Encephalitis (pronounced in-seh-fuh-LIE-tess) is an inflammation of the brain. It may be caused by a number of different factors. One of the most common causes is direct infection of the brain by a virus or bacterium. Inflammation can also occur as a complication of some other disorder, such as mumps (see mumps entry) or herpes simplex (see herpes infections entry). About two thousand cases of encephalitis are reported in the United States each year.
Inflammation of the brain is a reaction of the body's immune system. The immune system is a network of cells, tissues, and chemical substances designed to protect the body against invasion by foreign agents. Sometimes, a foreign agent gets directly into the brain. In the process of fighting off the foreign agent, brain tissues become swollen and inflamed. In other cases, the infection occurs elsewhere in the body, such as the throat or neck. The immune reaction to those infections can also cause inflammation of the brain.
There are more than a dozen viruses that can cause encephalitis. In some cases, the viruses are spread by direct contact between two people. In other cases, the viruses are transmitted by means of an animal or insect bite. Some common viruses and viral diseases that can cause encephalitis include:
Some of these viruses may infect the brain directly. In other cases, the infection spreads from another part of the body, as is usually the case with chickenpox, measles, mumps, rubella (see rubella entry), and Epstein-Barr virus. For example, a person may develop a case of chickenpox, then about five to ten days later, as an immune reaction to the chickenpox virus, the brain becomes inflamed and swollen.
Many forms of encephalitis are spread by the bites of insects or animals. Mosquitoes are common carriers of encephalitis viruses. They carry the viruses in their blood and saliva. When they bite a human, they may transfer the virus to the human's bloodstream. The virus multiplies and spreads throughout the body. When it reaches the brain, it may cause encephalitis.
Dogs, cats, mice, raccoons, squirrels, and bats are also carriers of encephalitis viruses. These animals also carry the virus in their blood and saliva. When they bite a human, they can transmit the virus to the human bloodstream.
One of the most serious forms of encephalitis is caused by the herpes simplex virus. The herpes simplex virus causes cold sores and genital herpes. Sometimes the herpes virus spreads directly to the brain, causing an encephalitis infection. About 10 percent of all encephalitis cases are caused by this virus. In untreated patients, the rate of death is 70 percent. The rate drops to 15 to 20 percent if patients receive treatment.
The symptoms of encephalitis range from very mild to very severe. They include:
Symptoms often progress rapidly. They change from being mild to severe within several days or even a few hours.
The first step in diagnosis may be a medical history. A doctor will try to determine if the patient has had recent contact with a virus that can cause encephalitis. The diagnosis can be confirmed with a variety of tests. These include:
The treatment used for encephalitis depends on the cause of the infection. Bacterial encephalitis can be treated with antibiotics. Antibiotics kill bacteria, but not viruses. Viral encephalitis can be treated with drugs that kill viruses. Relatively few drugs of this kind have been developed. Some antivirals that can be used are acyclovir (pronounced a-SIGH-klo-veer), ganciclovir (pronounced gan-SIGH-klo-veer), foscarnet (pronounced fos-KAHR-net), ribovarin, and azidothymidine (AZT, pronounced AZE-ih-do-thigh-mih-deen). These drugs are more effective with some forms of encephalitis than with others.
Other drugs are available for the treatment of the symptoms of encephalitis. For example, corticosteroids (pronounced kor-tih-ko-STAIR-oids) are used to reduce inflammation and swelling. Anticonvulsant drugs can be used to control seizures. Fever can be treated with aspirin or acetaminophen. Aspirin should not be given to childen due to the risk of Reye's syndrome (see Reye's syndrome entry).
Encephalitis symptoms may last several weeks. Most cases of encephalitis are mild, however, and patients recover quickly and completely. They experience no further problems after the disease has disappeared.
About 10 percent of all encephalitis patients die from the infection. The death rate varies, depending on the kind of encephalitis. For example, there aren't any effective treatments for eastern equine encephalitis and the death rate is usually about 30 percent. Herpes encephalitis has one of the highest death rates. With treatment, 15 to 20 percent of herpes encephalitis cases result in death. Without treatment, the number of deaths jumps to 70 to 80 percent.
Some people do experience long-term neurological damage (damage to the nervous system, including the brain) after having encephalitis. The effects include personality changes, memory loss, language difficulties, seizures, and partial paralysis.
There are two major ways to avoid encephalitis. One is to reduce the risk of getting the disease from another human who has been infected. Most infections of this kind are spread hand-to-hand or mouth-to-hand. To avoid transmission of this kind, a person should remember to wash his or her hands frequently during the day.
A second way to avoid encephalitis is to reduce the chance of being bitten by mosquitoes, rats, bats, and other animals that carry the disease. One should be aware when such animals may be around. For example, mosquitoes tend to be more common in warm, moist areas. They tend to be more active at dawn and dusk. A person who has to be outdoors during these times should try not to have bare arms and legs. Mosquito repellent should be used to prevent bites.
Vaccines (treatments that enable the body to build immunity to certain viruses) are available for some viruses, such as polio, herpes B, and equine encephalitis. A person who may be at risk for these viruses should have injections of the vaccines.
Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (404) 639–3311. http://www.cdc.gov.