Thousands of leeches are used for medical emergencies every year. The leech’s blood sucking accelerates healing on any deep wound but especially after re-attaching a severed limb or any other deep wound. The leech feeds off the oxygenated blood that would otherwise cause swelling and gangrene. The sucking assists the flow of blood and an anticoagulant in the leech prevents scabbing. The sucking is painless because the leech releases an anesthetic.
Of the 650 types of leeches, only a small number of Hirudo species are used, the most popular being the Hirudo medicinalis. They’re usually sold 20 to a pack, at US$7 apiece.
Also see Use of the medicinal leech (but please note that the graphics are not for the faint of heart).