January 29, 2009

Drench resistant worms in sheep - refugia

Sheep, farming, husbandry, animal health, disease, worms, internal parasites, refugia

By Dr Clive Dalton

What is 'refugia'?
  • The word ‘refugia’ appeared out of the blue with no explanation of where it came from. It has been around academia since at least 1976 when it was posed in relation to insect populations in Africa becoming resistant to insecticides.
  • Researchers talked about insects being ‘in refugia’ so now we talk about internal parasites being in refugia.
  • The academic definition of refugia is ‘the proportion of a worm population that cycles (breeds) and is not exposed to a particular drench chemical, so escapes genetic selection for resistance’.
  • The concept is very simple. Think of it as worms in a refuge hopefully avoiding resistance.
  • You don’t want worms inside the sheep or larvae on the paddock that are not killed by a drench. If you keep on drenching, the susceptible worms will die and the resistant ones will survive and breed.
  • So you need to keep a population of ‘susceptibles’ in the system somewhere, to keep on breeding with susceptibles which will then be killed by drench.
  • But you also want the susceptibles to breed with resistant worms to slow up their resistance developing, and hopefully be killed by drenching too. Let’s hope the crossbred don’t show hybrid vigour.
  • In 1981 researchers at Armidale pointed out that refugia will only delay the evolution of resistance where a large proportion of the population escapes exposure to the anthelmintic so the population continues to consist of mainly susceptible worms.
  • It’s important to keep remembering this.

The dilemma !
There's always a dilemma.
  • If you drench to kill worms in a flock to improve health and production (which is all good), then you cannot expect the flock to have a large population of worms in refugia especially as larvae on the paddock.
  • With extensive drenching you will end up with a large population of resistant worms as larvae on the paddock (which is not good) as you have killed all the susceptible ones.
  • As the refugia population cannot be measured, this means that things will vary a great deal, and it will change greatly between years and between farms.
  • The fear is that it may only be a very short-term solution in some flocks depending on how far drench resistance has developed.

What’s good for refugia?

  • Reduce the number of drenches given.
  • Extend the intervals between drenches.
  • Leave around 5-10 % of top lambs in a group undrenched.
  • Not to drench any animals in the top end of a mob, or only drench the tail-enders.
  • Not to drench animals above 12-18 months old as their immune system should be fully developed.

What’s bad for refugia?
  • Remember the dilemma. What’s good for worm control is bad for refugia.
  • Drenching animals on to a ‘clean’ pasture as this will allow the resistant worms to multiply.
  • Using only one class of stock on a paddock that are regularly drenched.

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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