The genus Onchocerca comprises approximately 30 species, all of which are obligate parasites of vertebrates and arthropods (Muller, 1979). Onchocerca volvulus is the only species to infect humans, the remaining members of the genus being almost exclusively parasites of wild and domesticated ungulates. Onchocerca parasites have skin-dwelling first-stage larvae, or microfilariae (mff), that are adapted to transmission by pool-feeding, haematophagous insects with scarifying mouthparts. Consequently, vectors of the various species are insects belonging either to the ceratopogonid midges or to the simuliids, commonly referred to as blackflies on account of the dark coloration of some (but by no means all) species. Blackflies are the vectors of human onchocerciasis (Bianco, 1991).