I cannot say I have asked about official about the wormer, but situations like with weeds and fungicides killing mold and mildew I have sat through many grower meeting with chemical company , and university professors all saying change every time . With luck using different chemistry it kills those that had immunity to what you used last time. So it was change fast and often. So I would think this is the same.I've also read that you should use one type of dewormer until it isn't effective then switch. A simple rotation may cause resistance to both types. ?