Read the link I included. It's an FDA approved treatment [1] particularly beneficial in limb transplants. FTL, blood pools in attached limbs before the network of blood vessels to recirculate it can form. The leeches extract that blood, preventing potential loss of the attached limb. Also, similarly, in skin grafts. And in lots of other diseases.

Maggots are used to remove necrotic tissues, because they consume it and leave living tissue untouched. Nasty, but quite effective.

Feel free to take a quick Google.

[1] http://www.nbcnews.com/id/5319129/ns/health-health_care/t/fd...