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Western Australia worms, flies and lice update - June 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

We may be into winter, and although the pasture is still very short in most districts, the winter worm pattern has started. Green feed and moderate to low temperatures means that not only can worm eggs develop to the larval stage, but the larvae can survive for weeks or even months.

Unless a particular pasture is known to be occupied by sheep that have low or zero worm egg counts, it must be assumed that there is potential for sheep to pick up worms on most paddocks. The extent of the risk depends mostly on the effectiveness of drenches back in summer or autumn, but predictions based on hindsight are dangerous. It’s time to commence monitoring every few weeks to check how worm burdens are running.

Weaners, hoggets: If there is no recent worm count information, check now and if no signs of worms, then check at 6-weekly intervals. Sheep can carry moderate burdens with few signs, but suffer an unrecognised growth rate penalty.

Ewes: Check counts 2–3 weeks out from lambing, and drench if higher than 100 eggs per gram (common from mid-winter onward). If they have already lambed, a drench at lamb marking is probably warranted if a significant proportion (say, more than 20%) are actively scouring, as this indicates many will have burdens that may detract from their milking ability.

Lambs: There should be no need for a drench on the marking cradle. Lambs don’t eat sufficient grass until 6 weeks or more to take in enough larvae to develop significant burdens, and it takes 3 weeks or more for larvae to become an adult worm. In the vast majority of cases, we don’t see average mob worm egg counts above about 200 eggs per gram until they are 10 or more weeks of age, and usually the first drench can wait until weaning (or 14–16 weeks, whichever comes first).

An exception to this is where there is poor worm control in the ewes, so that every mouthful of pasture brings in significant numbers of worms. However, if the ewes have had a pre-lamb drench, or a worm egg count showed them to have low burdens, there should be no need to drench lambs.

Scouring with low worm egg counts: A few reports have come in recently of scouring in 20–50% of a mob, but with very low worm egg counts. This probably indicates that the sheep took in sufficient larvae over a short period to incite an immune response at the gut level, which successfully expels the worms (hence low or zero egg counts), but causes enough gut damage to lead to scouring. The term for this is “hypersensitivity scouring”—similar to an allergic response, but it is short-lived (a few days in individual sheep, which appears as 3–4 weeks in a mob), and causes no observable production loss. It occurs mostly in adult sheep (which have previously developed a strong worm immunity), but can occur in hoggets. As there are few worms present, a drench has little effect.
Action: Hoggets (last year’s lambs): drench—even if scouring is due to the hypersensitivity response in many in the mob, some are likely to have significant burdens and hence warrant removing. Older sheep: check worm egg counts—if low (less than 100 eggs per gram on average), a drench is not justified. Scouring will halt in a short time, and they can be crutched, as this episode will ensure their full immunity has re-established and they should then remain relatively worm free.

Esperance: Nicole Swan, Swan’s Veterinary Services (

We have had rain and the pastures are picking up. Most farmers have finished seeding.

The majority of counts in May/June have been for ewes pre-lambing. Levels have, in most cases, been low. Farmers in coastal areas where barber’s pole can be a problem are advised to drench with closantal to avoid problems at lambing. Some mobs have required a pre-lambing drench. The drench history of these mobs varies. Some of these mobs had been drenched in autumn but onto contaminated pasture. Some had not been drenched or monitored since November 2014.

Hoggets that were treated with an effective drench late last year have low levels. 2014 drop weaners required drenching in most cases.

Farmers are reminded to start thinking about drench resistance testing on this year's weaners. This is a very important tool as it gives us crucial information for future worm control and to help preserve the effectiveness of the drenches we have available.

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