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Western Australia worms, flies and lice update - January 2015

Albany: Brown Besier, DAFWA (

Holidays in mid-summer also extends to sheep worms, provided there’s a good plan to keep them at low levels. Weaners and hoggets (red and yellow tags) should have been given summer drenches, which will generally have been a month or more ago.

Worm larvae don’t survive on hot dry pastures, so sheep with low worm burdens now won’t pick up a new burden at this time of year.

What can go wrong?

The main likelihood of problems is where summer drenches were not fully effective. If the drench was only, say, 90% effective, there are two potential bad outcomes:

  • Earlier onset of worm problems than usual: Most worms left in the sheep will still be there in autumn, so when pasture conditions change and worm larvae develop, you’ll have more worms ready to build up in winter than if the sheep were worm-free;
  • Increased drench resistance: These worms are by definition drench resistant (they’ve survived the summer drench), so the new year’s worm population starts from a resistant source, and ratchets up the resistance level on the farm.

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

How do we know if this is likely to occur? Obviously, it comes back to drench effectiveness: some drenches can be expected to be 100% effective (especially the newer types, released within the last 5 years). Against that, we know that drench resistance will mean the older types are almost certainly not adequate—this includes the white and clear drenches, and ivermectin. The problem is the drenches that are potentially effective, but may not be: abamectin, moxidectin, triple combination types.

Checking drenches is simple: just compare the worm egg count of a mob of sheep before and after drenching. Take dung samples when a mob is in for a drench and then collect samples from the paddock 10–14 days later. This only tests one drench type, but over a year or so, several types can be checked. The Rolls Royce is a full drench resistance test in spring time, with lambs.

As a rule, any worm egg count over 50 eggs per gram in weaners or hoggets in mid-late summer could set up the problems mentioned above. It is best to talk to an adviser if there are still counts above this level after a test.

Barber’s Pole worm: An exception to the above is in areas where there is some green pasture over summer (mostly close to the coast), where problems with Barber’s Pole worm can occur. We don’t see a lot of this these days, but it can occur in weaner sheep and especially after rainfall in summer. The best way to check is again to take worm egg counts, as heavy burdens can develop without much warning. If it looks to be an on-going problem, it’s best to talk to an adviser about a preventative program. 

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