Medicinal leech. Nurse using tweezers to drop a medicinal leech (Hirudo medicinalis) into a container. 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.