What is anthelmintic resistance?

Resistance is the heritable (and therefore genetic) ability of the worm to survive a dose of anthelmintic which would normally be effective. It could also be described as ‘drug tolerance’ in worms. A worm is said to be resistant if it survives exposure to the standard recommended dose of the anthelmintic and can than pass this ability on to its offspring.

In a population of worms on a farm, anthelmintic resistance is said to exist on that farm when more than 5% of the worms are ‘drug tolerant’. However, unless we test for resistance we would probably not notice any lack of effectiveness of treatment until their numbers had increased to 50% or more of the total worm population. At this stage the anthelmintic is killing so few worms that 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 we act the more impact we can have.
  2. Frequency of anthelmintic use. Every time we use an anthelmintic we 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. 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 reduce the number of resistant worms that the survivors produce is to make sure they breed with susceptible worms, as this will dilute their resistant genes.
Resistance develops over time.
Resistance develops over time.