Yeah, parasites are all over the place. However, I don't think I've had a single instance of them with feeder fish, but they say that those minnows are the dirtiest fish out there. Try to switch them over to rodents (pinkies/fuzzies depending on the size of the snake). There is a thread from yesterday or the day before about the process of switching them over. It works out best for everything except the rodents.

Question for all, what is the biggest rodent your garter ever ate?
e.g., hopper, small mouse, medium mouse, large mouse, jumbo mouse, medium rats, etc.