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Western Australia worms, flies and lice update - December 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

The main parasite control issue is to ensure that young sheep receive summer drenches, and for adult sheep, to ensure both good worm control and management of drench resistance.

Weaners and hoggets (blue and red tags): As outlined in the November newsletter, in most sheep areas of WA, young sheep should receive a summer drench. Lambs are still developing their worm immunity and in most parts of WA, they usually have significant worm burdens by early summer.

Some sheep from the previous year’s drop will also not have developed a complete immunity, and unless a worm egg count shows they have low counts, a drench is recommended.

However, in low-rainfall cropping zones, there may be no need for summer drenches to lambs, if they had a drench at weaning. Hoggets often also have low worm counts in areas of lower worm intake. A worm egg count will quickly show the need for treatment.

Adult sheep: in the mid-high rainfall regions in WA, two main strategies are recommended to ensure that some non-resistant worms are left in some mobs of sheep through summer. The aim is that non-resistant worm numbers increase in winter, and dilute any resistant worms that survive in summer drenched-sheep. There are two options:

1. Give an annual drench to ewes in autumn, not summer (March or early April, and before green pasture growth occurs).

  • If ewes are in good body condition (average condition close to 3, or more), they usually have low worm egg counts and tolerate any worms they may have. Drenching in early summer is therefore not usually justified on worm burden grounds, and it markedly increases the level of drench resistance.
  • However, if unsure (ewes are in poorer condition, or a suspicion that local conditions have favoured worm development) a worm egg count will quickly show the need for action. For ewes in average body condition below 2.5, a drench is recommended if counts are above about 250 eggs per gram, but a percentage should be left undrenched (see below).


2. An alternative to autumn drenching for adult sheep is to give a summer drench, but leave 15-20% undrenched.

  • This may apply either for ease-of-management reasons, or if worm egg counts are high (as above). The sheep to leave untreated are those in the best body condition score, at least score 3, but usually there will be plenty at score 3.5 or 4. Research over several years has shown that there is no loss of sheep production or increase in winter problems with this strategy.

With the onset of hot and dry weather, worm burdens will stay low through summer, and no further action is needed until autumn. At that time either autumn drenches should be given (if following that strategy), or if summer drenches were given, a worm egg count taken to check that all is on track.

As noted in past months, it is important that summer and autumn drenches are given with fully-effective drenches—any surviving resistant worms will have a head-start over others, and can rapidly increase the level of drench resistance.

For December 2015 state outlooks, please follow the links below:
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