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Western Australia worms, flies and lice update - September 2019

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

Recent dry conditions haven’t helped pasture growth, but worms will be active for some time yet.

Provided that there is some green pasture, warmer weather will accelerate worm egg development, and add flystrike to the risk agenda.

The main focus for worm control has to be on lambs, and very importantly, the opportunity to test for drench resistance.

Lambs at weaning

Unless specific worm control tactics have managed to keep worm burdens in the ewes very low, their lambs will usually pick up a significant number themselves by weaning age. Almost always, a drench at 12 to 14 weeks is recommended, and especially so if weaning is later than this.

For prime lambs, the same age guide applies, even if they are not to be weaned for some time. Recent research shows that by 20 weeks of age, worms can reduce prime lamb growth rates by 2 kg or more—a significant, and totally avoidable, economic loss.

If not sure that a drench is justified, check worm egg counts—but remember, low counts now typically increase to significant levels within a few weeks, or sooner.

And after drenching—get the lambs out of harm’s way by moving them out of the lambing paddock. If a prepared low-worm paddock (e.g., fodder crop or cattle paddock) isn’t on the agenda, look for pastures grazed by sheep with relatively low counts. Ideally, the next drench to lambs will be onto a crop stubble or dry summer pasture—and for prime lambs, hopefully, none.

Ewes after weaning

If the ewes are in reasonable body condition, their normal immunity to worms will have returned—why it is typically depressed over the lambing period remains an unsolved mystery, though a change in weight from channelling resources to the lamb is a likely factor. With spring pasture growth, they will continue to gain weight, and we usually find their worm egg counts are very low by early summer.

A drench under this scenario is rarely warranted. However, if the ewes are in a poorer condition—likely the case on some properties this year—taking worm egg counts will quickly sort out whether worms or the feed is the main issue.

Time for a drench resistance test!

The number of drench tests in recent years has stayed very low in Western Australia, but a test every couple of years is a good investment. Resistance can develop to even the most newly-introduced products, and flying blind may be a costly, although invisible, toll on sheep production.

A few resistance test tips,

Pick a representative mob: You could get a false picture of the resistance situation unless the sheep tested  carry the “farm mix” of worms. Best are lambs from ewes that have grazed several paddocks since the season’s break.

What drenches to test? A good start is to test the widely-used moxidectin. If fully effective, the more potent options will usually be as good or better. If moxidectin turns out to be resistant, the options have narrowed, and you’ll need to test the more potent drench types.

No recent drenches: The test mob should not have been drenched within the last 2 months. Any surviving worms will be resistant to that drench and bias the test result. Lambs at weaning age are ideal.

Are counts high enough? The ideal is an average of 300 eggs per gram or more. If a count shows they are lower than this, chances are they will increase if you re-test after a couple of weeks. If still low, talk to the laboratory about procedures to increase the sensitivity of the count.

Uniform mob weights: As the idea is to drench to the heaviest in the mob, you don’t want a big variation in test groups. For lambs averaging 30 to 40 kg, you don’t want much more than a 5 kg difference. 

Best of all, consult an adviser or veterinarian to plan the test most useful for your particular situation.

Blowflies about?

If flies are going to emerge as a problem this year, strikes will have started, or soon will, mostly depending on recent rainfall. It’s not too late for long-acting protective action, especially along the south coast, where the season is usually more drawn-out.

Esperance: Nicole Swan, Swan’s Veterinary Services (

For most properties in our district, September is weaning time. Pre-weaning worm egg counts (WEC) are recommended, however, in most cases, weaners will receive a drench onto the cleanest pasture available. This will ensure that they get the best start. The exception to this is if you want to conduct a drench resistance test (DrenchTest). If this is the case, the appropriate number of weaners should be left undrenched, and a WEC should be conducted to determine if there are enough eggs to conduct a test. For properties in areas susceptible to barber’s pole, it is recommended that the test group weaners are given a narrow spectrum closantel drench 10 days prior to the pre-DrenchTest worm egg count. This is so there are only scour worms present. If you are planning to do a DrenchTest, it is recommended that you contact your local veterinarian for specific advice about drench groups and protocols.

Overall, recent WECs have been low. A number of counts from orange tag hoggets have been conducted. These ranged from 0–190 eggs per gram (epg). Counts from white tag ewes averaged 515 epg. Drenching was recommended.

Looking forward, it is recommended that sheep are monitored pre-summer drenching to determine if drenching is required. Weaners and hoggets should receive a first summer drench (ideally onto stubbles) as experience has shown that if these classes of sheep do not have that, they have significant worm burdens later in the summer. Older sheep should be monitored. All mobs with counts greater than 200 epg should be drenched. Some sheep in each mob (5%) should be left undrenched to act as refugia and reduce the development of drench resistance. It is recommended to consult your veterinarian to determine the best drench for you to use at this time of year. A fully effective drench is required at this strategic time. In most cases, this will be Startect©, Zolvix©, or a combination ML drench.

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