache, or body aches. However, many people do not have any symptoms of encephalitis. In other cases, people may experience:

  • Speech or hearing problems
  • Double vision or vision loss
  • Sudden and/or high fever
  • Hallucinations
  • Personality or behavior changes
  • Loss of consciousness
  • Loss of sensation in some parts of the body
  • Alterations in thinking
  • Neck rigidity (stiffness)
  • Disturbances in motor function, including uncontrollable jerking movements of the head, trunk, or limbs (called myoclonic jerks)
  • Partial paralysis in the arms and legs
  • Impaired judgment
  • Tremors
  • Seizures
  • Memory loss
  • Coma

    Encephalitis can be dangerous in infants. Watch for fever, lethargy (weakness or drowsiness), poor feeding, vomiting, body stiffness, unexplained/unusual irritability or crying, and a full or bulging fontanel (the soft spot on the top of the head).

    Because people may have subtle symptoms of encephalitis, many cases may go undiagnosed. Several thousand cases are reported each year, but many more may occur since the symptoms may be mild at onset in some people.

    Who is more likely to get encephalitis?

    Anyone at any age can get encephalitis. People with weakened immune systems, including people living with HIV or 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 sharing toothbrushes or lipstick)

    For example, people sharing a household, at a day care center, or in a classroom with an infected person can become infected. 

    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

    Inflammation in the brain and spinal cord can be caused by infections such as viruses. Encephalitis can also be autoimmune when antibodies can attack brain cells. Infectious encephalitis is caused either by an infection invading the brain or a reactivation of a previous infection that damages the brain. Encephalitis can also arise when a virus or vaccine triggers a reaction that makes the immune system attack health brain tissue by mistake. This is referred to as autoimmune encephalitis. Autoimmune encephalitis can also be triggered by tumors or be primary. Although rare, there have been cases of post-infectious autoimmune encephalitis (where it is triggered by an initial response to an infectious agent).

    Infectious encephalitis

    Infectious causes of encephalitis include bacteria, viruses, fungi, and parasites. For some people, environmental exposure (such as a parasite), recent travel, or an immunocompromised state (such as HIV, diabetes, steroids, or chemotherapy treatment) are important risk factors.

    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.
    • Enteroviruses, a very common type of virus that mostly causes mild illness or respiratory infection and usually occurs in the summer and fall. 

    Some types of encephalitis include:

    Acute disseminated encephalomyelitis

    Acute disseminated encephalomyelitis (ADEM) causes an attack of inflammation in the brain and spinal cord that damages myelin. Myelin is the protective coating over nerves that helps with electrical nerve signaling. ADEM usually appears following a viral or bacterial infection. In very rare cases, ADEM can appear following a vaccination. A common cause of ADEM is myelin oligodendrocyte (MOG) antibody. It is sometimes misdiagnosed as the first severe attack of multiple sclerosis, since the symptoms and damage to the white matter may be similar. ADEM can affect anyone, but mostly affects young children.  Treatment is targeted at suppressing the immune system and reducing inflammation using anti-inflammatory drugs such as corticosteroids. Some people may need plasma exchange (plasmapheresis) or immunoglobulin therapy. Most people with ADEM begin to recover within days after treatment, and many will recover completely within six months. Rarely, ADEM can be fatal.

    Encephalitis lethargica

    Encephalitis lethargica is a rare disease with no known cause, but researchers suspect it may come from a virus. There was a pandemic of encephalitis lethargica (known then as “sleeping sickness”) from around 1916 to 1930, but no recurrence of the epidemic has since been reported. In some cases of encephalitis lethargica, people developed Parkinson’s disease after recovery—sometimes as long as a year after the illness. Levodopa and other drugs to treat Parkinson’s disease may be used to treat encephalitis lethargica, depending on the person’s symptoms. Progression of the disease varies depending on complications or other disorders the person may have.

    Herpes simplex encephalitis

    Herpes simplex encephalitis (HSE) is responsible for about 10% of all encephalitis cases. More than half of untreated cases are fatal. Most cases are caused by reactivation of an earlier infection with the herpes simplex virus, but some cases result from initial infection. Most people acquire herpes simplex virus type 1 (HSV-1, the cause of cold sores or fever blisters) in childhood.

    Cases of encephalitis due to HSV-1 can affect any age group, but is most often seen in people under age 20 or over age 40. This rapidly progressing disease is the single most important cause of fatal encephalitis. Symptoms 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 children 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 herpes simplex virus (HSV-2, 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 HSV-2 to her child at birth, through contact with genital secretions. In newborns, HSE symptoms such as lethargy, irritability, tremors, seizures, and poor feeding generally develop between four and 11 days after delivery.

