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Western Australia worms, flies and lice update - December 2016

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Early summer is a critical time for sheep worm control, but best-practice worm control is not as simple as giving summer drenches to all sheep on the farm.

Drench resistance and summer drenching

Removing worms in sheep in summer means that very few worm eggs will be dropped onto the pasture in autumn, when milder weather conditions allows them to resume development to the larval stage. With no worm larvae to pick up from the pasture, they should stay relatively worm-free for months (if the drench worked—more on that later).

However, we also know that any worms that survive summer drenches because they were resistant to that drench are given a head-start over other worms. If the worm population on the farm from early autumn onward originates from these drench-resistant worms, the overall resistance level increases, and worms will be harder to control later in the year.

The rapid development of drench resistance in Western Australia from the 1980s onward is largely blamed on summer drenching, as at the time we didn’t know of the resistance risk.

However, research since then has led to sustainable programs that can give us the best of both worlds. The key is to drench at the most appropriate time for different classes of sheep (see the link to your Program and Drench Decision Guide at the top of this page), and to always use a fully-effective drench.

Lambs and hoggets

Regardless of the resistance risk, young sheep (this and last-years lambs) are likely to be carrying significant worm burdens in early summer, and it’s important that these are removed so their growth rate is not affected.

They should receive a drench either as soon as pastures become fully dry, or onto crop stubbles.

If you used a pasture management move to keep burdens low, such as weaning lambs into a fodder crop or cattle paddock, a worm egg count will indicate the need for a drench. However, unless the counts are very low (less than 100 eggs per gram) a drench is usually warranted, even when these young sheep are in very good condition (such as prime lambs).

Mature sheep

Adult sheep in good body condition have both a strong resistance and tolerance (resilience) to worms. Generally, coming out of spring, and with lambs weaned, ewes have low worm burdens in early summer, and the few worms cause little ill-effect. After the current good pasture year, the ability of adult sheep to tolerate worms will be especially high.

Adult sheep are therefore at the centre of drench-resistance management strategies. The aim is to allow the survival of a small number of susceptible worms, so these can dilute the resistant worms likely to be present in summer-drenched younger sheep (provided they are moved onto pastures previously occupied by mature sheep). There are two main ways to go about this:

  • Autumn drenching: delaying drenching until mid-March or early April means that a small percentage of the worm eggs dropped onto pasture will develop to the larval stage. These can survive beyond that time, ensuring a small population of susceptible worms. This strategy has been used to good effect on many farms for many years, with no ill-effects on the sheep themselves, provided that they are in good condition over summer, and that this drench is not left too late. 
    (Editor’s note: The worms carried over won’t all be susceptible (you may already have some degree of drench resistance on your property), there will be some resistant worms among these, but the proportion of resistant worms in the carryover population in sheep that did not receive a summer drench will be far lower than in sheep that do receive a summer drench.)
  •  “Targeted treatment” at summer drenching: this means leaving a small percentage of adult sheep undrenched when treatments to the rest of the mob are given in summer. The worms that survive in these provide the undrenched population needed to dilute resistant worms in summer-drenched mobs of younger sheep. Trial work confirms that sheep in especially good body condition (3.5 or more) can stay undrenched with no danger to them, due to their high level of worm-tolerance. You need to leave a minimum of 10% of a mob undrenched, and ideally, 20% or more.

Drench effectiveness

No long-term worm preventative strategy will work if the drench doesn’t. Worm numbers will start to build up from mid-autumn onward, and lead to more severe problems in winter than necessary.

Worse, any resistant worms surviving the drench will boost the drench resistance level—and make it harder for a resistance management strategy in adult sheep to work.

The aim is to use drenches as close to “100%” as possible. In practice, on some properties this may only be achieved by use of multi-active (combination) products. Combination drenches also have their own benefits to slow the development of drench resistance.

How do we know whether the drench is fully effective or not? Staying away from the older drench types (white, clear and ivermectin) as single actives is a good start in WA, but resistance now affects the newer types. Drench test results show resistance to abamectin in about 50% of cases in WA, and over 25% for moxidectin. In a small number of tests, resistance even affected the “triple combinations”, though we are yet to confirm resistance here to the new drenches introduced within the last 5 years.

A full drench resistance test on young sheep will show the farm picture for all drench types. The next best is to do before-and-after DrenchChecks of worm egg counts when treating a mob, as over the course of a year, a number of drenches can be checked.

Talking to a sheep management adviser or rural veterinarian is recommended for planning resistance management strategies, and the WormBoss website gives full details of sustainable programs and testing for drench effectiveness.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Worm egg counts performed this month have, in most cases, been low.

All ewe mobs tested had counts below 100 eggs per gram (epg). For ewes other than lambs or hoggets, drenching is not recommended until levels are 200 epg or greater. For lambs, counts have been around 75–100 epg, confirming that a summer drench is required. As discussed in the last ParaBoss report, this is ideally onto stubbles. Unless current drench resistance test data supports otherwise, the newer drench groups in Zolvix® or Startect® are recommended for this drench. It is important if you are treating prime lambs to remember that Zolvix has an ESI of 115 days.

It is important that if we experience a significant summer rain event, as is common on the south coast, that sheep are monitored 3–4 weeks after the event. This is especially important for properties that have had barber’s pole problems in the past.

The lifecycle of the worms in sheep is approximately 21 days. Following rain, small brown stomach worm larvae that have survived in a faecal pellet during the dry summer months will resume development, and, as long as there is enough moisture on the pasture for them to survive, will be ingested by sheep and the life cycle will start again. Within 3 weeks, there will be adult worms in the sheep, which will then start to produce eggs.

Finally, I would like to wish everyone a Merry Christmas and a Happy New Year.

For December 2016 state outlooks, please follow the links below:
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