Hematophagy



Hematophagy (sometimes spelled haematophagy or hematophagia) is the habit of certain animals of feeding on blood (from the Greek words, haima "blood" and phagein "to eat"). Since blood is a fluid tissue rich in nutritious proteins and lipids that can be taken without enormous effort, hematophagy has evolved as a preferred form of feeding in many small animals such as worms and arthropods. Some intestinal nematodes, such as Ancylostomids, feed on blood extracted from the capillaries of the gut and about 75% of all species of leeches (e.g. Hirudo medicinalis), a free-living worm, are hematophagous. Some fish, such as lampreys, and mammals, especially the vampire bats, also practice hematophagy.

Mechanism and evolution of hematophagy
These hematophagous animals have evolved different specialized mouth parts and chemical agents for penetrating vascular structures in the skin of hosts, mostly of mammals, birds and fish. This type of feeding is known as phlebotomy (from the Greek words, phleps "vein" and tomos "cutting").

Once phlebotomy is performed (in most insects by a specialized fine hollow "needle" called proboscis which perforates skin and capillaries; in bats by sharp incisor teeth that act as a razor to cut the skin), blood is acquired either by sucking action directly from the vases, or from a pool of escaped blood, or by lapping (again, in bats). In order to overcome natural hemostasis (blood coagulation), vasoconstriction, inflammation and pain sensation in the host, biochemical solutions in the saliva for instance, for pre-injection, anesthesia and capillary dilation have evolved in different hematophagous species. In fact, new anticoagulant medicines have been developed on the basis of substances found in the saliva of several hematophagous species, such as leeches (hirudin).

Hematophagy can be classified into obligatory and optional practice. Obligatory hematophagous animals do not have any other type of food besides blood; one such species is Rhodnius prolixus (an assassin bug from South America). Contrast that with optional hematophages, like the many mosquitoes species, such as Aedes aegypti, which may also feed on pollen, fruit juice and other biological fluids besides blood. Sometimes, only the female of the species is a hematophage (this is essential for egg production and reproduction).

Hematophagy has apparently evolved independently in many disparate arthropod, annelid, nematode and mammalian taxa. For example Diptera (insects with two wings, such as flies) have eleven families with hematophagous habits (more than half of the 19 hematophagous arthropod taxa). About 14,000 species of arthropods are hematophagous, even including some genera that were not previously thought to be, such as moths of the genus Calyptra. Several complementary biological adaptations for locating the hosts (usually in the dark, as most hematophagous species are nocturnal and silent, in order to avoid detection and destruction by the host) have also evolved, such as special physical or chemical detectors (for sweat components, CO2, heat, light, movement, etc.).

Medical importance
The phlebotomic action opens a channel for contamination of the host species with bacteria, viruses and blood-borne parasites contained in the hematophagous organism. Thus, many animal and human infectious diseases are transmitted by hematophagous species, such as the bubonic plague, Chagas disease, dengue fever, filariasis, leishmaniasis, Lyme disease, malaria, rabies, sleeping sickness, St. Louis encephalitis, tularemia, typhus, Rocky Mountain spotted fever, West Nile fever and many others.

Among the hematophagous insects of medical importance are the sandfly, blackfly, tsetse fly, bedbug, assassin bug, mosquito, tick, louse, mite, midge, chigger, and flea.

Recently, hematophagous organisms have been used by physicians for beneficial purposes (hirudotherapy). Some doctors now use leeches to prevent the clotting of blood on some wounds following surgery or trauma. The anticoagulants in the laboratory-raised leeches' saliva keeps fresh blood flowing to the site of an injury, actually preventing infection and increasing chances of full recovery. In a recent study, a genetically engineered drug called desmoteplase based on the saliva of Desmodus rotundus (the vampire bat) was shown to improve stroke patients

Human hematophagy
Drinking blood and manufacturing foodstuffs and delicacies with animal blood is also a feeding behavior in many societies. For instance, cow blood mixed with milk is a mainstay food in the African Maasai. Some sources say that Mongols would drink blood from one of their horses if it became a necessity. Black Pudding is eaten in many places around the world. Some societies, such as the Moche, had ritual hematophagy, as well as the Scythians, a nomadic people of Russia, who had the habit of drinking the blood of the first enemy they would kill in battle. Some religious rituals underline the importance of metaphorical hematophagy, such as in the transubstantiation of wine as the blood of Jesus Christ during Catholic eucharist. Psychiatric cases of patients performing hematophagy also exist. Sucking one's own blood from a wound is also a behaviour commonly seen in humans, and in small enough quantities is not considered taboo. Finally, real or imagined, human vampirism has been a persistent object of literary and media attention.

Judaism, Islam, and Christianity forbid drinking of blood. There are references in the Old and New Testaments clearly prohibiting this practice (see, for instance, Genesis 9:4; Leviticus 3:17, 7:26, 17:12, 17:14, 19:26; Deuteronomy 12:16, 12:23, 15:23; 1 Samuel 14:33-34; Ezekiel 33:25; Acts 15:20, 15:29, 21:25).