Resistance to triple combination drenches is becoming more common, and likely to become widespread, veterinarian Sara Sutherland says.

Resistance to triple combination drenches is becoming more and more common, and will continue to become more and more common. We are seeing our first cases of lack of efficacy of the novel drenches Zolvix Plus and Startect.

Worms resistant to both the triples and novels has not been confirmed in New Zealand yet, but it would be naïve to assume these novel drenches will continue to work at a high level for the next 10-20 years. It is inevitable that there will be worms resistant to triples and novel drenches – but it’s not inevitable on your farm.

So what do you do once resistance has been diagnosed?

The first thing you should do is get good advice – talk to your vet or local Wormwise facilitator about your parasite management plan. This plan should include a number of different tools you can use in a number of different areas. Don’t focus on just one tool – in other words, don’t think that you are doing enough just by avoiding capsules. Instead, think about all of the things that you do to control worms, and all the things you are already doing that you can do just a little bit better.

I find it helpful to divide your worm tools into three areas – things that safeguard your drenches, things that reduce larval contamination on pasture, and things that improve the animal’s innate immunity to worms.

Refugia (providing a source of susceptible worms to dilute out the resistant ones, either by leaving lambs undrenched or grazing undrenched animals with lambs) safeguards drenches. So does using the most effective combination drench, limiting the use of drenches that are already ineffective. Crossgrazing with cattle reduces larval contamination. So does preventive drenching lambs every 28 days through the summer to avoid an autumn peak of worm larvae on pasture.

Choosing rams with breeding values for resistance or resilience, or reduced dag score, improves the animals’ innate immunity to worms so fewer of them need drenching. All three areas are important. When you are planning your parasite management plan, make sure you are using tools from all three areas at once.

Too much focus on safeguarding drenches can lead to high larval contamination of pasture, and too much focus on reducing larval contamination puts a lot of pressure on drenches, especially if they are already failing.

Some measures can affect more than one area. For example, targeted selective treatment – only drenching animals based on whether they need a drench – safeguards your drenches compared with blanket drenching, and reduces larval contamination compared with leaving all animals undrenched.

Using crops reduces larval contamination because the environment in a crop paddock is less friendly to worm larvae survival, and cultivation will destroy some (unfortunately probably not all) larvae on and in the soil. Using crops also improves a lamb’s innate immunity to worms, because of the higher protein and digestibility.

Better fed animals are more tolerant of worms, and high protein is more important than high energy.

Regardless of the other tools in your parasite management plan, monitoring is vital. This means doing a faecal egg count reduction test (FECRT) every 2-3 years so you know which drenches are still effective, AND regular drench checks (worm egg count 7-14 days after drenching) every season.

Make it routine to do a drench check after your first lamb drench in the spring, and again in the autumn, as well as after a “knockout” or “exit” drench. Assume that the situation will be dynamic on your farm – one good FECRT five years ago does not mean you are “safe”. Your situation will continue to change – and hey, if you’re using lots of these tools on your farm, maybe it’s getting better!

We are farming in a very different space now than 25 years ago – our genetics have improved, our knowledge of pastures and cropping has improved, and our parasite management needs to improve too. You can’t drench the way your old man did, because the worms in your lambs are not the same worms that were in your lambs when you were little. Look at how many tools you can incorporate into your system, and also look at how well you can use each tool. For example, if you are a lamb trader, running a small mob of fat ewes over your finishing country might not be enough refugia – how can you increase the amount of refugia you are providing?

Instead of looking for a silver bullet that will fix your worm problem, look at all of the tools that you can use, and think about how to use as many of them as possible in your farm system. Want to know more about what these tools are, so you can find out if there are some that you could be using? Head to or sit down and have a chat to your vet.

The next issue that needs addressing is one of communication. Do farmers selling trade lambs have an obligation to let the purchaser know their resistance status? What about stud flocks selling rams? Should farms with no evidence of resistance on their FECRT get a premium for selling lambs that are shedding susceptible worms, so that the purchaser can be confident they aren’t buying in resistance?

When the first cases of triple resistance appeared, some farmers didn’t want people to know their status, in case it reflected badly on their management or made it more difficult to sell their animals (or sell their farm!). Now that triple resistance is common, that is no longer the case. Open communication benefits everyone. Talk about worms. Don’t bury your head in the ground and pretend you don’t have to do anything – there are probably worm larvae in the ground looking back at you.

  • Sara Sutherland is a vet for Veterinary Services Wairarapa.