Last summer, the U.S. Food and Drug Administration approved a French firm's application for the use of leeches (Hirudo medicinalis) as a medical device. By definition, "a medical device is an article intended to diagnose, cure, treat, prevent, or mitigate a disease or condition, or to affect a function or structure of the body, that does not achieve its primary effect through a chemical action and is not metabolized." Especially useful in reconstructive surgery like regrafting amputated fingers and toes, the leeches emit an anesthetic and an anti-coagulant into the wound as they suck until the patient's blood flow takes over.
The FDA didn't approve any old leech. The medical variety, imported from France by Leeches USA Ltd., are carefully born and bred in carefully controlled environments to eliminate the possibility of infections. They may be pedigreed, but at $10 or so per critter they are also one of the cheapest therapies in modern healthcare.