    Mosquito-transmitted infectious encephalitis

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

    • Equine encephalitis affects both horses and humans.
      • Eastern equine encephalitis mostly occurs in the eastern U.S. with a few cases seen in the Great Lakes region. Only a few cases are seen annually in the United States. It mainly affects young children and adults over age 55. Symptoms are seen within four days or up to two weeks following transmission through the bite of an infected mosquito and include sudden fever, general flu-like muscle pains, and headache of increasing severity, followed by coma and death in severe cases. 
      • Western equine encephalitis is seen in farming areas in the western and central plains states. Symptoms begin within five to 10 days following infection. Children, particularly those under a year old, are affected more severely than adults and may have permanent neurologic damage. 
    • LaCrosse encephalitis typically occurs in the upper midwestern states but also has been reported in the southeastern and mid-Atlantic regions of the U.S. Most cases affect children under age 16. Symptoms, such as vomiting, headache, fever, and lethargy, appear up to 10 days following infection. Severe complications include seizures, coma, and permanent brain damage. About 100 cases of LaCrosse encephalitis are reported each year.
    • St. Louis encephalitis mostly commonly occurs in temperate regions of the U.S. The disease is generally milder in children than in adults, with older adults at highest risk of severe disease or death. Symptoms typically appear within seven to 10 days 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.
    • 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 people may develop a skin rash and swollen lymph glands, while others may not show any symptoms. Older adults and people with weakened immune systems are at highest risk of severe symptoms.

    Other causes of encephalitis, including the Zika virus, Japanese encephalitis, and Venezuelan equine encephalitis affect people in other parts of the world. Talk to your healthcare provider about potential risks if you travel outside the U.S.

    Tick-transmitted encephalitis  

    Some types of encephalitis are spread by ticks, including Tick-borne encephalitis (TBE virus) and Powassan virus.

    Subacute sclerosing panencephalitis

    Subacute sclerosing panencephalitis (SSPE) is a slow and persistent viral infection that affects the central nervous system (CNS). SSPE is related to measles and is considered a rare disease in countries where vaccination for measles is readily available and widely used. SSPE affects children and young adults. Most children with SSPE have a history of measles infection at an early age, usually younger than age 2, followed by a latent (symptom-free) period of six to eight years before neurological symptoms begin. Some children had asymptomatic measles infection before the age of first vaccination. Currently, there is no cure for SSPE, however measles vaccination can help prevent it. In advanced stages of SSPE, people may lose the ability to walk as their muscles stiffen or spasm. People with the disease will deteriorate physically until they fall into a coma and then a persistent vegetative state. People with SSPE usually die from fever, heart failure, or when the brain can no longer control the autonomic nervous system. In rare cases, people with SSPE may have a chronic, slowly progressive form, some with relapses and remissions.


    Autoimmune encephalitis

    Autoimmune encephalitis refers to a group of conditions that occur when the body’s immune system mistakenly attacks healthy brain cells, leading to inflammation of the brain. Some cases of autoimmune encephalitis are caused by certain medications or an autoimmune or rheumatological disorder that may in some instances be triggered by an initial response to an infection (post-infectious), such as post-herpetic NMDA receptor antibody encephalitis or anti-NMDA receptor encephalitis, or by a cancer—even one that is microscopic and cannot be found (so-called paraneoplastic neurological syndromes). Some types of autoimmune encephalitis include:

    COVID-19 encephalitis

    Although rare, some people with COVID-19 develop what appears to be encephalitis, which is caused by the SARS-CoV2 virus that enters and affects the brain or may be an autoimmune reaction to infection with the virus. Learn more about the nervous system and COVID-19.

    Limbic encephalitis

    Limbic encephalitis causes inflammation of the limbic system (the parts of the brain that control your emotions, behavior, motivation, and memory) and other parts of the brain. Symptoms of limbic encephalitis include confusion, memory loss, sleep disturbances, psychiatric symptoms (altered personality or behavior), and seizures. In many cases, it occurs when the immune system has a reaction to a cancerous tumor known as a neoplasm and can attack the brain. This is called paraneoplastic limbic encephalitis. Non-paraneoplastic limbic encephalitis is the term for limbic encephalitis without a neoplasm. Imaging studies, lab tests, and EEG (electroencephalography), which measures the electrical activity of the brain, can diagnose limbic encephalitis. Treatment involves medicines to prevent the immune system from attacking healthy cells and tissues by mistake (immunosuppressants) including corticosteroids, immunoglobulins (a blood product given through the vein), and plasma exchange (a machine separates the plasma (the liquid part of the blood) from the blood cells. Rituximab is a recent immune medication which targets cells in the body which produce antibodies.

