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:-