Chapter 19 - The equine encephalitides

Abstract

Abstract The viruses associated with the equine encephalitides – Eastern, Western, and Venezuelan equine encephalitides – are not primarily agents affecting horses, nor are they normally agents of humans. Rather, these are agents which circulate between insect vectors and reservoir species of birds or small mammalian hosts. Horses, humans, and certain other species are incidental targets of infection and are for the most part end hosts. Although these viruses are unusual causes of human illness, they are important pathogens because of their pathogenicity in individual patients and their potential to produce epidemic disease. Eastern equine encephalitis virus, although the least common of the three, is the most dangerous, with a relatively low rate of asymptomatic or minor illness compared to cases of encephalitis, and with the highest rate of mortality. Venezuelan equine encephalitis has traditionally caused the most extensive epizootic infections in horses and the most widespread epidemic infections in humans. Western equine encephalitis virus is widely disseminated in the United States west of the Mississippi River. Despite being present in a large geographic area, however, and despite its documented ability to cause epidemic disease, human infections with Western equine encephalitis virus have become increasingly less common, and no human case has been reported since 1994. This chapter will discuss these three viruses in terms of their virology and their ability to cause infection in animals and humans.