The evolution of benzimidazoles (BZ) resistance in Teladorsagia circumcincta was investigated in a controlled trial with lambs, submitted to different treatment regimens. Four paddocks were seeded with a T. circumcincta strain constituted by 25% of BZ-resistant nematodes. Ten permanent lambs were allocated to each paddock, from April to November in order to renew the contamination of pasture. Monthly, three tracer lambs were allocated in each paddock. BZ-resistant nematode frequency was determined (PCR diagnosis). The faecal egg count reduction test (permanent lambs) and the number of nematodes in lambs were also determined (permanent and tracer lambs). Four different regimens of treatments were performed: control, levamisole (a non-BZ drug), fenbendazole (a BZ drug), and an alternation of levamisole and fenbendazole every second treatment. The same protocol was repeated on two consecutive grazing seasons, increasing the number of treatments (3 in first year and 5 in second year). The proportions of BZ-resistant nematodes did not change during all the study in both the control and the levamisole paddocks, supporting an equal global fitness of BZ-resistant and susceptible nematodes. Thus, no reversion of BZ resistance is to be expected. In the alternated drug group and in the BZ treated group, BZ-resistant nematodes increased from 25% to 47% and to 78%, respectively. BZ resistance increased proportionally to the selective pressure (number of BZ treatments). The drug alternation is not a good solution to delay importantly the evolution of resistance when more than 25% of nematodes are BZ-resistant. This study is the first evaluation of BZ-resistance evolution (using individual genotyping) in controlled conditions. It showed that when a monogenic anthelmintic resistance is established at 25% in a sexually reproducing nematode population, it seems to be impossible to prevent its increase even when using limited number of BZ treatments.
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