Encephalitis is an inflammation of the brain; its severity can range from mild to extremely serious. Often the inflammation also affects the membranes that cover the brain and spinal cord (the meninges). Such cases are called meningoencephalitis. The usual cause of encephalitis is a viral infection.

What Is Encephalitis?

Encephalitis is an inflammation * of the brain that may have several different causes. One of the most common causes is infection with one of several types of viruses. Depending on the type of virus or other cause, the condition may be mild to severe. Most cases of encephalitis are so mild that people do not realize they have it. A person, often a child, may have fever, headache, nausea, or sleepiness—symptoms much like those of the the flu—that go away on their own. In more severe cases, the viral infection destroys so many nerve cells (neurons) in the brain that it can lead to seizures * , breathing problems, personality changes, and coma * . When white blood cells arrive to fight the virus, they may cause brain tissues to swell, which can destroy neurons or lead to bleeding within the brain. Permanent brain damage or even death can result.


Illustration by Frank Forney. © 2016 Cengage Learning®.

Some types of encephalitis are caused by arboviruses (AR-buh-VY-ruhsez), which are viruses that are spread from animals to humans when such blood-sucking insects as ticks and mosquitoes bite infected birds, rodents, and other small animals, which then bite humans. These infected animals are called vectors, or carriers of the virus. In the United States, these vectors include the following:

In Asia, Japanese encephalitis virus is common, with 30,000 to 45,000 cases reported each year in the region, usually in rural and agricultural areas. According to the World Health Organization, the fatality rate among those who experience symptoms and live in areas without intensive care facilities is between 20 and 30 percent. In addition, 30 to 50 percent of those who survive may have lasting effects on the central nervous system, including seizures. A vaccine to prevent this disease is available in Japan, China, India, Korea, and Thailand, but its high cost sometimes makes it unavailable to people of limited financial means. The Japanese encephalitis virus is related to the St. Louis virus, and it infects mostly pigs, ducks, and wading birds.

Medical professionals report several thousand cases of encephalitis every year in the United States, but health officials believe that many more cases—mainly mild ones—probably occur. Serious encephalitis is particularly common in people with weakened immune systems * , such as those with AIDS * .

Many viruses and a few other microbes can cause encephalitis. One of the most common—and most dangerous—causes is infection with the herpes simplex * virus. This virus rarely infects the brain, but when it does, it can be life-threatening.

Encephalitis may develop in a person who has meningitis (meh-ninJY-tis), which is an inflammation of the membranes surrounding the brain and spinal cord, called the meninges (meh-NIN-jeez). In addition, encephalitis can be a complication of such other infectious diseases as rabies, cytomegalovirus infection * , listeriosis * , syphilis * , or Lyme disease * . In people with a weakened immune system, such as those with HIV/AIDS, infection by parasites can lead to encephalitis, especially the parasite that causes toxoplasmosis * .

Several arboviruses (viruses spread by insects) cause encephalitis in horses or other animals, and these arboviruses may be spread to humans by mosquitoes. In the United States, this route of contagion was once rare, with a few hundred cases in an average year. During the late 1990s, however, the spread of West Nile virus led to an increase in mosquito-borne encephalitis in humans. In 2015, 48 states and the District of Columbia reported West Nile virus infections in people, birds, or mosquitoes. Overall, 2,060 cases of West Nile virus in people have been reported to the Centers for Disease Control and Prevention as of January 2016. Of this number, 1,360 people (66 percent) were classified as having meningitis or encephalitis.

In some cases, encephalitis develops after a person has a viral illness such as measles, chickenpox, mononucleosis, mumps, or German measles (rubella). This condition is called postinfectious or parainfectious encephalitis. Medical professionals cannot always pinpoint exactly why a particular person has encephalitis.

The acute phase of the disease, the time when symptoms are most severe, usually lasts up to a week. Lingering symptoms can continue for weeks to months. Most people recover from encephalitis completely, but some experience swelling of the brain that leads to permanent brain damage. These patients may face such long-term complications as learning disabilities, seizures, speech problems, memory loss, lack of muscle control, paralysis * , or coma. In rare cases in which brain damage is severe, death can result. Infants younger than 12 months of age and adults older than 50 years have the greatest risk of permanent brain damage and death from encephalitis.

How Common Is Encephalitis?

Encephalitis is fairly rare, affecting approximately 1 in 200,000 people in the United States each year. The most common causes of viral encephalitis in the United States are enteroviruses * and arboviruses.

