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Encephalitis

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 either bacteria or viruses, including viruses transmitted by mosquitoes or other arthropods (an arthropod is a group of animals including the insects and ticks). It can develop slowly or quickly, and it can be either very mild or quite severe, depending on the organism that is causing the infection.

Types & Causes

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

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

Herpes Simplex Encephalitis (HSE)
HSE is the most fatal encephalitis. It tends to develop in both very young and very old people, although it can develop at any age. Most people are exposed to the virus at some point in their life, and often the virus lives inside of 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.

HSE often causes swelling in the brain and significant neurological dysfunction.

Rabies
Rabies is a virus that is transmitted by mammals, especially bats, skunks, foxes and raccoons. If a rabid animal bites a person, the person may develop a rabies infection, including severe encephalitis. The encephalitis symptoms of a rabies infection include agitation, hallucinations, and muscle spasms. If left untreated, a person can develop seizures and coma.

Diagnosis

Medical History
As with all 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

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.

See also: More on Encephalitis

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