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Western Australia worms, flies and lice update - January 2017

WA WormBoss Worm Control Programs

WA WormBoss Drench Decision Guides





Albany: Brown Besier, Brown Besier Parasitology (

While many farmers are on summer holidays, in most cases the worms in their sheep are also taking it easy.

Hot and dry weather means that the vast majority of worm eggs dropped onto dry pastures die quickly, and never reach the larval stage that can infect sheep. Provided that worm control in the last couple of months has been on target, worms are not likely to be a significant risk at this time of year.

By this time of year:

  • Lambs (2016-drop black tags) and hoggets (2015-drop blue tags): should have received a summer drench, whether on pasture or as they went onto crop stubbles.
  • Adult sheep (red tags and older): if given a summer drench, it is recommended that 10–20% are left undrenched. Alternatively, all of this class should be left undrenched until late March or early April (“autumn drenching”).

The difference between drenching strategies for young sheep versus adults is to ensure the property drenching program as a whole is sustainable, but keeping susceptible sheep safe. Allowing some less-resistant worms to survive over summer means they will dilute resistant worms once worm egg development resumes in autumn, and will hence reduce the percentage of resistant types in worm burdens. Adult sheep in good body condition can tolerate small worm burdens, so some can be safely left undrenched.

Possible worm control break-downs

Unless summer and autumn drenches are fully effective, the annual worm control program can be derailed.

If these drenches are only, say, 90% effective, there could be two unwanted outcomes:

  • Earlier onset of worm problems than usual: Most worms left in the sheep will still be there in late autumn, when worm egg development on pasture is back at the peak rate. More worms are then likely to build up in winter than if the sheep had been worm-free by early autumn.
  • Increased drench resistance: As any drench-resistant worms that survive the summer drench are the basis of the new year’s worm population, the resistance level on the farm increases.

However, you won’t see any problem at the time if the drench was 90% effective, as the few worms left will have little immediate effect on the sheep. But given time, the problems outlined above could be costly.

It is wise to check worm egg counts of some mobs by mid-autumn at latest, with the goal of a mob average of less than 50 eggs per gram in weaners or hoggets. If counts are above this level, it is recommended that an adviser is consulted.

Checking drench effectiveness

Resistance now affects all drench types, except the newer types (released within the last 5 years), although even these should be checked at some time to confirm their effectiveness. The effect of the older drench types cannot be guaranteed—we have known for a long time that the white, clear and ivermectin drenches are unlikely to be highly effective, but resistance can also affect the newer ML drenches, abamectin and moxidectin. On a small percentage of properties, worms are even resistant to the “triple combinations”.

A simple check of drenches involves comparing the worm egg count of a mob of sheep before and after drenching. Take dung samples when a mob is in for a drench, then collect samples from the paddock 14 days later, a larval culture on these tests is also a good idea. This only tests one drench type, but over a year or so, several types can be checked. However, a full drench resistance test using lambs in spring time is recommended. As well as providing a more accurate picture of drench effectiveness, several drenches can be tested at the same time.

Barbers Pole worm: As an exception to the above recommendations, in areas where there is some green pasture over summer (mostly close to the coast), problems with barber’s pole worm can occur. We rarely see significant barber’s pole worm outbreaks these days, but it can occur in weaner sheep, especially after rainfall in summer, and heavy burdens can develop without much warning. The best way to check is with worm egg counts, at intervals over summer in sheep on green pasture. If it looks to be an on-going problem, it’s best to talk to an adviser about a preventative program.

Esperance: Nicole Swan, Swan’s Veterinary Services (

Worm Egg Counts (WECs) done in December ranged from 0 eggs per gram (epg) to an average of 200 epg with individual counts up to 600 epg.
Faeces from a mature ewe mob with the count of 200 epg was sent for further testing (larval differentiation). This mob had 72% barbers pole and so was drenched accordingly.

As stubbles run out it will be important to consider the worm risk potential of the pasture sheep are moved to. This will be influenced by the type of pasture—perennial or annual, the amount of recent rain, and the time and weather conditions since this pasture was last grazed by sheep.

Editor’s note: Paddocks become low-worm risk if not further contaminated by worm eggs for 3 months during summer and 6 months over winter, (or 2–5 months in very hot regions).

Young sheep should be monitored late February to assess their worm burden. This should be earlier if you consider that they are at a higher than normal risk of larval pickup.

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