Merck Manual

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John E. Greenlee

, MD, University of Utah Health

Reviewed/Revised Mar 2022 | Modified Sep 2022

Encephalitis is inflammation of the brain that occurs when a virus directly infects the brain or when a virus, vaccine, or something else triggers inflammation. The spinal cord may also be involved, resulting in a disorder called encephalomyelitis.

  • People may have a fever, headache, or seizures, and they may feel sleepy, numb, or confused.

  • Magnetic resonance imaging of the head and a spinal tap are usually done.

  • Treatment involves relieving symptoms and sometimes using antiviral drugs.

Encephalitis is most commonly due to viruses, such as herpes simplex, herpes zoster, cytomegalovirus, or West Nile virus. It can occur in the following ways:

Sometimes bacteria cause encephalitis, usually as part of bacterial meningitis (called meningoencephalitis).

Sometimes a brain infection, a vaccine, cancer, or another disorder triggers a misguided immune reaction, causing the immune system to attack normal cells in the brain (an autoimmune reaction Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more ). As a result, the brain becomes inflamed. If triggered by an infection, the disorder is called postinfectious encephalitis. Brain infections due to an autoimmune reaction sometimes develop in people who have cancer—a disorder called paraneoplastic encephalitis.

Types of Encephalitis

Infections that can directly lead to encephalitis can occur in epidemics or occasionally as isolated cases (sporadically).

Epidemic encephalitis

In the United States, the most common types of epidemic encephalitis are caused by one of the following:

Arboviruses are viruses transmitted to people through the bites of arthropods, usually mosquitoes, fleas, or ticks. (Arbovirus is short for arthropod-borne virus.) The viruses are transmitted to arthropods when arthropods bite infected animals or people. Many species of domestic animals and birds carry these viruses.

Epidemics occur in people only periodically—when the population of mosquitoes or infected animals increases. Epidemics tend to occur when arthropods are biting—for mosquitoes and ticks, usually during warm weather. Infection spreads from arthropod to person, not from person to person.

Many arboviruses can cause encephalitis. The different types of encephalitis that result are usually named for the place the virus was discovered or the animal species that typically carries it.

In the United States, mosquitoes spread several types of encephalitis, including the following:

  • La Crosse encephalitis is caused by the La Crosse virus (also called California virus). It is most common in the Midwest but can occur anywhere in the country. This encephalitis accounts for most cases in children. Many cases are mild and undiagnosed. Fewer than 1% of infected people die from it.

  • Eastern equine encephalitis occurs predominantly in the eastern United States. A few cases have occurred in the Great Lakes region. Eastern equine encephalitis affects mainly young children and people older than 55. In children younger than 1 year, it can cause severe symptoms and permanent nerve or brain damage. Over half of infected people die.

  • West Nile encephalitis, once present only in Europe and Africa, first appeared in the New York City area in 1999. It has spread throughout the United States. Several species of birds can be infected with the virus when they are bitten by an infected mosquito. This encephalitis affects mainly older people. This virus also causes a milder infection called West Nile fever, which is much more common. West Nile encephalitis develops in fewer than 1% of people who develop West Nile fever. About 9% of people with West Nile encephalitis die. However, those who have only West Nile fever usually recover fully.

  • St. Louis encephalitis occurs mostly in urban areas of central and southeastern states of the United States but also in western states. Infection is more common in the summer and is more likely to affect the brain in older people. Epidemics once occurred about every 10 years but are now rare.

  • Western equine encephalitis can occur throughout the United States but, for unknown reasons, has largely disappeared since 1988. It can affect all age groups but is more severe and more likely to affect the brain in children younger than 1 year.

A few types of encephalitis are spread by ticks. They include

  • Tick-borne encephalitis occurs in Northern Asia, Russia, and Europe. The infection usually causes a mild flu-like illness that clears up within a few days, but some people, usually those age 50 or older, develop more severe symptoms. Because many cases occur in Europe and Russia, a vaccine is available there.

