Epidemic viral encephalitis is usually caused by arboviruses, meaning arthropod-borne viruses, a diverse group ofviruses transmitted by insects, mites, or ticks. The four leading causes of epidemic encephalitis in the United States are all arboviruses transmitted by mosquitoes. The West Nile virus, also mosquito-borne, was introduced into New York from the Middle East in the summer of1999. Migrating birds soon spread the virus to 20 eastern states from Maine to Florida, with expansion of its range continuing. All these viruses are enveloped, single-stranded RNA viruses: LaCrosse encephalitis virus, bunyavirus family; St. Louis and West Nile encephalitis viruses, flavivirus family; and the eastern and western equine encephalitis viruses, togavirus family. ■ mosquitoes, p. 315
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