Localization and migration of benzimidazole resistant and susceptible adult Cooperia curticei in the small intestine of sheep after treatment with thiabendazole.

Four groups of three lambs per group were experimentally infected with Cooperia curticei susceptible (two groups) or resistant (two groups) to benzimidazoles, and distributions of adult worms in the small intestine were studied. For each Cooperia isolate, one group was treated with thiabendazole (TBZ) (5 or 50mg/kg bodyweight) 28 days after infection. In the two untreated groups, the population of C. curticei were present from the second to the tenth meter of intestine from the pylorus with a maximum in the sixth meter for both isolates. After treatment with TBZ, the size of the resistant worm population did not significantly decrease but a large number of worms were found towards the proximal sections of the intestine. In contrast, the susceptible population was reduced by about 40% but the surviving worms remained at this same site of predilection after treatment. Measurements of the concentration of TBZ and 5OH-thiabendazole (5OH-TBZ) in the intestinal segments do not indicate a clear relationship between the localization of worms and TBZ or 5OH-TBZ concentrations at least 12h after the anthelmintic treatment. The hypothesis of an enhanced expression of the mechanisms of resistance in the first few meters of small intestine is suggested.