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Drench resistance - so what?

Stephen Love, State Coordinator – Internal Parasites, I&D NSW Primary Industries, Armidale.

We have all heard about drench resistance. So what? Here are some thoughts on the matter.

The problem – solve or sidestep?

In the mid-1980s, with the introduction of programs such as WormKill, then DrenchPlan, there was a leap from ad hoc and tactical drenching to less frequent and strategic drenching. It was hoped that this would delay the onset of resistance.

Since the mid 1980s, there has been increasing emphasis on shifting away from drug-centred worm control to an integrated and ‘robust’ system employing several control methods as outlined in the current versions of the WormKill, DrenchPlan and FarWestWorm programs. (These can be classed as ‘modified’ strategic programs). However, it is likely that drenches will continue to be important in worm control in the foreseeable future.

There have been no enduring solutions to the problem of anthelmintic resistance. The comments (below) from Douglas Gray, formerly a Principal Research Scientist with CSIRO, are still relevant.

"The problem of preventing resistance gets smaller – still remaining unsolved – when there is less reliance on them for worm control. If there was a choice between solving the current problems or making them irrelevant … making them irrelevant is the most sensible option.

Since the late ‘60s and the appearance of BZ resistance, a huge effort has been directed at solving the drench resistance problem. From time to time I need to be reminded that worms are the problem, not anthelmintics. Reduced reliance on drenching makes drench resistance increasingly irrelevant". (The emphasis is mine)


Integrated Worm Management is:

  • The right drench at the right time.
    (Most producers do not accurately know which drenches work on their property)
  • Grazing management – creating safer pastures for susceptible sheep, notably lambing ewes and young sheep. The lambing paddock is usually the worst one for weaners. Rotational grazing generally produces better worm control than set-stocking.
  • Nutrition – good nutrition which may include ‘strategic’ supplementation for young sheep or pregnant ewes that fail to meet target weights or condition scores. Stunted weaners have poor immunity. Ewes with fat/condition score 3 – and their progeny – often have substantially lower worm burdens than ewes in poorer condition.
  • Flock and weaner management – ‘compact’ joining and lambing, and weaning by 12–14 weeks after lambing starts. Good ewe management to minimise the effects of the peri-parturient relaxation of resistance to worms.
  • Breeding resistant sheep - use rams that have favourable breeding values for resistance to worms, as well as production traits.

Fine-tuning based on sound information:

  • Regular worm egg count (WEC) monitoring (WormTests).
  • Regular resistance testing (DrenchTest (worm egg count reduction test)), ideally every two years, supplemented by regular “DrenchChecks’ (a WEC 10-14 days after a routine drench).
  • Expert advice.

Future strands:

  • Vaccines. Research on vaccines continues but there is little prospect of a commercial product in the near future.
  • Nematophagous fungi. These are fungi that trap and destroy worm larvae inside sheep faecal pellets. CSIRO and a commercial partner have researched a product using this technology (Not yet available).

Managing drench resistance:

  • Know which drenches are effective on your farm, by doing worm egg count reduction tests (WECRT; DrenchTest) every 2-3 years, with occasional DrenchChecks in between. (DrenchCheck is a worm egg count (WEC; WormTest) 10 days after a drench, and is a quick measure of drench efficacy).
  • Don’t use drenches unnecessarily, especially in adult animals.
  • WormTest first. Particularly avoid unnecessary drenches when there are few worms ‘in refugia’, i.e. few worm eggs and larvae on pasture due to prolonged hot, dry conditions, or due to management practices, e.g. harvesting a crop. Selection for resistance is stronger when there are few worms in refugia, because there are few, unselected worms on pasture to dilute the progeny of resistant worms surviving inside sheep after drenching.
  • Use drenches at the correct dose rate, using calibrated drench guns, and weighing a selection of sheep.
  • Consider using combinations of unrelated drenches, and rotate between drench types within season. This may mean rotating between different types of combinations if combinations are being used.
  • Avoid using long-acting drenches as a pre-lambing drench unless necessary. Good lambing paddock preparation reduces the need for long-acting drenches.
  • Reduce the need for drenching by using integrated worm management.