Sustainable control of nematode parasites – A New Zealand perspective

In many countries the presence of anthelmintic-resistant nematode parasites in sheep flocks is now the norm rather than the exception. Given this, resistance management should be an integral component of all parasite management plans. Production trials have shown a significant financial disadvantage from using an anthelmintic which has reduced efficacy due to resistance, and yet many farmers remain reluctant to test for the presence of resistant parasites.Much progress has been made in identifying practices which select for resistance, such as treatment on low-contamination pastures, use of long-acting anthelmintics and intensive grazing of monocultures of young livestock. The identification of high-risk practices has enabled alternatives to be found or, where no practical alternative is available, steps taken to mitigate the risk. Many resistance management strategies involve the deliberate retention of susceptible worm genotypes in refugia, and numerous approaches to achieve this have been evaluated. A major factor in determining how much refugia is required is the efficacy of the anthelmintic administered. Treatments which achieve high efficacy require less refugia to dilute the resistant survivors, and the most effective way to achieve high efficacy is to use combinations. Utilising refugia and combination anthelmintics can significantly slow the development of resistance. In New Zealand, an array of resistance management practices is now available, with many recommendations backed by empirical studies and evaluated on commercial farms. For sheep, the future focus will increasingly become one of extension and implementation of these practices on-farm.