Although the term "encephalitis" literally means "inflammation of the brain," it usually refers to brain inflammation resulting from a viral infection. The severe and potentially life-threatening form of this disease is rare.

What is it?

  • Although the term "encephalitis" literally means "inflammation of the brain," it usually refers to brain inflammation resulting from a viral infection. The severe and potentially life-threatening form of this disease is rare. Experts suspect that the actual incidence of encephalitis is probably much higher — but because most people have such mild signs or symptoms, many cases go unrecognized.
  • Encephalitis occurs in two forms — a primary form and a secondary form. Primary encephalitis involves direct viral infection of your brain and spinal cord.
In secondary encephalitis, a viral infection first occurs elsewhere in your body and then travels to your brain.
  • Seeing your doctor and receiving timely treatment is important because the course of the encephalitis is unpredictable.
  • Symptoms

    Most people infected with viral encephalitis have only mild, often flu-like symptoms, and the illness usually doesn't last long. In some cases, people might not have any symptoms. Possible symptoms include:

    More serious infections can cause:

    • Confusion and hallucinations
    • Personality changes
    • Double vision
    • Seizures
    • Muscle weakness
    • Loss of sensation or paralysis in certain areas
    • Tremors
    • Rash
    • Loss of consciousness
    • Bulging in the soft spots (fontanels) of the skull in infants


    The cause of encephalitis is most often a viral infection. Some examples include:

    • Herpes viruses
    • Arboviruses transmitted by mosquitoes, ticks and other insects
    • Rabies transmitted through animal bites

    Encephalitis takes two forms, categorized by the two ways that viruses can infect your brain:

    • Primary encephalitis. This occurs when a virus directly invades your brain and spinal cord. It can happen to people at any time of the year (sporadic encephalitis), or it can be part of an outbreak (epidemic encephalitis).
    • Secondary (postinfectious) encephalitis. This form occurs when a virus first infects another part of your body and secondarily enters your brain.

    Also, bacterial infections, such as Lyme disease, can sometimes lead to encephalitis, as can parasitic infections, such as toxoplasmosis, in people with weakened immune systems.

    Here are some of the more common causes of encephalitis:

    Herpes viruses

    Some herpes viruses that cause common infections may also cause encephalitis. These include:

    • Herpes simplex virus. There are two types of herpes simplex virus (HSV) infections. HSV type 1 (HSV-1) more commonly causes cold sores or fever blisters around your mouth. HSV type 2 (HSV-2) more commonly causes genital herpes. HSV-1 is the most important cause of fatal sporadic encephalitis in the United States, but it's also rare.
    • Varicella-zoster virus. This virus is responsible for chickenpox and shingles. It can cause encephalitis in adults and children, but tends to be mild.
    • Epstein-Barr virus. This herpes virus causes infectious mononucleosis (mono). If encephalitis develops, it's usually mild, but can be fatal in a small number of cases.

    Childhood infections

    In rare instances, secondary encephalitis occurs after vaccine-preventable childhood viral infections, including:

    • Measles (rubeola)
    • Mumps
    • German measles (rubella)

    In such cases, encephalitis may be due to hypersensitivity — an overreaction of your immune system to a foreign substance.


    Viruses that are transmitted by mosquitoes and ticks (arboviruses) have, in recent years, produced well-publicized encephalitis epidemics. Organisms that transmit disease from one animal host to another are called vectors. Mosquitoes are vectors for the transmission of encephalitis from small creatures — usually birds and rodents — to humans. This type of encephalitis is fairly uncommon.

