Encephalitis Overview

Encephalitis is an infection in the brain itself. (Meningitis is an infection in the meninges that cover the brain.)

Like meningitis, encephalitis can be caused by bacteria or viruses—including viruses transmitted by mosquitoes or other arthropods (a group of animals that includes insects and ticks). Encephalitis can develop slowly or quickly, and can range from mild to severe, depending on which organism is causing the infection.

Types & Causes of Encephalitis


An arbovirus (arthropod-borne virus) is a virus transmitted by an arthropod, including insects and ticks. There are many different types of arboviral encephalitis, which exist worldwide. Four types are found in the United States: eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis (SLE), and LaCross encephalitis (LAC). All of these types are transmitted through mosquitoes. Most cases of arboviral encephalitis occur during warm weather—when mosquitoes are active.

Most people who are infected with an arbovirus show no signs of infection or develop mild, flu-like symptoms such as fever and headache. Only a small percentage of infections develop into full-blown encephalitis, which is quite serious and can cause permanent neurological damage or death.

Herpes Simplex Encephalitis (HSE)

HSE is the most fatal type of encephalitis. It often causes swelling in the brain and significant neurological dysfunction. HSE is more common in the very young children and older adults, but can develop at any age. Most people are exposed to the virus at some point in their life, and the virus can live inside a person for many years before it begins to cause any problems. It is not clear why it causes encephalitis in some people but not others.


Rabies is a virus that is transmitted by mammals—especially bats, skunks, foxes, and raccoons. If a rabid animal bites a person and that person does not receive treatment, rabies infection—including severe encephalitis—may develop. Encephalitis symptoms of rabies infection include agitation, hallucinations, muscle spasms, seizures, coma, and death.

Diagnosis of Infectious Encephalitis

Medical History

As with other neurological disorders, a complete medical history is the first step in diagnosing encephalitis. It is essential that your physician know where you have traveled recently, any contact you had with unusual animals or ill people, and a history of insect bites.

Neurological Examination

A complete neurological exam should be done to look for any abnormalities that serve as clues about which specific area of the brain is affected.

Imaging Studies

Imaging studies are tests that provide computer images of the inside of the brain and involve either a CT scan (computer axial tomography) or MRI (magnetic resonance imaging). An MRI can often show damage to a particular brain area and may be more useful for diagnosing encephalitis.

Spinal Tap (Lumbar Puncture)

A spinal tap, or lumbar puncture, involves inserting a needle into the lower back (the lumbar area of the spine) and collecting some cerebrospinal fluid. The fluid is then sent to a lab where it is examined under the microscope and the organism causing the infection can be identified. Specialized molecular techniques are used to determine which particular virus is causing the encephalitis pictures.

Treatment for Encephalitis

The treatment for encephalitis depends on the cause.

Arboviral Encephalitis

There is no treatment for arboviral encephalitis, and there are no vaccines for the United States. In areas where arboviral encephalitis occurs, public health officials usually initiate insecticide spraying to kill the mosquitoes. In areas where mosquitoes known to be carrying an arbovirus live, people should do what they can to avoid being bitten (e.g., wearing insect repellant, avoiding going outside during times of the day when the mosquitoes are most active, etc.).

Herpes Simplex Encephalitis

This type of encephalitis can be treated with an antiviral drug called Acyclovir, which is given intravenously and is also used to treat genital and skin infections caused by a herpes simplex infection. Without treatment, herpes simplex encephalitis can (and probably will) be fatal, so treatment should be started immediately upon diagnosis.

Rabies Encephalitis

Rabies is a disease that is best treated through prevention. The animal that bit the person should be found. If it is wild, or if there is no proof that it has been vaccinated, it should be given to a pathologist who will kill the animal and examine its brain for the presence of the rabies virus. If the animal cannot be found, or if there is any question whatsoever about whether it is infected or not, the person should be treated immediately with human rabies immunoglobulin, an antibody that acts against the rabies virus.

Publication Review By: Stanley J. Swierzewski, III, M.D.

Published: 01 Jan 2000

Last Modified: 22 Sep 2015