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Western Australia worms, flies and lice update - March 2015

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, DAFWA (

Autumn is a critical time for sheep worm control, as the action taken now largely determines the risk of winter and spring worm problems.

After some late hot weather in mid-March, temperatures are dropping and who knows whether the recent spattering of rain will keep up? Either way, it’s a better time for worms than a month ago, as mild weather conditions allow development to commence. Conditions will become increasingly favourable as we get closer to winter, and if sheep are passing large numbers of worm eggs in late autumn, winter worm problems are highly likely.

All sheep on the farm should have low worm egg counts by the end of April at the latest.

Weaners and hoggets (red and yellow tags): These sheep should have had a drench in summer, and now have very low worm burdens. An exception is if the drench was not fully effective due to drench resistance, so the sheep continue to pass worm eggs. If the effectiveness of summer drenches is not known, dung samples should be taken for worm egg counts, to check the need for further treatment to ensure worm numbers are low.

Breeding ewes: Drenches to set up worm control in ewes for the year ahead are now recommended for autumn. Summer drenches have been shown to be a major cause of drench resistance, as resistant worms surviving these treatments are the main source of future worm burdens. Adult ewes in good condition should be drenched between the end of March and mid-April.

If it is suspected that they have low worm burdens and do not justify a drench, conduct a worm egg count to check. Drenches should be given if the result is over 100 eggs per gram, as even this low count can lead to significant pasture contamination with worm eggs. In most cases, counts are higher than this level.

If adult sheep were given a summer drench, a worm egg count is also recommended unless it is known the drench was highly effective. The drenching program should then be reviewed before next summer, to avoid increasing the drench resistance level.

What drench? Treatments at this time of year should be as close to fully effective as possible. Drench types expected to be in this category include the newer products monepantel and the derqantel-abamectin combination, and on the vast majority of farms the ‘triple combination’ drenches (mixtures of a white, macrocyclic lactone and either a clear or an organophosphate type). While resistance now affects all macrocyclic lactone (ML) drenches, tests indicate that moxidectin remains effective on about two thirds of properties, and abamectin on about one half. None of these other drenches (ivermectin, white and clear) are likely to give a useful result when used alone.

Unfortunately, there is no way of predicting how well various drenches are working on individual farms without some form of test. A drench resistance test conducted on lambs in spring will give the best indication, but a simple check can be done whenever a particular drench is used. This involves taking dung samples at the time the drench is given, and then 10–14 days later (from freshly-passed samples, directly from the paddock). Comparing the worm egg counts will indicate the drench effectiveness, and over a year, several types can be tested in this way.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Recent WEC have had varied results.

Ewes that received an effective drench in late November/December had low counts as we would expect and hope.

Results for weaners varied from 15–100 epg strongyles and 20–55 epg Nematodirus.
The weaners drenched onto pasture had the higher counts.

One mob of weaners was drenched with moxidectin onto stubbles. Their result of 50/40 epg is a cause for concern that there is resistance to this drench group.

It was recommended that all the weaners tested be drenched with a known effective drench (either Zolvix or Startect) prior to 1/4/15. This would ensure that resistant worms left from the summer drench do not populate the farm this winter.

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