January 31, 2009

Drench resistant worms in sheep – current myths & half truths

Sheep, farming, husbandry, health, worms, internal parasites, current practices, myths, half truths.

By Dr Clive Dalton

Myths & half truths won't clean up these backsides!
Dagging has been an accepted part of sheep farming since
the start of farming, but we can't afford the time, cost & physical pain
of dagging sheep any more.

Because of the way internal parasites operate in sheep, and the methods we have used in the past to control them, a lot of myths and half truths have become interwoven with good solid facts.

Myths are defined as ‘traditional narrative embodying popular ideas’ so some bits are true or half true, but others are certainly not. Myths are things we want to be true!


1. The more you drench, the quicker you’ll have worm-free stock.

Not true. You will never create worm-free stock and you would not want to either. Sheep need worms to challenge their immunity. But the more you drench the greater the chance of multiplying resistant worms.

2 You cannot farm without drench.

Not true. Some farmers are already doing this. They had no choice as they have already ‘hit the wall’ and used up all their drench options.

3. Drenching stock will eliminate worms from pastures.

It won’t. It will reduce larval numbers but to what extent will depend on a long list of factors.

4. If you don’t graze pastures for 3-6 weeks, all or most of the worm larvae will have died.

Not true. We now know that larvae can remain viable for up to 6 months, never mind 6 weeks.

5. Drenching hoggets at monthly intervals from weaning through autumn and winter with 5 to 7 drenches will make doubly-sure that they are worm-free as two-tooths.

Not true. Hoggets and two-tooths still produce worms long after the drenching stops.

6. If one drenching interval is not working, then shortening it will.

Not true. Drenching more often can easily build up drench resistance as resistant worms have a reproductive advantage.

7. Using long-acting drenches in sheep will delay the onset of drench resistance in worms.

Not true. Good evidence is building up that this will hasten the development of drench resistance.

8. Drenching ewes, especially with long-acting drenches, at pre-lambing and docking will prevent them infecting their lambs.

Not true. This will build up drench resistance in the flock even faster because the resistant larvae passed out by the ewes and recycled by the lambs will have a reproductive advantage.

9. Tapeworms have serious effects on the health of lambs.

Not true. Masses of research prove that tapeworms are not a serious problem. They look much worse than they really are.

10. Newly-drenched lambs should be immediately put on to clean pasture or special lamb blocks.

Not true. The chances are very high that the clean pastures will be contaminated with drench-resistant worms as all the susceptible worms will have been killed by the drench. They should be held for at least 4-5 days (some vets say 2 weeks) on old contaminated pasture before shifting them on to the lamb block.

11. Breeding is far too slow a way to fix the worm problem.

Not true. If you use an accurate way to identify worm resistance in both males and females and select flat out for this trait, you’ll be surprised at how fast things can change

12. New worm resistant genes, and developing new drenches and vaccines are just around the corner.

Pigs will fly! The current generation of farmers with average age 50+ should not hold their breath.

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.

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