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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.

The anthelmintic activities of nitroxynil, closantel and rafoxanide were tested in cattle against six- and eight-week-old infections of Fasciola hepatica. Administered by subcutaneous injection, nitroxynil and rafoxanide at dose rates of 10 and 3 mg/kg bodyweight respectively were 88.5 and 36.1 per cent effective in eliminating a six-week-old infection of F hepatica and 85.8 and 60.4 per cent effective against an eight-week-old infection. Closantel administered by intramuscular injection at a dose rate of 2.5 mg/kg bodyweight was 60.0 per cent effective against an eight-week-old F hepatica infection.

Three hill sheep flocks were used to study the effect of the strategic use of rafoxanide to kill liver flukes before they attained the adult egg-laying stage. The programme ran from 1973 to 1976. In November 1976 the programme was stopped in one flock but was continued in the other two, which were subsequently merged. The results of frequent whole flock monitoring since 1976, described here, showed that in both flocks the level of infection, as judged by the examination of faeces for the presence of Fasciola hepatica eggs, remained at a very low level. It is postulated that the adoption of a strategic dosing programme over a period of three to four years would limit egg deposition on pastures close to eradication levels and that several years without needing to use anthelmintics against liver flukes would ensue.