Tapeworm
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 Scolex of Tenia solium
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Subclass Cestodaria |
In biology, Cestoda is the class of ribbon-like endoparasitic flatworms, called cestodes or tapeworms, that live in the digestive tract of vertebrates as adults and often in the bodies of various animals (intermediate hosts) as juveniles.
In a tapeworm infection, adult worms absorb food predigested by the host, so the worms have no need for a digestive tract or a mouth. Even large tapeworms are made almost entirely of reproductive structures with a small "head" for attachment. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and anaemia; multiple infection can cause intestinal blockages. The disease caused by the worm in human beings is generally known as cestodiasis. Symptoms vary widely, depending on the species causing the infection, from simple upper abdominal discomfort and diarrhea to serious nervous disorders caused by toxin of the parasite. However, infestations are usually asymptomatic. Gravid proglottides (worm segments or eggs may be found in the stool of an infected person. Tapeworms can grow 15 to 30 feet (10 meters) in length.[1] The largest tapeworms grow up to 59 feet (18 meters)[1]. Most tapeworms enter humans through infected food, the same way they enter the secondary hosts like flea, pig, cow, etc.
There are two subclasses in class Cestoda, the Cestodaria and the Eucestoda. By far the most common and widespread are the Eucestoda, with only a few species of unusual worms in subclass Cestodaria. The cyclophyllideans are the most important to humans because they infect people, pets and livestock. Among them, four significant genera are Taenia, Diphyllobothrium, Echinococcus and Hymenolepis.
Types of Tapeworm
Taenia solium (pork tapeworm) and T. saginata (beef tapeworm) are the most common tapeworms of human beings. Their larval stage occuring intermediate hosts are pig and cattle respectively. A person can become infected by these parasites on consuming raw or undercooked meat (measly pork or beef) that has been infected by their larvae (cysticercus). The beef tapeworm is longer than pork tapeworm and lack rostellum and hooks on the scolex. Sometimes, these parasites may leave the usual digestive tract and migrate to the appendix, pancreas, or bile duct causing severe Sean Mullenish Diarrhea.
A dangerous complication of the parasite T. solium, Cysticercosis, may occur if due to reverse pesistalsis auto-infection takes place or the person as the secondary host comes to ingest onchosphore larvae in contaminated food and the larvae develop outside the intestinal tract. The released hexacanth larvae can move from the intestines to muscle tissue, bone marrow, fingers, and in some cases the central nervous system (neurocysticercosis). The latter infection can lead to seizures and other neurological problems.
A third type of tapeworm, Diphyllobothrium latum, is contracted by eating raw, infected fish. The fish become infected by eating infected crustaceans, which became infected by consuming untreated sewage. This tapeworm results in symptoms similar to those of T. saginata and T. solium, but can also include weakness and fatigue.
The hydatid worm, Echinococcus granulosus is a parasite of dog, cat, etc. and its itermediate host is cattle. However, humans may happen to ingest its onchophore larvae in contaminated food and drink or because of careless association with infected dogs and cats. Most harm is done by cysticercus larvae that develop in liver, lung and in a few cases in brain, heart, bone marrow, kidney, spleen, muscles, etc. by forming water bladder or hydatid cyst. A hydatid cyst may grow upto football size in 12 to 20 years in man, necessitating surgery.
Another common tapeworm of human beings is dwarf tapeworm, Hymenolepis nana. Same host serves for the development of larvae (cysticercus) and the adult without requiring intermediate host (monogenetic condition), however, certain rat fleas and beetles may work as intermediate host to facilitate the propagatioon of the parasite. In the monogenetic condition, onchophore larvae are ingested in contaminated food, released hexacanth larvae penetrate mucosa of villi, cysticercoid larvae develop in the villi and re-enter the lumen by rupture of villi. Due to auto-infection, cysticercoids attach themselves to the mucosa among the villi and develop into adults. This parasitism is asymptomatic unless there is heavy infection.
Adult morphology
Basically, adults of all tapeworms have same body structure consisting of a scolex, sometimes colloquially referred to as the "head," a neck and a strobila made of a few to large number of proglottids, which are sometimes called "segments." These parasites are given the name "tapeworm," because their strobila look like a strip of tape. As mentioned above, the pre-digested food in the host's small intestine is the chief source of nourishment for tapeworm. The general body surface of the parasite is greatly increased by the presence of microvilli. Therefore, the swift efficiency with which absorption takes place can be compared with the soaking action of blotting paper. Respiration of the tapeworms being mainly anaerobic or anoxybiotic and glycolysis being the chief respiratory pathway, the parasites lack respiratory organs too. When Oxygen become available, general body lining works as the respiratory surface. All cestodes have nerve rings and lateral nerve cords passing through out the length of the body. Their main excretory units are protonephric flame cells scattered all over the parenchyma of the body. The filtered excretory material is emptied into lateral longitudinal excretory canals extending the whole length of the body and thrown out through excretory pore at the end of the body.
Scolex
The Scolex or "head" is the anterior end of the worm and remain attached to the intestine of the definitive host. It is not bigger than the head of a pin, still it works as the holdfast of the parasite. In some groups, the scolex is dominated by bothria, which are sometimes called "sucking grooves," and function like suction cups. Other groups have hooks and suckers to help in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex, though they may have rostellum and hooks.
Though the scolex is often the most distinctive part of an adult tapeworm, diagnosis is carried out by identifying eggs and gravid proglottids in feces, as it remains hidden inside the patient.
Neck
The Neck of tapeworm is a well-defined, short, narrow and unsegmented region behind scolex. It is dorso-ventrally flattened and composed of a relatively undifferentiated mass of cells. It is the budding zone, growth zone, area of proligeration or area of segmentation, as it is here that new proglottids are formed.
Strobila
The strobila forms the main bulk of the body and is composed of a linear series of segments or proglottids arranged in a chain-like fashion. The number of proglottids varies from three in E. granulosus to 2,000 in T. saginata. However, they can be grouped into three different kinds, namely immature, mature and gravid proglottids.
Immature Proglottids are the anterior most one just behind the neck. They are shorter and broader and are devoid of reproductive organs.
Mature Proglottids occupy the middle part of the strobila and are squarish in outline. Tapeworms are hermaphrodite and protandrous, therefore, anterior mature proglottids consists of only male reproductive organs, while the posterior ones contain both male and female organs side by side. Thus a mature proglottid is a complete reproductive unit and produces eggs either by self-fertilization or cross-fertilization with other mature proglottids. It has been suggested by some early biologists that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids.
gravid proglottids occur in the posterior part of strobila and are longer than the width. These proglottids consist of no more reproductive organs excepting the highly branched uterus packed with ferlized eggs at different stages of development. The terminal gravid proglottids dettach from rest of the body either singly (e.g., T. saginata) or in small group (e.g., T. solium) by a process termed apolysis. This phenomenon serves to limit the length of the parasite and to transfer the developing embryo to exterior in feces of the host.
Treatment
CDC guidelines for treatment is a prescription drug called praziquantel. Praziquantel is generally well tolerated. Sometimes more than one treatment is necessary. [2]
Template:CDC
Reference
- ↑ Tapeworm Infection. Retrieved 2007-02-14.
- Campbell, Reece, and Mitchell, Biology, 1999
- Merck Manual of Medication Information, Second Home Edition, Online Version, Tapeworm Infection, 2005
- Mayo Clinic Website on infectious diseases, Mayo Clinic - Tapeworm Infection, 2006
See also
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