Over the years, parasites have developed the ability to survive different parasite-killing drugs. If a worm parasite develops the ability to survive the effects of a parasite killing anthelmintic drug, it is said to have developed "resistance" to that drug. Broad resistance to many of the older deworming drugs is also now evident. No single parasite control recommendation should be regarded as perfect and permanent.
For more information, please read the article Equine Parasite Control.