Internal parasites in sheep are a perpetual challenge to owners, especially for those operating in high rainfall or intensive conditions, and cost producers $369 million each year.

Resistance management (RM) is key, given major internal parasites have demonstrated the ability to rapidly develop significant levels of resistance to the commonly used drugs. This issue is compounded by the small range of drugs available to effectively treat these parasites, with only 8 classes of drench available plus the various combinations and mixes.
RM involves the strategic use of drugs in an effective Integrated Pest Management program which is designed to prevent pest populations from becoming a serious impediment to flock health. This is not easy as there are many variables in sheep flock care, with unpredictable climatic conditions and associated feed deficits worsening the situation through weight loss and grazing pastures to lower levels.
It is impractical to eradicate internal parasites in sheep, however by managing the pest populations, severe outbreaks can be minimised. This can be achieved by taking a holistic integrated approach to pest management rather than relying on one method of control. There are a number of factors to consider when deciding on a control program. Some of these are:

  • Lambing ewes : ewes become much more vulnerable to parasite challenge after lambing as their adult parasite resistance is diminished. It is important to consider this problem and take action before serious losses occur.
  • Young lambs: lambs must be treated with care especially at weaning time. They are exceptionally vulnerable to challenge by parasites at this time.
  • Weaner sheep : weaner sheep remain vulnerable to parasite challenge until they develop adult resistance.
  • Adult dry sheep , such as wethers, are less prone to serious problems from internal parasites unless they are farmed at high stocking rates
  • Current resistance status of drenches used on farm .

Wormboss is an extremely useful online tool to assist producers in their internal parasite management and includes regionally specific strategies.
Effective parasite control measures will vary according to the climatic conditions and stocking densities by region as shown on the Wormboss website, but there are some general principles you should consider:

  • treat the parasites when they are most vulnerable . In the hotter months fewer parasite eggs are able to survive outside the host. The strategic use of drenches is vital to the preservation of the few treatments still effective for most flocks;
  • regularly rotate between all known effective Mode of Action (MOA) groups and although there is no MOA code on product labels, there are 8 drench groups to select from;
  • ensure treatment is required by monitoring pest populations through the collection of faecal samples to determine worm egg counts (WEC);
  • generally short acting drenches are preferred as they are less likely to encourage the development of resistance;
  • monitoring your flock for drench resistance , and ensure that bought in stock are healthy (check Sheep Health Statements for freedom from OJD, Lice and footrot) and free of resistant worms through a quarantine drench;
  • “Quarantine” drench all sheep (including rams) new to the property. Use a combination of no less than 4 unrelated drench actives with at least one of these being the newest drench actives: monepantel (Zolvix®) or derquantel (with abamectin—Startect®). This can be done using multi-active (combination) and/or single-active products concurrently—up the race with one product, then up the race again with the next,” From ;
  • select rams with a negative WEC EBV ;
  • make sure that fresh paddocks are available for the flock after drenching ;
  • consider the use of refugia (sheep in the