Vector competence of South African Culicoides species for bluetongue virus serotype 1 (BTV-1) with special reference to the effect of temperature on the rate of virus replication in C. imicola and C. bolitinos.

The oral susceptibility of 22 South African livestock associated Culicoides species to infection with bluetongue virus serotype 1 (BTV-1) and its replication rate in C. imicola Kieffer and C. bolitinos Meiswinkel (Diptera: Ceratopogonidae) over a range of different incubation periods and temperatures are reported. Field-collected Culicoides were fed on sheep blood containing 7.5 log10TCID50/mL of BTV-1, and then held at constant different temperatures. Virus replication was measured over time by assaying individual flies in BHK-21 cells using a microtitration procedure. Regardless of the incubation temperatures (10, 15, 18, 23.5 and 30 degrees C) the mean virus titre/midge, infection rates (IR) and the proportion of infected females with transmission potential (TP = virus titre/midge > or = 3 log10 TCID50) were found to be significantly higher in C. bolitinos than in C. imicola. Results from days 4-10 post-infection (dpi), at 15-30 degrees C, shows that the mean IR and TP values in C. bolitinos ranged from 36.7 to 87.8%, and from 8.4 to 87.7%, respectively; in C. imicola the respective values were 11.0-13.7% and 0-46.8%. In both species the highest IR was recorded at 25 degrees C and the highest TP at 30 degrees C. The time required for the development of TP in C. bolitinos ranged from 2 dpi at 25 degrees C to 8 dpi at 15 degrees C. In C. imicola it ranged from 4 dpi at 30 degrees C to 10 dpi at 23.5 degrees C; no individuals with TP were detected at 15 degrees C. There was no evidence of virus replication in flies held at 10 degrees C. When, at various points of incubation, individual flies were transferred from 10 degrees C to 23.5 degrees C and then assayed 4-10 days later, virus was recovered from both species. The mean virus titres/midge, and proportion of individuals with TP and IR, were again significantly higher in C. bolitinos than in C. imicola. Also the infection prevalence in C. magnus Colaço was higher than in C. imicola. Low infection prevalences were found in C. bedfordi Ingram & Macfie, C. leucostictus Kieffer, C. pycnostictus Ingram & Macfie, C. gulbenkiani Caeiro and C. milnei Austen. BTV-1 was not detected in 14 other Culicoides species tested; however, some of these were tested in limited numbers. The present study indicates a multivector potential for BTV transmission in South Africa. In C. imicola and C. bolitinos the replication rates are distinct and are significantly influenced by temperature. These findings are discussed in relation to the epidemiology of bluetongue in South Africa.