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Western Australia worms, flies and lice update - March 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

The next month is the most important time of the year for ensuring good worm control over the coming winter and spring months. By mid-April, worm burdens in sheep and goats should be around the lowest for the year.

It’s all about the worm life cycle: in the summer months, any worm eggs shed in the dung struggle to survive, except in the odd spot where some green pasture has developed or survived. Sheep on dry summer pastures or crop stubbles can be very close to worm-free.

However, as temperatures fall and the first dribbles of rain arrive, conditions will start to look good for worms, with higher hatching rates of eggs and longer survival times of emerging worm larvae.

Lambs/weaners (green tags) and hoggets (orange tags):

These sheep should have received summer drenches or been managed to have minimal worm burdens in early summer, so worm counts should still be very low. If not, there’s a strong chance that those drenches were not fully effective, as there would have been very little pick-up of new worms from dry pastures.

Check worm egg counts now, and drench if the mob average is over 100 epg. Worm counts should then be checked every 4–6 weeks, to prevent rapid increases in worm burdens after the season’s break.


Mature sheep will fit into one of 3 worm-management categories: they’ve had a summer drench (going into a stubble paddock or dry pasture); they are due for (or have had) a routine autumn drench; or they may have no treatment at all planned. In all cases, the aim is to ensure that average worm egg counts in April are not more than 100 epg

Summer drenched ewes: Ideally, a percentage will have been left undrenched to ensure that some less-resistant worms remain in the mob. (See the WormBoss website for details of “sustainable summer drenching”). Worm counts should still be very low, but this is not certain: the drench may not have been fully effective, or more worms than expected may have survived. Worm counts should be checked from at least a couple of representative mobs.

Autumn drenches: these are best given between mid-March and mid-April. By then, a small population of worms not recently exposed to drenches will be developing on the pasture, as a small degree of worm development typically starts by late summer. These worms are a valuable resource, as they will dilute resistant worms left in other sheep classes.

As a refinement, checking worm egg counts of each ewe mob may allow some drench saving—often, some are still below 100 epg, and can be left undrenched. But if most mobs are over that figure, it’s an indication that this drench should be given as a routine.

No drench planned: In lower rainfall areas, especially where the majority of the property is cropped, worm burdens in mature sheep may stay low throughout the year. Drenching in this case may not be warranted and will add to drench resistance levels. Worm burdens are worth checking, as worms have a habit of surprising us with unexpected increases. If counts on several mobs over a couple of years are low, it’s an indication of a sustainable low-worm system.

Pre-lamb drenches: Where lambing is planned before mid-May, ensuring that worm egg counts are below 100 epg by mid-April removes the need for any further treatment to late-pregnant ewes.

Which drench?

It’s important that drenches are as close to 100% effective as possible at this time of year. Any worms that survive treatment now will be a major source of the worm population in the coming months. If you’re not sure of the drench resistance situation, checking worm counts after a drench is good insurance.

Esperance: Nicole Swan, Swan’s Veterinary Services (

There has been rain earlier this month and perennial pastures are taking off with the mild warm days we are having. These are perfect conditions for worms, especially barber’s pole. It is important to be vigilant over the next few weeks and, if in doubt, do a worm egg count.

Most worm egg counts we have done recently have been low. However, one property with scouring lambs had an average count of 235 epg ranging from 0–850 epg.

Ewe counts have overall been low with averages ranging from 50–87 epg. Mobs with counts over 100 epg should be drenched before the end of March. This will reduce contamination of pastures going into winter. Any larvae deposited after the beginning of April will survive on the pasture over winter.

Lambing will soon be starting; pre-lambing worm egg counts are recommended.

For March 2020 state outlooks, please follow the links below:
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