Parasites which are not killed by anthelmintics can become resistant. Possible causes of worm resistance are year-round dewormer treatments at regular intervals and underdosing of anthelmintics. With the year-round treatments animals were too frequently dewormed without checking what worms and how many worms were available.
Just a few anthelmintic drug classes are on the market and all classes show resistance. Especially for goats and sheeps resistance is a growing problem.
Process of parasitic resistance:
Check the worm resistance after two weeks of deworming with the McMaster method. A reduction of less than 90-95% indicates resistance.