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South Australia worms, flies and lice update - November 2020

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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

La Niña has made a pleasant change to the weather pattern this spring compared to recent years, although those who struggled to bale hay or have had a disjointed start to harvest would think otherwise. Above average rainfall — mostly State-wide for both September and October —has ensured abundant pasture growth and excellent livestock condition. From a health perspective it has limited worm larval pickup, but also dilutes trace element intake and increases the likely expression of footrot. As with most situations in agriculture you have to take the good with the bad.

Recent worm egg counts (WECs) indicate only a quarter of mobs with counts high enough to warrant a drench, and half of these in weaners. Some lamb mobs have had counts exceeding 700 eggs per gram of faeces, providing plenty of opportunities to do worm drench resistance testing to plan drench programs over the next three years. Many counts have been low to zero, mostly in ewes, indicating good immunity. However, this variability demonstrates the need to monitor if there will be a need for a first summer drench to minimise the worm population when they are most vulnerable. In addition, the significant likelihood of further rain, due to La Niña, raises the risk of Haemonchus (barber’s pole worm) infections and increased need for vigilance. Any rainfall episodes exceeding 20 mm during summer should be followed up with a WEC in susceptible mobs three weeks later, especially if on lighter soils. A less reliable measure is to drove the mob a few hundred metres and any sheep suffering from anaemia due to the blood sucking worm will soon lag behind. Fortunately, most commonly used drenches are effective against Haemonchus infections in SA, but they need to be timely to avoid deaths.

When drenching during summer it is always advised to use combination drenches with at least two and preferably three chemical actives. The type of drench used should not be repeated during this summer in order to minimise the risk of drench resistance developing. In addition, adopt the refugia strategy of either not drenching the top 20% of the mob or returning the mob to their paddock of origin for a week before moving into a new paddock. This is to ensure a population of “chemically susceptible” worms, i.e. those that have not been exposed to the drench chemicals recently used, will be carried into the new paddock.

The above average rainfall in spring also naturally encourages other parasites such as lice and flies. Those fortunate enough to shear in spring will have minimised the risk from these foes, but continued vigilance with summer rainfall is essential. If lice were detected at shearing plunge dipping six weeks later is a good strategy, unless they were backlined.

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