Is Encephalitis Contagious?

* , may disperse into the air when an infected person coughs or sneezes. Ticks spread Lyme disease, and humans can contract rabies * from the bite of infected animals including raccoons and bats.

How Do People Know They Have Encephalitis?

Symptoms of encephalitis range from mild to severe. They may appear suddenly and include fever, headaches, sensitivity to light, loss of appetite, and muscle pain. If the meninges are involved, the neck and back often feel stiff. In more serious cases, an individual may experience high fever, nausea (NAW-zee-uh), vomiting, confusion, double vision * , personality changes, problems with hearing and speech, hallucinations * , sleepiness, clumsiness, muscle weakness, loss of sensation, and irritability. In the most severe cases, seizures and loss of consciousness may occur. Encephalitis can be particularly dangerous in elderly people and in babies, who are more likely to experience permanent brain damage than adults. Any serious or severe symptoms require immediate medical attention.

How Do Doctors Diagnose and Treat Encephalitis?


To diagnose encephalitis, a doctor may use a number of techniques. Various blood tests may be ordered to look for encephalitis-related microorganisms in the blood or to determine whether a person's body is producing antibodies * in response to a specific virus or bacterium. For instance, a blood test for Lyme disease checks for antibodies against that particular encephalitis-causing microbe, which is a bacterium. Other tests used for diagnosis include the following:

■ A spinal tap, also called a lumbar puncture, is a procedure in which a sample of cerebrospinal fluid is taken from the area around the spinal cord to examine under a microscope for signs of infection.

■ One or more imaging tests using computed tomography * or magnetic resonance imaging * to look for signs of bleeding or swelling in the brain.

■ Samples with a cotton swab of fluids from the nose, throat, and rectum * may be acquired to test for the presence of certain viruses.

■ A brain biopsy * may be ordered, during which a small piece of brain tissue is removed with a needle so that it can be examined under a microscope.

These tests often cannot diagnose encephalitis positively, but they can help doctors rule out other possible causes of the symptoms. In many cases, doctors may never be able to identify the specific microbe causing the encephalitis.

Treatment * to treat such minor symptoms as fever and headache. A doctor may also prescribe antiviral medications that sometimes help prevent the spread of the virus and are important in the treatment of encephalitis caused by herpes simplex virus. Other medications may include anticonvulsants to help control seizures and corticosteroids (anti-inflammatory medications) to lessen swelling in the brain, which can lead to a dangerous increase in pressure within the skull.

Regional Forms of Encephalitis

Regional forms of encephalitis include the following:


Encephalitis may result from a variety of viruses, bacteria, and other organisms. These agents include the following:

If a patient is admitted to a hospital, medical professionals take special care to monitor body fluids. This information can help them prevent or control swelling of the brain. They may take various supportive measures, such as providing machines to help with breathing if needed, and caring for comatose patients to prevent bedsores * or other infections.

Can Encephalitis Be Prevented?

Childhood vaccines are available to prevent measles, mumps, chickenpox, and some of the other viral infections that cause encephalitis. To prevent encephalitis caused by West Nile virus or other viruses transmitted by mosquitoes, health officials urge people to avoid being outside at dawn and dusk, when mosquitoes are most active. They also recommend that people wear light-colored long-sleeved shirts and long pants; use insect repellent when they are outdoors in an area known to harbor mosquitoes; and empty any sources of standing water, such as birdbaths, children's wading pools, and flower pots, where the mosquitoes breed. Individuals can help prevent rabies by vaccinating pets and by avoiding contact with wild mammals; and they can help prevent

Lyme disease encephalitis by avoiding ticks. No Lyme vaccines are available as of 2016. In addition, individuals can prevent HIV infection and eliminate many cases of encephalitis by avoiding sexual contact and by never sharing needles.

See also AIDS and HIV Infection • Chickenpox (Varicella) • Cytomegalovirus (CMV) Infection • German Measles (Rubella) • Herpes Simplex Virus Infections • Immune Deficiencies • Lyme Disease • Measles (Rubeola) • Meningitis • Mononucleosis, Infectious • Mumps • Poliomyelitis • Rabies • Shingles (Herpes Zoster) • Syphilis • Toxoplasmosis • Viral Infections • West Nile Fever • Zoonoses: Overview


Books and Articles

Jackson, Alan C., editor. Viral Infections of the Human Nervous System. Basel, Switzerland: Springer, 2013.

