What is anthelmintic resistance?

For more detailed information, find the SCOPS Technical Manual in the resources section of this website.

Resistance is the heritable (and therefore genetic) ability of the worm to survive treatment with an anthelmintic. A worm is said to be resistant if it survives exposure to the standard recommended dose of the anthelmintic. Anthelmintic resistance is said to exist in a population of worms if more than 5% of the worms survive treatment. However, unless you test for resistance you will not notice the increasing lack of effectiveness of treatment until 50% or more of the worms survive, at which point the sheep are clearly not being effectively de-wormed and production losses are significant.

The graph shows this in terms of sheep performance versus the proportion of resistant worms on the farm. The percentage of resistant worms is increasing over time and production losses increasing as more and more worms survive treatment, causing damage to the gut of the sheep. In lambs this means reduced growth rates, but because the increase in resistant worms is slow and progressive, we either don't notice that performance is going downhill or blame other factors, such as trace element deficiencies, for the decline.

Factors affect the speed of resistance

There are five key factors which define the rate at which anthelmintic resistance develops. These factors are what the SCOPS principles are based on:-

  1. Proportion of resistant worms on a farm. As this gets higher the faster you head towards the red zone in the graph. This is why the sooner you act the more impact you can have.
  2. Frequency of anthelmintic use. Every time you use an anthelmintic you select for resistance because we kill susceptible worms and allow resistant ones to survive and breed.
  3. Efficacy of each treatment. Under-dosing or using an anthelmintic to which there is resistance present will give resistant worms even more chance to survive and breed, accelerating the pace towards the red zone.
  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 (e.g. lambs weaned on to a hay or silage aftermath) then the selection for resistance is high.
  5. Dilution of any worms that survive treatment with unselected worms. The best way to reduce the impact of surviving resistant worms is to make sure they breed with susceptible worms. This is why we recommend putting sheep that have been treated in quarantine on to pasture that has a worm burden so any survivors are not allowed to breed on their own.
Resistance develops over time.
Resistance develops over time.