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Controlling internal parasite (worm) burdens and delaying drench resistance

microscopeDrench resistance is a real threat to future worm control in goats, sheep and cattle.  About 80% of milking goat herds and 65% of sheep flocks may already be affected, and on some goat farms, resistance to all three drench families has been recorded.

Therefore it’s important that you manage your livestock so that you control worm burdens and at the same time try to prevent the development of drench resistance.  This involves a combination of management practices and strategic drenching using effective drenches or drench combinations.

(Note that you will find helpful articles explaining what is meant by ‘drench resistance’ and ‘drench families’ in the Lifestyle File.)

Managing the worm challenge.
  • Sheep and cattle less than 9 months old are the major contributors to internal parasite populations.
  • The number of Iambs carried into the autumn and where they graze have a big influence on future worm burdens.  Plan pasture rotations so that you do not graze Iambs during summer and autumn on pasture where ewes will be lambing the following spring.
  • When possible, reduce worm intake by grazing new pasture, hay/silage aftermath, and providing forage crops.
  •  Integrate grazing with other species (animals of one species can help ‘mop up’ the worm eggs of other species). 
  • Feed stock well.  Well fed adult sheep and cattle cope better with worm burdens
Delaying the development of drench resistance:
  • Avoid excessive or unnecessary drenching
  • Don’t underdose
  • Get drench effectiveness checked
  • Prevent the introduction of resistant worms
Avoid excessive or unnecessary drenching
  • Try to make the interval between drenches as long as possible, using FEC tests or liveweight gain monitoring to give you confidence.  The longer the gap, the lower the risk of selecting for resistant worms.  Drenching at less than 28-day intervals increases the risk of developing resistance.
  • It's ok to leave the healthiest ewes or Iambs in a mob undrenched.  This helps to maintain a reservoir of 'susceptible' worms in the population to dilute down any resistant worms.
  • Pre-Iamb drench treatments may not be cost-effective.
Don’t underdose
  • Base the dose volume on the heaviest animal in the mob.
  • If there is a wide amount of variation in body weight, split onto smaller groups and dose to the heaviest in each group.
Test drench effectiveness
  • Arrange for a faecal egg count test ten days after you've drenched to check that your drench is working. (See your vet.)
  • The first drench after weaning and the first autumn drench are good times to make the check.
  • A full drench test should follow if the faecal egg count test is not zero.
Prevent the introduction of resistant worms
  • Whenever they arrive on the farm, give all sheep and goats (and cattle under 2 years old) a quarantine drench.  
  • A suitable quarantine drench for sheep and cattle is a combination of all three broad-spectrum drench families. 
  • Take care when introducing goats.  They should be given 1.5 times the dose of a triple combination drench, repeated in about 24 hours.  Consult a veterinarian about what to use, and be accurate with the dose, as some drenches are toxic for goats.
  • After quarantine drenching, yard stock (with hay and water) for 24 hours (longer if pour-ons are used on cattle) and then let them out onto a 'wormy' paddock so that any resistant survivors are quickly diluted.
The genetic tool

In the future, you may be able to buy sheep that are resistant to worms.  Real progress has been made by some breeders in selecting for sheep that are naturally immune to worm parasites.


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