Ineffikacy of ivermectin againstCooperiaspp. infection in cattle

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Abstract
Ivermectin, either as an injectable or topical formulation, is used widely to control parasites in New Zealand cattle. These formulations have been well documented as having high efficacy against the major nematode species which infect tattle(1)(2)(3)(4), although the efficacy against adult Cooperia oncophora is reported to be 95%(5). This is considered to be one of the doselimiting species. There have been two reports of lower than expected efficacy of the topical formulation against Cooperia spp. in cattle. In a slaughter trial, Bisset et al. (6) found that ivermectin applied topically at the recommended dose rate of 500 𝛍g/kg failed to achieve a statistically significant reduction in the burden of Cooperia spp. worms compared with untreated calves. The geometric mean of adult Cooperia spp. worms present in the treated calves was 5788 compared with 7524 present in the untreated calves. This reduced efficacy was not attributed to acquired anthelmintic resistance because ivermectin had not been used on the farm previously. In Australia, Eagleson and Allertonc(7) found in controlled field trials that ivermectin applied topically at 500 𝛍g/kg ranged in efficiency in reducing faecal egg counts from 80.7% to 100% ; four out of 13 trials achieving less than 90% and a further four were in the range of 90–95% reduction. Larvae obtained from the faeces of ivermectin-treated cattle on days 11–14 after treatment were predominantly Cooperia spp.