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Western Australia worms, flies and lice update - November 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

The late spring and early summer period is critical for sheep worm control. Effective treatments at this time ensure that worms don’t reduce growth in lambs and hoggets, and also set the property up for good control in the following year.

Moving sheep into crop stubbles provides a natural break in the worm cycle, as worm pick-up ceases once the sheep move in. It’s a bit more complicated on pasture, as it needs to be thoroughly dry for about three weeks before becoming reasonably worm-free. The recent rains across the state will have kept worms in business longer than usual in some districts.

Current-year lambs (purple tags) and hoggets (green tags)

Both age-groups should receive a ‘summer drench’, and then be grazed on a low worm-risk pasture. As growing sheep, they can’t afford to have worm burdens holding them back at the time of year when pasture nutrition is declining.

As mentioned above, if not moved to a crop stubble, consider the worm risk of the pasture. In paddocks where some pasture has stayed green, the risk of worm pick-up will continue until early December, and longer if there is any further rain. Blowflies will also remain a threat for longer under these conditions.

If a drench is delayed because pastures are still green, there is a risk that ill-effects from worms may emerge before the treatment is given. Taking samples for worm egg counts (WECs) will show whether a treatment is needed at present, or can be delayed without adverse consequences.

Adult sheep

There is more latitude with older sheep, as due to their worm immunity they typically have far lower worm burdens than younger sheep, and are more resilient to the effect of worms.

Research over the years tells us that most ewe mobs have low average worm counts in early summer, once pastures have dried off. WECs can be taken to confirm this, but most mobs will have average counts below 200 eggs per gram, indicating little effect on the sheep.

This gives the opportunity to reduce the development of drench resistance. Allowing some less-resistant worms (i.e. not recently exposed to drenches) to survive offsets the increase in any resistant worms that survive in younger sheep.

There are two main options:

- Autumn drenching: delay drenching until between mid-March and mid-April, as prior to that, only a few worm eggs dropped onto pasture will reach the larval stage. However, as they are from a lower-resistance worm population, they will dilute resistant worms from other mobs.

- “Targeted treatment” for summer-drenched: leave a small percentage of adult sheep undrenched. Individual sheep in good body condition (3.5 or more) have a high level of worm-tolerance and can stay undrenched without problems. A minimum of 10% of a mob should be left undrenched, and ideally, 20% or more — those in especially good condition are easy to spot as you move along a race.

What drench?

All drenches given in summer should be as close to 100% effective as possible, to keep the number of surviving resistant worms to a minimum. However, this comes with an important proviso — some form of drench resistance management is essential, as drenches in summer add heavy selection pressure for drench resistance. The strategies mentioned above for ewes are a good start.

Barber’s pole worm

Late rains always increase the risk of outbreaks of barber’s pole worm disease (haemonchosis), as green pastures and warm weather favour barber’s pole development. The risk is highest in younger age groups, but attacks can affect all sheep, with little warning. WECs will indicate whether the usual summer drenching program will be sufficient, or additional barber’s pole worm treatment is a wise precaution.


For more information, see the WormBoss website and the Department of Primary Industries and Regional Development webpages.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Most producers are busy with harvest and haymaking. Recent rain events have made this frustrating for some. This rain will also increase the risk of a worm problem on properties with perennial pastures. The rain, together with warm days, is perfect for larvae to germinate and infect the pastures. On a lot of properties, the pasture is still relatively short. This will result in a greater pickup of worms per mouthful compared to grazing long grass.

On properties that have a history of barber’s pole we are approaching a strategic time of year. As annual pastures "hay off" drenching with closantel will help "vacuum up the larvae". If sheep are drenched with closantel, any larvae they ingest over the 3–4 weeks post drenching will die due to residual action of the drench. In this way grazing will help lower the burden of barber’s pole larvae on the pasture. As summer progresses three months of dry hot weather will further help to clean up the paddocks. This will not be as effective if we get summer rain.

As stubbles becomes available it is recommended that all weaners and hoggets are given a strategic, effective (95%) first summer drench onto stubbles. If this is done it is most likely that these sheep will make it through the summer without a worm problem. Older sheep should have a WormTest. Mobs with an average over 200 eggs per gram should be drenched, preferably onto stubbles also. It is important that a small percentage of each of the older sheep mobs is left undrenched (refugia). This will slow down the rate at which the worms develop drench resistance.

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