    NMDA receptor antibody encephalitis

    NMDA receptor antibody encephalitis or anti-NMDA receptor encephalitis occurs when the immune system makes antibodies that mistakenly attack proteins on the surface of nerve cells called NMDA (N-methyl-D-aspartate) receptors. Antibodies, also called immunoglobulins, are proteins which identify and help remove viruses and bacteria. This type of encephalitis has become more common. It sometimes develops after encephalitis due to herpes simplex virus or a non-cancerous tumor which is thought to stimulate the production of NMDA receptor antibodies. NMDA receptor encephalitis mostly affects children and young adults, and more females than males. Symptoms include seizures, confusion, and memory loss, as well as psychiatric symptoms like hallucination, agitation, and mood changes. About 10 to 20 days later, affected people may develop a movement disorder with writhing and twisting of the face, arms, and legs, and experience changes in blood pressure, heart rate, and temperature and lose awareness. Doctors can perform an NMDA receptor antibody test in the blood and spinal fluid to diagnose the condition. Treatment involves medicines to prevent your immune system from attacking healthy cells and tissues by mistake (immunosuppressants) and/or removal of the tumor (if present).     

    Rasmussen’s encephalitis

    Rasmussen's encephalitis (RE) is a very rare, chronic inflammatory neurological disease that usually affects only one hemisphere (half) of the brain. It mostly affects children under age 10 but can also affect teens and adults. RE has features of an autoimmune disease in which immune system cells enter the brain and cause damage. Children with RE frequently enter a phase of permanent, but stable, neurological deficits after eight to 12 months. In teens and adults, the disease may continue to progress slowly over the person’s lifetime. Anti-epileptic drugs may be prescribed but may not entirely control seizures. Recent studies have shown some success with immunosuppressants, in particular, corticosteroids, intravenous immunoglobulin, and tacrolimus. Surgery to control seizures may be recommended, particularly in children where recovery potential is high. For some children, surgery may halt disease progression and stabilize seizures. However, most people with RE are left with some paralysis, cognitive deficits, and problems with speech. In some very rare cases, the disease can progress to the opposite brain hemisphere.


    How is encephalitis diagnosed and treated?

    Preventing encephalitis

    There are several ways to prevent encephalitis caused by viruses and infections. These are good practices to prevent the spread of many illnesses. Avoid sharing food, utensils, glasses, and other objects with someone who may be exposed to or have an infection. Wash your hands often with soap and rinse under running water.

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

    Children and adults should get routine vaccinations for viruses that can cause encephalitis, including measles, mumps, polio, rabies, rubella, and varicella (chickenpox). People who are traveling to places such as parts of Asia, where tick-borne and Japanese encephalitis are found, should receive specific vaccines. To prevent encephalitis caused by the rabies virus, avoid contact with wild animals and vaccinate pets.

    Diagnosing encephalitis

    To diagnose encephalitis, a healthcare provider will conduct a physical exam and medical history to review activities of the past several days or weeks. They may ask about recent exposure to insects, ticks or animals, any contact with sick people, or recent travel, preexisting medical conditions, and medications. Depending on the findings of the physical exam or medical history, the healthcare provider may order 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 and cerebrospinal fluid can help detect and identify brain and/or spinal cord infection and determine the presence of auto-antibodies. 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 (lumbar puncture) and the fluid is tested to detect the presence of bacteria, blood, and viruses. 

    Brain imaging can reveal signs of brain inflammation, internal bleeding, or other brain abnormalities. It also can help doctors rule out other disorders that can cause similar symptoms, such as stroke and a brain tumor. CT (computed tomography) and MRI (magnetic resonance imaging) are routinely used to diagnose encephalitis.

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

    Treating encephalitis

    Treatment for encephalitis includes antiviral drugs, antibiotics, corticosteroids, and other drugs, depending on the cause. For severe types of encephalitis that do not have a vaccine or medication, supportive care (rest, nutrition, and fluids) can help the body fight the infection and relieve symptoms.

    People with symptoms of encephalitis should seek immediate, aggressive medical treatment from a healthcare provider.

