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Western Australia worms, flies and lice update - December 2017

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

WA state

With harvest complete on most properties, it’s time to think about Christmas holidays—but not before sheep worm control is up to scratch!

For effective worm control over the coming year, worm burdens must be at the lowest possible level by mid-autumn. However, managing drench resistance means that a few less-resistant worms need to survive into autumn to dilute resistant worms in summer drenched sheep.

Getting the balance right is the key to sustainable and effective worm control, and it means there are different rules for sheep of different ages.

Young sheep need summer drenches

Most young sheep mobs (this year, white and black tags) need a drench in late spring or early summer, either as soon as crop stubble are ready, or pasture paddocks have dried off. Removing worm burdens means there will be no adverse effect on growth rates, and as dry pastures carry few or no worm larvae, they will stay worm-free for a long period.

The main exception is where management policies are specifically aimed at keeping worm burdens low, such as weaning lambs onto a fodder crop or cattle paddock. However, a check of worm egg count is recommended, as it is essential that counts are very low (less than 100 eggs per gram)—even a small leakage of worms onto pastures in autumn may lead to winter worm problems.

In lower-rainfall wheatbelt areas, a drench to lambs at weaning may be sufficient to keep worm burdens low, especially if given late in the year. A routine summer drench will not always be needed for lambs in this zone, and often not at all for older sheep—this is easily shown by a worm egg count.

Mature sheep—a different story

Although good worm control is important across the entire flock, adult sheep are at the centre of drench resistance management strategies. The aim is to allow the survival of a small number of non-resistant worms, so these can dilute the resistant worms likely to be present in summer-drenched younger sheep.

Provided that adult sheep are in good body condition, they have a strong resistance to worms and tolerance of their potential ill-effects. After good spring nutrition, and with lambs weaned, ewes will have low worm burdens in early summer, and these few worms cause little ill-effect.

This year will be different for some ewe mobs, where spring pastures were poor—if the average ewe body condition score is less than 2.5, a summer drench to all is likely to be needed. In this case, a decision is best based on a worm egg count, and it will be worth discussing this with a veterinarian or sheep adviser.

As a rule, however, there are two main approaches to sustainable control in mature sheep—this generally applies to the ewes:

  • Autumn drenching: delay drenching until mid-March or early April, when a small percentage of the worm eggs dropped onto pasture will develop to the larval stage. These can survive beyond that time, ensuring a small population with fewer resistant worms in winter. This strategy has been used with success on many farms for some time, provided that the ewes were in good condition over summer, and that this drench is given by early April.
  • “Targeted treatment” at summer drenching: leave a small percentage of ewe mobs undrenched if they are drenched in summer. The worms surviving in these sheep provide an undrenched population to dilute resistant worms in summer-drenched mobs (such as younger sheep). Trial work confirms that individual sheep in good body condition (3.5 or more) can stay undrenched with no problems, due to their high level of worm-tolerance. 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.

A late rain worm risk?

If the heavy rains in mid-December have freshened-up any remaining green pasture, worm larvae will survive longer into summer than usual. This is most likely in coastal areas, and sheep grazing pasture paddocks may still pick up worms after drenching. In barber’s pole worm areas, there is a risk of losses from this worm, lasting until well after hot and dry conditions resume.

A worm egg count at 4–5 weeks after summer drenches are given, or by mid-January, will show whether worm control is on track, or if further drenches are needed for some mobs.

Drenches must be effective

No long-term worm preventative strategy will work if the drench doesn’t.  If worms are left after summer or autumn drenching, their numbers will start to build up after the season’s break—and as these worms have already survived a drench, the drench resistance level across the property will increase.

The aim is to use drenches as close to 100% effective as possible. We define drench resistance as less than 95% effectiveness, but this is a low figure for critical drenches, and ideally, drenches at this time of year should be 98% or more effective.

A full drench resistance test on young sheep (weaning is a good time) will show the farm picture for all drench types. The next best is to do before-and-after drenching checks of worm egg counts (DrenchCheck) when treating a mob, as over the course of a year, a number of drenches can be checked.

Talking to a sheep management adviser or rural veterinarian is recommended for planning resistance management strategies, and the WormBoss website gives full details of sustainable programs and testing for drench effectiveness.


Esperance: Nicole Swan, Swan’s Veterinary Services (

In the last month Harvest has kept everyone busy. Now that stubbles are available it is important to monitor sheep prior to moving to stubbles to ensure that we take advantage of this strategic time for worm control.

As a rule all weaners and hoggets should be drenched onto stubbles with an effective drench. This will increase the likelihood that they will make it through summer with no further drenching. However sheep grazing on perennial pastures or in the event of significant summer rain monitoring using WormTest is recommended.

For older sheep, drenching is warranted if counts are 200 eggs per gram (epg) or higher. If you have multiple mobs it is recommended that a number of representative mobs are tested and drenching based on this, if testing of every mob is not feasible.

Faecal worm egg count results conducted in the last month were varied. Most mobs did not require drenching. One mob had a count of 200 epg.  This mob was from a property on which a haemonchus lectin test had been conducted. This test showed that 50% of worm eggs were haemonchus. Therefore a broad-spectrum effective drench to kill all types of worms present was recommended for these sheep.

Lastly, I want to wish everyone a Merry Christmas and a Happy New Year.

For December 2017 state outlooks, please follow the links below:
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