Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. There are several causes, but the most common is a viral infection.
Encephalitis often causes only mild flu-like signs and symptoms — such as a fever or headache — or no symptoms at all. Sometimes the flu-like symptoms are more severe. Encephalitis can also cause confused thinking, seizures, or problems with movement or with senses such as sight or hearing.
In some cases, encephalitis can be life-threatening. Timely diagnosis and treatment are important because it's difficult to predict how encephalitis will affect each individual.
Most people with viral encephalitis have mild flu-like symptoms, such as:
Aches in muscles or joints
Fatigue or weakness
Sometimes the signs and symptoms are more severe, and might include:
Confusion, agitation or hallucinations
Loss of sensation or paralysis in certain areas of the face or body
Problems with speech or hearing
Loss of consciousness (including coma)
In infants and young children, signs and symptoms might also include:
Bulging in the soft spots (fontanels) of an infant's skull
Nausea and vomiting
Poor feeding or not waking for a feeding
When to see a doctor
Get immediate care if you are experiencing any of the more-severe symptoms associated with encephalitis. A severe headache, fever and altered consciousness require urgent care.
Infants and young children with any signs or symptoms of encephalitis should receive urgent care.
The exact cause of encephalitis is often unknown. But when a cause is known, the most common is a viral infection. Bacterial infections and noninfectious inflammatory conditions also can cause encephalitis.
There are two main types of encephalitis:
Primary encephalitis. This condition occurs when a virus or other agent directly infects the brain. The infection may be concentrated in one area or widespread. A primary infection may be a reactivation of a virus that had been inactive after a previous illness.
Secondary encephalitis. This condition results from a faulty immune system reaction to an infection elsewhere in the body. Instead of attacking only the cells causing the infection, the immune system also mistakenly attacks healthy cells in the brain. Also known as post-infection encephalitis, secondary encephalitis often occurs two to three weeks after the initial infection.
Common viral causes
The viruses that can cause encephalitis include:
Herpes simplex virus (HSV). Both HSV type 1 — associated with cold sores and fever blisters around your mouth — and HSV type 2 — associated with genital herpes — can cause encephalitis. Encephalitis caused by HSV type 1 is rare but can result in significant brain damage or death.
Other herpes viruses. These include the Epstein-Barr virus, which commonly causes infectious mononucleosis, and the varicella-zoster virus, which commonly causes chickenpox and shingles.
Enteroviruses. These viruses include the poliovirus and the coxsackievirus, which usually cause an illness with flu-like symptoms, eye inflammation and abdominal pain.
Mosquito-borne viruses. These viruses can cause infections such as West Nile, La Crosse, St. Louis, western equine and eastern equine encephalitis. Symptoms of an infection might appear within a few days to a couple of weeks after exposure to a mosquito-borne virus.
Tick-borne viruses. The Powassan virus is carried by ticks and causes encephalitis in the Midwestern United States. Symptoms usually appear about a week after a bite from an infected tick.
Rabies virus. Infection with the rabies virus, which is usually transmitted by a bite from an infected animal, causes a rapid progression to encephalitis once symptoms begin. Rabies is a rare cause of encephalitis in the United States.
Childhood infections. Common childhood infections — such as measles (rubeola), mumps and German measles (rubella) — used to be fairly common causes of secondary encephalitis. These causes are now rare in the United States due to the availability of vaccinations for these diseases.
Anyone can develop encephalitis. Factors that may increase the risk include:
Age. Some types of encephalitis are more common or more severe in certain age groups. In general, young children and older adults are at greater risk of most types of viral encephalitis.
Weakened immune system. People who have HIV/AIDS, take immune-suppressing drugs or have another condition causing a weakened immune system are at increased risk of encephalitis.
Geographical regions. Mosquito- or tick-borne viruses are common in particular geographical regions.
Season of the year. Mosquito- and tick-borne diseases tend to be more common in summer in many areas of the United States.
The complications of encephalitis vary, depending on factors such as:
The cause of your infection
The severity of your initial illness
The time from disease onset to treatment
People with relatively mild illness usually recover within a few weeks with no long-term complications.
Complications of severe illness
Inflammation can injure the brain, possibly resulting in a coma or death.
Other complications — varying greatly in severity — may persist for months or be permanent. These complications can include:
Weakness or lack of muscle coordination
Hearing or vision defects
The best way to prevent viral encephalitis is to take precautions to avoid exposure to viruses that can cause the disease. Try to:
Practice good hygiene. Wash hands frequently and thoroughly with soap and water, particularly after using the toilet and before and after meals.
Don't share utensils. Don't share tableware and beverages.
Teach your children good habits. Make sure they practice good hygiene and avoid sharing utensils at home and school.
Get vaccinations. Keep your own and your children's vaccinations current. Before traveling, talk to your doctor about recommended vaccinations for different destinations.
Protection against mosquitoes and ticks
To minimize your exposure to mosquitoes and ticks:
Dress to protect yourself. Wear long-sleeved shirts and long pants if you're outside between dusk and dawn when mosquitoes are most active, and when you're in a wooded area with tall grasses and shrubs where ticks are more common.
