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Zolvix Plus providing efficacy and easier management of drench resistance

Lewis Kahn1 and Nick Rolls2

1 ParaBoss Executive Officer;

2 Technical Services Veterinarian, Elanco Animal Health

 June 2017


Elanco Animal Health has recently launched Zolvix Plus in Australia; a new combination drench for sheep. Zolvix Plus combines monepantel (the active ingredient in Zolvix) with abamectin (a widely used macrocyclic lactone).

Zolvix (monepantel) as a single active has been available in Australia since late 2010. The release of the dual-active combination Zolvix Plus, will provide better drench resistance management. Combination drenches slow development of drench resistance because each drench group has an independent mode of action and there is little evidence for cross-resistance between groups.

Value of combinations for slowing drench resistance

Worm populations resistant to one drench group are not necessarily resistant to another. Exposing worms to both drench groups in a combination product provides the opportunity for one drench group to kill worms resistant to the other group.

The best time to use combination drenches is before resistance has emerged to any of the groups. In this matter, the launch of Zolvix Plus is timely because resistance to monepantel remains uncommon with very few properties affected.

Unfortunately, this isn’t the situation with abamectin, with a national summary of drench tests for the  period 2009–2012 indicating the presence of brown stomach worm resistant to abamectin on half of the properties that conducted a Drench Test. The situation with barber’s pole worm was more severe with abamectin resistance detected on approximately 80% of the properties.

Even if you already have resistance to abamectin, the combination with monepantel will still be beneficial for slowing the development of resistance to both drench groups. Modelling by Leathwick (2012) indicates that inclusion of a second active, even if its efficacy is as low as 50%, will still help to slow the development of drench resistance to the other fully effective active. In that model, the inclusion of a 50% effective active almost doubled the time before resistance rendered the drench of little value.

Know your drench resistance status

If resistance to abamectin is very severe on your property, be mindful that the efficacy of the Zolvix Plus combination product will more closely resemble the single active, Zolvix. Currently, Zolvix remains a highly effective drench active on the majority of properties.  However, where there is severe resistance to abamectin, concurrent use of additional drench groups (down the race again) may be beneficial in reducing selection pressure on monepantel over time.

Severe resistance to abamectin is most commonly observed in barber’s pole worm, where results from the national summary of drench tests indicated efficacy of less than 60% on approximately 60% of properties involved. In contrast, abamectin generally remains highly effective against other species and especially against black scour worm. The inclusion of abamectin with monepantel will still provide benefits for slowing development of resistance and provides a short-acting, broad spectrum combination treatment.

Managing drench resistance

Make sure that you use the key principles to manage drench resistance whenever possible.  They are equally important and slow the development of drench resistance.

  • Use drenches most effective on your property. Drenches that reduce worm egg count by at least 98% are preferred.
  • Use an effective combination of two or more drench groups, either in a multi-active product or using more than one product concurrently (up the race with one and then the other) to combine different drench groups.
  • Use short-acting treatments and restrict the use of persistent products for specific purposes and high worm-risk times of year.

Leathwick, D. (2012) Modelling the benefits of a new class of anthelmintic in combination.  Veterinary Parasitology, 186, 93-100.