Parasites costing New Zealand millions

Sheralee Cleland & Steve Lees demonstrate the CARLA Saliva Test, a tool to test for parasite resistance

The presence of worm parasites, or nematodes, is the biggest animal health issue for sheep (and most likely cattle) farmers in New Zealand. The cost of worms to the farming industry is estimated to be around $700 million per year.

Around 20 species of nematodes live in sheep, which makes the process of designing management strategies and preventing resistance to therapeutic drugs very difficult.

AgResearch’s long-term work has resulted in three broad approaches to managing anthelmintic resistance. The first is a strategy to identify and minimise high-risk parasite-control practices, the second is to identify practices that reduce resistance and the third is using combinations of drugs to combat resistance.

Minimising risk

One of the most common ways that farmers can select strongly for drug resistance is to treat adult sheep with long-acting products just prior to lambing – especially if whole flocks are treated. Only treating some flocks, or leaving the bigger, better conditioned animals untreated, allows some susceptible worms to survive and so reduces the selection rate of resistant worms.

Other practices identified as causing drug resistance include separating ewes and lambs at weaning and ‘finishing’ the lambs in isolation from other sheep, using long-lasting athelmintics and failing to apply adequate quarantine procedures when importing livestock onto a farm.

Maintaining ‘refugia’

One strategy for managing anthelmintic resistance is to maintain what are known as ‘refugia,’ whereby the majority of sheep are drenched for worms but a portion of the population is left without treatment. This strategy retains populations of drench-susceptible worms that can cross with resistant ones and dilute resistance.

Drug combinations

The third strategy that scientists are investigating is using combinations of
drugs to combat resistance. Combinations of drugs not only slow the development of resistance, they also help farmers maintain worm control in the presence of resistance to one or more classes of anthelmintics.

Breeding for resistance

AgResearch offers toolkits to help farmers select parasite resistance breeding stock. Follow the links below for more information about the following products: