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Western Australia worms, flies and lice update - February 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Across the state:

Worms won’t be on the radar on most farms at this time of year, and the generally hot and very dry conditions will have been bad news for most pests. However, things could change if there’s some early rainfall—and importantly, it’s time to make sure that worm control is on track as we head into autumn.

Worms are survivors

Worm control in all livestock in Western Australia—sheep, cattle, alpacas, goats —revolves around the long period of hot and dry summer conditions.

However, moisture is the key. If there’s enough summer rain to fuel a green pick—even if mostly weeds—some worm development is likely. If it’s a false break and the green dries off, worm development may be short-lived, but if the green continues, it’s usually a longer worm year than usual.

Even if a normal year eventuates and it’s not green until late autumn, worm eggs begin to develop once temperatures start to drop. While the pasture is dry, the worm larvae stay inside the dung pellet, with some surviving for a few weeks before emerging onto the pasture once green growth begins. Small worm numbers in autumn can then lead to significant burdens in winter.

Dodging a worm problem

Good worm control is relatively simple: sheep must be carrying few or no worms by the time we reach early autumn.

This should be no problem for sheep that received an effective summer drench— that applies mostly to lambs and hoggets. They won’t have picked up worms from dry pastures or crop stubbles, so they should be fairly worm-free.

However, the big question is: was the drench actually effective? Drench resistance affects all but the newest drench types, and unless you know the drench worked, it’s possible a few worms survived in the sheep. Worm burdens are usually small in summer and there are no visible signs, but this changes once conditions start to favour worms—either early rains or the usual autumn break—and the worm life cycle resumes.

The simple answer is to do some worm egg counts on summer drenched mobs— if average counts are more than 50 eggs per gram (a very low figure), re-treatment is usually warranted.

What about drench resistance management?

Summer drenching all sheep on the farm has long been known as a major cause of resistance. Any worms surviving summer treatments are the foundation of worm populations in the coming months, which explains the early development of resistance in Western Australia.

Resistance management strategies are best employed in adult sheep, as we can’t take risks with the growth of young sheep. The main avenues have been:

  • delay drenches until autumn, to deliberately let a few worms not recently exposed to drenches survive; or
  • leave some adult sheep not drenched if the mob is drenched in summer.

Both strategies aim to ensure some less-resistant worms survive to dilute any resistant ones left in summer drenched sheep.

However, we don’t want the “good” (susceptible) worms to multiply to harmful levels. So, it’s important that mobs intended for autumn drenches are treated somewhere between mid-March and mid-April (at the latest). If they look as though they won’t have any worms to make a drench worthwhile, do a worm egg count to check.

For mobs where some sheep were left undrenched, the average mob count should be checked now. Most times, it will be low, as adult sheep usually have very low egg counts in summer, to begin with. But it’s wise to be sure.

What about summer-green areas?

As mentioned, green pasture and worms go together. In areas where green pasture survives through summer—perennial pastures, big areas around creek lines and dam seepages—the worm risk is higher. The biggest risk is barber’s pole worm, the warm-weather worm mostly in higher rainfall and coastal areas, though sometimes a lot further from the coast.

The main barber’s pole worm areas are usually well-known, and problems are typically sporadic. But worm egg counts at intervals are especially necessary in these areas, as outbreaks often occur with little warning, and burdens of other worm types can also increase rapidly once there is some rainfall. The good nutrition in summer-green situations should help by keeping sheep in better body condition, but worm egg counting is the key to avoiding unpleasant surprises.

The bottom line

Worm egg counts need to be low as we come into autumn. It’s important to make sure by ensuring routine autumn drenches are given on time and checking worm egg counts. Worm control for the coming year starts now!

And another thing … sheep lice

Shearing coming up? If chemical treatments are needed, make sure they do the job. Resistance by lice is common to some chemical groups (the synthetic pyrethroid and insect growth regulators). Several off-shears chemical groups can eradicate lice, but no long-wool products are registered for complete lice removal, so it’s worth getting it right at shearing.

The LiceBoss website has chapter and verse on factors affecting the lice risk and treatment decisions: see, or for information on all sheep and goat parasites:


Esperance: Nicole Swan, Swan’s Veterinary Services (

This summer has been very dry. The lack of summer rain is good for worm control, but not so great for stock feed. Even Kikuyu paddocks are looking pretty dry.

Despite this, we have had a couple of mobs of lambs with high worm egg counts. The lambs were drenched with a moxidectin at weaning (spring) and moved onto pasture. The weaners had an average worm egg count of 945 eggs per gram (epg) with other counts up to 4,000 epg. It is most likely, given the location of this property, that the majority of worms were barber’s pole (Haemonchus). The property is a sheep-only operation.

As a comparison, a mob of weaners from a different property, also in a coastal location, were drenched in mid-December with Startect® onto barley stubbles. These sheep had 0 epg. This shows the importance of:

  • The timing of the drench
  • The type of grazing sheep are moved onto after drenching—pasture (clean) versus pasture (dirty) versus stubbles
  • The choice of drench and its effectiveness to kill the worms on the property. In the case above, there hadn’t been any recent testing to check the effectiveness of moxidectin.

Weaners and hoggets are more susceptible to worms than other classes of sheep. It is essential they receive a summer drench even if one was given 2–3 months earlier at weaning. This drench greatly reduces the likelihood of a worm burden over summer. Monitor these classes of sheep in late February/March to determine the need for a second summer drench.

For February 2019 state outlooks, please follow the links below:
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