Western equine encephalomyelitis (WEE) virus belongs to the genus Alphavirus of the family Togaviridae. WEE virus is present in the Americas and can cause disease in both humans and equids with encephalitis in most clinical cases. WEE is typically maintained in nature by alternating between vertebrate hosts and vector mosquitoes. Encephalitis caused by this virus occurs sporadically in horses and humans from mid-summer to late autumn in temperate regions but can occur year-round in tropical regions, depending on climate conditions that support the presence of the mosquito vector. Clinical disease in horses is characterised by fever, anorexia, and depression. In severe cases, it can progress to hyperexcitability, blindness, ataxia, severe mental depression, recumbency, convulsions, and death. WEE virus can cause a subclinical or mild disease with less than 30% mortality. The principal reservoir hosts for the virus are passerine birds. Most infections in birds are nonclinical, but WEE virus has been reported to cause disease in poultry, game birds and ratites. Horses and humans are incidental dead-end hosts for WEE virus. WEE vaccines are safe and immunogenic.