After a first description in 1975 of a strain of Haemonchus contortus resistant to the benzimidazole anthelmintics in South Africa, further strains of the same species developed resistance to rafoxanide and closantel, and a strain of Ostertagia species became resistant to the benzimidazoles. Recently five further strains of H contortus have been found which show resistance to these anthelmintics. Of these, four showed varying degrees of resistance to ivermectin and one was resistant to three anthelmintic groups. One of the strains became resistant to ivermectin very rapidly despite effective rotation of this anthelmintic with chemically unrelated anthelmintic compounds.
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