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 initially develop in birds. Insects that feed on the infected blood from a diseased bird (or reservoir ) carry the virus, and transmit it when they bite a susceptible host (such as an animal or a human). Because these insects play an role 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.