VECTOR COMPETENCE OF CULICOIDES-BOLITINOS AND CULUCOIDES-IMICOLA FOR SOUTH-AFRICAN BLUETONGUE VIRUS SEROTYPE-1, SEROTYPE-3 AND SEROTYPE-4

The susceptibility of field-collected Culicoides bolitinos to infectio n by oral ingestion of bluetongue virus serotypes 1, 3 and 4 (BLU 1, 3 and 4) was compared with that of field-collected C. imicola and labor atory reared C. variipennis sonorensis. The concentration of the virus per millilitre of bloodmeal was 10(5.0) and 10(6.0)TCID(50) for BLU 4 and 10(7.2)TCID(50) for BLU 1 and 3. Of 4927 C. bolitinos and 9585 C. imicola fed, 386 and 287 individual midges survived 10 days extrinsic incubation, respectively. Midges were assayed for the presence of vir us using a microtitration assay on BHK-21 cells and/or an antigen capt ure ELISA. Infection prevalences for the different serotypes as determ ined by virus isolation ranged from 22.7 to 82.0% in C, bolitinos and from 1.9 to 9.8% in C. imicola; infection prevalences were highest for BLU 1, and lowest for BLU 4 in both species. The mean log(10) TCID50 titre of the three BLU viruses per single fly was higher in C, bolitin os than in C. imicola. The results suggested that C. bolitinos populat ions are capable vectors of the BLU viruses in South Africa. A high co rrelation was found between virus isolation and ELISA results for the detection of BLU 1, and less for BLU 4; the ELISA failed to detect the presence of BLU 3 in infected flies. The C. v. sonorensis colonies ha d a significantly lower susceptibility to infection with BLU 1, 3 and 4 than C. bolitinos and C. imicola. However, since infection prevalenc e of C. v. sonorensis was determined only by ELISA, this finding may m erely reflect the insensitivity of this assay at low virus titres, com pared to virus isolation.