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Western Australia worms, flies and lice update - August 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

With winter behind us, spring will bring maximum pasture growth, but unfortunately, it’s also the best time of the year for parasites. While more rainfall is needed in most districts, what has arrived will be more than enough to keep worms and blowflies in the game.

A lot of lambs will be weaned in the next few weeks, so it’s important that they are not held back by worm burdens. And it is also time to ensure that flystrike management is planned ahead of the main risk period.

Lambs and worms

Lambs are at the most worm-susceptible time of their lives from about 3 to 9 months of age, for two main reasons:

  • They are now fully-dependant on grazing, and as they take in larger amounts of pasture, the number of worm larvae they can potentially ingest as a result also increases. 
  • They do not develop their adult worm immunity until they have experienced a period of worm intake and a consistently low exposure will allow immunity to develop without ill-effects in the lambs. This ideal situation does not always apply!

Whether lambs are exposed to many worm larvae or relatively few depends on how heavily worm-contaminated the paddock may be, determined by the season and worm control measures in the ewes. However, we generally don’t know this, so we have to assume that worms will be a risk from now until the pasture dries off, or the sheep go onto crop stubbles.

Action: In almost all cases, it must be assumed that lambs have significant worm burdens by 12–16 weeks of age, or that significant burdens will develop by that time. A drench should, therefore, be given if lambs are weaned at about this time, ideally at 12–14 weeks.

If lambs are not to be weaned then (such as prime lambs), a drench at this time is still recommended. Meat and Livestock Australia (MLA)-supported research in Western Australia has shown that significant reductions in final slaughter weight can occur if moderate worm burdens persist in growing lambs, but often these will be “invisible” and the usual signs of worms won’t be seen. Waiting until prime lambs are weaned before thinking about worms can be costly.

As an exception to the above, if programs to reduce worms ahead of lambing were followed, worm burdens in lambs may be low. This could be where ewes were drenched and moved to low-worm paddocks, or where long-acting drenches were used.

Worm egg counts on the lambs will tell the story—but don’t be fooled by “lowish” counts. A mob-average count as low as 100 eggs per gram would not indicate that worms are an immediate problem, but immature worms may be developing inside the animal, and worm counts may become significant within a short time.

If no drench is given at this time, make sure that further worm egg counts are taken over the weeks following weaning (or from 12 weeks onward), to keep lamb growth rates on track.

Drench resistance testing

Lambs are the ideal age group for checking the effectiveness of drenches. More details of resistance tests will be discussed next month, but if you’re planning a test shortly, head to “Tests” on the WormBoss website.

Managing dags

Scouring and dags are a bane of the life of sheep farmers, especially in the higher rainfall parts of the state. The problem is at the maximum in springtime when the blowfly risk is also at its peak, as dags are the main factor attracting flies.

A new review of the causes of scouring and how to manage it (“Dealing with Dags”), has just been released by Australian Wool Innovation—check the September issue of ‘Beyond the Bale ’.

Flystrike management

Plans should be in train for dealing with the usual annual fly risk—the relatively dry year may reduce the problem in some areas, but it is always unpredictable and at times a disaster.

Many factors determine the best approach, based on the size and timing of the flystrike risk, various sheep factors, and the management options that are practical on the particular property.

There’s a good discussion of this on the FlyBoss website, starting with “Prepare a flystrike management plan”, on the front page.

For August 2019 state outlooks, please follow the links below:
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