Deworming, unfortunately N. America, and her prairies and grass-systems, is the old grey mare that ain’t what she used to be! Bison now must negotiate and thrive in an eco-system either surrounded by exotic things, or including them. With these exotic domestic species, comes internal parasites that that have adapted to hosts that have been selectively bred and cared for continuously. The bison are still wildlife wherever they roam, and they are pristine remnants of long ago when there were more hoof prints than foot prints. Deworming is now required to help the bison with health and resilience and can be accomplished with various anthelmintic drugs. Pasture rotation has been and still remains the best wormer. This also represents our ability to graze bison as they would have 300 years ago, on virtually worm-larvae free grasses, or fresh ground.