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Western Australia worms, flies and lice update - December 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





WormTest with the Best

The following are successful participants in the 2019 ParaBoss WEC QA and have demonstrated proficiency in worm egg counting.

Albany: Brown Besier, Brown Besier Parasitology (

An early finish to the season generally means that worms also go on holiday, for a while at least. Whether or not they come back in a big way next year depends on how they are managed in summer.

At this time of year, most sheep will be grazing crop stubbles or dry pastures. Many will have had a drench in recent weeks, or a drench is planned. How can you avoid the downsides of under-drenching (less-effective worm control) or over-drenching (increased drench resistance risk)? Is it worthwhile to check worm egg counts as a guide?

Weaners and hogget-age sheep usually have worm burdens large enough by the end of spring to warrant a drench, without further checking. These worms need to be removed or they will reduce growth rates of young sheep and set up pastures with worms in the next year.

However, here’s a few situations where young sheep may have low worm burdens in December, and may not need drenching:

  • in lower-rainfall areas (wheatbelt and drier), a drench at weaning a few weeks ago may have done the trick
  • lambs moved into a prepared low-worm paddock after a weaning drench in late spring
  • lambs from ewes given long-acting drenches pre-lambing, especially for late-lambing ewes.

In cases like this, a worm egg count on a couple of mobs will show whether a drench in early summer is or is not justified, but if it is, make sure you check again in 4–5 weeks, as immature worms may have developed by then.

Adult sheep show an opposite trend. In most cases, their worm burdens are too low to justify a drench in early summer. Yes, it’s convenient to drench them as they go onto a stubble paddock, and it puts an end to potential worm problems for some months, but it also increases the level of drench resistance in next year’s worm population.

So, it’s definitely worth checking worm burdens in early summer in adult sheep: counts are often so low that there’s no way a drench is needed. In that case, leave it until autumn (from mid-March on), and either give a routine drench, or re-check worm egg counts.

If an autumn drench is not easy to fit into your management system, it’s still important that some worms are left in adult sheep in summer. In areas that are not a high risk for barber's pole worm, leaving 10–20% of a mob undrenched (refugia strategy) goes a long way towards reducing the development of drench resistance. Let’s say that a mob of ewes has an average count of 100 eggs per gram in December (pretty usual): leaving 20% undrenched means a mob average of 20 eggs per gram. Why would you drench the whole mob? Really, any of the ob can safely be left undrenched, but most people feel better not drenching the better-conditioned sheep.

I’d still check the egg counts of a mob or two in mid-March, but after a couple of years, the pattern will be clear, and the system that works best for the property will emerge. This should answer the question: how do we get the best possible worm control based on low worm burdens in summer, but also reduce increases in drench resistance?

If it’s too late to make changes this year, plan for some resistance management steps next year—the WormBoss website has chapter and verse to help.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Most farmers have now finished harvest, so stubbles are readily available for grazing by newly drenched sheep.

Worm egg counts done recently have been low. Older sheep have not required drenching as counts have been below 200 eggs per gram (epg).

In this region, lambs and hoggets should be drenched even if counts are low as these classes of sheep are more susceptible to the sub-clinical effects of a low worm burden. It has also been found that if young sheep do not receive a first summer drench, they are likely to have significant worm burdens towards the end of summer.

Older sheep should be drenched if counts are >200 epg.

If you are drenching onto stubbles or clean pasture, ensure you are using an effective drench (one that reduces the worm burden by more than 95%) for your property. It is also essential to leave a small percentage of the mob undrenched. This strategy will ensure that, if some worms are resistant to the drench and not killed, the resistant population is diluted out on the pasture by the mixed population of worms left in the undrenched sheep. This is this refugia strategy.

Be aware that if your stock is on perennial pastures that summer rain can increase the risk of a worm problem, especially in barber’s pole risk-areas. It is recommended that under these conditions that a worm egg count is done 3–4 weeks after significant rain.

Wishing everyone a Merry Christmas and a Happy New Year.

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