The oral susceptibility of South African livestock-associated Culicoides species to selected orbiviruses

Culicoides (Diptera, Ceratopogonidae) midges play an essential role in transmitting orbiviruses. Recent outbreaks of bluetongue (BT) in Europe highlighted the risk for introduction and rapid spread of vector-borne diseases outside their traditional boundaries, and increase international interest in arbovirus epidemiology. African horse sickness virus (AHSV) and bluetongue virus (BTV) cause diseases of high socio-economic impact, especially on international trade. Identifying potential vectors is crucial for the implementation of integrated control measures, disease risk analysis and management. Determination of oral susceptibility of Culicoides species to infection with orbiviruses provides valuable data for assessing vector competence. The aim of this work was to 1) determine oral susceptibility of livestock associated midges to infection with wild-type and live-attenuated strains of BTV and AHSV; 2) evaluate the efficiency of different light sources for the collection of Culicoides species; 3) compare laboratory blood-feeding methods; 4) identify potential vectors and 5) determine field infection prevalence in Culicoides species in the winter rainfall region of South Africa during an outbreak of AHS. Although black light collection did not have any influence on the agegrading determination of a Culicoides population, it was more effective than white light for the collection of adult Culicoides midges. Cotton wool feeding yielded lower infection rates than membrane feeding due to the greater blood meal size taken by Culicoides females fed through the latter. Virus recovery of reference and vaccine strains of BTV was higher in Culicoides bolitinos than in Culicoides imicola. There was no significant difference between the oral susceptibility of C. bolitinos and C.-imicola for the various AHSV isolates used. Virus recovery of the vaccine strain of AHSV serotype 7 (AHSV-7) from C. imicola at Onderstepoort was higher than that of the field strains of AHSV-7. Culicoides imicola from the eastern Free State was more susceptible to AHSV than in Gauteng. Both BTV and AHSV were recovered from non-Avaritia Culicoides species. These results indicate BT and AHS to be multi vector diseases and add to the complexity of the epidemiology of orbiviruses. True assessment of vector competence might be difficult to assess if it would require some level of real-time monitoring, e.g. testing local Culicoides populations using variants of orbiviruses in current circulation. Light trap surveys in the winter rainfall area of South Africa during an outbreak of AHSV demonstrated that C. imicola is superabundant and occurs in numbers to be equal or even higher than that in the summer rainfall areas. Results of oral susceptibility studies and the recovery of different orbiviruses, indicate this population of C. imicola to be highly vector competent.