For more detailed information, see SCOPS Technical Manual, Chapter 1.
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:-
To see why anthelmintic resistance is a threat to farmers, watch the following video.