Vector studies on Rift Valley Fever virus in South Africa.

Abstract

SUMMARY Attempts to isolate Rift Valley fever virus from wild-caught haematophagous flies were made over an 8-year period in the coastal I·owlands of Natal, where the virus causes enzootic infection of cattle, and on infected farms in the inland plateau region of South Africa during epizootics in 1974/1975. In Natal the virus was not isolated from 71 561 mosquitoes, 10514 Culicoides spp., or from fewer numbers of simuliid, phlebotomine and tabanid flies. On the plateau there were 12 isolations from Culex theileri and 6 from 3 other mosquito species. There were no iso­ lations from 12 368 Culicoides spp., and smaller num­ bers of simuliids, tabanids and Stomoxys flies. In labo­ ratory transmission tests the virus was transmitted by 7 mosquito species: Cx. theileri, Cx. univittatus, Cx. quin­ quefasciatus, Aedes juppi, Ae. lineatopennis, Ae. aegypti and Eretmapodites quinquevittatus. Two mosquito species, Ae. caballus and Cx. rubinotus, became infected but failed to transmit by !lite. Cx. theileri was shown to be the most efficient vector. The tick, Ornithodoros savignyi, failed to become infected or to transmit virus by bite. It is concluded that Cx. theileri is the main epizootic vector among sheep and cattle on the plateau, where it probably causes some human infection. In the same region Ae. juppi and Ae. lineatopennis are believed to act as vectors of lesser importance. Failure to isolate the virus from mosquitoes in enzootic foci in Natal is in accordance with previously observed low isolation rates from mosquitos in such localities. It is concluded that there is no definitive evidence to indicate that mosquitoes are concerned in viral maintenance.