Before reading ‘Preventing Parasite Resistance’, you can read our previous article ‘Parasite Resistance: A Simple Explanation’ for a refresher.
Summary
- It’s important to read the active ingredients of products, not just the brand or product name.
- A drench is considered ineffective when 10-15% of worms within the population have developed resistance to it.
- Effective combination drenches, or the simultaneous use of two separate drenches, are better at preventing drench resistance.
There are many different drenches on the market, with lots of different brand names and product names. However, there are a few things the different products can have in common: the active ingredients. Active ingredients including abamectin, moxidectin, albendazole, levamisole, dicyclanil, fluralaner, etc., are classed into different drench groups depending on their mode of action (e.g., how they kill their target parasite).
A drench is considered ineffective once 10-15% of worms within a population have developed resistance to it, which can occur at an alarmingly fast rate if producers consistently rely on a singular drench group or active ingredient to treat parasite burdens. To protect the efficacy of the active ingredients that we currently have access to, we must be smart about the way we use them to control worms and other parasite burdens.
When choosing a drench to use, producers must consider:
- Using an effective combination drench.
- Ensuring the active ingredients being used are high in efficacy.
- Using short-acting treatments, saving long-acting products for high-risk times.
The Power of Combination Drenches for Preventing Parasite Resistance
Combination drenching means you are drenching your stock with multiple active ingredients of different chemical groups at one given time. For example, drenching your ewes with Vetmec LV + Selenium (abamectin ML) and Vetmec Dual (albendazole BZ & levamisole LEV) is considered a combination drench as it contains 3 active ingredients, each from a different drench group.
Combination drenches slow down resistance development by reducing the number of resistant worms that survive a round of drenching. To put this in perspective, a worm may be resistant to abamectin, but when drenched with an albendazole & levamisole product, it would be knocked down by the albendazole/levamisole. And vice versa. The greater the number of active ingredients in a drench, the smaller the likelihood of resistant parasites surviving. This is because they would have to change multiple things about their physiology at once to be able to survive attacks on multiple ‘parts’ of their body.
So what combination drenches are available?
CATTLE
Vetmec LEV Pour On for cattle makes it very easy to make any of your current drenching programs a combination strategy! Use in conjunction with Vetmec Injection, Vetmec F Injection or Vetmec Pour-On to create a dual active drench:
SHEEP
When you combine our Vetmec LV + Selenium with our Vetmec Dual, it creates a super easy triple active combination drench.
*Note: these combination strategies focus on gastrointestinal worms.
Can I Rotate Drenches Instead?
The short answer is, it’s best not to.
Research tells us that the continuous use of one active ingredient is the fastest way for resistance to develop within your worms. Rotating the drench active ingredients you use year by year, drench by drench, or mob by mob, is often wrongly assumed to be effective in reducing resistance developing in worms within your herd or flock. While this has been a common practice, and assumption, for Australian sheep and cattle producers for many years, it is not as effective as preventing resistance as using combination drenches.
When we talk about rotating drenches we mean the practice of changing the drench group each time you buy more drench. This includes rotating between drenches with totally different drench groups (e.g., ML, BZ, LEV, OP, SI), NOT actives of the same drench group (e.g. ivermectin, abamectin, moxidectin).
For example, you may have treated your calves with an ivermectin (ML) drench at marking and then changed to a levamisole (LEV) drench at weaning. The continuous rotation or chopping and changing between single active drenches will NOT effectively delay the development of drench resistance within your worm populations. Whereas, combining ivermectin and levamisole at both of those drenching events would decrease the risk of resistance developing significantly.
The goal is to keep resistance in worm populations below 15-20%. The blue line in the figure above shows the speed at which resistance can occur when relying on only one active ingredient, reaching 20% in less than 5 years. The red line shows two actives, reaching 20% resistance in about 10 years. And the green line represents three active ingredients, taking 30 years to get close to 20% resistance. While giving combination drenches may be a little bit more expensive in the short term, it will be worth it in the long run by preventing resistance.
Whilst combinations play a very significant role in the mitigation of parasite populations developing resistance on your property, it helps to employ other important practices including worm testing and grazing management.
If you would like to talk about parasite management plans and other strategies for preventing parasite resistance, call 1800 243 683 to chat with one of our parasite advisors.
Definitions
ML – Macrocyclic lactones (any ‘mectin)
BZ – Benzimidazoles (albendazole in Vetmec Dual)
LEV – Levamisole (in Vetmec LEV or Vetmec Dual)
OP – Organophosphate (napthalophos in PoleAxe)
SI – Spiroindole (derquantel in Startect)
References
Drench Rotation Versus Combinations to Combat Drench Resistance | Wormboss
Sheep drench combinations, resistance and ‘refugia’ | DPI NSW