Apply mosquito repellent. Chemicals such as DEET can be applied to both the skin and clothes. To apply repellent to your face, spray it on your hands and then wipe it on your face. If you're using both sunscreen and a repellent, apply sunscreen first.
Use insecticide. The Environmental Protection Agency recommends the use of products containing permethrin, which repels and kills ticks and mosquitoes. These products can be sprayed on clothing, tents and other outdoor gear. Permethrin shouldn't be applied to the skin.
Avoid mosquitoes. Refrain from unnecessary activity in places where mosquitoes are most common. If possible, avoid being outdoors from dusk till dawn, when mosquitoes are most active. Repair broken windows and screens.
Get rid of water sources outside your home. Eliminate standing water in your yard, where mosquitoes can lay their eggs. Common problems include flowerpots or other gardening containers, flat roofs, old tires and clogged gutters.
Look for outdoor signs of viral disease. If you notice sick or dying birds or animals, report your observations to your local health department.
Protection for young children
Insect repellents aren't recommended for use on infants younger than 2 months of age. Instead, cover an infant carrier or stroller with mosquito netting.
For older infants and children, repellents with 10% to 30% DEET are considered safe. Products containing both DEET and sunscreen aren't recommended for children because reapplication — which might be necessary for the sunscreen component — will expose the child to too much DEET.
Tips for using mosquito repellent with children include:
Always assist children with the use of mosquito repellent.
Spray on clothing and exposed skin.
Apply the repellent when outdoors to lessen the risk of inhaling the repellent.
Spray repellent on your hands and then apply it to your child's face. Take care around the eyes and ears.
Don't use repellent on the hands of young children who may put their hands in their mouths.
Wash treated skin with soap and water when you come indoors.
Your doctor will start with a thorough physical examination and medical history.
Your doctor might then recommend:
Brain imaging. MRI or CT images can reveal any swelling of the brain or another condition that might be causing your symptoms, such as a tumor.
Spinal tap (lumbar puncture). A needle inserted into your lower back removes a small amount of cerebrospinal fluid (CSF), the protective fluid that surrounds the brain and spinal column. Changes in this fluid can indicate infection and inflammation in the brain. Sometimes samples of CSF can be tested to identify the virus or other infectious agent.
Other lab tests. Samples of blood, urine or excretions from the back of the throat can be tested for viruses or other infectious agents.
Electroencephalogram (EEG). Electrodes affixed to your scalp record the brain's electrical activity. Certain abnormal patterns may indicate a diagnosis of encephalitis.
Brain biopsy. Rarely, a small sample of brain tissue might be removed for testing. A brain biopsy is usually done only if symptoms are worsening and treatments are having no effect.
Treatment for mild encephalitis usually consists of:
Plenty of fluids
Anti-inflammatory drugs — such as acetaminophen (Tylenol, others), ibuprofen (Advil, Motrin IB, others) and naproxen sodium (Aleve) — to relieve headaches and fevers
Encephalitis caused by certain viruses usually requires antiviral treatment.
Antiviral medications commonly used to treat encephalitis include:
Some viruses, such as insect-borne viruses, don't respond to these treatments. But because the specific virus may not be identified immediately or at all, doctors often recommend immediate treatment with acyclovir. Acyclovir can be effective against HSV, which can result in significant complications when not treated promptly.
Antiviral medications are generally well tolerated. Rarely, side effects can include kidney damage.
People who are hospitalized with severe encephalitis might need:
Breathing assistance, as well as careful monitoring of breathing and heart function
Intravenous fluids to ensure proper hydration and levels of essential minerals
Anti-inflammatory drugs, such as corticosteroids, to reduce swelling and pressure within the skull
Anticonvulsant medications, such as phenytoin (Dilantin), to stop or prevent seizures
If you experience complications of encephalitis, you might need additional therapy, such as:
Physical therapy to improve strength, flexibility, balance, motor coordination and mobility
Occupational therapy to develop everyday skills and to use adaptive products that help with everyday activities
Speech therapy to relearn muscle control and coordination to produce speech
Psychotherapy to learn coping strategies and new behavioral skills to improve mood disorders or address personality changes
Preparing for an appointment
Serious illness associated with encephalitis is usually severe and relatively sudden, so seek emergency care. The emergency care team will likely include specialists in infectious diseases and in the brain and nervous system (neurologist).
Questions from your doctor
You may need to answer these questions, or answer them on behalf of your child or another person with severe illness:
When did the symptoms begin?
Have you recently started taking any new medications? If so, what is the medication?
Have you been bitten by a mosquito or tick during the past few weeks?
Have you traveled recently? Where?
Have you recently had a cold, flu or other illness?
Are you up to date on your immunizations? When was your last one?
Have you had any exposure to wild animals or known toxins recently?
Have you had unprotected sex with a new or long-term sexual partner?
Do you have a condition or take any medications that result in a weakened immune system?