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Western Australia worms, flies and lice update - May 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

It’s the merry month of May, and lambs are hitting the ground in droves! Unfortunately, worms seem to know that, and are getting ready to hitch a ride in young animals that haven’t developed a strong worm immunity.

Although rain has been patchy across the state, the cool conditions and moisture at ground level means that worm eggs will be developing to the larval stage. Once there is some pasture growth, grazing sheep (and goats, and cattle) will pick up worm larvae as they graze.

Whether significant worm burdens will develop depends mostly on the effectiveness of worm control in the previous months. Worm counts in all classes of sheep should be very low—close to zero—at the end of April, so worm development starts from a low base. The key to good worm control in the lambs is to ensure effective worm management in ewes around lambing time.

Is a pre-lambing drench to ewes needed?

Worm burdens in ewes should be very low at lambing, as the higher their worm egg output, the greater the worm risk to lambs. Worm counts of ewes should ideally be a mob average of less than 100 eggs per gram prior to lambing.

However, giving a routine pre-lamb drench can significantly increase the level of drench resistance. The likelihood that a drench to the ewes is necessary depends on worm control action in the previous weeks or months.

Ewes last drenched in summer

If the ewes have last been drenched as they went onto crop stubbles (or earlier in the year), or onto dry summer pastures, their counts should be very low. However, if that drench was not fully effective, or some had been left undrenched (the recommended drench resistance management tactic), counts will probably be positive – but will they be high enough to warrant a drench?

The only way to know is to do a worm egg count check, at about 3 weeks before lambing is due—this is to give time to yard the mob without danger of miscarrying.

(Note: ewes should be kept off pasture for the very minimum time, to avoid the possibility of metabolic diseases.)

Ewes drenched in autumn

The aim of an early autumn drench to adult sheep is to reduce the pasture contamination with worm eggs before the seasonal worm increase in late autumn (it is also less selective for drench resistance than summer drenches). In ewes due to lamb in May or early June, it should serve as a pre-lambing drench.

If an autumn drench was given when pastures were generally dry, and the drench is known to be fully-effective (based on the type of drench, or better, on a test result), no pre-lamb drench should be needed. If not sure, a worm egg count will tell the story—unexpectedly high counts are often due to drench resistance.

Ewes that have lambed or are due in the next week

It’s too late to drench once lambing has started or is due shortly, but unless there is good reason to believe ewes have low worm counts, they should be checked before lamb marking. A count of more than about 250 epg would suggest that a drench to the ewes at marking time is worthwhile to reduce the worm exposure of the lambs. (Worm counts post-lambing are almost always higher than before lambs drop, as ewes temporarily lose their worm immunity at that time.)

Samples are best collected from the paddock, with the size of the dung pellets used as the guide to whether samples are from ewes or from the lambs.

Long-acting or short-acting drenches?

Long-acting capsules or injections to ewes can prevent worm development for many weeks or longer, so pastures do not increase in worm contamination, often with significant benefits in the lambs. However, they also have the potential to increase the level of drench resistance unless managed to minimise this.

If pre-lambing worm control is effective and pastures carry few worm larvae (usual in WA in autumn), short acting drenches will usually be effective. Long-acting types are most useful where the pasture is expected to be heavily worm-contaminated, and ewes are likely to pick up a significant worm burden if only a short-acting drench is given before lambing. The additional cost of long-acting treatment may be justified if it prevents a reduction in lamb growth rates due to continual worm challenge. The WormBoss website has detailed information on the management of long-acting drenches to get the best effect with the least increase in drench resistance (from the WormBoss homepage: Tests and Tools/Management Tools/Drenches/Effective use of long-acting drenches).

Esperance: Nicole Swan, Swan’s Veterinary Services (

Rainfall last week was varied over the district. Some areas had 30 mm rain while others had none. This has a big impact on worm numbers. This week will be warm. Areas that have good moisture in combination will warm sunny weather will be the perfect environment for larvae to hatch.

Both Haemonchus (barber’s pole) and scour worms have a life cycle of around 21 days.

Therefore, eggs will be hatching to larvae about a week after the rain, and these will be eaten with the pasture by the sheep. Therefore at 3-4 weeks after a rain event we should start to see an increase in worm egg counts. As a rule of thumb, conditions that are conducive to pasture growth are ideal for worms. Because of this, any eggs laid on the pasture in the autumn will survive and become the source of a worm burden over the winter months.

If you are unsure of the worm status of your flock, worm egg counts (WEC) are recommended. Worm egg counting is recommended pre-lambing, pre-marking, pre-weaning, pre-summer drench time as well as whenever sheep are looking lethargic, in poor condition or scouring. It is also recommended before moving sheep to a paddock that you are preparing to be a clean or low worm-risk pasture. Worm egg counts require minimal effort on farm to obtain very helpful information. For details on how to collect samples for a WEC you can contact your veterinarian or look on the WormBoss website for ParaBoss-endorsed WEC providers.

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