Discovery of a Culicoides imicola-free zone in South Africa: preliminary notes and potential significance.

In December 1993, a light-trap survey was made of the Culicoides found at eight horse stables and dairies in the sandy dune field west of Port Elizabeth, South Africa. While it was notable that Culicoides numbers were low (4749) and that the diversity was poor (15 species), the most remarkable fact to emerge, was that C. imicola, the only proven vector of the virus of African horse sickness (AHS), was entirely absent. Though not abundant, C. bolitinos, a sister species of C. imicola, was overwhelmingly dominant (91.7%). Its larvae and pupae develop exclusively in the dung of cattle, but it is a species that is not implicated in the transmission of animal viruses. Elsewhere in South Africa, a frost-free climate, good rainfall and a plentiful supply of livestock would normally lead to the development of large foci of C. imicola. That this is not the case in the Port Elizabeth (P.E.) area is most likely owing to the winds inhibiting adult flight and the sandy soils being nutrient-poor and too well-drained to sustain Culicoides larvae. Studies are needed to confirm that sandy soils cannot sustain C. imicola. If so, the sandy coastal areas hold promise for quarantining against AHS.