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Western Australia worms, flies and lice update - September 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

With the weather at last starting to warm across WA, pasture growth will be at its maximum, but both sheep worms and blowflies will be active. So far across the state, we’ve heard only of odd strikes on individual sheep, and whether it’s a bad fly season will depend mostly on how long significant rains persist. Many will remember the bad fly season a few years ago, when rain into December kept the problem alive for longer than usual, justifying the use of long-acting preventative treatment back in spring.

Weaning drenches for lambs

  • A weaning drench given to lambs is always recommended. This is the most worm-susceptible stage of their lives, and will have had plenty of worm exposure at this time of the year. In most cases, immunity to worms does not fully develop until lambs are well over 6 months, and some individuals in a mob will take much longer.
  • Many prime lambs will not be weaned until they are over 20 weeks, when the first draft goes for slaughter. However, these will also often benefit from a drench by 14–16 weeks. Recent MLA-supported studies in WA show that the effects of worms depend largely on growth rates, and rapidly-growing lambs (over about 250 grams per head per day) can generally tolerate moderate worm burdens. However, where the pasture quantity or quality is restricting growth rates, or there is a heavy worm challenge, worms can hold them back. A drench at 14–16 weeks is a good precaution, unless you can make a decision based on lamb growth rates and worm egg counts.
  • There may be less need for a drench to lambs at this time where ewes have been managed to lamb onto a worm-free pasture, either by planning ahead for low worm-risk paddocks, or by giving ewes a long-acting drench before lambing. Both can achieve the aim of keeping worm problems in lambs to the minimum, although they can also add to the development of drench resistance. However, you should still check the lamb worm egg counts to be sure. (Also, it’s a good idea to talk to an adviser about how to get the best from worm control with the least drench resistance downside, especially from long-acting drenches.)
  • There is usually no need to drench ewes when the lambs are weaned. The lambs will have been off them for some time by then, allowing their worm immunity to return to its highest level. With good spring nutrition, their ability to tolerate worms will also be at its maximum.  Worm burdens and their effects are generally low from now until well into autumn, and it’s an opportunity to avoid giving a drench. If there are concerns about a particular mob, a worm egg count will soon sort this out.

Checking for drench resistance

  • Lambs are the best group to test for drench effectiveness. A full drench resistance test can be done by keeping, say, 100 lambs back, and testing for several drench types at the same time. Contact your local vet or worm egg count provider to set this up, and DAFWA can also provide a testing service.
  • Another option, needing less time and work, is to DrenchCheck the drench used when a mob is to be treated. This involves taking dung samples for a worm test when the mob is drenched, then again 10–14 days later (from the paddock—no need to yard the sheep for this sampling). Comparing the worm egg counts will indicate the drench effectiveness, and over time, a number of drenches can be checked this way. 
  • A good start is to test moxidectin (oral), as this is a benchmark: resistance is now present on about one-third of WA sheep properties. If moxidectin is working well, next check abamectin (resistance present on about half of farms), but if moxidectin is no-go, check the “triple combination” (abamectin plus a white and clear drench).

Checking drench effectiveness is a small effort for the effect, as there is usually no sign after drenching of how effective it may have been—but down the track, it could make a major difference in sheep performance. 


Esperance: Nicole Swan, Swan’s Veterinary Services (

This month we have continued to get rain and sunshine so the pasture is flourishing and it is perfect weather for worms. It will soon be time for weaning; pre-weaning worm egg counts are recommended on both the weaners and ewes. Properties that have not conducted a drench resistance test in the last 2 years are strongly encouraged to do so. Tests can be tailored to the individual property based on the resistance history of previous tests. It is best to contact your veterinarian to organise this. If possible, weaners should be drenched onto a low worm-risk (’clean’) paddock. For a paddock to be clean it should not have had sheep grazing on it for at least 6 months or ideally drench onto stubble, though this is unlikely to be available yet.

Weaners should be drenched with an effective drench. If drench resistance status is unknown then consider using the newer drenches (Zolvix® or Startect®) that are most likely to be fully effective.

Recent worm egg counts of ewes drenched pre-lambing in March/April have been low. This shows the effectiveness of drenching at this time of year prior to conditions being favourable for larvae to survive over winter.

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