Practising doctors don't do research, they rely on reports given to them by pharmaceuticals. Pharma research make its way into medical text books.
Most doctors think that patients who tell them things in conflict to what their medical text say are either dumb or crazy. Even if those information were published decades ago.
Mean while in the real world, anthelminthic drugs are distributed to farms around the world. Doctors who think that these things do not develop resistance to drugs over time are naive. They exist in large numbers and reproduce in large numbers. Each time you exterminate 900 thousand out of a million and a 100 thousand survive and mate, what happens...