I have been feeding night crawlers as a main food source for years and haven't seen any ill effects to my garters. They are all healthy and grow strong. This also goes for my wild population which is large. All the snakes are big and healthy on a main diet of night crawlers. Is there a risk? Yes but I feel it is small in comparison to other wild food sources. For example, I've never found a wild T. radix with subcutaneous worms but have found them in my captive snakes(3 cases). My guess is they came from pet store feeder guppies or gambusia.