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Western Australia worms, flies and lice update - February 2021

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Summer rainfall can throw a spanner into the works for worm control, as we mostly rely on the hot dry conditions to sterilise pastures of worms and give months of good worm control.

So what does the rain across much of the state in early February mean for worm control? Depending on the location, any pasture growth that followed could be a flash in the pan, or it could hang on for some time.

For worm development, green is good. It indicates enough moisture for worm egg development, and if weather conditions remain mild, the resulting worm larvae can survive for some time.

However, there was a spell of hot weather in late February, which will have wilted the pasture growth in many districts and cooked most of the larvae that developed after the rain. A quick reversion to dry pastures means little worm risk.

In contrast, green pasture along creek banks and dam seepages provides good conditions for worm survival, and perennial pastures in higher-rainfall areas are even more favourable. Even if larvae are present for only a few weeks, it may mean heavier pasture contamination with worms than in most years, reducing the effectiveness of the summer worm wipe-out.

On top of this, the worm picture will be complicated if drench resistance affected drenches given as sheep went onto crop stubbles or dry pastures. If some worms survived, they could kick off worm problems later in the year, and also increase the level of resistance in the worm population.

Assessment and action

If sheep aren’t carrying worms, it doesn’t matter how favourable pasture conditions are for worm development. If you are sure that drenches given earlier in summer were fully effective, there is no need  to change your worm program. (That’s a big “if”, and a check of worm egg counts (WECs) is always a good idea.)

Similarly, if green growth never transpired or faded quickly, there’s no real change to the worm risk.

However, the likelihood of a significant worm intake increases where drench resistance reduces drench effectiveness, and some green areas persist for a few weeks. In this case, it will be worth checking WECs about four weeks after growth first appeared (it takes this time for worm larvae to develop to egg-laying adult worms). Drench any mobs where the average count is around 100 eggs per gram or more.

For ewes which were on track for an autumn drench (mid-March to early April), and some green pasture has persisted, bring the drench forward and give by mid-March. Then check WECs every few weeks, as further worm pick-up is likely.

As always, the worm risk is especially significant where the blood-sucking barber’s pole worm occurs. Kikuyu grass pastures on the south coast have had a major boost, and the same will apply to perennial pastures elsewhere. From a month after the rains, be on the lookout for weak and wobbly sheep (a sign of anaemia) and maybe “bottlejaw” (fluid swelling under the jaw) —hopefully before any sheep deaths. Far better, take samples for WECs every 3–4 weeks to check whether there is a real danger.

Obviously there are many variations regarding how this could play out, and seeking some help from an adviser or vet is a good move. The WormBoss website will also help you plan the best approach.

Summer rains are a mixed blessing in our environment: as filling dams and maybe some useful pasture is often offset by rapid weed growth, and a bigger worm risk!

Esperance: Nicole Swan, Swan’s Veterinary Services (

We have done a number of worm egg counts (WECs) in the last few weeks of both lambs and maiden ewes. In the majority of cases counts are high enough to warrant drenching now or in the next few weeks. In all cases the maiden ewes had not been drenched for 10– 12 months. These ewes had been drenched in early 2020 with Startect™ (an effective drench).

One aim of ‘summer’ drenching with an effective drench is to ensure that sheep have a low worm burden at the start of winter. Any worms that are carried into winter will become the source of pasture contamination. Ideally sheep would go into winter with a worm burden under 100 eggs per gram.

If sheep receive an effective drench at the beginning of summer and are drenched onto stubbles, WECs should remain low, however summer rain and the need to move sheep as feed diminishes means that this is not always so.

Therefore it is important that sheep are monitored late summer/early autumn to ensure good worm control over the winter months.

On properties with autumn lambing, monitoring in late March may double as a pre-lambing WEC check.

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