A number of diseases, such as St. Louis, Japanese B, and equine encephalitis, which are caused by arthropod-borne viruses (abbreviated “arboviruses”). In their most severe human forms, the diseases invade the central nervous system and produce brain damage, with mental confusion, convulsions, and coma; death or serious aftereffects are frequent in severe cases. Inapparent infections are common.
The arbovirus “group” comprises more than 250 different viruses, many of them differing fundamentally from each other except in their ecological property of being transmitted through the bite of an arthropod. A large number of arboviruses of antigenic groups A and B are placed in the family Togaviridae, in two genera, alphavirus (serological group A) and flavivirus (serological group B). Still other arboviruses, related structurally and antigenically to one another but unrelated to Togaviridae, are included in the family Bunyaviridae, consisting chiefly of the numerous members of the Bunyamwera supergroup—a large assemblage of arboviruses in several antigenic groups which are cross-linked by subtle interrelationships between individual members. The nucleic acid genomes of all arboviruses studied thus far have been found to be RNA.
Members of serological group A include western equine encephalitis, eastern equine encephalitis, and Venezuelan equine encephalitis viruses; and Mayaro, Semliki Forest, Chikungunya, and Sindbis viruses, which have nonencephalitic syndromes. Group A viruses are chiefly mosquito-borne. Serological group B viruses include Japanese B, St. Louis, and Murray Valley encephalitis viruses (mosquito-borne), and the viruses of the Russian tick-borne complex, some of which produce encephalitis (Russian spring-summer), whereas others cause hemorrhagic fevers (Omsk, Kyasanur Forest) or other syndromes, such as louping ill. Also in group B are the nonneurotropic viruses of West Nile fever, yellow fever, dengue, and other diseases. See Yellow fever
There is no proved specific treatment. In animals, hyperimmune serum given early may prevent death. Killed virus vaccines have been used in animals and in persons occupationally subjected to high risk. A live, attenuated vaccine against Japanese B encephalitis virus, developed in Japan, has been used experimentally with some success, not only in pigs to reduce amplification of the virus in this important vertebrate reservoir but also in limited trials in humans. In general, however, control of these diseases continues to be chiefly dependent upon elimination of the arthropod vector. See Virus