Encephalitis (en-sef-uh-LIE-tis) is inflammation of the brain. There are several causes, but the most common is 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 senses or movement.
Rarely, encephalitis can be life-threatening. Timely diagnosis and treatment are important because it’s difficult to predict how encephalitis will affect each individual.
The number of new cases a year of acute encephalitis in Western countries is 7.4 cases per 100,000 population per year. In tropical countries, the incidence is 6.34 per 100,000 per year. In 2015, encephalitis was estimated to have affected 4.3 million people and resulted in 150,000 deaths worldwide. Herpes simplex encephalitis has an incidence of 2–4 per million population per year.
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.
Types and Causes
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.
The patient typically has a fever, headache, and photophobia (excessive sensitivity to light). There may also be general weakness and seizures.
Less common symptoms
The individual may also experience nuchal rigidity (neck stiffness), which can lead to a misdiagnosis of meningitis. There may be stiffness of the limbs, slow movements, and clumsiness. The patient may also be drowsy and have a cough.
More serious cases
In more serious cases, the person may experience very severe headaches, nausea, vomiting, confusion, disorientation, memory loss, speech problems, hearing problems, hallucinations, as well as seizures and possibly coma. In some cases, the patient can become aggressive.
Signs and symptoms in infants
Initially, encephalitis is harder to detect in young children and babies. Parents or guardians should look out for vomiting, a bulging fontanel (the soft area on the top center of the head), incessant crying that does not get better when the baby is picked up and comforted, and body stiffness.
Diagnosis and test
Your doctor will first ask you about your symptoms. They may perform the following tests if encephalitis is suspected.
Spinal tap or lumbar puncture
In this procedure, your doctor will insert a needle into your lower back to collect a sample of spinal fluid. They will test the sample for signs of infection.
Brain imaging with CT scan or MRI
CT scans and MRI detect changes in brain structure. They can rule out other possible explanations for symptoms, such as a tumor or stroke. Certain viruses have a tendency to affect certain areas of the brain. Seeing what parts of your brain are affected can help determine what type of virus you have.
CT image of encephalitis
An EEG uses electrodes (small metal discs with wires) attached to the scalp to record brain activity. An EEG does not detect the virus that causes encephalitis, but certain patterns on the EEG may alert your neurologist to an infectious source of your symptoms. Encephalitis can lead to seizures and coma in later stages. That’s why the EEG is important in determining the areas of the brain that are affected and the types of brain waves that occur in each area.
A blood test can reveal signs of a viral infection. Blood tests are rarely performed alone. They usually help diagnose encephalitis along with other tests.
In a brain biopsy, your doctor will remove small samples of brain tissue to test for infection. This procedure is rarely performed because there’s a high risk of complications. It’s usually only done if doctors can’t determine the cause the brain swelling or if treatment isn’t working.
Treatment and medications
Treatment for mild encephalitis usually consists of:
- Bed rest
- Plenty of fluids
- Anti-inflammatory drugs such as acetaminophen (Tylenol, others), ibuprofen (Advil, Motrin IB, others) and naproxen sodium (Aleve) to relieve headaches and fever
Encephalitis caused by certain viruses usually requires antiviral treatment.
Antiviral medications commonly used to treat encephalitis include:
- Acyclovir (Zovirax)
- Ganciclovir (Cytovene)
- Foscarnet (Foscavir)
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 behavioural skills to improve mood disorders or address personality changes
Children and adults should avoid contact with anyone who has encephalitis.
Controlling mosquitoes (a mosquito bite can transmit some viruses) may reduce the chance of some infections that can lead to encephalitis.
- Apply an insect repellant containing the chemical, DEET when you go outside (but DO NOT use DEET products on infants younger than 2 months).
- Remove any sources of standing water (such as old tires, cans, gutters, and wading pools).
- Wear long-sleeved shirts and pants when outside, especially at dusk.
Children and adults should get routine vaccinations for viruses that can cause encephalitis. People should receive specific vaccines if they are traveling to places such as parts of Asia, where Japanese encephalitis is found. Vaccinate animals to prevent encephalitis caused by the rabies virus.