The arthropod-borne viruses, the so-called arboviruses, are transmitted between vertebrates by blood-feeding arthropod vectors of which mosquitoes transmit the !argest number of vlruses. The viruses multiply in both vertebrates and vectors. Normally, the viruses must produce viraemia in vertebrates to lnfect vectors, and must infect the salivary gland of vectors with excretion of virus in the saliva to infect vertebrates.
The arboviruses belong to several virus families and genera, and within these taxonomic categories various informal groups and subgroups are recognised. The latter two categorics are based on the immunological relationship of each virus to one or more others within the particular group. These relationships are important in serological diagnosis as thev can confuse the interpretation of the immune response in patients who have had sequential infections by two or more related viruses.
Twenty on mosquito-borne viruses have been isolated in southern Africa, of which 9 are known to have caused an illness in humans. In addition, dengue virus, probably introduced into southern Africa, has infected man in Natal. With all these viruses man is regarded as an incidental host in that humans are not usually concerned in the maintenane of the virus in nature. None the less, in the case of some viruses humans become highly viremic and readily infect mosquitoes. The occurence of high viraemia in humans does have the important effect of permitting human outbreaks to proceed independently of any transmission cycles involving animals which may have initiated the outbreak.
The viruses and their dieases are discussed under the respective viral families and genera.