Equine viral encephalitis.

The most important neurotropic viral infections of the horse are the arthropod-borne encephalitides. These include Venezuelan encephalitis (VE), eastern encephalitis (EE) and western encephalitis (WE), which are found in the Americas, and Japanese B encephalitis which occurs in the Far East. All the viruses cause encephalitis in man. Between 1969 and 1972 an epidemic of VE occurred in Central America. In 1971 the disease was reported in Texas, where it was brought under control by the vaccination of susceptible horses with an attenuated live virus vaccine and by the reduction of the mosquito population with insecticides sprayed from aircraft. A high titre viraemia occurs with VE virus in the horse and epidemics are maintained by a mosquito/horse cycle; infection of man and other species is incidental. EE and WE have been recognised as separate diseases since 1933 and in the U.S.A. horses are protected by routine vaccination. Epidemics of these diseases are routine vaccination. Epidemics of these diseases are now uncommon. In contrast with VE, both EE and WE viruses are maintained by a bird/mosquito cycle. The viraemia in the horse is generally considered insufficient to infect mosquito vectors; the horse is a "dead end host". Several species of mosquito can act as vectors of VE, WE and EE. The extension of other arthropod-borne diseases to areas originally outside their geographical distribution (e.g. bluetongue in sheep) serves to illustrate the potential of VE, WE and EE to cause disease on other continents.