    In the United States, the following types of mosquito-borne encephalitis occur:

    • Eastern equine encephalitis. This infection generally afflicts horses and birds, especially birds that live near freshwater swamps. It can also affect humans, although fewer than 10 cases are reported in most years. Eastern equine encephalitis outbreaks occur most commonly in the eastern United States. Although some people experience it only as a mild illness, eastern equine encephalitis is fatal in about one-third of the cases. Symptoms of eastern equine encephalitis usually appear three to 10 days after a bite by an infected mosquito.
    • Western equine encephalitis. Most reports of western equine encephalitis come from the central and western Plains of the United States. Like eastern equine encephalitis, this infection affects horses and, rarely, humans. It flourishes in birds that live near irrigated fields and farming areas. Symptoms appear between five and 10 days after a bite. Western equine encephalitis is less likely to be fatal than is its eastern cousin, but can result in brain damage and other major complications, particularly in infants.
    • St. Louis encephalitis. This virus is transmitted to mosquitoes by birds. The mosquito vector of St. Louis encephalitis breeds in areas of standing water, including polluted pools, roadside ditches and containers such as birdbaths, flowerpots and discarded tires. Symptoms appear within a week to 10 days. The mortality rate can be as high as 30 percent for this type of encephalitis. Older adults have a higher mortality rate from St. Louis encephalitis.
    • La Crosse encephalitis. This virus is named for La Crosse, Wis., where the virus was first recognized in 1963. It's most common in the hardwood forest areas of the Upper Midwest and in Appalachia. Unlike other forms of viral encephalitis, this virus is passed to mosquitoes from chipmunks and squirrels. La Crosse encephalitis usually affects children and is rarely fatal. Generally, fewer than 100 cases are reported annually. Symptoms appear five to 15 days after a bite by an infected mosquito.
    • West Nile encephalitis. This virus first appeared in the United States in 1999 and spread across most of the country over the next several years. The virus is similar to other encephalitis viruses in that birds are its main animal hosts. However, in rare cases, it's possible for the disease to spread from person to person through organ transplant, blood transfusions or breast-feeding, or from mother to unborn child. Symptoms of West Nile encephalitis are generally mild, but the disease can be severe, especially in older adults and those with weakened immune systems. Symptoms appear within five to 15 days of being bitten by an infected mosquito.

    Risk factors

    Mosquitoes don't discriminate, so anyone can develop viral encephalitis. But some factors put you at greater risk:

    Age. Some types of encephalitis are more prevalent or more severe in young children or older adults.

    Weakened immune system. If you have an immune deficiency — for example, because of AIDS or HIV — or you're going through cancer therapies or organ transplantation, you're more susceptible to encephalitis.
    Geographic regions. Visiting or living in areas of the country where mosquito-borne viruses are common increases your risk of epidemic encephalitis.

    Outdoor activities. If you have an outdoor job or open-air hobby, such as gardening, jogging, golf or bird-watching, be extra careful during an encephalitis outbreak.
    Season of the year. The warm months of summer are the prime mating time for birds and mosquitoes. As a result, mosquito-borne diseases tend to be more prevalent in late summer to early autumn.


    Severe viral encephalitis can cause:

    • Seizures
    • Respiratory arrest
    • Coma
    • Death

    In those who've had severe encephalitis, some problems may last for a year or more, including:

    • Fatigue
    • Weakness
    • Depression
    • Personality changes
    • Gait problems
    • Memory difficulties

    Some complications may be permanent, such as memory loss, the inability to speak coherently, lack of muscle coordination, paralysis, or hearing or vision defects.


    Diagnosing encephalitis may involve:

