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Western Australia worms, flies and lice update - October 2017

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

WA State parasite outlook:

As in every year, late spring is often a challenging time of year for parasite control, with a high risk of both worm and flystrike problems. However, from recent reports and worm egg counts there’s no doubt that worm control has proven particularly difficult this year on many properties.

This is mostly due to the very early rains, which meant that many sheep started the year with higher worm burdens than usual, but the generally lighter bodyweight of sheep due to poor pasture growth has also had a big effect. The increased susceptibility to worms and decreased tolerance to their effects largely would explain the especially severe problems we’ve seen in some mobs of ewes during lactation, and in lambs from marking onward. 

Depending on the location, worms and blowflies will remain a threat for another 4–6 weeks, or until pastures dry off and crop stubbles start to become available. The parasite risk will remain while pastures stay green, and a drawn-out finish to the season would prolong the need for vigilance. 


A drench is almost always needed when the oldest are 14–16 weeks of age. However, as many sheep producers have found so far, this drench may be needed earlier than usual this year—leaving it until the normal time of weaning may be too late.

Prime lambs are just as much at risk from worms as Merinos, and don’t be fooled if they seem to be growing at reasonable rates. A moderate worm burden can cause an invisible growth rate loss of 10% or more, and much more if they are left untreated for too long, and worms lead to scouring. If you plan to leave the lambs with the ewes beyond 16 weeks, either give a routine drench or check worm egg counts by this time at the latest. 

This may be a year when more drenches are needed for effective worm control than in more normal pasture years. Ideally, if lambs are drenched at 14–16 weeks and moved to a high-nutrition pasture, they won’t need re-treatment until a summer drench. However, in this year in particular, a worm egg count at 4–5 weeks after a drench is recommended to check whether worms are under control, or further treatment is needed.


As mentioned last month, drenching is rarely justified in ewes at lamb weaning or before summer, as their worm burdens generally drop rapidly once they have stopped lactating and are on good pastures.  We typically see average mob worm egg counts down to 100–200 eggs per gram by early December, even if they have been higher during lactation.

However, with the higher worm risk this year, checks of worm egg counts in ewes are recommended in late spring. If a drench is needed due to high counts then, this is likely to keep counts low from then until summer, but a check in December would be recommended to guide the drenching policy from then on. That will guide the choices of either a summer or an autumn drench to ewes.

The aim will then be to minimise worm burdens in summer and autumn, but also to manage the worm population to reduce the development of drench resistance. This is based on diluting any drench-resistant worms with other worms not exposed to drenches over summer, and is achieved by leaving a small percentage of adult sheep undrenched in summer, or drenching all adult sheep in autumn. We will discuss worm management in ewes over summer in more detail in the newsletter next month, and WormBoss also explains these strategies.

Drench resistance testing.

Young lambs are the ideal group for checking for drench effectiveness, as they usually have significant worm burdens—this is especially likely this year.

A full drench test is recommended, so that all drench options can be compared in a single test. However, a short-hand way to check drench effectiveness is a “14-day DrenchCheck”, where dung samples for worm counts are taken when a mob is drenched, and then again 14 days later. There’s no need to yard the mob for the second lot of samples—they can be picked up off the ground (provided they are fresh). Comparing the worm egg count results gives a guide to the drench effect, and using different drenches in different mobs allows you to check several drenches quite quickly.

Whether doing a drench resistance test or a drench check, the method and the results are best discussed with an adviser. Knowing drench effectiveness is only part of the equation, as how and when drenches are used is at least as important.


The blowfly risk will remain or increase if the rains persist while the weather warms.

In many cases, a pre-emptive treatment with a long-acting product will have been given by now, but if you are still to do this, or are waiting to see which approach to take, there are some very useful decision guides. The FlyBoss website has full information on the chemical options, and some very useful modelling tools to estimate the relative effects of different options, depending on the location and time of year.

As well as this, the new DPIRD phone app, “FlyStrike Assist” is a rapid and easy-to-use guide to the length of protection and withholding periods for different treatments:

And if not sure of the options for lice control, LiceBoss has all the information needed to decide on the need or otherwise of treatment at shearing, and the range of effective product choices.  


Esperance: Nicole Swan, Swan’s Veterinary Services (

We have continued to have a wet spring. However in the last week the sun has been shining and the pastures are thriving. A lot of farmers have started harvest and so stubbles should soon be available to ‘drench sheep onto’ if required.

Faecal worm egg counts (WormTest) have varied from low (0 eggs per gram (epg) for ewes that received Cydectin LA in March) to high (3,150 epg in a dying weaner). Two mobs of weaners given Weanergard in July had average counts of 30 epg and 50 epg respectively. These sheep will be drenched onto stubbles when available.

Weaners and hoggets that are moving onto stubbles, should be drenched on entry to the paddock. This will provide them with a worm free environment for much of the summer. Older sheep should be monitored with a WormTest prior to moving. If counts are 200 epg or greater they should be drenched. It is important to drench with an effective drench. Otherwise the resistant worms left behind will be the basis for the population of worms next year. In an ideal world the drench used would be 100% effective, but, at less than 100% efficacy, we can be sure that a very low, undetectable level of resistance will start to develop over time.

If all mobs on the farm require drenching it is important that a small percentage (2–5%) of older sheep in each mob are left undrenched. This will provide a mixed population of worms to dilute out any "Super worms" which may be resistant to the drench that is used. This concept is called Refugia.

Editor’s note: in a coming edition we will publish the new recommendations from Brown Besier for Targeted Treatment, i.e. leaving some adult sheep undrenched. But here is a sneak preview extract (please note this is only on adult sheep):

“Percentage to leave undrenched: 10–20% of the mob, leaving a mean of 50 epg or less after treatment (if mean mob egg counts are suspected to be relatively high, check before implementing targeted treatment and base the percentage on this).”

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