A comparison of the vector competence of the biting midges, Culicoides (Avaritia) bolitinos and C. (A.) imicola, for the Bryanston serotype of equine encephalosis virus

Equine encephalosis virus (EEV) is widespread and prevalent in southern Africa. In this study, the oral susceptibility of Culicoides (Avaritia) imicola Kieffer (Diptera: Ceratopogonidae) to EEV was confirmed. In addition, C. (A.) bolitinos Meiswinkel, collected in the high-lying eastern Free State, South Africa, was systemically infected with the Bryanston serotype of EEV after feeding through a membrane on artificially infected equine blood containing 4.7 log(10) PFU/mL of EEV. The mean infectivity of Bryanston virus in C. bolitinos increased from 1.2 log(10) PFU/midge, in midges assayed for virus immediately after feeding on the blood-virus mixture, to 3.1 log(10) PFU/midge in midges assayed after 10 days' incubation at 23.5degreesC. Elevated virus infectivity titres, found in individual infected C. bolitinos, suggested that this Culicoides species is a vector of EEV. This bovine dung-breeding Culicoides species may play an important role in transmitting EEV in the cooler parts of southern Africa, where it can be the most abundant Culicoides species collected near livestock. In the present study the prevalence of infection obtained for C. bolitinos (2.2%) with the Bryanston serotype of EEV was significantly lower than that of C. imicola (18.4%). After incubation, the Bryanston serotype of EEV was also isolated from one of 110 C. onderstepoortensis Fiedler assayed. However, the virus titre in this midge was 1.2 log(10) PFU/midge, which is not different from the titre that would be expected immediately after feeding on the blood-virus mixture. Culicoides species that survived the incubation period and that were negative for the presence of Bryanston virus were C. magnus Colaco (96), C. bedfordi Ingram & Macfie (95) and C. pycnostictus Ingram Macfie (45).