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Western Australia worms, flies and lice update - April 2021

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





All regions: Brown Besier, Brown Besier Parasitology (

It’s been an amazing start to the season! Provided you were out of the path of Cyclone Seroja, it’s looking good for crops and pastures, with decent rains every few weeks since February across most of the agriculture areas.

As noted earlier, it’s also looking good for worms of sheep (and goats, and cattle). The green growth and cooler temperatures mean that worms will be enjoying high egg-hatching rates, and the resulting larval stages will stay viable for long periods.

Putting it all together, we can expect an earlier and probably more severe worm year than normal. It will be essential to stay on top of worm burdens as the year unfolds, by monitoring worm egg counts (WECs) at suitable intervals.

WECs should be checked now, in a mob or two of all sheep classes. Any with more than about 100 eggs per gram (epg) should be drenched, as it’s important that worm burdens come off as low a base as possible now that conditions are favourable for worm development.

Weaners and hoggets (purple and green tags)

Monitoring should be at about five to six weekly intervals. Shorter intervals mean that you won’t pick up developing worm burdens, but if too long, you could miss a significant rise in worm numbers. Once there is a consistent green mat of pasture, the maximum ‘tolerated’ worm count associated with scour worms should be around 250 epg (mob average). Setting the bar much lower means a lot of drenching – with the major downside of drench resistance development, as well as cost and effort. But letting counts go much higher risks production loss, and in young sheep, a good chance that counts will increase and lead to visible signs of worm infections.

Choosing lambing paddocks

Ewes should have the lowest possible worm burdens when they lamb, as counts typically increase immediately after the lambs are born. Hence, it’s important that the number of worm larvae on the pasture is low before the lambs are dropped.

This means making an estimate of the likely worm situation on candidate paddocks. In WA, there is a big change from summer-dry conditions (very few worms) to the development of green pasture. Ideally, the sheep on potential lambing paddocks will have low worm burdens over summer or early autumn, so there will be little chance of a big increase since then.

This mostly depends on the effectiveness of drenches in summer or early autumn — if a significant number of worms survived due to drench resistance, paddocks could be relatively wormy before lambing starts. In most cases, this is only obvious in hindsight, once high WECs are seen in both ewes and the lambs.

Pre-lamb drenches

If needed, these should be at a maximum of three weeks from the lambing date, and ideally one to two weeks out. If given much earlier, the ewes could pick up a replacement worm burden and the benefit of the drench is much reduced. This applies especially to short-acting drenches, but also to the long-acting moxidectin injection, as the period of prolonged effect is as only about seven weeks for some important worm types.

As with younger sheep, a drench would be recommended at about 100 epg or higher. This is well below a level with any effect on ewe health, but this is about the lambs, not their mums.

Barber’s pole worm areas

As discussed last month, this could be a good year for barber’s pole worm, as there will be a window of time for the larvae to develop on the pasture before it becomes too cold. Where there’s been a history of outbreaks and sheep deaths, the risk can be removed with a pre-lamb treatment with closantel, a mid-length drench specific to barber’s pole worm.

Where the barber’s pole worm history is more sporadic, WECs at more frequent intervals than in normal years would be good insurance, as counts typically rise very rapidly.


Esperance: Nicole Swan, Swan’s Veterinary Services (

Worm egg counts (WECs) over the last month have been variable. The majority of mobs tested required drenching. Monitoring at this time of year is important as drenching of sheep with counts above 100 eggs per gram (epg) will greatly reduce the contamination of pastures and therefore reduce the winter worm burden, especially of lambing paddocks.

In some enterprises this will coincide with a pre-lambing monitor.

If drenching is required it is recommended to use an effective drench (>95% efficacy). If drench information is not available then drenching with Startect™ or Zolvix™ Plus is recommended, both are newer drench actives in a combination with abamectin.

Now that we have had some rain but the days are still warm, larvae will hatch and thrive on the green pasture. Strategic drenching, good pasture management and monitoring of at-risk sheep, especially weaners and pre-lambing ewes is recommended. If you are in a barber’s pole susceptible area pre-lambing monitoring is especially important, as close to lambing a ewe’s natural immunity wanes and worm numbers can rapidly multiply.

As lambing approaches it is worth considering getting organised to conduct a drench resistance test (DrenchTest) on this year’s weaners. The information gained from this is invaluable, ensuring that appropriate drenches are used on farm. Using an ineffective drench is not only a waste of time and money but also accelerates the development of resistance. Undrenched weaners are required for this test. More information can be found on the WormBoss website.

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