Western equine encephalitis (WEE) is caused by a zoonotic arthropod-borne virus, WEE virus, which belongs to the Togaviridae family and genus Alphavirus. WEE virus is closely related to other arboviral members in the Togaviridae family, namely eastern equine encephalitis and Venezuelan equine encephalitis. Human cases of WEE have occurred in the western and midwestern United States, western Canada, and South America (Guyana, Ecuador, Brazil, Uruguay, and Argentina).
The enzootic cycle of WEE is maintained between passerine birds as the primary reservoirs and the mosquito vector Culex tarsalis. The epizootic transmission to horses and humans is mediated by specific mosquito bridge vectors such as Ochlerotatus melanimon in California, Aedes dorsalis in Utah and New Mexico, and Aedes campestris in New Mexico. Most cases of WEE infection have occurred in summer, from June to September in the United States and a month later in Canada. Similar to Venezuelan equine encephalitis virus, WEE virus can be transmitted through aerosols and so could be used as an agent in bioterrorism.
There has been a significant decline in human cases of WEE since 1988, and no cases have been reported since 2001 according to the US Centers for Disease Control and Prevention (CDC). This decline in incidence is thought to be due to improved mosquito control, a decline in the horse population, and high vaccination rates in horses.
Most human cases of WEE are asymptomatic. In symptomatic patients, a nonspecific febrile syndrome occurs after a 5-10 day incubation period.
Less than 1 in 1000 adult patients develop neuroinvasive encephalitis, but the risk is much higher in children (1:50) and infants (1:1). Typical features of encephalitis are headache, nausea, vomiting, neck stiffness, altered mental status, and seizures. Children and infants tend to have more severe neurologic symptoms.
The overall case fatality rate is estimated to be between 3% and 7% and is especially high in the extremes of age, ie, the young and the very elderly. Among survivors, neurologic sequelae are more common in infants younger than 1 year (sequelae such as intellectual disability, developmental delay) and in elderly patients (sequelae such as parkinsonism, dementia).
ICD10CM: A83.1 – Western equine encephalitis
SNOMEDCT: 47523006 – Western equine encephalitis
Differential Diagnosis & Pitfalls
The major differential diagnoses are several mosquito-borne arboviral encephalitides commonly seen in the United States.
West Nile virus (WNV) encephalitis (see WNV, viral encephalitides) – There is widespread distribution in the continental United States, and it is the most common arboviral infection in the United States. The majority of cases occur in late summer through fall. Neuroinvasive WNV can present as encephalitis, meningitis, flaccid paralysis, or a mixed pattern.
St. Louis encephalitis (SLE) – Elderly patients are more prone to symptomatic infections. Prior to the onset of neurologic symptoms, patients experience fever, generalized malaise, and myalgias. Some patients have dysuria, hematuria, or sterile pyuria. Neurologic manifestations include meningitis and encephalitis. Patients may present with myoclonus, tremors, opsoclonus, ataxia, or nystagmus. Cerebrospinal fluid (CSF) studies show lymphocytic pleocytosis. Magnetic resonance imaging (MRI) findings are nonspecific, although MRI may show T2 signal intensity in the substantia nigra.
Eastern equine encephalitis (EEE) – Most human cases in the United States occur along the Atlantic and Gulf coasts during summer. It has the highest mortality rate (as high as 75%) among arboviral encephalitides in the United States. Typical MRI brain findings include hyperintense signal abnormalities in bilateral symmetric basal ganglia, thalami, and brain stem.
Venezuelan equine encephalitis (VEE) – It has a widespread geographic distribution from Florida to South America. It has an average case fatality rate of 10% but neurologic recovery is excellent in survivors. VEE virus could be utilized as a biowarfare weapon since it is infectious via aerosols.