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Western Australia worms, flies and lice update - February 2020

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Continued hot and dry summer weather will be keeping parasites at bay for the present, and the recent downpours in some locations will probably be a flash in the pan unless any weed or pasture growth stays green.

However, while worms and blowflies shouldn’t be causing trouble at present, they’ll be re-grouping for an appearance later in the year. Plans for control action in autumn should be on the agenda.

Worm control in Mediterranean climatic areas is based on making sure that worm egg counts of sheep are as low as possible by mid-autumn. Until the end of summer, any worm eggs dropped onto the ground usually die within a few hours and have no immediate effect. However, once summer temperatures begin to drop, and there’s some moisture in the system—rains or over-night dews—worm eggs will start to develop to the larval stage.

Although worm egg survival is usually patchy until mid-autumn, some of the worm larvae that do develop will remain in dung pellets until well into winter. By the end of April, the seeds for the worm situation in winter and spring will be sown, even if the pasture is still totally dry.

So, will worm egg counts in sheep (and goats) on your property be close to zero by mid-April?

If lambs and hoggets had an effective summer drench, they should be worm-free at present, and stay that way until early winter. Assuming that the drench did the job (no drench resistance), a check of worm counts can wait for a month or two.

Adult sheep are different—we don’t want all of them to be worm-free in summer. It’s essential that some non-resistant worms survive into early autumn, to dilute more resistant worm populations in other mobs. Small worm burdens won’t cause problems in ewes or adult wethers, so they are at the core of drench resistance management.

At this time of year, adult sheep should be managed under one of two basic strategies:

  • mobs where a percentage were deliberately left undrenched when the others received summer-drenches (often as they went into crop stubble paddocks);
  • mobs deliberately not drenched in summer, with an autumn drench planned for between mid-March and mid-April.

Both approaches will keep some less-resistant worms in the system and help prevent or reduce resistance development.

For ewe mobs not drenched since winter or spring (often, not since pre-lambing), a check of worm egg counts is recommended in early autumn—a drench may be advised.

For mobs where all ewes were drenched as they went into stubble paddocks or onto dry pastures, there will be a risk that the level of drench resistance has increased. Avoiding the drench types used at that time for the following year will help delay further resistance to them, although the seeds for resistance to those drenches will remain. Seeking advice for drench resistance management, and checking recommendations on WormBoss, is strongly recommended!

Esperance: Nicole Swan, Swan’s Veterinary Services (

Recent worm egg counts have been variable. Average counts have ranged from zero to 625 eggs per gram (epg), with individual counts as high as 1650 epg.

The higher counts were from sheep on coastal properties that received recent rains. As these sheep were not been scouring, it was considered most likely that the worm egg count was due to a significant Haemonchus contortus (barber’s pole) burden. However, these producers did not want to carry out any further testing, so sheep were drenched with a broad spectrum anthelminthic that also provided some prolonged cover against barber’s pole.

As we get closer to the end of summer, especially in areas of the district that have had a significant amount of rain, farmers must remain vigilant for barber’s pole. It is recommended that worm egg counts are done on a selection of mobs 3–4 weeks after a rain event to determine if worms are a problem.

Regardless of whether there is rain or not, sheep should be tested at the end of summer (usually March) to check worm egg count levels. Any worms at this time of the year will be the source of contamination of pastures going into autumn and winter.

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