Transmission potential of South African Culicoides species for live-attenuated bluetongue virus

Field-collected Culicoides were fed on sheep blood-virus mixtures, each containing one of four live-attenuated vaccine strains of bluetongue virus (BTV), namely: BTV-1, BTV-4, BTV-9, and BTV-16. A South African field isolate of BTV-1 was used as the non-attenuated control virus. Titres of vaccine strains in blood meals ranged from 5.1 to 6.1 log(10)TCID(50)/ml; the titre of the field isolate of BTV-1 was 7.1 log(10)TCID(50)/ml. Recovery rates of vaccine viruses from Culicoides assayed immediately after feeding varied from 0% to 10.6%. This indicates that virus concentrations in blood meals were too low to ensure that all individuals ingested detectable amounts of virus. Thus, the oral susceptibility of Culicoides to infection with BTV vaccine strains determined in this study might be an underestimation. Of a total of 6 540 Culicoides that survived a 10-day extrinsic incubation period at 23.5 degrees C, 124 tested positive for BTV; 65 individuals yielded vaccine strains, and the remaining 59, the field isolate of BTV-1. Infection prevalences with the vaccine viruses ranged from 11.0% in C. bolitinos fed on blood containing 6.1 log(10)TCID/ml of BTV-1 down to 0.3% in C. imicola fed on a blood containing 5.3 log(10)TCID/ml of BTV-4. The infection rate for C. imicola and C. bolitinos fed on the field isolate of BTV-1 was 9.5% and 36.0%, respectively. In most infected midges the replication levels of vaccine strains were below the postulated threshold for a systemic infection with an orbivirus as previously calculated in the larger American vector, C. sonorensis (>2.5 log(10)TCID(50)/midge) but some individuals replicated BTV vaccine strains to high titres. This carries an implication that if ruminants become viraemic after vaccination with live-attenuated BTV vaccines, they might act as a source for the infection of Culicoides vectors.