  • Powassan virus infection occurs mainly in Canada and in the Great Lakes region and northeastern United States. Powassan virus has also caused cases of encephalitis in Russia. The virus is similar to the one that causes tick-borne encephalitis in Europe. Powassan virus infection usually causes mild or no symptoms. However, the infection can also cause severe encephalitis with headache, vomiting, seizures, loss of coordination, speech problems, or coma. About 10% of people with severe encephalitis die. Powassan virus is spread by the deer tick, which also transmits Lyme disease. In Lyme disease, the tick must be attached for 24 to 48 hours to spread disease. In contrast, Powassan virus infection may be transmitted if an infected tick is attached for as little as 15 minutes. The vaccine that is effective against tick-borne encephalitis in Europe and Russia is not effective against the Powassan virus.

  • Colorado tick fever occurs in areas of western United States and Canada that are 4,000 to 10,000 feet above sea level. Colorado tick fever causes a flu-like illness. Occasionally, people with Colorado tick fever develop meningitis or encephalitis. Colorado tick fever rarely causes death. It is rarely transmitted by blood transfusion.

Several viruses that cause encephalitis were once present in only a few parts of the world but now are spreading, probably because travel has increased. These viruses include

All are spread by mosquitoes.

The Chikungunya virus was first identified in Africa but has spread to Southeast Asia, India, China, some parts of Europe, the Caribbean, and Central, South, and North America. Most people with chikungunya disease feel better within a week. However, chikungunya disease can lead to severe encephalitis and even death, especially in infants and people over age 65.

The Japanese encephalitis virus is a common cause of encephalitis in Asia and the western Pacific. In the United States, Japanese encephalitis occurs only in travelers who acquired the virus in areas of the world where the virus is common.

Venezuelan equine encephalitis occurs mainly in parts in South and Central America. The Venezuelan equine encephalitis virus caused an epidemic of encephalitis in Texas in 1971 but now rarely causes encephalitis in the United States. It occurs mainly in travelers returning from areas where the virus is common.

The Zika virus was first identified in the Zika Forest of Uganda, then spread to the South Pacific islands, then South America, Central America, the Caribbean, Mexico, and Florida. Zika infection may cause fever, joint and muscle aches, headache, and a red, bumpy rash. Having Zika virus infection during pregnancy can cause microcephaly Microcephaly Microcephaly is an abnormally small head. Often the head is small because the brain is small and abnormally developed. Microcephaly can be caused by many disorders, including genetic abnormalities... read more Microcephaly and severe brain damage in the baby.

Sporadic encephalitis

Reactivation of a previous infection

Autoimmune encephalitis

After people have certain viral infections or get certain vaccines, the body’s immune system sometimes attacks the layers of tissue that wrap around nerve fibers (called the myelin sheath Overview of Demyelinating Disorders Most nerve fibers inside and outside the brain are wrapped with many layers of tissue composed of a fat (lipoprotein) called myelin. These layers form the myelin sheath. Much like the insulation... read more ) in the brain and spinal cord—an autoimmune reaction Autoimmune Disorders An autoimmune disorder is a malfunction of the body's immune system that causes the body to attack its own tissues. What triggers an autoimmune disorder is not known. Symptoms vary depending... read more . The attack occurs because proteins in myelin resemble those in the virus. As a result, nerve transmission becomes very slow. The resulting disorder, called acute disseminated encephalomyelitis Acute Disseminated Encephalomyelitis (ADEM) Disorders that cause demyelination and have no known cause are called primary demyelinating disorders. Demyelination is the destruction of the tissues that wrap around nerves, called the myelin... read more , resembles multiple sclerosis Multiple Sclerosis (MS) In multiple sclerosis, patches of myelin (the substance that covers most nerve fibers) and underlying nerve fibers in the brain, optic nerves, and spinal cord are damaged or destroyed. The cause... read more except that symptoms do not come and go as they do in multiple sclerosis. The viruses most often involved include enteroviruses Overview of Enterovirus Infections Enteroviruses are a group of viruses. Enterovirus infections affect many parts of the body and may be caused by any of several different viral strains. Enterovirus infections may be caused by... read more Overview of Enterovirus Infections , Epstein-Barr virus Infectious Mononucleosis Epstein-Barr virus causes a number of diseases, including infectious mononucleosis. The virus is spread through kissing. Symptoms vary, but the most common are extreme fatigue, fever, sore throat... read more Infectious Mononucleosis , hepatitis A Hepatitis A Acute hepatitis A is inflammation of the liver that is caused by the hepatitis A virus and that lasts less than 6 months. Hepatitis A is usually spread when people ingest something that has... read more or hepatitis B Hepatitis B, Acute Acute hepatitis B is inflammation of the liver that is caused by the hepatitis B virus and that lasts from a few weeks up to 6 months. Hepatitis B is spread through contact with blood or other... read more virus, human immunodeficiency virus Human Immunodeficiency Virus (HIV) Infection Human immunodeficiency virus (HIV) infection is a viral infection that progressively destroys certain white blood cells and is treated with antiretroviral medications. If untreated, it can cause... read more Human Immunodeficiency Virus (HIV) Infection (HIV), and influenza viruses Influenza (Flu) Influenza (flu) is a viral infection of the lungs and airways with one of the influenza viruses. It causes a fever, runny nose, sore throat, cough, headache, muscle aches (myalgias), and a general... read more . Before vaccination of children became widespread, the viruses that cause measles, rubella, chickenpox, and mumps used to be common causes of acute disseminated encephalomyelitis. This type of encephalitis can also occur in people with cancer or other autoimmune disorders.

