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Western Australia worms, flies and lice update - November 2017

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

WA State worm outlook

With harvest underway or starting across the state, worms are on the slide,      but will remain a risk for some time, especially in lambs. For all sheep, it’s time to lock in your summer-autumn worm control plans, as this is the basis of worm control over the next year. It’s also an opportunity to check drench effectiveness in sheep that do need a drench at this time.

Worms in lambs – don’t take your eye off the ball!

The worm risk reduces as the pasture dries, but:

  • green feed means some worm larvae will usually be present, even after a run of high temperatures
  • some worms will remain on pasture for 2-3 weeks after the pasture has dried off

Summer drenches should therefore be given a couple of weeks after the pasture is totally dry, or as the lambs go onto a crop stubble.

Ideally, lambs won’t need a drench between now and a summer drench, but that depends on how quickly the pasture is drying off, and whether we have late rains that keep it green.

If you’ve had a wormy year, counts are still likely to be higher than usual. If the lambs have not had a drench for the last 4-5 weeks, a worm egg count will show whether treatment is needed (say, over 250 eggs per gram, average), or if it can be delayed until crop stubbles or dry pastures are ready.

This is especially important in prime lambs, if not due to be sent off for another few weeks. A moderate worm burden can reduce final weights, even if the lambs seem visually to be in good condition.

Summer drenching – there are options

Annual worm control programs for sheep (and goats) in WA revolve around ensuring that worm burdens are at the lowest possible level by mid-autumn, so worms don’t take off after the season’s break. This means action to remove worms in early summer if burdens are high at that time, or by autumn at latest. There are several options and points to consider:

Weaners (white tags): these almost always need a routine summer drench. Except for very early-drop lambs, they will still be developing their worm immunity, and in most parts of Western Australia they will still have significant worm burdens in early summer.

Hoggets (black tags): if late-born (July 2016 on) they often also need a summer drench, as their worm exposure will be less than if dropped earlier. However, worm immunity and worm burdens can’t be easily predicted at this age, so a worm egg count is recommended to show whether a summer drench is needed. If no worm egg count check is done, a drench is a good precaution.

Adult sheep: worm burdens will usually be low in blue-tags and older sheep by late spring, except where ewes had a rough time with worms over the lambing time, or where the nutrition was very poor (obviously the case on many properties this year).  

This governs the need or not for a summer drench—it should not be automatically given to adult sheep. Although young sheep generally need summer drenching, we know that this increases drench resistance, as the resistant worms surviving that treatment will be the source of future worm populations. Ideally, we use adult sheep to provide a small population of worms not exposed to drenches, to dilute resistant worms surviving in summer drenched-mobs (refugia).

There are 2 main options for achieving this:

  • Drench adult sheep in autumn (early April at latest), not in summer. In most years for most mobs, their worm burdens by December won’t justify a drench, but counts can rise over summer, so an autumn treatment will pre-empt pasture contamination by worm eggs ahead of winter.
  • Drench in summer, but leave 10-20% of the mob undrenched. There are always plenty of sheep in good body condition (3.5 plus), and these won’t be affected by worms, so not drenching them will not reduce flock production. 

How to choose the best path for your situation?  For ewes (and adult wethers) in good body condition, in most cases either option could be used, as their counts are unlikely to be significant from now onward. However, ewes that have had a battle with worms during the year, or where they are well down on their normal body condition, counts may well be higher than usual.

Where there is a query on the best option, check worm egg counts. If counts are around 200 eggs per gram or more (mob average), a drench is needed, but leave 10–20% of the best-condition sheep undrenched.

If counts are low, the option of a summer or autumn drench is open. The main point is that somewhere, a small population of worms not exposed to a drench during the summer months should be allowed to survive. 

Drench choice

Summer and autumn drenches should be as close to 100% effective as possible.

Recent drench resistance tests indicate that the commonly-used abamectin is often no longer fully-effective, and without checking, it should not be relied upon. Resistance is less common to the more-potent moxidectin (25-30% of tests), but it should also be checked.

Effective combination drenches are recommended as fewer worms are resistant to a number of drenches.

Beyond these, there are several categories in which resistance is still very low, or not reported in WA:

  • pre-mixed “triple combinations” based on abamectin (with a white drench and either a clear drench or naphthalophos);
  • pre-mixed “triple combination” based on moxidectin (with a white and clear);
  • derqantel-abamectin combination (Startect®);
  • monepantel (Zolvix®) or monepantel-abamectin (Zolvix® Plus).

Checking for drench resistance

Ideally, a full drench resistance test (DrenchTest) where several drench types can be compared side-by-side should be done every 2 years on lambs at weaning. (Talk to an adviser to arrange this, or check the WormBoss website.)

However, as a simple guide to drench effectiveness carry out a DrenchCheck, which involves checking worm egg counts before and after a drench gives an approximate result. Samples can be taken when a mob is yarded for a drench, and then from the mob in the paddock 2 weeks later.  Doing this over a few mobs and at various times of the year will allow you to check a number of drench types with little effort.


Esperance: Nicole Swan, Swan’s Veterinary Services (

This month has been a mixed bag as far as worm egg count [WormTest) results go. Results have ranged from 0 eggs per gram (epg) to 12000 epg.

One mob of ewes tested last month had a Worm Egg Count (WEC) of 305 epg. None of these sheep were scouring, even though there was one high count out of the 10 tested. This property is in a barber’s pole endemic area and it was recommended that these sheep be drenched with Closantal and retested in 10-14 days. The post drench count was 0 epg confirming that the eggs seen in the WormTest were indeed due to barber’s pole (Editor’s note: Closantel is specific for barber’s pole worms). 

Another case was from a mob of 900 weaners. They had been drenched onto pasture a month earlier, and more recently, moved onto a standing oat crop. At the time of inspection, some were scouring badly with liquid scours, and 25 had died. Two of the dead lambs were brought into the clinic for post mortem. They had no faeces in their rectums and the post mortem revealed a large burden of brown stomach worms in their abomasums. A WEC of the liquid gut contents showed a very high worm burden of 12000 epg.

With stubbles becoming available, it is important that weaners and hoggets are drenched with an effective drench (Drench Test) before being moved onto them. Unless information regarding drench efficacy is available it is recommended that you use Startect® or Zolvix®. Other classes of sheep should have a WEC done and drenched if the mob average is greater than 200 epg.

Consulting your veterinarian to tailor your drenching requirements to suit your specific enterprise is highly recommended.

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