Western equine encephalitis is a rare mosquito borne disease that affects both humans and horses. Although the virus that causes the disease is closely related to the one that causes Eastern equine encephalitis, the diseases can be distinguished by their causative agent, although they are transmitted in an identical manner.
Despite its name, and the fact that it is more prevalent in the United States west of the Mississippi River (where it was first identified) the condition is found worldwide anywhere that swampy conditions provide habitat for mosquitos.
As is suggested by its name, the disease manifests as encephalitis but because there is no effective vaccine or pharmaceutical treatment, treatment can only be supportive until the patient's own immune system can overcome it. It is far more dangerous to horses than to humans.