Sounds like a big ol' case of parasites to me.
I would use ivermec, worm them clean their cages and scrub them the next day. Then wait a week worm them again and scrub the cages again. wormer does not kill the eggs so the initial worming only gets the actual worms so you have to wait for the eggs that may be present to hatch and worm them again before the new hatched worms are able to reproduce.
To prevent worms in your fryers it's always a good idea to treat your breeding stock twice a year or so as even if they don't get to go outside on the gound they can get worms from hay, and greens, and other animals (like barn cats, and rodents) that may be present in their environment, even from you if you handle another animal then a rabbit.
Selecting resistant brood stock is always a good idea as well.