An autoimmune encephalitis may also develop if the immune system produces antibodies that attack proteins on the surface of nerve cells called N-methyl-d-aspartate (NMDA) receptors. The resulting encephalitis is called anti-NMDA receptor encephalitis. Some evidence suggests that anti-NMDA receptor encephalitis is a more common type of encephalitis than was previously thought. It occasionally develops after encephalitis due to the herpes simplex virus, even when that encephalitis was successfully treated.

COVID-19 encephalitis

Rarely, people with COVID-19 COVID-19 COVID-19 is an acute respiratory illness that can be severe and is caused by the coronavirus named SARS-CoV-2. Symptoms of COVID-19 vary significantly. Two types of tests can be used to diagnose... read more develop what appears to be encephalitis. (COVID-19 is caused by the newly identified severe acute respiratory syndrome coronavirus, or SARS-CoV2.) This encephalitis may be caused by the virus that enters and affects the brain. But it may also be an autoimmune or a partly autoimmune reaction.

Symptoms of Encephalitis

Before symptoms of encephalitis start, people may have digestive symptoms, such as nausea, vomiting, diarrhea, or abdominal pain. Or they may feel as if they are getting a cold or the flu and have cough, fever, a sore throat, a runny nose, swollen lymph nodes, and muscle aches.

Symptoms of encephalitis include

  • Fever

  • Headache

  • Personality changes or confusion

  • Seizures

  • Paralysis or numbness

  • Sleepiness that can progress to coma and death

People may vomit and have a stiff neck, but these symptoms tend to be less common and less severe than when caused by meningitis.

Encephalitis due to the herpes simplex virus causes headache, fever, and flu-like symptoms at first. People also have seizures, sometimes accompanied by strange smells (such as rotten eggs), vivid flashbacks, or sudden, intense emotions. As the encephalitis progresses, people become confused, have difficulty speaking and remembering, have repeated seizures, then lapse into coma.

HIV-associated encephalopathy can cause gradual personality changes, problems with coordination, and dementia.

If the spinal cord is affected, parts of the body may feel numb and weak. Which parts are affected depend on which parts of the spinal cord are affected (see figure Where Is the Spinal Cord Damaged? Where Is the Spinal Cord Damaged? Where Is the Spinal Cord Damaged? ). If the infection is severe, people may lose sensation, become paralyzed, and lose control of the bladder and bowels.

Recovery from viral encephalitis may take a long time. Some people do not completely recover. The likelihood of dying depends on what the cause is and how quickly the infection is treated.

Did You Know...

  • Long after a case of measles or chickenpox, the virus can be reactivated and cause inflammation in the brain.

Diagnosis of Encephalitis

  • Magnetic resonance imaging

  • A spinal tap

Doctors suspect encephalitis based on symptoms, especially if an epidemic is in progress. Magnetic resonance imaging (MRI) and a spinal tap are usually done.

