Western Equine Encephalitis virus (WEEV) is a mosquito‑borne alphavirus that causes encephalitis in horses and humans. It circulates primarily in western North America and parts of South America and is transmitted by mosquitoes, with Culex tarsalis serving as the principal vector.
Epidemiology and Virology
WEEV is an enveloped, positive‑sense RNA virus in the genus Alphavirus of the family Togaviridae. Phylogenetic analyses indicate that it arose from recombination between an ancestral Sindbis‑like virus and an Eastern Equine Encephalitis virus‑like virus. Its genome encodes structural proteins that form an icosahedral nucleocapsid surrounded by a lipid envelope. WEEV circulates in an enzootic cycle between passerine birds and Culex tarsalis mosquitoes, which are common in irrigated agricultural regions and freshwater marshes of western North America. Other mosquito species such as Aedes melanimon and Aedes dorsalis may act as secondary vectors. Horses and humans are dead‑end hosts; they can develop high viremia but do not contribute significantly to transmission. Cases occur during the summer and early autumn when vector populations are abundant. Although equine vaccines exist, there is no licensed human vaccine or specific antiviral therapy. WEEV incidence has declined markedly since the mid‑2oth century for reasons that are not fully understood. The virus remains enzootic, and periodic epizootics can occur under favorable ecological conditions.
Outbreaks and Clinical Features
Large outbreaks of Western Equine Encephalitis occurred in the 1930s through the 1950s, causing thousands of equine deaths and hundreds of human cases across the western United States and Canada. Human infection begins after an incubation period of 5‑10 days with fever, headache, nausea and malaise, and may progress to encephalitis with altered consciousness, seizures and coma. Case‑fatality rates in humans are estimated at 3–15%, lower than those of Eastern Equine Encephalitis virus but still substantial. Survivors, particularly young children, may have residual neurologic deficits. Horses typically show fever, anorexia, ataxia and eventually recumbency due to encephalomyelitis. Because WEEV infections have become rare in recent decades, clinicians may be unfamiliar with its presentation. Control measures focus on vaccination of horses, surveillance of mosquito populations, and reduction of vector breeding sites through water management and insecticide application. Personal protective measures, including repellents and protective clothing, help reduce human risk. Western Equine Encephalitis virus is an example of how ecological and evolutionary factors influence the emergence and decline of arboviral diseases. Continued surveillance and mosquito control are essential to prevent resurgence of this historically significant pathogen. Related Terms: Eastern equine encephalitis virus, Venezuelan equine encephalitis virus, St. Louis encephalitis virus, West Nile virus, Alphavirus