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Western Australia worms, flies and lice update - July 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

It’s turning out to be a wormy year, no doubt driven by the early pasture growth, which gives worms more time to increase in numbers. Whether the pasture is still short or growth is reasonable, the moisture at ground level will mean worm larvae survive for some months. Cold weather is no barrier—a wormy paddock now will be a wormy paddock until October at least.

The likelihood of worm problems in winter mostly depends on how well worms were controlled back in autumn, and it’s hard to predict with hindsight. The main aim now is to stay on top of worms for the next 3–4 months, by monitoring with worm egg counts. 

Lambs: As noted last month, lambs shouldn’t need a drench until about 12 or more weeks of age—unless worm control in the ewes was very poor and the pastures are knee-deep in worm larvae. (There’s more detail below on worms in lambs at marking.)

Hoggets: If more than say 10–15% are scouring, give a drench to all. It’s almost certainly due to worms, and scouring indicates that their immunity is developing. Chances are that most hoggets in good condition will handle worms for the rest of the season without much further scouring. However, they should be checked every 6 weeks or so to see whether low-level burdens may be affecting production.

Ewes: Visually check ewes for signs of worms, and if significant scouring is present, take a worm egg count. If counts warrant treatment, it may be feasible to drench at lamb marking. Once the lambs are 8–10 weeks old, ewes recover their worm immunity and worm burdens will reduce from then on.

Why is there no point in drenching lambs on the marking cradle?

This question often comes up: wouldn’t it help the odd lamb that had picked up a few worms before marking?—especially if a few are scouring.


  • To pick up worms, lambs need to take in grass that carries worm larvae. Up to about 6 weeks of age they are mostly dependant on milk from the ewes, and even though we see them grazing from a very early age, the actual amount they take in is very small.
  • It takes about 3 weeks from ingesting worm larvae before they become an adult worm. Hence, any worms taken in at say Week 2 will not be at the adult stage until Week 5, and it takes some time of grazing for the numbers to build up to a significant level.

Putting this together, the only way lambs can take in enough worm larvae to have real problems by the time of marking (unless delayed until well after 2 months of age) would be if the pasture was massively contaminated with larvae. This only happens when there’s been a real failure of worm control—either the ewes have very large worm burdens, or other sheep grazing the paddock earlier had similarly heavy burdens. Worm egg counts of lambs on the cradle are therefore almost always very low.

Scouring is sometimes seen at a very young age (2–3 weeks on), but is more likely to be due to the disease coccidiosis. This very rarely causes deaths, but if the ewes are in very poor condition, poorly-nourished lambs can suffer badly. If significant scouring is seen in pre-marking lambs, it needs sorting out by checking with a vet.

Bottom line: unless there is a suspicion that worm control in the ewes was pretty poor, there will be no need for a drench to the lambs at marking. Leave it to weaning (or 14–16 weeks if not weaned by then—including prime lambs). If worm egg counts of lambs showed significant worm burdens, or they responded to a marking drench, something is wrong with the worm control program, and needs changing for future years.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Esperance has been cold and wet for the last week. Pastures are growing well and we are expecting some more sunshine over the weekend.

Worm egg counts have been variable ranging from 40 epg to 300 epg. Those ewes with higher counts had not been drenched for a long time. Mobs drenched in November with Trigard (a triple active) had counts between 100–140 epg.

Worm egg counts on scouring 4 week old lambs showed 0 epg as expected, but a high burden of coccidia. These sheep were on poor quality pasture and so ewes probably had low milk production and necessitated early grazing of the lambs.

With lambing coming to an end the next event will be lamb marking. Lambs should not require drenching at marking as they should be too young to have picked up a significant burden. This time is, however, a good opportunity to monitor the ewes for increased worm burdens post lambing. If counts are high ewes should be drenched to ensure that the pasture worm burden is as low as possible for the lambs when they start to graze.

Now is the time to start thinking about conducting a Drench Resistance Test at weaning. Depending on the drench resistance history of the farm the test can be tailor made to ensure that the drench groups tested are appropriate. Contacting your veterinarian 4­–6 weeks pre-weaning will ensure that you have all the information you require as well as adequate samples of drench to conduct the test at weaning.

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