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Western Australia worms, flies and lice update - February 2018

Albany: Brown Besier, Brown Besier Parasitology (brown@bbpara.com.au)

Autumn is a make-or-break time for sheep (and goat) worm control, when worm egg counts should be at their lowest for the year. Worm control action from about mid-March to mid-April will largely determine whether or not significant worm problems occur in winter and spring.

The key factor is the change in environmental conditions at pasture level. While the weather is consistently hot and dry, almost all worm eggs passed onto pasture in sheep dung die quickly and few reach the larval stage. Increasing moisture and milder temperatures during March allow a slow increase in the worm egg survival rate—daytime temperatures may still be high, but night temperatures are cooler, and pasture moisture increases with heavier dews.

In most years, by the end of March, a significant percentage of worm larvae will be developing, and the total number of larvae depends on the number of worm eggs initially dropped onto pasture. Even moderate worm egg counts can be significant and lead to an excessive intake of worm larvae from the pasture as the sheep graze. The rate of worm turn-over increases as conditions become milder, and worm burdens in sheep will be significant by early winter if they are off a relatively high base in early autumn.

Effective but sustainable worm control—a balancing act

As well as maximising worm control effectiveness, this is also a key time for managing drench resistance. It has long been known that when sheep are drenched in summer (typically onto crop stubbles), the only worms that survive will be worms resistant to that drench. These will be few in number if the drench used was highly effective, but it is never zero, and we blame “summer drenching” for the rapid development of resistance to the ML drenches in WA.

So, we want to ensure that at least some of the worms that inevitably survive through summer are non-resistant—or as close to this as we can get. However, changes to traditional “summer drenching” are recommended only for adult sheep, as if they are in good body condition they will handle the small worm burdens typical of this time of year. 

Current recommendations for sustainable drench management:

  • Young sheep (weaner and hoggets): give a drench in summer unless you know from a worm egg count that a particular mob has an average worm egg count of less than 50 eggs per gram. This is a very low figure and is unlikely to be achieved unless sheep are in a low-rainfall area, or if low-worm pastures have been specially prepared since weaning. If no drench has been given since November or before, drench now or check the worm egg counts!
  • Adult sheep (mostly ewes): make sure a small number of worms that have not been exposed to a drench since winter or spring survive past autumn. There are two possible ways to go:
    • Autumn drenches: drench all sheep in each mob in autumn between the end of March and mid-April. There will be some low-resistant worm larvae on pasture by then, but not enough to cause problems downstream. They will dilute resistant worms that survived in summer-drenched younger sheep as these mobs move around the property. (You can check worm egg counts, but typically they will be above the 100 eggs per gram cut-off recommended for an autumn drench, unless managed to have lower burdens.)
    • Partial-mob drenches in summer (obviously too late now, but consider it for next year): leave 10–20% of the mob undrenched – select the best-condition sheep for this. Research shows no harm to them or mob production over the year, but the few worms left have a useful resistance-dilution effect.

What drench? 

Treatments at this time of year must be as close to fully effective as possible. The figure of 95% worm egg count reduction used for drench resistance tests is too low—aim to use a drench with efficacy of 98% or more. On the majority of WA farms, this means that abamectin, long a mainstay of drenching, won’t do the trick any more. Unless a drench test shows it to be fully effective, reserve abamectin for use in combinations.

This still leaves plenty of options on most farms:

  • Moxidectin: still effective on about two thirds of properties, but should be tested;
  • Triple combination” drenches (mixtures of a white, macrocyclic lactone (ML) and either a clear or an organo-phosphate type). Ideally the drench should be tested before use as a few cases of resistance have been seen—especially if the ML component is abamectin rather than moxidectin;
  • Derquantel-abamectin combination;
  • Monepantel or monepantel plus abamectin.

There’s 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 then14 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.

Barber’s pole worm

Where the pasture has stayed green through summer, especially with a boost from the rains a few weeks ago, the barber’s pole worm risk will be greater than in normal years. It’s not possible to predict when or whether an outbreak is likely in any one mob, and worm egg counts are the only way to check. Where counts are very high, treatment with the long-acting drench closantel may be a wise precaution, as while warm weather continues, large barber’s pole worm burdens can develop rapidly.

Sheep lice

As in last month’s newsletter, autumn shearing means lice treatments in many flocks.

Make sure that:

  • Treatment is necessary
  • Treatments are effective
    • Is resistance likely or not to the chemical used?
    • Will the application method be effective?

LiceBoss is the go-to website for factors affecting the lice risk and treatment decisions: see www.liceboss.com.au, and for information on all sheep and goat parasites: www.wormboss.com.au. (For the complete range of websites: www.paraboss.com.au.)


Esperance: Nicole Swan, Swan’s Veterinary Services (nicole@swansvet.com)

Esperance has had a dry summer. The usual January rain did not eventuate and now pastures are dry and worm problems are few. However this week the district has received 50–130 mm rain. This is great for those with perennial pastures and running short on water. With the continued warm temperatures the rain is also perfect for worms, especially barber’s pole (Haemonchus). Farmers will need to be vigilant over the next 3–6 weeks. It is recommended that in high-risk areas farmers collect samples in the next 3–4 weeks for worm egg counts (WormTests) to check for worms. Although barber’s pole is most likely to cause deaths, scour worms may also be a problem, especially in young sheep.

As a rule it is recommended that worm egg counts are done in March as this is an important time to control worms prior to winter when conditions for worms will remain favourable. Weaners and hoggets should be tested and drenched if they have a count of 100 eggs per gram (epg) or greater.

Adult sheep: if they did not receive a summer drench then they should be drenched mid-March to early April. Alternatively, sample a few mobs, and drench if over 100 epg. This strategy is effective if ewes are in good condition. If not, then testing earlier may be warranted.