MRI can sometimes detect abnormalities in certain areas of the brain. These abnormalities may help confirm the diagnosis of encephalitis and/or suggest which virus is causing encephalitis. If MRI is not available, computed tomography (CT) may be done. MRI and CT can help doctors exclude disorders that can cause similar symptoms (such as stroke Ischemic Stroke An ischemic stroke is death of an area of brain tissue (cerebral infarction) resulting from an inadequate supply of blood and oxygen to the brain due to blockage of an artery. Ischemic stroke... read more Ischemic Stroke and a brain tumor Overview of Brain Tumors A brain tumor can be a noncancerous (benign) or cancerous (malignant) growth in the brain. It may originate in the brain or have spread (metastasized) to the brain from another part of the body... read more ). These tests can also check for problems that can make doing a spinal tap dangerous.

A spinal tap Spinal Tap Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Imaging tests commonly used to diagnose nervous system (neurologic) disorders... read more Spinal Tap (lumbar puncture) is done to obtain a sample of cerebrospinal fluid, which flows through the tissues (meninges) that cover the brain and spinal cord. Normally, the spinal fluid contains very few white blood cells. But when the brain and meninges are inflamed, the number of white cells in cerebrospinal fluid increases.

To identify the virus causing encephalitis, doctors take samples of blood and cerebrospinal fluid and test them for antibodies to the virus when the person is sick and later when the person is recovering. Sometimes techniques are used to grow (culture) viruses in the cerebrospinal fluid so that they can be identified more easily. Some enteroviruses Overview of Enterovirus Infections Enteroviruses are a group of viruses. Enterovirus infections affect many parts of the body and may be caused by any of several different viral strains. Enterovirus infections may be caused by... read more Overview of Enterovirus Infections (such as those that can cause polio-like illnesses) can be cultured, but most other viruses cannot.

The polymerase chain reaction (PCR) technique is used to identify many of the viruses that can cause encephalitis. PCR, which produces many copies of a gene, is used to detect the genetic material of these viruses in a sample of cerebrospinal fluid. Prompt identification of the herpes simplex virus by PCR is difficult, so treatment is usually started immediately if the herpes simplex virus is thought to be the cause. Prompt treatment is essential because the encephalitis it causes is destructive and, if untreated, is often fatal. Immediate treatment can help lessen the severity of symptoms and prevent death.

Rarely, a sample of brain tissue is removed and examined under a microscope (biopsy) to determine whether herpes simplex virus or another organism is the cause.

Sometimes, even after extensive testing, no virus, bacteria, or other cause of infection is identified. In such cases, the cause may be autoimmune or cancer-related (paraneoplastic) encephalitis because tests cannot always confirm those disorders.

Treatment of Encephalitis

  • Depending on the possible cause, an antiviral drug, an antibiotic, corticosteroids, and/or other drugs

  • Measures to relieve symptoms and, if needed, provide life support

If herpes simplex virus and varicella zoster virus cannot be excluded, the antiviral drug acyclovir is given. Acyclovir is effective against herpes simplex and herpes zoster viruses. Cytomegalovirus encephalitis can be treated with the antiviral drug ganciclovir. Foscarnet is an alternative, which can be used alone or with ganciclovir. Sometimes several antibiotics are also given in case the cause is bacteria.

For HIV-associated encephalopathy, a combination of drugs used to treat HIV infection (antiretroviral drugs Antiretroviral Treatment of Human Immunodeficiency Virus (HIV) Infection Antiretroviral medications used to treat human immunodeficiency virus (HIV) infection aim to do the following: Reduce the amount of HIV RNA (viral load) in the blood to an undetectable amount... read more ) helps the immune system function better and delays the progression of the infection and its complications, including dementia.

Autoimmune encephalitis is usually treated with the following:

For other viruses and most other causes, no specific treatment is available. Treatment usually involves relieving symptoms (such as seizures and fever) and, when necessary, providing life support (for example, with a breathing tube) until the infection subsides—in about 1 to 2 weeks.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Sitavig, Zovirax, Zovirax Cream, Zovirax Ointment, Zovirax Powder, Zovirax Suspension
Cytovene, Vitrasert, Zirgan
Deltasone, Predone, RAYOS, Sterapred, Sterapred DS
A-Methapred, Depmedalone-40, Depmedalone-80 , Depo-Medrol, Medrol, Medrol Dosepak, Solu-Medrol
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