Ruiz, Andrew, and Douglas Fleming, eds. Encephalitis, Encephalomyelitis and Encephalopathies: Symptoms, Causes and Potential Complications. Hauppauge, NY: Nova Science Publishers, 2013.


Centers for Disease Control and Prevention. CDC Health Information for International Travel, “Chapter 3: Infectious Diseases Related to Travel.” http://wwwnc.cdc.gov/travel/yellowbook/2016/infectious-diseases-related-to-travel/tickborne-encephalitis (accessed March 16, 2015).

Centers for Disease Control and Prevention, Division of Vector-Borne Diseases (DVBD). http://www.cdc.gov/ncezid/dvbd/ (accessed June 3, 2016).

National Institute of Neurological Disorders and Stroke. “Meningitis and Encephalitis Fact Sheet.” http://www.ninds.nih.gov/disorders/encephalitis_meningitis/detail_encephalitis_meningitis.htm (accessed March 16, 2016).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: http://cdc.gov (accessed March 16, 2016).

World Health Organization. Avenue Appia 20, 1211 Geneva 27, Switzerland. Telephone: 41-22-791-2111. Website: http://who.int/en/ (accessed March 16, 2016).

* inflammation (in-fla-MAY-shun) is the body's reaction to irritation, infection, or injury that often involves swelling, pain, redness, and warmth.

* seizures are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness. Also called convulsions.

* coma (KO-ma) is an unconscious state like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.

* immune system (im-YOON SIStem) is the system in the body composed of specialized cells and the substances they produce that helps protect the body against disease-causing germs.

* AIDS (or acquired immune deficiency [ih-myoon dih-FIH-shensee] syndrome) is an infection that severely weakens the immune system; is caused by the human immunodeficiency virus (HIV).

* herpes simplex (HER-peez-SIMplex) is a virus that can cause infections of the skin, mouth, genitals, and other parts of the body.

* cytomegalovirus infection (sy-tuh-MEH-guh-lo-vy-rus) (CMV), is a common infection that usually causes no symptoms. It poses little risk for healthy people, but it can lead to serious illness in people with weak immune systems.

* listeriosis (lis-teer-e-O-sis) is a bacterial infection that can cause a form of meningitis in infants, and produces other symptoms in children and adults.

* syphilis (SIH-fih-lis) is a sexually transmitted disease that can lead if untreated to serious lifelong problems throughout the body, including blindness and paralysis.

* Lyme (LIME) disease is a bacterial infection that is spread to humans by the bite of an infected tick. It begins with a distinctive rash and/or flulike symptoms. In some cases, it can progress to a more severe disease, with complications affecting other body organs.

* toxoplasmosis (TOX-o-plaz-MOsis) is a parasitic infection that usually causes no symptoms in healthy people, but can cause serious problems in unborn babies and people with weak immune systems.

* paralysis (pah-RAH-luh-sis) is the loss or impairment of the ability to move some part of the body.

* enterovirus (EN-tuh-ro-VY-rus) is a group of viruses that can infect the human gastrointestinal tract and spread through the body, causing a number of symptoms.

* mucus (MYOO-kus) is a thick, slippery substance that lines the insides of many body parts.

* rabies (RAY-beez) a viral infection of the central nervous system that is usually transmitted to humans by the bite of an infected animal.

* double vision is a vision problem in which a person sees two images of a single object.

* hallucinations (ha-LOO-sin-AYshuns) occur when a person sees, smells, tastes, or hears things that are not really there. Hallucinations can result from nervous system abnormalities, mental disorders, or the use of certain drugs.

* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by such microorganisms as bacteria and viruses.

* computed tomography (kom- PYOO-ted toe-MAH-gruh-fee), or CT, is a technique in which a machine is used to acquire scans of the body to create a three-dimensional image.

* magnetic resonance imaging (or MRI) uses magnetic waves instead of x-rays to scan the body and produce detailed pictures of the body's structures.

* rectum is the final portion of the large intestine that connects the colon to the outside opening of the anus.

* biopsy (BI-op-see) is a test during which a small sample of skin or other body tissue is removed and examined for signs of disease.

* acetaminophen (uh-see-tehMIH-noh-fen) is a medication commonly used to reduce fever and relieve pain.

* bedsores are skin sores caused by prolonged pressure on the bony prominences of the hips, sacrum, heels, elbows, and shoulders; typically are seen in people who are confined by illness or paralysis to beds or wheelchairs. Also called pressure sores.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)