Rabbits are very vulnerable to worm infestation due to egg laying insects as well as other parasites that can make you very sick. Once we get that first frost, just like any insect the cold (frost) usually kills them off. If you cook the meat thoroughly you can eat rabbits all times of the year. But like others said, Grandpa always said first snow, so that's my rule of thumb if I was to go out and hunt them.