Medically Reviewed By William C. Lloyd III, MD, FACS

What is encephalitis?

Encephalitis is a potentially serious disease marked by irritation and inflammation of the brain. This swelling or inflammation is called cerebral edema and can lead to the destruction of nerve cells, bleeding into the brain (intracerebral hemorrhage), and brain damage.

While encephalitis can be a mild disease, people most at risk of developing a serious case of encephalitis include infants, young children, the elderly, and people with weakened immune systems. Encephalitis is rare in the United States.

Encephalitis is most often caused by viral infections. You can be exposed to viruses and other pathogens that cause encephalitis in various ways. Viruses, bacteria and parasites can be transmitted by breathing in air droplets from an infected person, swallowing contaminated food or drink, or making person-to-person contact. Encephalitis can result from viruses that cause childhood infections, such as measles and mumps. These illnesses used to be very common, but are less frequent now because of childhood immunizations. Pathogens can also be transmitted by a bite from a rabid animal or from a tick, mosquito, or other blood-sucking insect.

Once any of these pathogens enter your body, they can spread to the bloodstream where they are carried to the nervous system. In the nervous system, they multiply and cause infection and inflammation of the brain. Other causes of encephalitis include autoimmune disorders, allergic reactions to vaccines, and certain cancers.

Typical symptoms of encephalitis include a low-grade fever, mild headache, neck stiffness, low energy, and poor appetite. Treatment of encephalitis varies depending on the type and severity of encephalitis. Almost all people with mild cases of encephalitis will recover, but more severe cases of encephalitis can be life threatening if they are not quickly diagnosed and treated.

Encephalitis is a disease that can cause serious complications, such as problems with vision, memory and speech, and can be life threatening in some cases. Seek prompt medical care if you have been exposed to someone who has encephalitis.

Seek immediate medical care (call 911) if you, or someone you are with, have symptoms of encephalitis, such as fever, severe headache, change in consciousness, hallucinations, seizure, muscle weakness or paralysis, changes in mental function, and changes in speech, hearing or vision.

What are the symptoms of encephalitis?

Symptoms of encephalitis vary depending on the type of encephalitis and individual factors. Cases can range from mild to severe, with mild cases resembling a cold or the flu. Symptoms of mild cases of encephalitis can also mimic symptoms of other diseases, such as influenza or meningitis.

Common symptoms of encephalitis

Common symptoms of mild cases of encephalitis include:

  • Generally feeling ill
  • Low-grade fever
  • Neck stiffness
  • Poor appetite

    Other symptoms of encephalitis

    Other symptoms that are common to a wide variety of conditions and may also occur with encephalitis include:

    • Altered mentation
    • Sensitivity to light
    • Vomiting

      Because these vague symptoms may be easily confused with other conditions, it is important to contact your health care provider for proper diagnosis.

      Symptoms that might indicate a serious of life-threatening condition

      In some cases, symptoms of encephalitis progress rapidly and can lead to irreversible brain damage or death within days. Seek immediate medical care (call 911) if you, your child, or someone you are with, have any of these symptoms:

      • Change in consciousness, such as loss of consciousness (passing out) and unresponsiveness

      • Change in mental status or sudden behavior change, such as confusion, delirium, lethargy, hallucinations or delusions

      • Muscle weakness or paralysis

      • Problems with walking, talking, hearing, speech or vision

      • Seizure

      • Severe headache

      • Stiff neck or back

      • Sudden memory loss

      Symptoms of encephalitis in infants

      Recognizing encephalitis in infants can be difficult. Seek immediate medical care (call 911) if your baby has the following symptoms:

      • Body stiffness or limpness

      • Constant crying that may worsen when the infant is moved or picked up. Crying may be combined with poor feeding and projectile vomiting.

      • Full or bulging soft spot (bulging fontanel)

      What causes encephalitis?

      In the United States, the most common cause of encephalitis is a viral infection by enteroviruses, herpes simplex viruses, or arboviruses. In most cases, these viruses result in a cold-like or mild illness, but once in the body, these pathogens can spread to the bloodstream where they are carried to the nervous system. In the nervous system, they multiply and cause infection and inflammation of the brain.

      Common causes of encephalitis

      Common causes of encephalitis include:

      • Arboviruses and bacteria are transmitted by bites from blood-sucking insects, such as mosquitoes and ticks. Arboviruses include West Nile virus, Powassan encephalitis, St. Louis encephalitis, and Eastern equine encephalitis virus. Bacteria that cause Lyme disease and Rocky Mountain spotted fever can also be spread by insect bites. Similarly, the rabies virus can be transmitted by bites from infected animals.

      • Enteroviruses are very common viruses that spread by hand-to-mouth contact, coughing, and contact with fecal matter of an infected person (such as changing the diaper of a baby infected with the virus). Enterovirus infections normally cause upper respiratory symptoms or flu-like symptoms.

      • Herpes viruses are transmitted through contact with the herpes rash or sores as well as other close contact and sexual intercourse. Herpes viruses include varicella zoster (the virus that causes chickenpox), Epstein-Barr virus (the virus that causes mononucleosis), herpes simplex 1 (the virus that causes cold sores), and herpes simplex 2 (the virus that causes genital herpes). Acyclovir is effective against herpes simplex and is promptly administered in suspected herpes virus cases.