    Antiviral drugs such as acyclovir and ganciclovir can treat infectious encephalitis caused by a virus. However, for most encephalitis-causing viruses, there are no specific treatments available. Antibiotics can treat infectious encephalitis caused by bacteria. 

    Immunosuppressant drugs and screening for underlying tumors when appropriate can treat autoimmune encephalitis. Acute disseminated encephalomyelitis is treated with steroids and intravenous immunoglobulin (IVIG).

    Medications can also help treat some symptoms of encephalitis. Anticonvulsants can stop or prevent seizures. Corticosteroids can reduce brain swelling. If a person is irritable or restless, they may be prescribed a sedative. Or the person may take acetaminophen to relieve a fever or a headache. People having difficulty breathing may need artificial respiration.

    Once the acute illness, which normally lasts for 1 to 2 weeks, is under control, comprehensive rehabilitation should include cognitive rehabilitation and physical, speech, and occupational therapy if brain function is severely affected.

    The outlook for people with encephalitis generally depends on the type of encephalitis, the severity of the illness, and how quickly the person gets treatment. In most cases, people with very mild encephalitis that doesn’t last long can make a full recovery, although the process may be slow and take several months, and cognitive issues can persist.

    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, memory loss, or death. People with serious cases of encephalitis may need long-term therapy, medication, and supportive care. 

    What are the latest updates on encephalitis?

    NINDS, a component of the National Institutes of Health (NIH), the leading supporter of biomedical research in the world, is the primary federal funding agency on encephalitis and other neurological disorders. As part of its mission, NINDS conducts and funds research on encephalitis and other related disorders that affect the brain and nervous system. 

    NMDA receptor antibody encephalitis is one of the most common causes of autoimmune encephalitis. NINDS-supported researchers are studying inebilizumab, a promising therapeutic monoclonal antibody for the treatment of NMDA receptor antibody encephalitis to promote better long-term outcomes in the disorder, determine more meaningful cognitive endpoints, and identify better biologic biomarkers to predict outcome. The results of the trial will immediately impact patient care and facilitate the design and implementation of future clinical trials in people with autoimmune encephalitis.

    Autoimmune-mediated limbic encephalitis can lead to epilepsy. Prior research has emphasized paraneoplastic causes, in which the body’s immune response to a tumor cross-reacts with proteins normally expressed in the brain, which then become the target of an autoimmune attack. However, recent case reports are raising awareness of non-paraneoplastic autoimmune-mediated limbic encephalitis and seizures.

    The location of nasal neurons and their exposure to the outside environment make them an easy target for infection by airborne viruses, but viral respiratory infections rarely make their way from the olfactory bulb to the rest of the brain, where they could cause potentially fatal encephalitis. Using fluorescent microscopy, NINDS researchers found that a viral infection that started in the nose was halted right before it could spread from the olfactory bulb to the rest of the central nervous system. They showed that CD8 T cells, which are part of the immune system responsible for controlling viruses, can act as a front-line defense mechanism in protecting the brain after infection of nasal tissue. Additional tests showed that microglia, immune cells within the central nervous system, took on an underappreciated role of helping the immune system recognize the virus and did so in a way that limited the damage to neurons themselves.

    NINDS, along with other ICs, is promoting research on Myalgic Encephalitis/Chronic Fatigue Syndrome (ME/CFS) through support for Collaborative Research Centers that work independently and collaboratively to define the causes of, and develop improved treatments for, ME/CFS. The NIH Intramural Study on ME/CFS, an intensive study of a small number of people with post-infectious ME/CFS (PI-ME/CFS), revealed a number of biological changes in people with the condition that warrant further exploration in larger and varied cohorts. A working group of the National Advisory Neurological Disorders and Stroke (NANDS) Council has developed an ME/CFS Research Roadmap to guide future research on ME/CFS that will lead to better understanding and treatment. Next, the Trans-NIH ME/CFS Working Group, with representatives from NIH institutes, centers, and offices, will review the Roadmap and identify how best to implement research to move the suggestions forward.

    More information about encephalitis research supported by NINDS and other NIH Institutes and Centers can be found using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies. RePORTER also includes links to publications and resources from these projects.

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

    Learn About Clinical Trials
    Clinical trials are studies that allow us to learn more about disorders and improve care. They can help connect patients with new and upcoming treatment options.

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

    All types of participants 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 Clinicaltrials.gov.

    Where can I find more information about encephalitis?

    Information may be available from the following resources:

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