Changes in resistance of Ostertagia spp. to thiabendazole following natural selection or treatment with levamisole

Abstract

Two strains of Ostertagia spp. were monitored for changes in the degree of resistance to thiabendazole (TBZ) following either no anthelmintic selection or treatment with levamisole (LEV) over a 4-year field study or during six generations in the laboratory. Initially, the TBZ resistance ratio of one strain was estimated by egg hatch assays to be 14 times that of a similar unselected strain. After 4 years in the field the ratio was reduced to 8.8 by one LEV treatment during the summer each year and to 4.5 by 3-weekly LEV treatment. However, a controlled anthelmintic efficiency assay in the third and fourth year, using TBZ at a dose rate of 88 mg kg −1 in naturally infected animals, indicated that the level of resistance was still too high for adequate control by TBZ. No significant reversion occurred in this strain when TBZ treatment of grazing sheep ceased, or in the laboratory when passaged in the absence of anthelmintic, or selected with LEV. This suggested that LEV did not select in vivo against TBZ resistance but rather, the use of LEV in the field altered the population dynamics so that natural selection favoured the TBZ susceptible phenotype during free-living development and survival, especially over summer. After six generations of no anthelmintic selection in the laboratory, no significant reduction of TBZ resistance occurred in another strain of Ostertagia, but selection with LEV, in contrast to the first strain, produced a significant reversion in the degree of TBZ resistance. However, the level was still eight times that of a susceptible strain. In both strains, selection with LEV produced a reduction in TBZ resistance too slowly to consider the reintroduction of this compound for parasite control. It would be prudent, therefore, for producers relying on anthelmintics as the main defence against parasites, to introduce a multiple anthelmintic attack programme before the degree of resistance to one compound reaches a high level.