Occurrence of bluetongue virus in Culicoides midges collected at Mnisi, Mpumalanga Province, South Africa.

Abstract

Background: Bluetongue (BT) is a non-contagious, vector-transmitted disease caused by the bluetongue virus (BTV) of the orbivirus genus in the family Reoviridae. BT is a disease mainly of ruminants with sheep being the most susceptible. The vector-borne spread of BTV by adult females of specific Culicoides species comprises the most important route of transmission of BT. The occurrence of different Culicoides vectors and BTV serotypes varies throughout the world. Throughout the world, sequential identification of BTV is performed. At present, BTV is considered present on all continents except AntarcticaAt present, there are 26 different serotypes of BTV recognised, with the majority being originally of African origin. Aim of the study: The aim of this project was to identify which Culicoides species are present in different areas of Mnisi within the Mpumalanga Province in South-Africa and to determine whether BTV is present in those Culicoides midges. Methods: Biting midges of the genus Culicoides were caught using Onderstepoort black light traps. A total of 48 collections were made at 10 different trap sites in Mnisi during 13 sampling nights between June 18 and July 10 2013. The midges were allocated in pools of approximately 200 midges per pool. One pool per trap site were morphologically pre-identified to species, gender and parity level by an entomologist. Midges were homonogised, RNA extraction was performed by adding Trizol reagent to the insect debris. RNA purification of the Trizol reagent extracts was performed by using Qiagen RNeasy Minelute columns according to the protocol of the manufacturer. To determine the presence of BTV, a RT-PCR assay using BTV segment 10 as target was used. Results: A total of 18 561 midges were captured. After differentiation of 1 614 midges (8.7% of total midges caught), 20 Culicoides species were recognized, with C. imicola being the dominant species. A total of 48,5% of differentiated midges were either parous (26,6%), bloodfed (0,7%) or gravid (21,2%), with the remaining being nulliparous (35,4%) or male (16%). The qRT-PCR tested positive for BTV segment 10 in 63 out of 82 midge pools. A total of 19 samples did not show amplification of BTV. Conclusion: The parity rate found in this research are quite high, which serves as an indicator of increased survival rates in a population. Therefore, midges might have been feeding frequently on nearby livestock. The qRT-PCR tested positive for BTV segment 10 in 63 out of 82 midge pools. A total of 19 samples did not show amplification. The infection rate of BT in Culicoides midges in this study is estimated to be 0,73. These results show that BTV is present within Mnisi. However, positive qRT-PCR results do not reflect that vector competent Culicoides midges are present to transmit BT to other animals since midge species were not tested separately. To address the question whether vector competent Culicoides midges are present within Mnisi, virus isolations of individual BTV-positive midges and oral susceptibility studies are indicated.