Western Equine Encephalitis (WEE) virus causes acute meningoencephalitis in horses and humans and until the 2002 introduction of West Nile Virus was the primary mosquito-borne encephalitis type found in Colorado. The disease is especially severe in infants and children. About one third of the cases reported have been in children under five years old. The onset of the illness is quite sudden, and involves a period of lethargy, fever, headache, vomiting and stiff neck. The symptoms of acute illness include confusion, disorientation, stupor and coma. The acute phase will generally last three to ten days. WEE Transmission Cycle Adult patients will generally recover with little residual neurologic damage. However about half of the infected infants will suffer permanent sequelae, including progressive retardation and major motor disorders. The case fatality rate is approximately three to four percent. The virus has been reported throughout the United States and southern Canada, however human and equine disease occurs almost exclusively in the western U.S. and Canadian provinces. WEE virus exists primarily as an infection of bird populations and is transmitted from bird to bird by several species of mosquito. Wild birds serve as the basic viral reservoir during the epidemic season. The Culex tarsalis mosquito is the classic vector, and is responsible for transmitting the disease to horses and man. Horse cases routinely occur in Colorado and human cases, although more rare, do periodically occur.

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