That’s certainly already the case. Ivermectin has lost effectiveness against particularly insidious parasites such as Haemonchus Contortus (barber pole worm). We’ve resorted to other treatments such as Moxidectin, Levamisole, etc, all of which are more expensive and harder to dose safely and effectively.

I can totally understand how Ivermectin got its following— it’s an amazing general purpose medicine around the farm. I generally use it responsibly by verifying the presence of parasites before administering (which can be literally any animal on the farm with almost any parasite) … but then there are times such as when my daughter had a single pet chicken (among many healthy ones) that wasn’t looking so hot … Whereas previously I’d just dispatch the animal and be done with it, now I give a dose of Ivermectin and isolate for a few days before calling in the grim reaper. So far the success rate is something like 90% where now my 9 year old daughter administers it herself. While I acknowledge this isn’t the most responsible, I can imagine others have had similar experiences which explain some of the popularity of Ivermectin with rural folk.