You can get Eprinex from Tractor Supply Store or online from many different places. Eprinex is probably the safest to use as there is no withdrawal time for eggs or slaughter. It is a cattle wormer. Worm infestations can do many different things in chickens, there are many variations of each type of worm and they each attack a different part of the body, ie: crop, gizzard, throat, intestines, reproductive system, you name it, there is a variety of worm that will attack it. Best of luck with whatever you decide to do.