As far as parasites- I like to freeze feeder fish for a few weeks before I use them, to kill at least some types of parasites. I actually freeze most feeder animals including frogs and lizards.
I put feeder fish in ice cube trays of water covered with plastic wrap- one feeding per cube. Its very convinient, and for the most part, any snake that takes live anything for me, usually takes f/t as well. Right now, I only use them for scenting mice.
I actually re-use the same frozen treefrog over and over for scenting mice for my ribbon. I just rub the front half of the pinky against the frozen frog, and throw the frog right back in the freezer.