Resistance which is the ability of parasites to survive being treated with a wormer. Due to historic over use of some active ingredients and other issues worms may still survive despite the use of the correct dose and efficient animal health planning. These worms then reproduce subsequent generations of worms will be also be resistant which in turn can lead to worms with even more special powers of resistance.
One way to check for wormer resistance is to perform a feaeal egg count reduction test. To do this the numbers of worm eggs in faeces before and after administration of a wormer needs to be ascertained but ensuring that the period of activity of the individual chemical is taken into account which basically means taking into account which active ingredient was initially used and then deciding on when to retest.
A reduction in egg numbers of less than 90-95% indicates resistance. This is where the help of a specialist SQP can help in determing the next steps taking into account treatment history, pasture management, individual horse details and assesing the risk level.