Selection studies on anthelmintic resistant and susceptible populations of Trichostrongylus colubriformis of sheep

Waller P. J., Dobson R. J., Donald A. D., Griffiths D. A. and Smith E.F. 1985. Selection studies on anthelmintic resistant and susceptible populations of Trichostrongylus colubriformis of sheep. International Journal for Parasitology15: 669–676. A T. colubriformis population (BCK), formerly resistant to benzimidazole anthelmintics, but now highly resistant to levamisole after 6 years exposure to this drug alone in the field, was passed through 12 generations in the laboratory in three separate lines exposed either to selection with thiabendazole or levamisole, or to no selection. Another population (McM) not previously exposed to these anthelmintics was treated similarly in two lines, selected with thiabendazole or not selected.Selection with thiabendazole resulted in a return of benzimidazole resistance in the BCK line which occurred faster than in the McM line, but a similar level of resistance was reached in each by the twelfth generation. Resistance ratios in both selected lines compared with the unselected McM line were less than 20: 1, and only 1.5 times the recommended dose rate of thiabendazole was required to remove more than half of the resistant population. This suggests that a polygenic vigour tolerance rather than a specific resistance had been selected.In the case of levamisole resistance, the BCK population was found to contain two distinct subpopulations, one susceptible and the other highly resistant. Resistance ratios for the highly resistant subpopulation were greater than 4000: 1, implying a specific resistance controlled by a major gene. During the 12 generations of levamisole selection, the proportion of resistant phenotypes fluctuated about an average level of 70%, suggesting that susceptibility alleles were being maintained in the population through superior heterozygote fitness. This conclusion is supported by a significant decline in levamisole resistance in the absence of levamisole selection. Moreover, thiabendazole selection hastened the reversion to levamisole suceptibility.The results provide support for the reintroduction of a benzimidazole anthelmintic to control this helminth population, and for a slow rotation in the use of drugs with different modes of action.