Well over 350 arboviruses are distinguishable by serological procedures. Theiler and Downs list 293 as of 1973, allotting these to groups, illustrates the complexity of the subject, with 24 groups and the Bunyamwera supergroup, which includes another ten groups. In addition, there are four unassigned viruses in the Bunyamwera supergroup and 61 entirely ungrouped viruses. Since this compilation , there have been several new groupings proposed and several recently characterized viruses have been added, either to known groupings or to the "ungrouped" category. The American Committee on Arthropod-Borne Viruses published the Catalogue of Arthropod-Borne Viruses of the World, compiled by R. M. Taylor, in I967, and this was supplemented by additional listings in 1970 and 1971. A complete updated revision of the catalogue appeared in 1975. The above publications provide a mass of digested Information on viruses individually , including data on arthropod vectors, vertebrate hosts, isolation localities, pathogenesis and pathology, and laboratory propagation in vertebrates and cell cultures, and also provide condensed bibliographies for each virus.
The term arbovirus is defined in the ecological sense to include any virus of vertebrates that is biologically transmitted by arthropods. Arthropods include mosquitos, sandflies (Phlebotomus and related genera and Culicoides and related genera), hard (ixodid) and soft (argasid) ticks, and, of lesser importance to arbovirus epidemiology, horseflies (Tabanidae), blackfies (Simuliidae), and mites. Limited explorations of fleas, lice, hippoboscids, muscids, streblids, and nycteribids have not yielded encouraging leads. Biological transmission implies an obligatory phase of virus multiplication in the arthropod, before transmission to the next host. Mechanical transmission of an arbovirus can of course occur if an arthropod bites a viremic host with the arthropod mouthparts or foregut still virus contaminated. Such a mechanism has been postulated for hepatitis B virus transmission.
Mechanical transmission could in extraordinary arbovirus epidemic situations, be a significant adjunct to biological transmission, although this has never been demonstrated in an epidemic.