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Western Australia worms, flies and lice update - January 2021

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

For the majority of sheep farmers in WA, worms and blowflies are well off the radar at the moment. Hot and dry conditions don’t allow much survival of any parasite that needs moisture and moderate temperatures.

Unless there’s some summer rainfall, or you have a lot of perennial pasture, worms should stay quiet for a good couple of months.

At this time of year, sheep given a summer drench should have close to zero worm burdens, although there’s two reasons that this may not be the case:

  • If there was any green pasture left in the paddock after the drench, some worms may have been picked up, and a check of worm egg counts (WECs) is a good idea.
  • If there was some resistance to the drench used, some worms will have survived.

However, chances are that worm numbers are very low at present, and there’s little likelihood of worm disease or even production loss.

That could change once weather and pasture conditions change, and it’s essential that sheep are not putting out worm eggs at that time. Not only will any worm eggs lead to possible problems, but they will come from drench-resistant worms, and increase the level of resistance in the new year’s worm population.

This means that it’s important to keep an eye on the weather from the worms’ point of view. By the end of March at latest, WECs should be checked to ensure there’s no need for a follow up drench where worms remain (except where an autumn drench is planned for ewes).

Summer rainfall can also fire up a worm population if green pasture growth persists for a week or more. In that case, a check of WECs about four weeks later will show whether many worms were picked up, and whether a treatment is worthwhile.

Barber’s pole worm is a potential problem where perennial pastures survive over summer. If summer rain freshens-up green pastures, there’ll be a bigger risk of problems. A periodic check of WECs is always a good idea in barber’s pole areas, whether or not there is summer rain.

While hot and dry conditions are bad for worms, they are survivors, and are ready to take advantage of any changes in the weather!

Esperance: Nicole Swan, Swan’s Veterinary Services (

Harvest is now over and stubbles are available to drench sheep onto if required. Stubbles are the cleanest paddocks available, so the use of a strategic effective drench gives us the best chance of significantly reducing worm numbers on the farm and in the sheep.

When first grazing stubbles there are no worms for sheep to ingest. If sheep are drenched onto stubbles with a drench of poor efficacy, these stubbles will quickly become contaminated and sheep will most likely require drenching again during the summer.

If sheep are drenched with an effective drench (efficacy  at least 98%) the effect is three fold:

Firstly, the majority (ideally almost 100%) of worms and larvae within the sheep will be killed.

Secondly, as the life cycle of the worms is around 21 days, it will take a number of weeks for worm numbers to increase. In addition, if the weather is hot and dry and regrowth is slow, a high number of larvae that hatch will die before being ingested by the sheep.

Thirdly, any worms that are not killed by the drench and have larvae that survive to be ingested will make up the population of worms that can potentially infect winter pastures. The problem with this is that this population of worms is resistant to the effective drench (Super worms). For this reason it is important that a percentage of sheep on the farm (either 2–5% of every drenched mob or 100% of mobs with lower worm burdens) are left un-drenched. This is called refugia, and it ensures that any resistant worm populations are diluted out by the mixed population left in the un-drenched sheep. This will prolong the efficacy of the drench.

If there is any significant summer rain it is important to monitor mobs four weeks later. This is especially the case in mobs grazing pasture paddocks or in stubble paddocks with significant regrowth.

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