Vector competence of selected South African Culicoides species for the Bryanston serotype of equine encephalosis virus

Equine encephalosis virus (EEV) was recognized and described in the Republi c of South Africa in 1967 and subsequent serological studies have shown thi s orbivirus to be both widespread and prevalent in southern Africa. In the present study it was shown that wild-caught Culicoides (Avaritia) imicola K ieffer (Diptera: Ceratopogonidae) can become infected with and permit the r eplication of the Bryanston serotype of EEV following membrane-feeding on i nfective blood containing 5.0 log(10) plaque-forming-units (PFU)/ml. The me an prevalence of Bryanston virus infection in C. imicola after 10 days extr insic incubation at 23.5 degrees C was 22.3% (23/103). The mean infectivity of Bryanston virus in the infected C. imicola increased from 1.3 log(10) P FU/midge, in insects assayed immediately after feeding on the brood-virus m ixture, to 2.6 log(10) PFU/midge in insects assayed after incubation. The v irus concentration in individual C. imicola infected with the Bryanston ser otype of EEV ranged from 0.7 to 3.6 log(10) PFU/midge. Bryanston virus titr es higher than 2.5 log(10) TCID50, found in individual C. imicola, suggest that this species may be able to transmit this virus to susceptible hosts. Prevalence of virus infection in C. imicola was determined by PFU and micro titration assays on both BHK and Vero cells and confirmation of the Bryanst on serotype of EEV was determined by plaque inhibition. No virus replicatio n could be demonstrated in 102 C. nivosus tested after the incubation perio d, suggesting that not all Culicoides species are equally susceptible to Br yanston virus infection. Other Culicoides species that survived the incubat ion period and that were negative for the presence of Bryanston virus were C. pycnostictus (42), C. leucostictus (7), C. magnus (2), C. bolitinos (1) and C. bedfordi (1).