Orbivirus detection from Culicoides collected on African horse sickness outbreaks in Namibia.

African horse sickness (AHS), a non-contagious infectious disease caused by a RNA virus in the Orbivirus genus within the Reoviridae family affecting all equids, is endemic in sub-Saharan Africa. The virus is transmitted by some species of biting midges in the genus Culicoides (Diptera: Ceratopogonidae). In April 2011, 8 Culicoides collections were performed in 6 districts of 4 regions of the Republic of Namibia (Africa), all within a 400 km radious from the capital Windhoek. Six farms - Khomas (Windhoek and Steinhausen), Erongo (Karibib and Omaruru), Otjozondjupa (Okahandja), and Omaheke (Gobabis) involved in the AHS outbreaks, were sampled. Overall 194,211 Culicoides were collected and identified. Culicoides imicola was largely the most abundant species at all farms (99.4%). A total of 18,687 parous and gravid Culicoides females were assayed for AHS virus (AHSV) by real time RT-PCR. Of the 248 assayed pools, 227 consisted of C. imicola, 13 of Culicoides pycnostictus and 5 of Schultzei complex. Only 1 pool each of Culicoides nivosus, Culicoides leucostictus, and Culicoides tropicalis was assayed. Of the 248 pools examined by real time RT-PCR, 81 tested positive for AHSV, all consisting of C. imicola collected at Omaruru, resulting in a field vector infection rate of 0.91%. No viable AHSV could be isolated from 88 of the tested pools (n = 1,463). However, bluetongue virus (BTV) serotype-1 and 10 were isolated from 3 of these pools, each consisting of 100 C. imicola collected at Windhoek. The present study confirms the relative low infection prevalence in field collected Culicoides and the strict relationship between the high abundance of C. imicola and outbreaks of AHSV.