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Dosing guidelines to prevent anthelmintic resistance

by Frank Hynes

The biggest problem facing roundworm control in sheep has been the development of resistance to anthelmintics among worm populations in many sheep flocks. Resistance means that the worms are resistant to the dose and are not killed by it.

The primary aim should be to have clean or safe pastures as much as possible (i.e. pastures with low parasite challenge). Dosing should be based on faecal egg counts. It is believed that dosing followed by a direct move to ‘clean’ pasture may be highly selective for resistance. Worms that survive treatment (hence resistant) will contribute significantly to the population that subsequently arises on the ‘clean’ pasture. Various alternatives to this direct ‘dose and move’ approach which will reduce the selection for resistance have been suggested. These alternatives include;

  1. Delay the move until 4-7 days after dosing.
  2. Move to clean pasture for a few days before dosing.
  3. Some of the lambs (10%) may not be dosed at all.

Dosing should also be delivered effectively, in particular to avoid under-dosing:

  • Dose according to the heaviest lamb in the group (not the average)
  • If there is a large weight variation, split into two groups to avoid overdosing
  • Ensure the dosing gun is calibrated to deliver the correct dose

Finally, the importance of adhering to withdrawal dates must be stressed. Particular attention is needed with lambs approaching slaughter weights. If there is a possibility that they will be slaughtered before the expiry of the drug withdrawal period the dose should not be given or an alternative product could be used