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Western Australia worms, flies and lice update - April 2018

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Across Western Australia:

The first winter-pattern fronts have started to appear—will we see good rains in the next few weeks to set up a good pasture and cropping year? Unfortunately, there’s no such uncertainty for sheep worms, as the mild temperatures and moisture at ground level from dews and small rain events mean good conditions for their development.

As noted last month, effective worm control depends on having the lowest possible number of worms in sheep at this time of year, provided it’s compatible with drench resistance management strategies (more below). An increasing number of worm eggs will now be developing to the larval stage on the pasture, ready to move onto green pasture and wait to be eaten by grazing sheep.

The extent of worm problems later in the year is directly related to worm egg counts in sheep in late autumn. I use a figure of 100 eggs per gram as a maximum for all sheep classes at this time—a drench to reduce them is needed if above this. This figure is well below the level where worms are likely to be causing production loss, but our aim now is prevention, not treatment.

The worm risk varies greatly between districts and individual properties, with a larger risk where there’s been early green growth. In these situations, there will be a greater likelihood that a pre-emptive treatment is needed, and only worm egg counts will tell the story.

Weaners and hoggets

If summer drenches were effective, worm egg counts should be low or zero at this point, provided pastures were mostly dry through summer.

However, to achieve this result drenches need to be close to 100% effective. If less than this, some resistant worms will have survived. These are likely to fuel significant worm burdens in winter, and worse, those worms will be more worm resistant than if an effective drench had been used.

Which drenches are likely to be fully effective?

At present, very few cases of drench resistance have been seen to the “triple combination” types, and none in Western Australia to the newer drenches introduced over the last few years.

However, resistance is very common to the older drench types, and we can no longer expect that abamectin is sufficiently effective. Abamectin resistance is present in over 60% of tests, and to moxidectin in at least 25% of tests.

Action recommended: if summer drenches were given on paddocks with some green pasture growth (including perennial pastures), or with drenches not known to be fully effective (now including abamectin and moxidectin given as single-actives), a worm egg count is a good precaution now to avoid disappointing worm control in winter.


The main priority now is to ensure that worm burdens are not too high ahead of lambing. Significant worm egg counts in ewes mean likely worm problems in both them and their lambs, especially for ewes in lower than optimal body condition.

For ewes given an autumn drench: if an effective drench (see above for guidelines) has been given sometime from mid-March onward, and pastures were dry at the time, no further treatment is likely to be needed for ewes due to lamb in April or May.

If lambing later, or you are not sure the autumn drench was effective, a worm egg count taken three weeks before lambs are due will check whether an extra treatment is needed (counts should be less than 100 eggs per gram, as a mob average). Pre-lamb drenches can have a big role in increasing the resistance level, so it is important they are not given unless necessary.

For ewes drenched in summer: as with younger sheep, the effectiveness generally depends on the performance of that drench.

For ewes where a small proportion of the mob was deliberately left undrenched (recommended to reduce drench resistance development), worm egg counts will be positive, though usually low.

In either case, a worm egg count will show whether further treatment is needed—this applies to ewes due to lamb at any time (not just in winter, as with autumn-drenched sheep).

Barber’s pole worm

We occasionally see barber’s pole worm disease in late autumn, almost always where a large part of the property is under perennial pasture, and where summer and autumn worm control was not effective.

For ewes, a pre-lamb worm egg count check is a good precaution to avoid possibly severe consequences for both ewes and their lambs. Younger sheep should be checked as well, as summer drenches are never as effective in areas of higher rainfall where barber’s pole worm occurs compared to where summer pastures are totally dry.


Esperance: Nicole Swan, Swan’s Veterinary Services (

The majority of worm egg counts conducted in the last month have been low.

  • Ewes that did not receive a summer drench should by now have received an autumn drench.
  • Mobs that had a summer drench should now be monitored.
  • Mobs with average counts greater than 100 eggs per gram should be drenched.

At this time of year any eggs that have hatched into larvae will survive to populate the pasture over winter.

Farms with a history of barber’s pole infestations should be diligent with pre-lambing monitoring. In sheep with barber’s pole there will be a rapid rise in the worm burden as ewes get close to lambing (peri-parturient rise). On properties with a history of haemonchosis (barber’s pole outbreaks) it is recommended to consider a pre-lambing drench with closantel, which will give 7 weeks protection from reinfection from the pasture. Other options include capsules or the long acting moxidectin injectable.

Heavily pregnant ewes can be prone to pregnancy toxaemia if stressed in the last few weeks of pregnancy. This needs to be a consideration when handling late pregnant ewes for drenching. It is a compromise between monitoring and drenching as close to lambing as possible, and reducing the risk of pregnancy toxaemia due to moving, yarding and handling.

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