Viral Encephalitis

  • Basics

    Encephalitis is a general term that means inflammation of the brain.

    Encephalitis can occur two different ways: from direct infection of the brain, or from a previous infection that causes your immune system to attack your brain. Direct infection can be caused by many different agents, including bacteria, fungi, and viruses. This article will explore only viral causes of encephalitis. There are many encephalitis-causing viruses, so only causes, symptoms, and treatment related to the most common viruses will be discussed.

  • Causes

    Encephalitis can result from certain viral infections passed between humans, or from contact with infected animals or insects.

    Encephalitis can develop 5 to 10 days after some childhood illnesses (for example, chickenpox, measles, mumps, rubella, and polio). Fortunately, it is not common to get encephalitis after a viral illness. On rare occasions, encephalitis can occur weeks, months, or years after the initial viral infection. Vaccine programs have markedly reduced some of these diseases in the developed world.

    There are many viruses transmitted between humans that can lead to encephalitis. Viruses from the herpes group are familiar examples. Chickenpox, cytomegalovirus, and Epstein-Barr virus (the cause of infectious mononucleosis) are herpes viruses. All herpes viruses may cause encephalitis occasionally, but herpes simplex virus 1 is the most common culprit.

    The herpes simplex virus 1 causes herpes simplex encephalitis.

    Herpes simplex viruses occur throughout the world. About 90% of adults age 50 or older have been exposed to herpes simplex virus 1 at some time during their lifetime. Herpes simplex virus 1 is most commonly known for causing cold sores (fever blisters) on the lips or around the mouth area. The most common cause of non-epidemic (not affecting a large number of people at once) encephalitis in developed countries is the herpes simplex virus. About 30% of herpes encephalitis infections result from initial contact with the virus. However, 70% of encephalitis cases occur years after the first infection. This happens when virus particles that have been dormant in the brain since the initial infection become active. While it is not known what causes the herpes simplex virus to reactivate, it is thought that a weakened or suppressed immune system may play a role.

    Ticks or mosquitoes spread certain encephalitis-causing viruses to people.

    Insect-transmitted viruses are the most common cause of epidemic viral encephalitis in the U.S. Viruses that can be transmitted to humans from mosquitoes or ticks are called arboviruses. Mosquitoes and ticks acquire these viruses from feeding on an infected host. Depending on the specific virus, birds, horses, small animals, or rodents can act as hosts. After the mosquito bites an infected host, the virus multiplies inside the mosquito. When it feeds or bites again, the mosquito can transmit the virus to the person or animal it bites. (For additional information, see the Patient Guide Insect Stings and Bites).

    The most common insect-borne encephalitis in the U.S. is St. Louis encephalitis. It can occur in people of all ages, in either rural or urban areas.

    The St. Louis encephalitis virus is transmitted by mosquitoes that bite infected birds before biting humans. Older adults infected by St. Louis encephalitis have a greater chance of their illness being serious. While overall only 7% of those with St. Louis encephalitis die, 20% of those over the age of 60 do not survive. For those who live, recovery tends to be slow. Recovering patients suffer from unstable emotions, trouble concentrating, forgetfulness, and tremors. Most cases of St. Louis encephalitis occur in mid- to late-summer, or whenever the mosquito population of the particular region is active.

    California encephalitis is caused by an infection with the LaCrosse virus.

    California encephalitis is found mainly in midwestern, north central, and eastern regions of the U.S. In areas where it is commonly found, as many as 20% of the human population have antibodies to the virus. This means they have been exposed to the virus in the past; however, most of those exposed did not develop an illness. The virus exists in small woodland mammals like chipmunks and squirrels, and is transmitted by mosquitoes to humans. The disease occurs primarily in children under 15 years of age. Illness is rare and the mortality (death) rate is low. Unfortunately, about 10-15% of California encephalitis survivors may have lasting difficulty with thought, movement, and overall functioning.

    Western equine encephalitis was common in western and central areas of the U.S. and Canada from the 1930s to the 1950s, but is less common now.

    Horses are the primary carriers of this virus. Mosquitoes bite infected horses, then pass the virus on to other horses and humans. Children typically suffer more severely from western equine encephalitis, with 3% dying and 5% to 30% suffering permanent impairment.

    Eastern equine encephalitis occurs primarily in swampy areas along the eastern and Gulf coasts of the U.S. from June through October.

    Eastern equine encephalitis has a high fatality rate, and there is a high risk of neurological problems in those who survive. Only 38 cases of eastern equine encephalitis were reported in the U.S. between 1988 and 1994. It has also been found in some inland midwestern states, and may occur year-round in southern states. Older adults are more at risk to become ill with eastern equine encephalitis than younger individuals who are exposed.

    The West Nile virus has recently emerged in the U.S..

    Common in Africa, Asia, and the Middle East, the West Nile virus first appeared in the Americas in New York City during the summer of 1999. The virus is carried by bird populations, and is passed on to humans through mosquito bites. Encephalitis caused by the West Nile Virus is uncommon and usually mild, but can be fatal. During the first New York outbreak, 62 patients suffered from severe disease, and seven people died during the summer and fall of 1999. As of 2005, the virus had infected people in 48 states and Washingon, DC. (More information on West Nile Virus is available in the patient guide West Nile Virus).

    Other types of insect-transmitted encephalitis are found in different parts of the world.

    Powassan is a rare type of encephalitis transmitted to humans from infected birds or small mammals by a tick. Powassan is found in the northern U.S. and in Canada. Venezuelan equine encephalitis primarily occurs in Central and South America. Japanese encephalitis occurs throughout Asia, and tick-borne encephalitis occurs in rural areas of Europe and Russia. These are just a few examples of infectious diseases that you may encounter when traveling abroad. If you plan to travel outside of the U.S., it is important to discuss region-specific health concerns with your clinician. For more information in infectious diseases outside of the U.S., see the Patient Guide International Travel Advice.

    Encephalitis can also be caused by rabies.

    Bats, foxes, and skunks are some of the common carriers of the rabies virus. You should never handle a dead animal with your bare hands, because diseases (including rabies) can be transmitted even after death. Many people are under the misconception that rabies is only caught through a bite from an infected animal. However, simply having a cut or sore exposed to the saliva of an infected animal can transmit the disease. Airborne transmission of rabies has been proven in caves heavily infested with bats. In cabins and in houses where bats have invaded the attic or other areas, humans have been bitten while they sleep. Although common in other parts of the world, relatively few human cases of rabies occur in the U.S., due to widespread animal vaccinations.

    If a bite or contact with a possibly infected animal occurs, medical attention should be sought immediately. A person must receive immunization against rabies within two days of exposure to the virus to prevent the disease. Once symptoms of the disease begin, it will likely be fatal.

    (For more information on bite trauma, consult the Patient Guide Animal and Human Bites).

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