Drench resistance is a feature of the worms themselves and should be distinguished from ‘treatment failure’, which occurs when a drench treatment has failed to control worms due to factors such as:
- Treatment not reaching the worms e.g. when cattle spit out the drench, or a pour-on is not absorbed due to a dirty coat or heavy rain fall after application.
- The drench used does not target the worm species present e.g. levamisole used to treat inhibited small brown stomach worm (Ostertagia ostertagi).
- Operator error or mis-application.
- Use of expired products.