The way I see it is there is always a risk no matter what your feeding. I view slugs as a treat. They seem like they would be very fattening for the snakes. I know Snap loves them, and my wild caughts will take them without much of a problem. I'm cautious about feeding my snakes worms, too. I make sure that the worms have been eating nutritious things before I get them near my snakes. When I catch worms, I put them in a well ventilated container with some left over organic veggie scraps and some bone meal or finely crushed eggshells, with some carefresh and a little bit of clean soil. After a week, a one inch worm shoots up to 4 inches-6 inches. It doesn't lower the risk of parasites, but it makes them healthier, in my opinion.