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Restoring dewormer efficacy

A stud sheep producer in Victoria, Australia successfully introduced worms to his farm in order to restore dewormer efficacy. The farm raises "Nudie" sheep, naturally-shedding sheep that don't require shearing. The sheep have been selected to be genetically highly worm resistant, using EBVs.

According to the producer, the stud sheep have not had any drench for the past three years and prior to that just once a year at lamb marking. The commercial flock sheep are drenched at marking if they are under condition score 3. About 5 to 10 percent get drenched. The ewes get their last drench at 1.5 year of age when they wean their first lamb(s). All lambs are drenched as required or based on a worm egg count test, and ewe lambs might have a pre-joining drench if needed. Mature ewes are culled from the stud if they need a drench.

Twice in the past three years, the farm has purchased worms from Resurrect Refugia and drenched lambs with worms. According to the farmer, “With our low drench requirement these worms have taken over our original worm population, making all chemical drench groups effective again.”

From 2020 to 2023, efficacy of white dewormers (benizimidazoles) has improved from 72 to 96 percent. For abamectin, it went from 86 to 99 percent, and from 88 to 100 percent for levamisole.

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