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Sustaining the Effectiveness of Wormers in Sheep


At this time of year sheep farmers will be using wormers or anthelminthics to control gut worms in sheep. Enda O'Hart, Drystock Advisor, Teagasc Castlerea, has clear advice in just four actions that sheep farmers can take to prolong the efficacy of commonly used wormers on the farm

At this time of year sheep farmers will be using wormers or anthelminthics to control gut worms in sheep. Most sheep farmers are aware that there are problems around the build-up of resistance of gut worms to these anthelminthics and this problem is increasing year on year.

Available Wormer Products

There are three commonly available products to treat gut worms in sheep namely

  • White Drenches (Benzamidozoles)
  • Yellow Drenches (Levamisole)
  • Clear Drenches (Macrocylic Lactone)

Prescription-Only medicines

There are also two other products available:

  • Orange Drenches (Amino Acetronitrile Derivites)
  • Purple Drenches (Spiroindoles)

BUT these products are prescription-only medicines and are only available through your vet and are not for wide scale use.

Four Key Actions

There are four key actions that will help to maintain susceptible worm populations on your farm.

Don’t dose adult ewes for stomach worms unless there is a demonstrated need

Adult ewes are immune to stomach worms and don’t need dosing. In some cases lactating yearling ewes and thin/compromised mature ewes may warrant a dose. Do consult your vet or adviser if you have thin mature or yearling ewes that may need treatment. Leaving mature ewes undosed ensures you maintain a susceptible worm population on your farm.  Avoid using injectable macrocylic lactone products for external parasite control like sheep scab as this product will kill gut worms in all sheep if used, hence use plunge dipping instead.

Only use White Drenches to treat nematodirus in lambs

The white drenches do effectively treat nematodirus in lambs and no resistance to this product for nematodirus control has been reported. As white drenches have low efficacy for other stomach worms it is better to hold off using the yellow and clear drenches for later in the season.

Quarantine Drench to prevent buying in resistant worms

All sheep that are purchased or moved into the flock should receive a quarantine drench as these sheep could harbour a population of resistant worms in their gut. Consult with your vet about the best choice of wormer to be used to ensure a full kill of the worms present in these sheep. These sheep should be housed for 48 hours after dosing and released onto ground frequently grazed by sheep.

Carry out a Drench test

Farmers need to know what products actually work on their farm so consult your vet/adviser about completing this test.  Use faecal egg count tests from weaning onwards to determine the need for dosing.

By following these four steps you can prolong the efficacy of commonly used wormers on your farm and not find yourself in a situation where you have wormer failure due to resistance.

Teagasc Advisors are regular contributors of articles to Teagasc Daily. You can contact any of our Teagasc offices using this link Teagasc Advisory Regions here