Lewis Kahn¹, Stephen Love² and Brown Besier^3
1 ParaBoss Executive Officer
² Veterinarian/Parasitologist NSW Department of Primary Industries
³ Principal Veterinary Parasitologist, Department of Agriculture and Food, Western Australia.

Introduction

Startect® is a new sheep drench from Zoetis expected to be released September 2014. The drench is a combination of a novel (brand new) active, derquantel, from the spiroindole (SI) group and abamectin, which is an active from the ML (or mectin) group. Startect® has a number of attributes that will benefit worm control and management of drench resistance, but there are some considerations.

How to get the most out of combination drenches

The value of combination drenches is that they slow development of drench resistance. Also, combinations of older actives (e.g. BZ or white drenches and LEV or clear drenches), will likely be more effective than either active when used on its own.

Combination drenches slow the development of drench resistance because each drench group has an independent mode of action and there is little evidence for cross-resistance between groups.  This means that worm populations that develop resistance to one drench group do not automatically become resistant to other drench groups.

This is good news.  It means that each active in a multi-active drench has the potential to kill worms that are resistant to the other active.  This has two benefits

1.     Multi-actives slow development of drench resistance. This works best with higher efficacy of actives that all target the same worms and have the same persistency.

2.     Multi-actives increase the efficacy of older actives to which resistance has started to develop.

How does this apply to Startect and all combination drenches?

Modelling by Leathwick (2012) indicates that inclusion of a second active in a dual-active drench—where a new active is fully effective—will help to slow development of drench resistance even when its efficacy is 50%.

The benefit for slowing drench resistance to the new active, from inclusion of a second active, increases as the efficacy of the second active increases (Figure 1). For example, if used as a single-active drench, the model predicted that efficacy of the new active will fall below 95% after about 7 years of usage. This assumes that drench efficacy falls below 95% when the frequency of resistant (R) genes in a worm population exceeds 25%. See legend to Figure 1 for usage.

If used as a dual-active drench, where the second active was 50% or 80% effective, the model predicted that efficacy would fall below 95% after about 13 or 24 years respectively.  The model could not detect the development of resistance to the new active where the second active was 95% effective. 

 

Figure 1: Likely development of resistance (R) to a new and fully effective drench active if used as a single-active drench or as a dual-active drench where the second active has an efficacy of 50%, 80% or 95% against the worms targeted by the new active. Predictions from a model where lambs were drenched 6 times at 28 day intervals with drenching starting at weaning into a clean (low worm-risk) paddock. For full details see Leathwick, D. (2012) Veterinary Parasitology, 186, 93-100.

Abamectin resistance

This modelling is particularly relevant to Startect because abamectin resistance in Barber’s Pole worm (83% of Australian sheep farms) and Small Brown Stomach worm (49% of Australian sheep farms) was recently reported by M. Playford and colleagues in a national survey of drench resistance. Abamectin resistance in Barber’s Pole worm in the New England region of northern NSW can be severe where efficacy, i.e. reduction in worm egg count after treatment, is less than 20% on some farms.

Producers need to know the efficacy of abamectin to get the most out of this new dual-active combination drench, Startect®.  More generally, they will benefit from knowing the efficacy of individual drench groups in order to get the most out of all combination drenches.  This can be done by conducting a DrenchTest or a DrenchCheck-Day10.

When these drench tests show that the efficacy of abamectin is:

• greater than 50%, then modelling indicates it is likely to significantly slow development of resistance to derquantel (the new active in Startect®).  The higher the efficacy of abamectin the longer it will take for worms to develop resistance to derquantel. Importantly, derquantel will protect abamectin from worsening efficacy.
• lower than 50%, then Startect® will resemble an effective single-active drench and resistance to derquantel will develop more rapidly. Co-treatment with an unrelated effective active should then be considered as a means to slow development of resistance to derquantel. This entails going down the race again with a drench containing an effective active/s from a different drench group/s and/or doing treatments in periods of the year when any resistant survivors of treatment are least likely to survive on pasture.

Other notes about combination drenches

Some drenches contain active ingredients from a number of drench groups. These are often referred to as multi-active or combination drenches and include dual-active (contain actives from two drench groups), triple-active (three groups) and four-way-active (four groups).

Only those drenches that contain two or more active ingredients that target the same worms can be considered as combination drenches. For example, a drench that includes levamisole and fenbendazole: these both target scour worms, barber's pole worm and other roundworms.

In contrast, a drench that contains two or more active ingredients, but the actives target different worms, is called a mixture. These give the convenience of a single drench when quite different worms are targeted; however, they should be considered 'single-active' against each worm. For example, a drench that contains praziquantel and ivermectin is targeting only tapeworm with the praziquantel and only roundworms with the ivermectin.

A drench being a multi-active product does not guarantee it will be effective.  Unfortunately, most combination products were released after years of use as the single-active.  The only way to know the drench resistance status on your farm is to conduct a DrenchTest or DrenchCheck-Day10. Some animal health advisors and rural merchandise stores are able to assist with these tests.