Experimental assessment of the vector competence of Culex (Culex) neavei Theobald with West Nile and Sindbis viruses in South Africa.

Culex univittatus is the maintenance vector of West Nile (WN) and Sindbis (SIN) viruses on the inland plateau of South Africa and also transfers infection to man. Both viruses have frequently been isolated from this mosquito and human immunity is high on the plateau. In the Natal lowlands where Cx univittatus is replaced by Cx neavei, apparently the main vector there, immune rates are low and few isolations have been made from this species. Vector competence experiments were carried out with Cx neavei to compare with those previously done with Cx univittatus. It was thought that a difference in such competence might explain the contrasting epidemiological situation in the two regions. After infective feeds on chicks with high viraemias of each virus, Cx neavei showed a high transmission rate. However, the 50% infection thresholds determined by exposing birds with differing viraemias to mosquitoes of this species, were 4.4 logs per ml (WN) and 5.8 logs (SIN). These thresholds are both higher than those for Cx univittatus. It is concluded that Cx neavei is a poorer vector of both viruses which accounts for the lower viral activity, including the lower incidence of human infection, in the Natal lowlands.