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Western Australia worms, flies and lice update - October 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

A dry season is drawing to a close, but the worm-risk is very variable across the state. Dry pastures and hot weather means little or no worm pick-up, but where there is still any green pasture, some worm-risk remains.

The main parasite issues now are making sure that lamb growth is not held back by worms, and planning end-of-year worm control to ensure sheep have low worm burdens over the dry months.

Worm control in lambs

Summer drenches”, given as sheep go onto a crop stubble or onto pasture, once it has completely dried off, are the cornerstone of worm control well into the next year.

Lambs almost always benefit from this treatment, but a big question is: will they need a drench between weaning and summer treatments? Ideally, this will be avoided, but it should not be at the expense of lamb growth.

If the lambs were drenched some weeks or more ago, there’s a fair likelihood a drench will be needed, and a worm egg count will quickly tell the story. However, if counts suggest the need for treatment, can it wait until crop stubbles are ready or the pasture dries off? I’d suggest that if counts are lowish, say an average of 250–300 eggs per gram, the answer is probably yes.

If summer drenches are some time away, there may be no choice but to drench. However, there can be quite a difference between mobs of lambs on the same property, and checking more than one mob will show this.

If the lambs were drenched more recently, there’s a good chance they won’t need a drench until summer drenches are given. The main spoiler for this is where some green pastures survive, or if late rains occur—not that this looks likely at the moment.

Prime lambs

A real soapbox of mine in recent years has been: make sure prime lambs don’t suffer an invisible growth check due to moderate worm burdens!

Fast-growing lambs will handle worms with little penalty, but where the feed doesn’t allow ideal growth rates, worms can take a significant toll. Cross-bred lambs, in particular, often look to be going well, but in fact, could be a couple of kilograms (kg) heavier.

Where lambs won’t be turned off for more than a couple of weeks, a wormtest is good insurance, as almost any weight gain will pay for a drench. (A note of caution: check drench withholding periods (WHP) before treatment. A few drench types likely to be effective have short Export Slaughter Intervals [ESI], but for others, it maybe 28 days or longer.)

Planning for summer-autumn worm control

Drenches given during the dry season, or as it starts, have 2 main purposes: to prevent production loss over the summer period, and to ensure worm burdens are as low as possible before the onset of the new season. Getting this right depends on drenching (provided it’s needed) at the best possible time and using a highly effective drench. Getting either of these wrong can mean a significant loss to worms in both the short and long terms.

Drench timing

Lambs and hoggets: A summer drench (usually November or December) will almost always be necessary for current-year lambs (green tags), as they are at the most worm-susceptible time of their lives. Hogget-age sheep (orange tags) will also often need a summer drench, but not always, especially if from an early drop. A check of worm egg counts will sort this out.

Adult sheep: Worm burdens are generally low coming into summer, which gives an opportunity to manage drench resistance. The aim is to ensure a small portion of the worm population is not exposed to drenches in summer, so some non-resistant worms survive over summer and dilute resistant worms in the following year.

There are two main options for this, depending on what suits the property management plan. Drenches to adult sheep (mostly ewes) can either be delayed until autumn (more on this in later newsletters) or leaving some sheep undrenched.

If drenching ewes in summer or as they go onto a crop stubble, the recommendation is to leave 10–20% of the mob undrenched. Better, base the percentage on a worm egg count—if the average is less than about 100 eggs per gram, a bigger number can be left untreated. The sheep left undrenched should be in good body condition (3.5 plus), as these won’t be significantly affected by worms.

Drench choice

Drenches given onto the dry feed should be as close to 100% effective as possible.

It’s still not too late to test for drench effectiveness, especially if lambs are to be drenched shortly. The WormBoss website has full details of drench resistance test methods, but a drenchcheck is easily done by comparing worm egg counts before and after a drench is given, with the second lot of samples collected from sheep in the paddock.

If you do nothing else to test drenches, at least check the worm egg counts of a couple of mobs of lambs, at 10–14 days after drenching. Chances are that they will have had some worms, and unless counts are close to zero, the drench wasn’t good enough. If this is the case, talk to an adviser to decide on the most appropriate drench choice, and when re-treatment should be given.

Barber’s pole worm

We usually see a few cases of barber’s pole disease in lambs late in the year in the higher rainfall parts of the state. The risk is probably lower this year due to the dry season, but it can never be discounted—especially if there are showers of rain before the pasture really dries off.

Worm egg counts every few weeks are a wise precaution against this unwanted visitor, and also check that other worms are under control. If the environment suits barber’s pole worm, which needs warm-weather moisture above all else, there’ll be a high risk of other worms as well.

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