Culicoides imicola, vecteur du virus de sérotype 8 de la fièvre catarrhale ovine

Résumé

Based on its abundance near livestock and wide geographical distribution, one of the most important Culicoides virus vector species of orbiviruses is the Afro-Asiatic species C. (Avaritia) imi­cola. It is considered a proven vector of bluetongue virus (BTV) and is one of the world’s most widely distributed Culicoides spe­cies. Strong correlations were found between the presence of C. imicola and outbreaks of BT and African horse sickness (AHS) in Africa and BT in Southern Europe. In 2006, BT serotype 8 (BTV- 8) was responsible for the first outbreaks ever of BT recorded in Northern Europe. Outbreaks first appeared in the Netherlands and Belgium, and subsequently spread to Germany, France, and Luxemburg. The virus apparently overwintered in Northern Europe and, during 2007-2008, spread across the whole of Europe. The outbreaks in Northern Europe happened in the absence of C. imicola. Members of the Culicoides (Culicoides) pulicaris and C. (A.) Obsoletus complexes have been implicated in BTV transmission. In the present study, the oral susceptibil­ity of C. imicola and other South African livestock associated Culicoides species to various isolates of BTV-8 were determined by artificial feeding. During summer 2008, a Belgium and a recent (2004) South African field isolate of BTV-8 were fed to Culicoides collected at two localities in South Africa. Adult Culicoides midges were collected at the Agricultural Research Council; Onderstepoort Veterinary Institute (ARC-OVI) and at Clarens, in a cooler mountainous area of South Africa. At ARC-OVI the dominant species was C. imicola and at Clarens it was C. (A.) bolitinos. The South African field isolate was fed at con­centrations of 5.8 and 7.8 log₁₀TCID₅₀/mL in the blood meal and the Belgium isolate at 6.8 log₁₀ TCID₅₀/mL. The following summer (2009) the same South African (6.8 log₁₀ TCID₅₀/mL) and Belgium (6.8 log₁₀ TCID₅₀/mL), a Dutch (4.8 log₁₀ TCID₅₀/ mL) and a Spanish (6.1 log₁₀ TCID₅₀/mL) isolates of BTV-8 were fed to Culicoides species collected at ARC-OVI. In the same season, the South African and Belgium isolates were also fed to midges collected at Clarens. Virus recovery, after an incubation period of 10 days at 23.5°C in C. imicola, was low for all iso­lates for both years. The South African isolate (5.8 log₁₀ TCID₅₀/ mL) and the Spanish isolate of BTV-8 could not be recovered from 420 and 492 midges tested after incubation. The recov­ery rate for the other isolates ranged from 2/403 (0.5%) (2008: SA isolate) to 1/683 (0.1%) (2009: SA isolate). These relatively low recovery rates make direct comparison between isolates problematic. Virus recovery from C. bolitinos fed on the South African isolate (7.8 log₁₀ TCID₅₀/mL) was significantly higher than any of the other isolates. During 2008 the Belgium iso­late could be recovered from 4/229 (1.7%) C. bolitinos tested. Other Culicoides species from which BTV-8 could be isolated included C. (Meijerehelea) leucostictus, C. (A.) gulbenkiani and C. (unplaced) angolensis. Viral concentrations in infected C. imi­cola ranged from 0.7 to 2.4 and from 0.7 to 3.4 log₁₀ TCID₅₀/ mL in C. bolitinos. These results are in agreement with previous studies indicating a similarly low susceptibly in C. imicola for the reference strain of BTV-8. The relatively low oral susceptibility to near refractory status of C. imicola as determined for some of the isolates in this study will easily be compensated for by the high abundance of C. imicola in South Africa. Cumulative laboratory oral susceptibility results from South Africa indicate a consistently higher susceptibility in C. bolitinos for BTV as well as a multi-vector potential for BTV as well as AHSV. The unique biology of potential vector competent Culicoides species emphasizes the complex epidemiology of these diseases.