Arboviruses that cause encephalitis are primarily found in three virus families: Togaviridae, Bunyaviridae, and Flaviviridae. In the United States, the main mosquito-borne encephalitis strains are Eastern equine, Western equine, St. Louis, La Crosse, and West Nile. Equine encephalitis causes disease in both humans and, as its name implies, horses. Powassan encephalitis is a less common tick-borne flavivirus that occurs primarily in the northern United States. Japanese encephalitis is the most common form of viral encephalitis to occur outside of the United States. It is endemic in rural areas in east, south, and southwest Asia, especially China and Korea. Venezuelan equine encephalitis is found in South and Central America.
Different arboviruses cause different forms of encephalitis. Although the overall disease is the same, there are subtle differences in symptoms and the type of brain damage they produce.
Common Forms of Mosquito-Borne Arbovirus Encephalitis
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Eastern Equine Encephalitis
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Virus Family
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Togaviridae (genus Alphavirus)
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U.S. Geographic Areas
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Atlantic and Gulf coasts, in New England, and around the Great Lakes. States most affected are Florida, Georgia, Massachusetts, and New Jersey.
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Symptom Onset
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Symptoms appear 4 - 10 days following infection and can range from mild flu-like symptoms to full-blown encephalitis.
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Incidence and Mortality Rates
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About 6 cases are reported each year. About a third of people who contract EEE die from it. Children are more likely to survive but also to suffer complications afterward.
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Age Risk Groups
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Adults over age 50 and children under age 15.
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Western Equine Encephalitis
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Virus Family
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Togaviridae (genus Alphavirus)
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U.S. Geographic Areas
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Farming areas in western and central Plains and Rocky Mountain states west of the Mississippi.
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Symptom Onset
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5 - 10 days following infection.
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Incidence and Mortality Rates
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Very rare. Mortality rate is 3 - 4%; 30% of survivors have complications afterward. Most severe in children, especially those younger than 1 year. Infants may suffer permanent neurological damage.
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Age Risk Groups
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Infants younger than 12 months.
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St. Louis Encephalitis
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Virus Family
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Flaviviridae (genus Flavivirus)
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U.S. Geographic Areas
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Takes its name from an epidemic in St. Louis, but has occurred throughout the U.S., especially central and southern states, as well as parts of Canada, Caribbean, and South America.
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Symptom Onset
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5 - 15 days following infection.
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Incidence and Mortality Rates
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About 108 cases are reported annually. Mortality rate range between 5 - 30%, with highest rates among elderly. About 5% of survivors suffer complications afterward.
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Age Risk Groups
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Elderly adults (over age 60) are at highest risk, and the disease is most severe in this age group. Younger people usually experience mild, flu-like symptoms.
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La Crosse Encephalitis
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Virus Family
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Bunyaviridae (genus Bunyavirus)
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U.S. Geographic Areas
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Occurs most frequently in upper Midwestern, southeastern (Appalachia), and mid-Atlantic states. Most cases have occurred in Ohio and Wisconsin. Unlike other encephalitis viruses which originate in birds, La Crosse encephalitis is transmitted to mosquitoes from infected chipmunks and squirrels.
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Symptom Onset
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5 - 15 days following infection.
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Incidence and Mortality Rates
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About 80 - 100 cases reported each year. Mortality rates are less than 1%. More common and severe in children under age 16.
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Age Risk Groups
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Children younger than 16 years.
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West Nile Encephalitis
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Virus Family
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Flaviviridae (genus Flavivirus).
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U.S. Geographic Areas
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Cases have been reported throughout the mainland United States.
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Symptom Onset
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3 - 14 days following infection.
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Incidence and Mortality Rates
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In 2010, 981 cases of WNV were reported to the CDC, with 45 deaths. Of all the reported cases, 39% were due to West Nile fever and 61% to meningitis and encephalitis. However, most cases of West Nile virus do not produce symptoms, and are not reported, so these numbers imply a more worrisome picture than actually exists. In fact, fewer than 1% of people who are infected with WNV go on to develop neurological disease.
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Age Risk Groups
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Adults over age 50.
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