Author’s Note: My grandfather and his neighbors in southern Idaho lost several work horses to “brain fever” in the 1930s. My grandfather’s family managed to save one mare; they tied her in the barn, supported her with a sling, and secured ice packs to her head during her fever and delirium. She survived, but was never normal. My father, whose chores as a young boy included driving the work teams, didn’t enjoy working with her; she had a tendency to travel crookedly, and it was difficult for her to turn to the left. Today the mare probably would have been diagnosed with encephalitis.
Eastern, Western, and Venezuelan equine encephalitis (EEE, WEE, and VEE) are arboviruses–meaning they are transmitted by blood-feeding insects or ticks–and these pathogens can cause fatal neurologic disease in horses. EEE, WEE, and VEE are all alphaviruses that are a part of the Togavirus family (Togaviridae), and collectively they are termed the equine viral encephalomyelitides. West Nile virus is also an arbovirus, but it is a member of the Flaviridae family, which is named after yellow fever virus. We will confine our in-depth discussion to the Togaviruses in this article.
All of these viruses have public health implications because they are zoonotic, meaning they can be communicable from animals to humans under natural conditions. EEE, WEE, and WNV can be spread from birds to humans via mosquitoes. Infected horses cannot pass these diseases to other horses, animals, or humans, however, so equine infection is generally indicative of a virus being active in a geographic area. EEE is a reportable disease in most states for this reason. VEE, however, is even more serious, as it can be transmitted from horses to mosquitoes to humans.