Well if a rat ate rat poison, and a dog ate that rat, the dog could get poisoned. It makes sense that a dog that ate an animal that ate holly could become poisoned the same way. If it was me, I wouldn't take the chance. I believe the entire plant is poisonous, but don't quote me on that. I really don't know much about holly, just that you don't want it around if you have dogs. I wasn't really aware that it was creeping into other people's yards, but I guess when people let it run wild, it can get out of control and like a weed, infect anything it can reach. As much as I have read about holly lately, I guess the idea that it's more of a fungus than a plant?