'Knock-out' drenching - stopping those drench survivors

Drench resistance to multiple drug classes is the ‘new norm’ on New Zealand (NZ) sheep farms, especially in the North Island.

The diagram below comes from ‘Update on Anthelmintic Resistance’; McKennna 2018, courtesy Gribbles Veterinary Pathology.

Drench resistance

The numbers at the top of each bar represent the total number of drench tests (Faecal Egg Count Reduction tests/FECRT) in the 2016/2017 season, and the different bars are the different drench chemicals tested.

Just over 50% of farms showed resistance in one or more worm species to abamectin and the ‘standard’ double combination of BZ (white drench) and levamisole.

Over 75% of farms had resistance to white drench and/or ivermectin alone. 11% of farms nationally had resistance to a triple combination.

Those of you who’ve done FECRTs in recent years will know where you sit on this scale. For those who haven’t – presume there are resistant worms surviving your routine drenches.

On a breeding farm where lambs are grown out on permanent pasture over the summer/autumn, lambs will typically receive monthly drenches from weaning through till the start of winter. Based on the data above, there is a reasonable chance that resistant worms could be surviving those routine treatments.

This is where knock-out drenching comes in. Knock-out drenching is a strategy where a treatment of known high/likely efficacy is used to take out resistant worms that may have established a breeding population during a period of treatment with another products.

In most cases, a knock-out drench would be one of the ‘novel’ anthelmintics; Zolvix® or Startect®.

Treating lambs in this way has been shown in NZ modelling studies to be very helpful in slowing resistance development, where other measures to delay resistance are also in place.

The timing of the knock-out drench is a bit of a compromise. Leave it too late and there could already be significant autumn contamination by resistant worms; earlier on, there are more lambs to treat, plus the risk of further autumn contamination when you rotate back to a more routine drench. Speak to one of us about the timing for your own situation.

A final comment on Knock-out drenching

We cannot deal with drench resistance by simply throwing more drench at it.

Knock-out drenching can be a useful tool in the toolbox. However, it will have the most impact if combined with the other known management strategies for slowing resistance (including having a resilient farm system where nutrition and grazing management do a lot of the work for you).

Don’t hesitate to get in touch with us regarding drench-testing, knock-out drenching, or indeed any other aspect of sheep worm management!

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