[Formendacil]

I think the use of the terms "leech" and "leechcraft" have more to do with Tolkien evoking the ancientry of Gondor than using bloodsucking worms medicinally.

And! A little bit of quick googling "etymology of leech" leads me to believe we have a classic case of Tolkien attempting to reintroduce aan old meaning to a modern form: "leech" with the archaic meaning of "doctor" apparently has quite a few cognates in Germanic tongues.

That said, the descent of elven derived healing into bloodsucking worms does sound like a bit of a metaphor for the fall of Numenor.