    • Spinal tap (lumbar puncture). One common way to diagnose encephalitis is to analyze the cerebrospinal fluid surrounding your brain and spinal cord. A needle inserted into your lower spine (below the level of the spinal cord) extracts a sample of fluid for laboratory analysis, which may reveal the presence of an infection or an increased white blood count — a signal that your immune system is fighting an infection. Your cerebrospinal fluid may also be slightly bloody if hemorrhages have occurred. Diagnosis of herpes simplex encephalitis can be difficult, but advances in using sensitive DNA methods have allowed detection of the virus in spinal fluid.
    • Electroencephalography (EEG). This procedure measures the waves of electrical activity produced by your brain. It's often used to diagnose and manage seizure disorders. A number of small electrodes are attached to your scalp with paste or an elastic cap as you recline. You remain still during the test, but at times you may be asked to breathe deeply and steadily for several minutes or to stare at a patterned board. At times, a light may be flashed in your eyes. These actions are meant to stimulate your brain. The electrodes pick up the electrical impulses from your brain and send them to the EEG machine, which records your brain waves on a moving sheet of paper. An abnormal EEG result may suggest encephalitis, but a normal result does not rule out the disease.
    • Brain imaging. A computerized tomography (CT) or magnetic resonance imaging (MRI) scan may reveal swelling of your brain. Or it may reveal another condition with signs and symptoms that are similar to encephalitis, such as a concussion. If encephalitis is suspected, brain imaging is often done before a spinal tap to look for evidence of elevated intracranial pressure.
    • Brain biopsy. Very rarely, if diagnosis of herpes simplex encephalitis isn't possible using DNA methods or by CT or MRI scans, your doctor may take a small sample of your brain tissue. This sample is analyzed in the laboratory to see if the virus is present. Your doctor may also try treatment with antiviral medications before suggesting brain biopsy.
    • Blood testing. Your doctor can confirm the presence of West Nile virus in your body by drawing a sample of your blood for laboratory analysis. If you have West Nile virus, an analysis of your blood sample may show a rising level of an antibody to the virus, a positive DNA test for the virus or a positive culture of the virus.

    Treatments and drugs

    Treatment for mild cases mainly consists of:

    • Rest
    • A healthy diet, including plenty of liquids
    • Using paracetamol) to relieve headaches and fever

    More serious cases of encephalitis can be difficult to treat because the viruses that cause the disease generally don't respond to medications. However, some viruses, particularly the herpes simplex virus and varicella-zoster virus, respond to antiviral drugs such as acyclovir (Zovirax). If you have one of these kinds of virus-induced encephalitis, your doctor will likely start treatment with acyclovir immediately. Another antiviral that's sometimes used is ganciclovir.

    In addition to antiviral medications, your doctor may also treat you with:

    • Anti-inflammatory drugs, such as corticosteroids, to help reduce swelling and pressure within your skull
    • Anticonvulsant medications to stop or prevent seizures

    After the illness, you may also need physical and speech therapy.


    The best way to prevent viral encephalitis is to avoid the viruses that lead to this disease. That means taking steps to prevent genital herpes, for one. It also means making sure you and your children are immunized against viruses that can cause encephalitis, such as chickenpox, measles (rubeola), mumps and German measles (rubella).

    To protect yourself and your family against mosquito-borne encephalitis during an outbreak of the disease:

    • Dress to protect yourself. Wear long-sleeved shirts and long pants if you're outside between dusk and dawn.
    • Apply mosquito repellent. The Environmental Protection Agency has found only two products — DEET and picaridin — to be effective at controlling insect bites. When you're going to be outdoors, especially at dawn and dusk when mosquitoes are more active, apply a product containing one of these ingredients to the outside of your clothing and to your exposed skin. Don't spray insect repellent on your face; spray it on your hands and then apply it to your face. Don't use DEET on the hands of young children because they may put their hands in their mouths or eyes. The American Academy of Pediatrics advises parents not to use insect repellents on infants younger than 2 months of age. Instead, cover your infant's stroller or playpen with mosquito netting when outside.
    • Avoid mosquitoes. Refrain from unnecessary activity in places where mosquitoes are most prevalent. Additionally, avoid being outdoors from dusk till dawn whenever possible — that's the time when mosquitoes are most active.
    • Keep mosquitoes out of your home. Repair holes in screens on doors and windows.
    • Get rid of water sources outside your home. Eliminate standing water in your yard, where mosquitoes can lay their eggs. Empty birdbaths, drains, wheelbarrows and flowerpots, and remove old tires and unused containers that might hold water. Drain puddles when possible. Clean your gutters and drain flat roofs regularly.
    • Take advantage of nature's mosquito-controlling creatures. Fill ornamental pools with mosquito-eating fish. Consider placing houses for other mosquito eaters, such as bats, in your yard.
    • Look for outdoor signs of viral disease. If you notice sick or dying birds, report your observations to your local health department.

    A vaccine is available to protect horses from West Nile virus. No vaccine is available for humans, but researchers are working to develop one.