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Western Australia worm update - July 2012

Nicole Swan, Swan’s Veterinary Services, Esperance (

Not much to report here. We have had a fair bit of rain and this week will be quite sunny (20°C) which may get the worms going. Most counts have been low.

Brown Besier, DAFWA, Albany (

Now that mid-winter is upon us, sheep worms will be a risk for the next few months. Unlike other seasons, worm development is not limited by environmental condition – cool temperatures favour worm egg development, and even overnight frosts have only a marginal inhibiting effect. Once worm larvae develop in the dung pellet, the green cover and plentiful moisture means that they can move onto herbage, where they wait to be taken in by grazing sheep.

The extent of the potential worm problem is governed only by the level of pasture contamination with worm eggs, and this is largely determined by effectiveness of the summer-autumn drenching program. Drench resistance is technically considered present where a drench gives less than a 95% reduction in a worm egg count for a particular worm type, but unless summer and autumn drenches are close to the “100%” level, enough worms may survive to start a significant worm cycle by early winter. Visible effects of worms will start about a month earlier where a summer drench to young sheep was only 95% effective, compared to a fully-effective drench.

However, the effectiveness of drenches is rarely known for certain. Resistance is well-established to all drenches other than the newly-released Monepantel (“Zolvix”). We can no longer assume that Moxidectin (“Cydectin”, “Moximax”, “Moxitak”) will be fully-effective, and resistance has been shown to the “triple combination” drenches (a white, clear and ML drench).


  1. Do worm egg counts on hoggets not drenched within the last 6 weeks.
  2. Visually check ewes for signs of worms and if significant scouring is present, do a worm egg count. It may be feasible to drench at lamb marking – or if the count is low, the problem won’t be drench-responsive.
  3. Plan a drench resistance test on weaners in spring.

Unfortunately, the dry periods at this time of year that are bad news for newly-emerged crops have no negative effect on worms. Vigilance for worm problems is needed until late spring, and longer where green pasture continues.

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