How do leeches suck our blood without causing any pain?

            Leeches, carnivorous or bloodsucking worms, live as external parasites attaching themselves to a host and sucking blood. Actually, land leeches feed only on the blood of mammals. These flattened ringed worms, measure from 5mm to 46cm in length and are equipped with sucking disks at both the anterior and posterior ends. In some leeches, the anterior mouth contains three toothed plates with which the animal pierces the skin of its prey.

            First, with its 3 jaws set with sharp teeth, it makes a Y-shaped incisor in the flesh. Its saliva contains substances that anaesthetize the wound area, dilate the blood vessels to increase blood flow and at the same time prevent the blood from clotting.

            Hence the victim is often unaware that he has been bitten until blood is discovered running from the wound. Also the site of bite is directly connected to the crop of the leech through its buccal system.

            In the crop or pouch, food can be stored for several months. Land leeches await their victim in damp vegetation poising one end in the air.

            Leeches were once used by physicians and barbers for bloodletting and are still used for this purpose in some regions of the world. In modern medicine, bloodletting is no longer practiced, but leeches continue to be used to relieve blood congestion in certain delicate operations, where such use is less likely to cause infection than other techniques, according to the encyclopedia.

Blood ingested by leeches is mixed with salivary juices containing an anticoagulant substance known as hirudin (which can be extracted and has been used in medicine to prevent blood clotting. It is employed as an anticoagulant in surgical operations and has been recommended for the prevention of phlebitis and post-operative pulmonary inflammations.

            Hirudin has been synthesized by recombinant DNA technique. The blood passes into a dilated, branched stomach, or crop, where it is stored for several months before being completely digested. A leech consumes about 3 times its weight in one feeding and then subsists for months on the stored food.