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Western Australia worms, flies and lice update - November 2016

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides

Albany: Brown Besier, Brown Besier Parasitology (

With sustained dry weather and increasing temperatures, the end to a long worm season is close. Once sheep are on dry pastures, and are not carrying significant worm burdens, worms should not be an issue for some months (unless we have a return of the rains we did last summer!)

Summer drenches—get the timing right

Lambs and yearlings (last-year’s lambs) should receive a fully-effective drench once they are on a paddock where they won’t pick up any more worm larvae.

This is easy where they will go onto a crop stubble after drenching, which are close to 100% worm-free.

The main caution: don’t take your eyes of the worm-control ball before a summer drench is given. In much of the state, late rainfall and mild temperatures will mean that worm larvae are still present on pastures, and they certainly will be wherever some green growth remains.

Where a late harvest delays summer drenching, a check of worm egg counts will show whether a drenching can safely be delayed, or whether counts are already high. With good conditions for worms over the course of the year, many lamb mobs will have higher worm burdens than usual.

However, if summer drenches are given too early to sheep still on pasture, they could pick up a significant worm burden after that drench. Ideally, this is avoided by planning pasture moves for lambs from weaning onward, but in some cases, a late spring or early summer drench may not be enough, and a second treatment is needed later in summer. A worm egg count check will tell the story, which may be different in a “good” worm year (like this) compared with years with a lower worm pressure.

Prime lambs— wormier than they should be?

Lambs destined for slaughter often carry low-level worm burdens that reduce their growth rates, but are not visible as the usual signs of worms. Recent research shows that worms are a lamb growth risk from about 14 weeks of age onward, and in many cases a drench before the first draft is sent off will be beneficial.

Don’t let worms hold back lamb growth rates—checking worm egg count may show that a routine pre-turn off drench is needed, or that paddock moves effectively keep worm numbers low.

Drenching adult sheep

For many years we have known that drenching all sheep on the property in summer is a prime cause of drench resistance in WA, and that adult sheep can usually be left undrenched in summer without ill-effects. This led to the “autumn drenching” strategy, where ewes are drenched in late March or early April, rather than in summer.

An alternative way to go is to drench ewes in summer, but leave 10–20% of each mob undrenched—so called “targeted treatment”, as the aim is to avoid a drench to the best-condition sheep.

This year, adult sheep will be in very good condition going into summer, and will resist worms well. Either strategy—autumn drenching or a targeted treatment summer drench—will have a major impact against the development of drench resistance, but without an effect on sheep production.

Drench resistance testing

Many lamb mobs will have relatively high worm egg counts if not drenched for a few weeks—they will be ideal to use for a drench resistance test.

If not planning a full test of several drench types side-by-side, checking worm egg counts before and after a drench will give a fair indication of its effectiveness. As resistance increases, drenches believed to still be effective may not have held up, and a check is a precaution against the failure of summer drenches.

Esperance: Nicole Swan, Swan’s Veterinary Services (

With harvest started there should soon be stubbles available to drench sheep onto. This provides an excellent opportunity to use an effective drench that will then keep sheep relatively worm free over the summer period. If stubbles are limited, weaners, then hoggets, then maiden ewes should be given priority.

Weaners and hoggets should always receive a first summer drench, preferably onto stubbles. Older sheep should be monitored and drenched if faecal worm egg counts (WEC) are 200 epg or more. An effective drench should be used at this time. This is important as any resistant worms left in the sheep will provide the population of worms infecting the stubbles, increasing the level of resistance on the property and reducing the drench options available in the future. It is important that you consult your veterinarian about the best drench to use for your property.
If an effective summer drench is used, then worm burdens should remain low throughout the summer period. The only exception to this is if we get summer rain. This is especially the case if sheep have been moved to pasture.

Pasture paddocks are only considered clean if they have been sheep free for at least 6 months over winter, or for 3 months of hot dry weather.

Following summer rain it is important to do a WEC 3–4 weeks later. This is particularly the case for properties with a history of barber’s pole.


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