The virtual absence of Culicoides imicola (Diptera: Ceratopogonidae) in a light-trap survey of the colder, high-lying area of the eastern Orange Free State, South Africa, and implications for the transmission of arboviruses.

Altogether 52 078 Culicoides biting midges of 35 species were collected during February 1990 and 1993 in 40 light-trap collections made on 17 cattle and/or sheep farms in the Bethlehem and Fouriesburg districts of the colder, high-lying eastern Orange Free State. Culicoides (Avaritia) bolitinos was by far the most abundant species, representing 50.9% of all specimens collected. Culicoides (A.) imicola, considered to be the most common stock-associated species in the summer rainfall areas of southern Africa, and the only proven vector of bluetongue virus (BTV) and African horsesickness virus (AHSV) in the subregion, was uncommon, comprising only 1.4%. While AHS is apparently absent, BT and bovine ephemeral fever (BEF) are endemic in this cooler, high-lying area of South Africa. The virtual absence of C. imicola implies that other Culicoides species, such as C. bolitinos and C. cornutus, may be involved in transmitting BT virus (and perhaps BEF) in the eastern Orange Free State, and possibly elsewhere in Africa. Virus isolation attempts made on 45 single species pools of C. bolitinos, C. pycnostictus, C. milnei, C. leucostictus, C. zuluensis and C. gulbenkiani were, however, negative. Finally, 20 of 28 blood-engorged Culicoides of 11 species, which were tested against cattle, sheep, horse, pig and bird antisera, tested only positive against cattle antisera.