Nurse using a medicinal leech on patient. Nurse using tweezers to place a medicinal leech (Hirudo medicinalis) onto a patient's finger. The patient severed the tip of his finger in an accident. Leeches are parasites that feed on blood. They attach themselves to the skin using suckers and create a wound with three sharp jaw plates, which contain over 600 teeth. Their saliva contains hirudin, a chemical which prevents blood from clotting. A normal wound may bleed for up to 20 minutes before clotting, but a leech wound can bleed for up to 10 hours. Leeches are used in modern medicine to drain blood from haematomas (accumulations of blood) and to draw blood into transplanted tissues until circulation is established.