Besides occasional goldfish, I used wild frogs, trout fingerlings, amphibians and larvae, night crawlers, everything found where the pair was found. In essence, their natural food which is probably riddled with parasites. Snakes probably had them too, but the snakes were very large, healthy, and lived 20 years. They had already acquired resistance. When you introduce a wild food source they wouldn't normally be eating, they could encounter a parasite for which they have no resistance. As far as the bad enzyme goes, balance is key. They will do in a pinch but balance with thiamine rich foods.

If all your snakes fed and/or housed together are wild, like that pair was I'd say it's not a problem to feed them food from their wild habitat. They either already have parasites or have resistance. Just don't want that stuff in with the generations removed as it could wipe them out.