Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision Sep 2020| Content last modified Sep 2020
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What is encephalitis?

Encephalitis is inflammation of your brain, usually caused by an infection.

  • Encephalitis can be caused by a virus directly infecting your brain or when a virus or a misguided immune reaction triggers brain inflammation

  • Typical symptoms include a fever, headache, and feeling sleepy or confused

  • Doctors do an MRI and spinal tap

  • Doctors treat your symptoms and sometimes give antiviral medicines or corticosteroids

What causes encephalitis?

The most common cause of encephalitis is a

  • Viral infection

Many different viruses can cause encephalitis. Some of these include:

Some of these viruses come from bug bites, such as from mosquitoes or ticks. Rabies comes from animal bites.

Sometimes a virus mistakenly causes your immune system to attack tissue in your brain (autoimmune reaction). As a result, your brain becomes inflamed. This usually happens a few weeks after you get certain kinds of viral infections.

What are the symptoms of encephalitis?

Early symptoms depend on what caused the encephalitis. Many viruses first cause stomach and cold (flu-like) symptoms. You may feel sick to your stomach and throw up. You may have a sore throat, runny nose, and cough.

When your brain becomes inflamed, you usually have:

  • Fever

  • Headache

  • Sleepiness

  • Personality changes or confusion

With severe encephalitis, you may have seizures or go into a coma. Some people die.

How can doctors tell if I have encephalitis?

Doctors suspect encephalitis based on your symptoms. Doctors then usually do a:

  • MRI

  • Spinal tap (use a long needle to get spinal fluid from your lower back)

Rarely, doctors need to remove a sample (biopsy) of brain tissue and do tests on it to find the cause of the encephalitis.

How do doctors treat encephalitis?

For some kinds of viral encephalitis, doctors give you antiviral medicines. They may give you corticosteroids if encephalitis is caused by an autoimmune reaction.

Some causes of encephalitis have no specific treatment. Doctors treat symptoms and offer life support (such as a breathing tube) until the infection goes away. This usually takes about 1 to 2 weeks.

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