Encephalitis: Viral - Causes

Highlights	Risk Factors	Vaccinations
Introduction	Prognosis	Prevention
Causes	Diagnosis	Resources
Symptoms	Treatment	References

Causes

Arboviruses

Arboviruses, including the West Nile virus, are transmitted by blood-sucking insects such as mosquitoes and ticks.Most of the time,the viral infections initiallydevelop in birds. Insects that feed on the infected blood from a diseased bird (or reservoir ) carry the virus, andtransmit it when they bite a susceptible host (such as an animal or a human). Because these insects play anrole in the disease-transmission process, they are referred to as vectors.

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Arboviruses multiply in blood-sucking vectors, nearly always mosquitoes. There is no evidence that these infections can be transmitted casually from one infected person or animal directly to another uninfected person without passing through a mosquito (or tick) first. (Although, a small number of West Nile virus cases have occurred through blood tranfusions, organ transplantation, and breast feeding.) It should be stressed that only about 10% of people who are infected by an arbovirus develop encephalitis and that symptoms arise in only about 1% of those infected.

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.

Differentarboviruses cause different forms of encephalitis. Although the overall disease is the same, thereare subtle differences in symptoms and the type of brain damage they produce.

Common Forms of Mosquito-Borne Arbovirus Encephalitis
Eastern Equine Encephalitis
Virus Family	Togaviridae (genus Alphavirus)
U.S. Geographic Areas	Atlantic and Gulf coasts, in New England, and around the Great Lakes. States most affected are Florida, Georgia, Massachusetts, and New Jersey.
Symptom Onset	Symptoms appear 4-10 days following infection and can range from mild flu-like symptoms to full-blown encephalitis.
Incidence and Mortality Rates	The most serious of the US arboviruses and fortunately rare. Only 200 cases have been confirmed since 1964 with an average rate of 4 cases per year. Mortality rates are approximately 35-50%. Children are more likely to survive but also to suffer complications afterward.
Age Risk Groups	Adults over age 50 and children under age 15.
Western Equine Encephalitis
Virus Family	Togaviridae (genus Alphavirus)
U.S. Geographic Areas	Farming areas in western and central Plains and Rocky Mountain states west of the Mississippi.
Symptom Onset	5-10 days following infection.
Incidence and Mortality Rates	Very rare. There was only one case reported between 1995 and 2000. 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.
Age Risk Groups	Infants younger than 12 months.
St. Louis Encephalitis
Virus Family	Flaviviridae (genus Flavivirus)
U.S. Geographic Areas	Takes its name from an epidemic in St. Louis, but outbreaks have occurred in wider geographic areas, especially in midwestern and southeastern states, and can occur in rural or urban areas. As of 2000, the highest numbers of total cases have been reported in Texas (970), Illinois (695), Ohio (440), Indiana (368), and Florida (379).
Symptom Onset	7-10 days following infection.
Incidence and Mortality Rates	Although over 4500 cases have been reported since 1964, the average number of cases has been declining with a yearly average of only 11 cases between 1995 and 2000. Mortality rate of between 3 - 30%, with about 5% of survivors suffering complications afterward.
Age Risk Groups	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.
La Crosse Encephalitis
Virus Family	Bunyaviridae (genus Bunyavirus)
U.S. Geographic Areas	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.
Symptom Onset	5-10 days following infection.
Incidence and Mortality Rates	An average of 70-100 cases reported per year. Mortality rates are less than 1%. More common and severe in children under age 16.
Age Risk Groups	Children younger than 16 years.
West Nile Encephalitis
Virus Family	Flaviviridae (genus Flavivirus).
U.S. Geographic Areas	Cases have been reported throughout the mainland United States. In 2004, majority of West Nile encephalitis cases occurred in California, Arizona, Texas, and Louisiana.
Symptom Onset	2 -15 days following infection.
Incidence and Mortality Rates	In 2005, 2819 cases of WNV were reported to the CDC, and 105 people died. (The average age of those who died was 57 years.) Of all the reported cases, more than half (53%) were due to West Nile fever. Forty-two percent of those who contracted WNV had more severe conditions, such as meningitis and encephalitis. However, most cases of West Nile virus do not produce symptoms, and are not reported, so these numbers paint a more worrisome picture than actually exists. In fact, experts think that less than 1% of people infected with WNV go on to develop severe neurological disease.
Age Risk Groups	Adults over age 50.