The susceptibility of Ostertagia and Cooperia to morantel tartrate after extended exposure to the morantel sustained release bolus.

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Abstract The effect of continued use of the morantel sustained release bolus on the susceptibility of Ostertagia and Cooperia to morantel tartrate was investigated under laboratory and field conditions. The efficacy (ED 90 ) of morantel tartrate against a laboratory strain of Ostertagia was determined by dose response titration in controlled experiments before and after five passages through calves in the presence of boluses modified to release morantel on a continuous basis at rates determined to be sublethal to Ostertagia . There was no significant difference between initial susceptibility (ED 90 2.7mg/kg) and final susceptibility (ED 90 3.3mg/kg). In a field study, the morantel sustained release bolus was administered at turnout to a group of 10 cattle that grazed pasture which had been grazed by bolus treated cattle for the previous three seasons. An equal group of nontreated cattle grazed an adjacent identical pasture which had been grazed by nonmedicated control cattle for the previous three seasons. Tracer calves as well as principal cattle grazing the treated and control pastures were housed for three weeks following which half of the cattle from each pasture were treated orally with 2.5 mg/kg morantel tartrate. Efficacy comparisons of worm counts between morantel treated and nontreated animals and from the tracer calves grazing treated and control pastures demonstrated that the efficacy of morantel against the population of Ostertagia exposed to the bolus was equal to the efficacy against the unexposed population. Under the conditions of these studies the susceptibility of Ostertagia to morantel was not altered by the repeated application of the morantel sustained release bolus in four successive seasons.