      Less common causes of encephalitis

      There are other less common infectious causes of encephalitis, such as cytomegalovirus, which usually do not cause disease unless a person has a weakened immune system. Similarly, parasitic infections, such as amoebiasis and toxoplasmosis, are more likely to occur in a person with a weakened immune system. Other less common bacterial causes include syphilis and tuberculosis.

      Other causes of encephalitis include:

      • Autoimmune disorders

      • Certain cancers

      • Reactions to certain vaccines

      What are the risk factors for encephalitis?

      Encephalitis can occur in any age group or population. However, infants, young children, the elderly, and people with weakened immune systems are at highest risk of developing encephalitis. A number of factors increase the risk of catching encephalitis, although not all people with risk factors will develop encephalitis. Risk factors for encephalitis include:

      • Crowded conditions, such living or working in military barracks, prisons, refugee camps, day care centers, or college dormitories

      • Exposure to a person with encephalitis

      • Exposure to blood-sucking insects that may be carriers of disease

      • Poor hygiene, such as not washing your hands frequently and thoroughly, or sharing unwashed drinking glasses, water bottles, dishware, or personal items, such as lip balm and toothbrushes

      Reducing your risk of encephalitis

      You can lower your risk of catching or spreading encephalitis by :

      • Avoiding contact with a person who has encephalitis

      • Avoiding touching the eyes, nose and mouth, which can transmit pathogens from the hands into the body

      • Covering the mouth and nose with the elbow (not the hand) or a tissue when sneezing or coughing

      • Getting vaccinated as recommended by your health care provider to prevent encephalitis. These include vaccines for chickenpox, measles, shingles, and certain vaccines that are recommended when traveling to developing countries.

      • Guarding against insect bites by wearing long-sleeved shirts and pants, applying insect repellent, and removing areas of standing water. Check with your pediatrician before applying insect repellent to an infant or young child.

      • Not sharing unwashed drinking glasses, water bottles, dishware, or personal items, such as lip balm and toothbrushes

      • Vaccinating animals

      • Washing hands with soap and water for at least 15 seconds several times throughout the day and after contact with a person who has encephalitis

      How is encephalitis treated?

      Treatment of encephalitis varies depending on the type of encephalitis, your age, and other factors. General treatment of all forms of encephalitis includes easing symptoms and keeping your body well hydrated so you can keep your body strong and get the rest you need to recover. Treatment includes:

      • Acetaminophen (Tylenol) or ibuprofen (Motrin, Advil) and other medications as recommended to relieve headache, pain and fever

      • Close monitoring and possibly treatment of people who have had close contact with a person with encephalitis, even if there are no symptoms

      • Fluid administration by mouth or intravenously, if needed for dehydration

      • Rest

      Treatment of severe cases of encephalitis

      A severe case of encephalitis is a medical emergency because it can rapidly result in permanent, serious complications and death within days. Hospitalization in an intensive care setting is generally required and treatment may include:

      • Anticonvulsants to prevent seizures

      • Corticosteroids and osmotic agents to reduce brain swelling

      • Immediate administration of intravenous antibiotics if bacterial encephalitis is suspected

      • Immediate administration of intravenous antiviral medications for viral encephalitis (herpes simplex susceptible to antivirals)

      • Life-support measures if necessary

      • Sedatives to help a person rest and reduce restlessness and agitation, which will minimize increased pressure in the brain (intracranial pressure)

        What are the potential complications of encephalitis?

        People with mild cases of encephalitis usually recover fully within two to four weeks with appropriate rest and supportive treatment. However, encephalitis, especially severe cases, can lead to serious and life-threatening complications. In some cases, death can occur in a matter of days. You can help minimize your risk of complications by following the treatment plan you and your health care professional design specifically for you. Complications of encephalitis include:

        • Coma

        • Memory loss

        • Permanent behavioral and cognitive changes

        • Permanent neurological damage, such as blindness, hearing loss, brain damage, or paralysis

        • Seizures

        Was this helpful?
        1. Encephalitis. Medline Plus, a service of the National Library of Medicine National Institutes of Health. http://www.nlm.nih.gov/medlineplus/ency/article/001415.htm.
        2. Encephalitis. The Nemours Fondation. http://kidshealth.org/teen/infections/bacterial_viral/encephalitis.html#.
        3. Meningitis and Encephalitis Fact Sheet. National Institute of Neurological Disorders and Stroke. http://www.ninds.nih.gov/disorders/encephalitis_meningitis/detail_encephalitis_meningitis.htm.
        4. Collins RD. Differential Diagnosis in Primary Care, 5th ed. Philadelphia: Lippincott, Williams & Williams, 2012.
        5. Tierney LM Jr., Saint S, Whooley MA (Eds.) Current Essentials of Medicine (4th ed.). New York: McGraw-Hill, 2011
        Medical Reviewer: William C. Lloyd III, MD, FACS
        Last Review Date: 2021 Jan 18
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