Changing Worm Behaviour Calls For Updates To Worming Practices.
26th August 2019

A lot has been said about the escalating levels of wormer resistance in sheep worms around the world, and Northern Ireland is no different. The most recent study in Northern Ireland was published in 20131, which showed alarming numbers of farms with detectable resistance to the three older wormer groups (Grp1 BZ 81%, Grp2 LV 14%, Grp3 ML ivermectin 50%, Grp3 ML moxidectin 62%). Since then, anecdotal reports that a large number of farms in NI have been relying heavily on Grp3 ML products, especially moxidectin, for worm control, can only have made these numbers worse.

But why should we take notice when there are no obvious problems on many farms? The impact of a developing resistance problem creeps up slowly on the farm. As the levels of resistance increases, the number of worms surviving treatment increases, and this has a major impact on lamb growth rates and productivity long before there are enough worms left behind for an obvious treatment failure. The “left over” worm burden can reduce lamb growth rates by up to 50% BEFORE there are enough worms present to cause any visible signs of illness2.

What can we do now? Reducing reliance on wormers in general, and any one group in particular is a good start. Routine dung sampling of lambs for worm egg counts to determine the need for treatment (rather than routine treatment), will help to avoid unnecessary treatments, as will only using targeted treatments for adult ewes. (See for a video guide collecting samples for worm egg counts or for details on targeting treatments for adult sheep). Knowing which wormers are likely to work best at different points in the grazing season (as the balance of different worm species changes through the season) is also essential for effective worm control now that resistant worms are likely to present on the vast majority of sheep farms. The simplest way to keep an eye on this is by following every worm treatment with a drench check (worm egg count). Samples collected 7 days after treatment with a Grp2 LV product, or 14 days after treatment with Grp1 BZ or Grp3 ML products, will show how many worms have been left behind, whether further work to establish the level of resistance is necessary, or whether a retreatment with a different wormer group is called for.

Inclusion of the newer actives (Grp4 AD, monepantel, and Grp5 SI) now will also help to slow the development and spread of resistant worms. Currently, only Zolvix™ (Grp4 AD) is easily available, and there are two distinct times when Zolvix should be used on every farm. Firstly, to replace one dose of an older group wormer for all remaining lambs after weaning, in the latter part of the grazing season, as a break dose to remove the worms left behind by other treatments. This allows the lambs to make the most of late season grass and reach their growth potential, and also reduces the build up of resistant worms on pasture for the following grazing season. Secondly, for all incoming or returning sheep as a farm protection (quarantine) dose, to prevent the importation of resistant worms. (see for a detailed quarantine protocol)

There’s a lot to think about in adapting worming plans to keep up with how the worms are changing. However, there is always good advice on hand from the team of fully qualified SQP prescribers at Fane Valley stores, to help guide treatment choices and implement effective worm control strategies for best performance from your flock.



  1. McMahon et al, Veterinary Parasitology 195 (2013) 122– 130
  2. SCOPS manual

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