Hornwort is a bryophyte, like mosses and liverworts, so it should be safe in itself. However, you are talking about feeding something from outside, which could contain not only parasites, but some pretty nasty bacteria or diseases. There has been a school of thought for a long time to NOT let crickets roam in live, planted tanks for long because the crickets could feed on toxic plants, such as pothos, philodendrons, diffenbaechia, ect, which would lead to frogs eating said crickets and themselves being poisoned. This is a similar situation, with the exception that MOST bryophytes are not toxic to most animals.
I'm sorry, but assuming parasites would not survive in a roaches stomach is just plain naive. It's safer to assume that any parasite can survive more than what we would expect. After all, they are parasites for a reason.
That being said, drying and freezing would be the best bet, if using hornwort. Fecal samplings should also be the order of the day if you have already fed frogs feeders that have eaten the hornwort. If they are clear, then you found yourself a sustainable food source for your feeders
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