Are you up for a little scientific reading? https://www.sciencedirect.com/topics/medicine-and-dentistry/coccidiostat
This details what @Susan Skylark has been posting brief bits of information about. It's hard to say what is causing resistance in your flock to these coccidia drugs as there are many factors involved such as your locale and your flock management practices.
But you have correct information about there being strains of coccidia that opportunistically take advantage of drug resistance, three to be exact. Read through that paper, and you might find a clue as to why resistance is happening in your flock.
This details what @Susan Skylark has been posting brief bits of information about. It's hard to say what is causing resistance in your flock to these coccidia drugs as there are many factors involved such as your locale and your flock management practices.
But you have correct information about there being strains of coccidia that opportunistically take advantage of drug resistance, three to be exact. Read through that paper, and you might find a clue as to why resistance is happening in your flock.