Birds that are kept on the same soil, in a pen or run for example, are more prone to get reinfected. One roundworm can lay thousands of eggs in one day. Birds peck the soil and pick up infective eggs starting the worms lifecycle all over again. Birds that are kept in cages with wire bottoms are rarily worm infected simply because they are off the ground. Free ranging birds get worms too, if their feet touch the ground...they will get worms eventually.
It's always recommended to rotate wormers. I've rotated with valbazen, safeguard, added pyrantal pamote and have used zimectrin gold for tapeworms. These wormers usually have 2 to 3 year expiration dates.
As far as resistance goes, the best example I can think of is antibiotics given to patients by doctors for every nitnoid sneeze or hangnail that they might have. Alot of antibiotics for people are useless because of the indiscriminate administering and use of these drugs. It wasnt until a few years ago they've become aware of human resistance and are cracking down. More recently the Feds are cracking down on too much antibiotics being administered to livestock in commercial operations, maybe you heard about it in the media. I suppose the long term effect is showing itself in humans.
