If they free range and are not in a controlled run area they WILL have worms.. Any animal that lives outside eat bugs and worms, bugs and worms have worm eggs and the cycle continues. The problem is different meds take care of different worms, so there is no preventative measures. My preventative measures are acv in the water, vitamin supp. in the water, garlic in their mash in the am, and cleaning of their stool frequently..I also worm 2-3 times a year depending on the weather ( due to more worms from the rain).