HOW LONG BEFORE RESISTANCE MAKES IT IMPOSSIBLE TO CONTROL SOME FIELD STRAINS OF HAEMONCHUS-CONTORTUS IN SOUTH-AFRICA WITH ANY OF THE MODERNANTHELMINTICS

This paper describes an exceptional spectrum of multiple anthelmintic resistance in two strains of Haemonchus contortus in South Africa, one from Howick in KwaZulu-Natal, and the other from Badplaas, in Mpumala nga. Apparently for the first time, a helminth strain is described wit h resistance to compounds from all five of the modern anthelmintic gro ups used for nematode control in sheep; also, two strains of H, contor tus show resistance to the two substituted phenols, dinitrophenol and nitroxynil, Only closantel at 5 mgkg(-1) of nine compounds tested appe ars to have undiminished efficacy against the Howick strain, but even in the case of closantel, the residual activity and minimal effective level need to be tested before it can be concluded that its efficiency is unaffected, The exceptional resistance of the Howick strain is dem onstrated by the fact that sequential daily drenching of sheep infecte d with the strain, with levamisole at 18 mg kg(-1), oxfendazole at 20 mgkg(-1), levamisole at 20 mgkg(-1) and a mixture of fenbendazole at 1 0 mgkg(-1) plus trichlorfon at 132 mgkg(-1) on the fourth day, failed to clear sheep of the infection. There are strong indications that sid e-resistance occurs between dinitrophenol and nitroxynil, on the one h and, and the salicylanilides, on the other, and it is suggested that, before long, strains of H. contortus will be found with high levels of resistance to all the currently available anthelmintics. (C) 1997 Els evier Science B.V.