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There are several types of encephalitis viruses. Encephalitis is inflammation of the brain that can result in either mild flu-like symptoms or more severe and life-threatening ones, such as seizures and paralysis. Encephalitis viruses include those spread from mosquito or tick bites, the rabies virus, and several types of the herpes virus.
One of the more common encephalitis viruses is the herpes simplex virus. Herpes simplex 1 is usually responsible for cold sores in people. A person can get encephalitis after an initial exposure to herpes simplex 1 or can acquire it after the virus has been in the body for a while and becomes reactivated. Herpes simplex encephalitis is more common in young people less than age 20 or people more than age 40. It is usually very severe and can be fatal.
Two other types of herpes viruses, Epstein Barr and varicella-zoster, are also encephalitis viruses. Epstein Barr is the virus that usually causes mononucleosis, while varicella-zoster virus usually causes chickenpox in children and adults and shingles later in life. Most cases of encephalitis that result from Epstein Barr or varicella-zoster are mild.
Other encephalitis viruses are spread to humans from ticks or mosquitoes. These viruses are known as arboviruses. Powassan virus, or POW, is spread to humans from the black-legged tick, which is found in North America and in parts of Asia. Although cases of encephalitis from POW are very rare in the United States and in Canada, they are usually fatal in 10 percent of cases and have permanent neurological effects in about 50 percent of cases.
A number of encephalitis viruses are spread from mosquitoes, including the St. Louis virus, West Nile virus, and La Crosse virus. The viruses usually start in birds and are then passed to humans through mosquitoes, which act as vectors for the viruses. Usually encephalitis viruses from mosquitoes are mild and occur in older adults.
Several viruses that were previously common in childhood could also lead to encephalitis, such as measles, polio, and mumps. Those viruses usually cause a secondary form of encephalitis, as the person will suffer the symptoms of the virus first and then have signs of encephalitis. Vaccinations greatly reduce a person's risk of getting any of those viruses.
Encephalitis viruses are easily avoided. If a person plans to spend time outside in an area with ticks and mosquitoes, he should wear long pants, long-sleeved shirts, and other protective clothing. Mosquito repellent is a must. People should also avoid others who have signs and symptoms of encephalitis and should not share food and drink with people who are ill.
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