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Western Australia worms, flies and lice update - December 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





South Australia worms, flies and lice update - December 2020

Albany: Brown Besier, Brown Besier Parasitology (

Parasite control in WA in early summer is dominated by the change in the landscape from spring: once crop stubbles become available, worms are on notice. Once the header moves out, drenching sheep as they move in means that worms shouldn’t be a problem for many months.

However, as mentioned in previous articles, ‘summer drenching’ is a two-edged sword. If drench resistance means that not all worms were killed, the surviving ones may cause problems later on. Any worm eggs dropping onto the ground die quickly while conditions are dry and hot, but from early autumn onward some eggs will develop to worm larvae, ready for sheep to take in.

That’s Problem Number 1: the new year’s worm population starts off earlier, and at a higher level, than if the drench was effective. Worm problems in winter are likely to be bigger than they should be.

And Problem Number 2: those worms survived over summer because they were resistant to the drench. The population they kick off in the new year will have a higher level of drench resistance than previously, and worm control will be more difficult.

Even where the drench seems to be fully effective, a few resistant worms inevitably survive and increase the drench resistance level over time.

Putting all this together means it’s important to get summer drenching right:

  • Young sheep are the priority for summer drenching — growing animals can’t afford the drag of a worm burden;
  • Older sheep are more forgiving and not all will need a drench, so they can be used to help manage drench resistance;
  • Any drenches at this time of year must be as close to 100% effective as possible
  • Drench onto pasture paddocks — if there’s any green left, some worms can survive.

Current and last year’s lambs should be drenched onto crop stubbles or totally dry pastures, unless a worm egg count (WEC) shows close to a zero count. Some hogget mobs are likely to have low counts and not need a drench, but you need to be sure.

As often mentioned in previous articles, we have the two options for older sheep:

  • Delaying drenching until between mid-March and mid-April — if uncertain that this is wise, check their WECs. Ewes in good condition and well past lamb weaning commonly have average counts of less than 100 eggs per gram, and won’t benefit much from a drench. We still want to restrict the contamination of pastures with worm eggs in autumn, but delaying the drench until after summer puts far less pressure on the worm population for resistance than a summer drench.
  • The ‘targeted treatment’ strategy: this means deliberately leaving a small percentage of adult sheep undrenched, so they seed the pastures with eggs from worms that haven’t seen a drench recently. These eggs won’t start development until autumn, but from then on they provide a source of less-resistant worms, which will help dilute resistant worms that survive in sheep given summer drenches.

Drench choice is obviously critical: the older drench types (white, clear, ivermectin and abamectin) have had their day and should not be contemplated alone for summer drenches. Moxidectin may still work (although should be checked), but combination drench types are strongly recommended. They are far more likely to be fully effective, and the combination of two or more drench types helps delay the development of resistance.

Whatever drench is used, it’s good insurance to check that it worked. Taking dung samples from WECs from a couple of mobs at 14 days after drenching will quickly show whether Problems 1 and 2 are a real risk this year.

For December 2020 state outlooks, please follow the links below:
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