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South Australia worms, flies and lice update - October 2020

SA WormBoss Worm Control Programs

SA WormBoss Drench Decision Guides





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

The spring flush has arrived in abundance following exceptional rainfall events during September and October from both the north and west. Coinciding with the consequent feed excess worm egg counts have been modest with only the occasional lamb mob exceeding the drenching threshold. These modest counts indicate a potential reduced need for a summer drench in adult sheep. All age groups should be checked for worm burdens before deciding on the need for a drench going into summer.

This seasonal circumstance highlights the need to seek a diagnosis for scours in lambs before assuming worms to be the cause. A drench at weaning is still recommended unless monitoring indicates a negligible worm burden. However, scouring often coincides with abundant lush feed in late winter/spring due to a range of non-parasitic causes, such as bacterial infections, trace element deficiencies and nutritional excesses. It is much more strategic to first seek a diagnosis rather than drench and hope it works.

Regardless of seasonal conditions and time of year maintaining refugia is strategic in the maintenance of a susceptible worm population and minimising the risk of drench resistance developing. Refugia involves either not drenching the healthiest 20% of a mob, or returning the mob post-drenching to the paddock they had been grazing for a further week before moving to the next paddock. Note, this principle is should be directed at adult sheep rather than lambs. Either tactic will ensure they take worm larvae or adult worms to the next paddock that have not been exposed to the current drench chemicals used. A little worm contamination in the fresh paddock is a good outcome as long as they are mostly drench susceptible worms.

The abundant rainfall combined with warmth provide ideal conditions for all microbial and insect life leading to a high risk for flystrike, dermo and fleece rot. Lice will also be on the rise following winter and so spring shearing provides the ideal opportunity to minimise the risk of these conditions. Alternatively, crutching and monitoring is essential to prevent potential production loss.

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