Combination drenches—benefits and efficacy

WormBoss recommends using effective drenches and where possible, these should be combinations.

So what is a combination drench? This type of drench contains two or more active ingredients that each target the same worms.

What are the benefits of using combinations drenches?

  1. Combining actives can substantially slow the development of drench resistance to those actives on your farm, allowing you more drench choices for more years.
  2. On many farms a combination is likely to be more effective than using the individual actives (unless the individual active is fully effective). This will give a better kill of the worms in your animals.

However, don’t assume that any combination drench will be fully effective on your property.  Predicting the effectiveness of combinations can be done by testing the effectiveness of single actives on your property.

WormBoss has developed the WormBoss Combination-Drench Efficacy Calculator, to calculate the predicted efficacy of various combination drenches, if you have efficacy results for single actives. It's incredibly easy to use.


Why are combinations generally more effective?

While drench effectiveness is based on how much a drench reduces the worm egg count (because that’s what we can measure), for simplicity, this example refers to worms being killed.

Let’s consider 3 single drench ingredients, commonly called "actives" (all from different drench groups) that were tested in a DrenchTest on your property and call them A, B and C.

  • A is 90% effective (i.e. the drench kills 90% of the worms present)
  • B is 80% effective
  • C is 70% effective.

If A, B and C are in a combination drench you can think of the combination working this way:

If the animal being drenched had 10,000 worms in it…

  • Active A kills 90% (9000 worms) and leaves 1000 worms.
  • Active B then kills 80% of the 1000 worms, leaving 200 worms.
  • Active C then kills 70% of the 200 worms, leaving 60 worms.

The actives don’t really take turns, they are acting on the worms simultaneously, but this explanation is easier to follow.

The combination drench has killed 9,940 worms from the original 10,000 worms, meaning it was 99.4% effective.

In this example, only 60 worms from this animal survive, reproduce and spread resistant worm eggs onto the pasture. Using only one of the individual drenches would allow between 1000 and 3000 resistant worms in the one animal to survive.

Note that some anthelmintic products have more than one active, but are not combinations. These are the products where the actives target different types of worms (e.g. one active targets round worms, the other targets tapeworms). These products are best described as anthelmintic mixtures.

How is drench resistance development slowed with combinations?

Both modelling and on-farm trials demonstrate that using combinations prolong the life of the individual actives within them.

Each drench group works in a different way to kill worms: a worm that is resistant to a drench has a “resistant gene”. But it’s important to note that the resistant genes for each drench group are not the same. The gene for resistance to drench A is different to the gene for resistance to drench B, and so on.

Therefore, the chance of a worm having resistant genes to a number of drench groups is much lower than being resistant to just one drench group. This leaves fewer resistant worms to reproduce. And therefore, further development of resistance is slower than with single active drenches.

That doesn’t mean you can simply replace your single actives with combos and drench away with no other regard for managing drench resistance. You should still use a range of strategies to manage resistance.

Regional WormBoss programs advise the importance of minimizing worm challenge to susceptible stock by preparing low worm-risk paddocks for them and buying rams with genetic resistance (negative WEC ASBV) to worms.

Predicting the efficacy of combination products from their single actives

The WormBoss Combination-Drench Efficacy Calculator has been created to make drench resistance tests and understanding your results easier.

You can now predict the efficacy of any drench combination if you have conducted a DrenchTest with the individual actives.

For a comprehensive DrenchTest, WormBoss recommends testing these actives (groups):

  • levamisole (LV)
  • albendazole OR fenbendazole OR oxfendazole (one of the ‘white’ or BZ drenches)
  • abamectin (ML)
  • moxidectin (ML)
  • naphthalophos (OP)
  • closantel (SA)
  • monepantel (AD)
  • derquantel (SI) + abamectin (ML) (Derquantel alone, is not available for testing).

Note: praziquantel and triclabendazole are not included, as they do not target roundworms.

The Combination-Drench Efficacy Calculator

This calculator is a simple online tool. Open the page and simply enter your efficacy results for any of the single actives and the calculator will automatically calculate a prediction of efficacy for those combinations for which you have provided results for all of their single-active ingredients.