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South Australia worms, flies and lice update - January 2021

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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

Windy but relatively mild temperatures and no rain sums up the end of December and first half of January. These conditions, but more so the sustained high meat prices and many reports of excellent harvests, has led to a buoyant outlook in 2021.

Recent worm egg counts (WECs) indicate modest worm burdens in some weaners and older ewes across the state, that are sufficient to warrant a strategictactical summer drench. Lack of recent rain appears to have limited the incidence of Haemonchus (barber’s pole worm) and fly infestations so far this summer.

Monitoring WECs through January and February is an important strategy to assess the risk of significant worm contamination developing in autumn through early winter — even more so with high density grazing in feedlots and confinement. In order to minimise the risk of drench resistance developing, particularly in summer, it is recommended to use drenches with different chemical actives at each drench. This is to minimise the exposure of worm populations to the same chemicals in consecutive treatments — especially if that chemical was used in the last three months, as this is the average life expectancy of the most common worms.

The dilemma arises that summer drenching is generally discouraged as it promotes drench resistance; i.e. any worms that survive a summer drench will be directly responsible for the majority of the (drench resistant) worm population in the following winter. However, delayed drenching will encourage a larger proportion of worms surviving through to winter either in the gut or on pasture and so causing a higher worm risk in winter.

The overriding principles of worm management remain paramount:

  • Monitor WECs frequently to decide if drenching is necessary i.e. every 4–6 weeks in weaners and 2–3 months in adults
  • Rotate between drench groups in summer to ensure worms are not exposed to the same chemicals in consecutive drenches. This is not critical when grazing green feed as most of the worm population is surviving on the ground
  • Summer drench only if monitoring indicates the need
  • Adopt refugia management, i.e. either delay moving a mob to a clean paddock for a week post-drenching, or only drench the ‘most needy’ 80% of the mob. This is to promote the transfer of drench-susceptible worms to the clean paddock to dilute the impact of any drench-resistant worms that survive drenching.

The same principles apply to worm control in goats, but the limited choice of registered drenches and greater risk of drench resistance dictates greater care with choice of chemicals and preventing meat residues. Goats metabolise chemicals much quicker than sheep and so there is less time for them to have an effect.

Spring and summer shearing largely limits lice and fly risks. The reliance on chemical control and prevention is limited due to widespread resistance and so treating for lice at shearing should be based on evidence of lice pre-shearing or the risk of a lice incursion. Close monitoring for the presence of lice and flies throughout the year as well as good biosecurity are two vital strategies to minimising the economic and welfare impact of these and other diseases. Biosecurity specifically means keeping your sheep isolated from neighbours and avoiding the purchase of diseased sheep. The other critical strategy is attention to the genetic merit of rams and ewes purchased — especially in relation to disease susceptibility.

For January 2021 state outlooks, please follow the links below:
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