Trial shows sheep farmers can slow or reverse worm resistance

Using multi-active wormers in lambs to kill as many resistant worms as possible has been shown to slow or reverse wormer resistance, according to recent trial results. 

Researcher Dave Leathwick, principal scientist at Agresearch, New Zealand, found when a multi-active wormer containing ivermectin and levamisole was used on lambs in a three-year field trial, it slowed the development of resistance, with the best response shown in groups that also had 10% of animals left untreated.

See also: More advice on effective worm control

In another study of 20 commercial farms, chosen because of their worm resistance, the effectiveness of the multi-active drench improved.

Dr Leathwick said: “Even in the case where farms have resistance, multi-actives are still beneficial to a lesser extent. What this tells us is the best time to start using multi-actives is before you have resistance.”

Protecting susceptible worm populations

Speaking at a meeting on the future of sheep worming last Tuesday (4 August), Dr Leathwick said farmers in the UK needed to look at protecting some of the susceptible worm population. This would allow the susceptible worms to outcompete with the worms resistant to treatment.

The purpose of the second drug in the multi-active treatment is to kill those worms resistant to the first drug, which slows resistance development because the multi-active kills more resistant worms.

By reducing the number of resistant worms any resistant survivors will be more likely to breed with susceptible worms, which will dilute their resistant genes.

For example, Dr Leathwick said by using a single active treatment farmers may be able to kill up to 98% of worms, but it’s the remaining 2% that are the resistant ones and those are the ones farmers need to worry about.

When using a multi-active, if the second drug is also 98% effective – but crucially is also effective against those that survived the first drug – this results in 99.96% of worms killed or a a 50-fold decrease in the number of resistant worms that have survived treatment.

So identifying the worm species on farm and testing the effectiveness of drenches used against these is vital to allow farmers to make an informed decision on treatment, he added.

“The cost of using a drench that isn’t working properly is far higher than the cost of a test to see if it is working properly.”

Lesley Stubbings of Scops agreed that using an effective treatment was key.

“We know a large proportion of UK sheep farmers are using wormers that are not fully effective and as a result are getting reduced lamb performance. Testing to see if your wormer is working is a critical first step along with making sure that the dose rate is correct and administered properly.”  

The Sustainable Control Of Parasites in Sheep (Scops) guidelines recommend using one of the two newest drenches, called 4-AD also known the orange drench (single active), or 5-SI also known as the purple drench (multi-active), plus moxidectin as a quarantine treatment in order to kill all worms to avoid importing resistant ones.  

“Using only one product risks allowing worms that are resistant to that product surviving and entering the flock,” she said.

Resistance questions answered

What is worm resistance?

Resistance is the genetic ability of the worm to survive a dose of anthelmintic, which would normally be effective. Resistance is said to exist when more than 5% of the worms are drug tolerant. As more worms become resistant, animal performance will be compromised.

How does resistance develop?

Sustainable Control of Parasites in Sheep (SCOPS) says there are five key factors that contribute to the rate resistance develops.

  1. Proportion of resistant worms on a farm – as this gets higher the poorer lamb/sheep performance will get.
  2. Frequency of anthelmintic use – every time farmers use an anthelmintic they select for resistance because they kill susceptible worms and allow resistant ones to survive and breed.
  3. Efficacy of each treatment – under-dosing or using an anthelmintic where resistance is present will give resistant worms even more chance to survive and breed.
  4. Proportion of the total worm population in the animal at the time of treatment – this is very important because if a large proportion of the worms are in the sheep compared to on the pasture (eg. lambs weaned on to a hay or silage aftermath) then the selection for resistance is high; if the proportion in the sheep is low then the effect is much less.
  5. Dilution of any worms that survive treatment with unselected worms – the best way to limit reduce the number of resistant worms the survivors produce is to make sure they breed with susceptible worms because this will dilute their resistant genes.

What are multi-active worm treatments?

Multi-active drenches contain two or three different classes of active ingredient that act on different worm species. It is not the combination of a wormer with something like a flukicide as these two products work on different groups of parasite.