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Western Australia worms, flies and lice update - March 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Across the state:

Rainfall over the past month has been patchy through the agricultural areas, but the basic messages apply across the board: worm counts in sheep should now be as close to zero as possible.  Low worm counts now mean that future worm burdens will come off a low base, and the worm risk in winter and spring will be reduced.

Where recent rains have led to some pasture growth—whether it persists is another matter—it’s especially important that sheep are not dropping significant numbers of worm eggs onto the ground. With milder temperatures, a large percentage of eggs will start to develop to the larval stage, and the annual worm cycle will begin. However, even where paddocks are still totally dry, worm egg development will be starting.

Current recommendations

The advice given last month still applies but is more urgent: A drench is needed for any mobs where you are not sure worm counts are very low, an average less than 100 eggs per gram for adult sheep, or less than 50 eggs per gram for weaners. By next month, a drench will have a limited effect in preventing winter worm burdens.

A worm egg count 10 to 14 days after drenching will quickly indicate whether good control has been achieved, or a follow-up treatment is needed.

The main reason for continued high worm burdens is that the drench was not fully effective, due to resistance by the worms. A small “leakage” of worms into autumn due to survival after drenching could have unhappy consequences later in the year.

What’s the drench resistance situation?

At a minimum, drenches at critical times (summer and autumn) should be 95% effective, and ideally closer to 100%. Resistance to the older drench types has been widespread and severe for many years, but not everyone is aware that resistance is now common to the widely-used abamectin.

Abamectin has been a good work-horse for many years, but drench tests results suggest there may be resistance on close to two-thirds of Western Australian sheep farms. In most cases, it is not yet severe, although this can vary. Unless shown to be fully effective in a test, abamectin is probably best reserved for use in combination products.

So far, resistance appears to be only at low levels to the “triple combination” drenches that contain abamectin, but testing is recommended. Resistance has not been reported in Western Australia in the most recently-released drenches, but it is wise to include them in a resistance test or a check of drench effectiveness.

Are pre-lambing drenches necessary in early-lambing ewes?

Even a low count will be magnified during lactation because ewes spontaneously lose their immunity for about 2 months after lambing. The worst effects may be in their lambs, which pick up worms from a paddock contaminated with worm eggs by their mothers.

Worm burdens in ewes in late April or May are often too low to justify a pre-lambing drench, regardless of whether they were last drenched in autumn or summer. However, as noted above, this may not be the case, due to drench resistance. A worm egg count will quickly sort this out.

If needed, these drenches are best given about 2–3 weeks before the anticipated start of lambing. If average egg counts are over about 100 eggs per gram, the mob should be drenched. In barber’s pole worm areas, a test for worm species is recommended if the count is over 200 eggs per gram, and if mostly barber’s pole, a long-acting drench may be recommended.

This is a critical time for worm control, and getting it right now will reduce the downstream impact of worms on sheep profitability, and the need for further drenching.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Since my last report we have had about 10 mm rain, and despite this, pastures are still pretty dry, and feed is getting short in some parts of the district.

Worm egg counts have been low for most mobs tested. The mobs that were tested were drenched in December with Startect® or Triguard®.

Another mob of ewes, last drenched in October with Zolvix® onto Kikuyu pastures, had an average worm egg count of 245 eggs per gram (epg) and therefore required a pre-lambing drench.

Lambing for some properties will start in April. It is recommended that a worm egg count is done 2–4 weeks pre-lambing. The closer to lambing, the more accurate the count as an indication of the need to drench, but it also imposes a higher risk of pregnancy toxaemia due to the stress of yarding. Therefore most farmers tend to do a pre-lambing count 3–4 weeks prior to lambing, and if a drench is required, ewes can be safely yarded and drenched. Closer than 2 weeks before lambing is considered risky.  

For those lambing later in the year, an autumn count is recommended as any worms present after the cooler weather sets in will survive to populate the pasture over winter. This monitor is especially important in mobs that did not receive a first summer drench. This is an opportunity to reduce the winter worm burden on the property.

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