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South Australia worms, flies and lice update - August 2016

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides

Adelaide: Colin Trengove, Sheep Health Lecturer (UA Roseworthy campus) (

Above average winter rainfall has continued for most of the state and, as an example, Adelaide in mid-August is 100 mm ahead of its 550 mm average annual rainfall. The accompanying cold temperatures have not been conducive to pasture growth or late lamb survival, but elevated soil moisture augers well for good spring growth of pastures and livestock.  

The predominant activity has been marking and weaning of autumn/early winter lambs and the majority of worm egg counts in the last 3 weeks indicate significant worm burdens in lambs pre-weaning. Lactating ewe counts have mostly remained low indicating pre-lambing drenching and subsequent nutrition have been effective. It also highlights that even low levels of pasture contamination are sufficient to produce significant worm burdens in lambs pre-weaning. Lambs are born with the same innate immunity as ewes, but have not had time to acquire immunity against worms, and so most infective worm larvae on pasture are likely to develop into mature egg-laying adults within 3 weeks of ingestion. An adult female scour worm can lay 200 eggs/day during their 3-month lifespan and so there is great potential for worm burdens to build up rapidly during the conducive winter conditions. That is why a weaning drench is the most important drench in a sheep’s life.  

Moving weaned lambs to the most abundant and nutritious pasture that is also relatively worm-free is critical to maintaining their health and optimising growth rate. They do not begin to acquire immunity against worms until at least 6 months old and so maintaining low worm exposure during this period is fundamental to health. Fortunately, spring pastures should have adequate length i.e. >4 cm and so a reduced chance of worm larvae pick-up.  

It is common practice to use a combined 6-in-1 vaccine and drench at weaning. Weanerguard is the only product on the market that offers this combination and has the macrocyclic lactone (ML), moxidectin, as its chemical active. If low worm-risk paddocks have been prepared for weaning a short-acting combination drench is all that is required and preferred at weaning rather than a medium acting chemical such as moxidectin. However, if Weanerguard is used it is preferable to ensure that the next sheep to graze the “weaning paddocks” have moderate worm burdens and were not last drenched with an ML (i.e. moxidectin, ivermectin, abamectin). This strategy is to minimise the risk of developing drench resistance by repeated exposure of worms to the same chemical. 

Ewes prior to weaning should be monitored to see if they need a drench. As already indicated, their previous management combined with their immunity often means they have maintained low worm counts and do not need drenching. Given adequate access to spring pastures they should soon regain condition ready for the next joining.

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