I also switched from shavings to sand, which seemed to help, and cut down on how much "treat" I gave them in their run, to reduce wild bird contact. (Sparrows squeeze right through our poultry netting top cover.)
With carcinogens, you WON'T "see any problems." The damage is cumulative, and it also takes time, even years in humans, for the "damage," i.e. cancer, to show. I use ivomec also; I discard eggs for 2 weeks after the last treatment, myself.
I had to use ivermectin EVERY WEEK for an entire winter, plus Sevin on roosts and bedding, and used pyrethin on coop in warm enough weather AND bathed each chicken when it was warm enough. Also used Poultry Protector spray on vents when it was warm enough (in addition to everything else) and it...