Epidemiology and vectors Vet. Ital., 40 (3), 296-302 296 Veterinaria Italiana, 40 (3), 2004 Multiple vectors and their differing ecologies: observations on two bluetongue and African horse sickness vector Culicoides species in South Africa R. Meiswinkel (1) , K. Labuschagne (2) , M. Baylis (3) & P.S. Mellor (3) (1) Instituto Zooprofilattico Sperimentale dell’Abruzzo e del Molise ‘G. Caporale’, via Campo Boario, 64100 Teramo, Italy (2) Onderstepoort Veterinary Institute, Private Bag X05, Onderstepoort 0110, South Africa (3) Institute for Animal Health, Pirbright Laboratory, Pirbright, Surrey GU24 0NF, United Kingdom Summary Blacklight traps were used to collect Culicoides biting midges weekly between September 1996 and August 1998 at 40 sites distributed equidistantly across South Africa. The seasonal and geographic prevalences of 86 species of Culicoides were elucidated simultaneously, and included C. imicola Kieffer and C. bolitinos Meiswinkel the principal vectors of bluetongue (BT) and African horse sickness (AHS) in the region. These two species were amongst the most prevalent Culicoides to be found and, together, comprised >50% of the more than three million biting midges captured. The data are presented as coloured matrices, and are transformed also into inverse distance weighting (IDW) interpolative maps. The data reveal that the prevalence of each vector is somewhat fractured and it is posited that this is (in part) due to significant differences in their respective breeding habitats. The results illustrate also that the presence of multiple vectors (in any region of the world) will complicate both the epidemiology of the orbiviral diseases they transmit and the formulation of rational livestock movement and disease control strategies. This is especially true for southern Europe where the recent devastating cycle of BT has been shown to involve at least three vectors. Finally, the influence that man has on the development of large foci of vector Culicoides around livestock may be less important than previously suggested but must be investigated further. Keywords Culicoides vectors – Culicoides imicola – Culicoides bolitinos – Seasonal distribution – Geographic distribution – Interpolative maps – South Africa – Vector. Introduction Pantropically some 30 species of Culicoides biting midges are involved in the transmission of orbiviral diseases injurious to livestock; these diseases include bluetongue (BT) and African horse sickness (AHS). Because of their negative economic impact there is a great need to model the distributions of the insect vectors involved, not only for disease control, but also for predictive purposes. To obtain data that are reliable, modellers depend heavily upon an accurate taxonomy as each vector species, in occupying a specific niche in nature, possesses a unique biology that determines its distribution in both space and time. However, accurate large-scale species-linked data sets are surprisingly few for Culicoides. Equally cogent is that these diseases are, in all regions, being transmitted by more than one vector species. Not only does this complicate disease epidemiology, but also invalidates (partially) ‘single vector’ risk models. As to the involvement of multiple vectors, South Africa is a case in point: 60 years were to pass before it was realised that C. imicola is not the only vector of BT and AHS (5) but that C. bolitinos is also involved (11). To assess objectively whether the relative abundances of these two species, and their seasonal and geographic prevalences, differed significantly, a network of 40 blacklight traps was established and operated weekly across South Africa from September 1996 to August 1998. The data reveal the two species to occur widely but profound differences