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 Scolex of Taenia solium
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Subclass Cestodaria |
In biology, tapeworms or cestodes comprise a class (Cestoda) of ribbon–like endoparasitic flatworms that live in the digestive tract of vertebrates as adults and often in the bodies of various animals (intermediate hosts) as juveniles.
Among the most common tapeworms in humans are the pork tapeworm, the beef tapeworm, the fish tapeworm, and the dwarf tapeworm. Infections involving the pork and beef tapeworms are also called taeniasis.
Tapeworm infestation, being tied to the human diet, are preventable diseases, avoidable through responsible actions of individuals and societies. Preventive measures include proper cooking of meat, such as beef, pork, and fish and proper means of dealing with human and animal wastes. Eating of sushi (raw fish) requires careful procedures for preparation.
Overview
As flatworms (phlum Platyhelminthes), the soft flatworm body is ribbon-shaped, flattened flattened dorso-ventrally (from top to bottom), and bilaterally symmetric. They are acoelomates that are characterized by having three germ layers (ectoderm, mesoderm, and endoderm) and lacking respiratory and circulatory systems.
Cestoda is one of four classes of flatworms, the others being Trematoda (flukes), Monogenea, and Turbellaria. While turbellarians, such as planarias, tend to be free-living, the cestodes, trematodes, and monogeneans live as parasites in or on other animals.
Cestodes are covered with a cuticle (tough but flexible, non-mineral covering), and lack a true circulatory or respiratory system; they do have a bilateral nervous system.
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 (an order of Eucestoda) are the most important to humans because they infect people, pets, and livestock. Among them, four significant genera are Taenia, Diphyllobothrium, Echinococcus, and Hymenolepis.
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 can grow 15 to 30 feet (10 meters) in length. The largest tapeworms grow up to 60 feet (Kimball 2006). Most tapeworms enter humans through contaminated food, the same way they enter secondary hosts like flea, pig, cattle, and so forth.
Adult morphology
Adult tapeworms typically have the 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.
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.
The parasites lack respiratory organs as well, with respiration of the tapeworms being mainly anaerobic or anoxybiotic, with glycolysis being the chief respiratory pathway. When oxygen become available, the general body lining works as the respiratory surface.
All cestodes have nerve rings and lateral nerve cords passing through outthe 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 remains attached to the intestine of the definitive host. It is not bigger than the head of a pin, yet it works as the hold-fast 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 the scolex remains hidden inside the patient.
Neck
The neck of a tapeworm is a well-defined, short, narrow, and unsegmented region behind the scolex. It is dorso-ventrally flattened and composed of a relatively undifferentiated mass of cells. It is the budding zone, growth zone, area of proliferation 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 two thousand in T. saginata. However, they can be grouped into three different kinds, namely immature, mature, and gravid proglottids.
Immature proglottids are the anterior most ones 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 (male and female sex organs in the same individual) and protandrous (male maturing first), therefore, anterior mature proglottids consist 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 than the highly branched uterus packed with fertilized eggs at different stages of development. The terminal gravid proglottids detach 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.
Life cycle
Excepting a few, most tapeworms are digenetic, which means completing the adult stage and sexual reproduction in the primary host and the larval stage in secondary host.
The break-off gravid proglottids of the adults contain thousands of fertilized eggs with onchosphere larvae. On reaching the ground, the proglottids eventually disintegrate and the larvae are set free.
The secondary hosts (pig for T. solium, cattle and buffalo for T. saginata) become infected on consuming food contaminated with the larvae. In the stomach of the secondary host, the larvae lose their protective sheath due to the proteolytic enzymes and the hooked hexacanth larvae hatch out. Hexacanths pierce the mucosa of the intestine to enter the blood stream and make a voyage through different organs of the body, finally landing in the striped muscle of the host. They settle there to develop into bladder-worm or cysticercus.
Feeding on such infected meat without proper cooking leads to the entry of the parasite into the primary host. Cysticercus become active in the small intestine, develop scolex, and transform into miniatures of the adult tapeworm. With the help of scolex, they remains attach to the intestinal mucosa in between the villi and repeat the cycle.
Tapeworm infestations
Adult tapeworm infection is the infection of the digestive tract by parasitic cestodes. Tapeworm larvae are sometimes ingested by consuming undercooked food. Once inside the digestive tract, the larva grows into an adult tapeworm, which can live for years and grow very large. Additionally, many tapeworm larvae cause symptoms in an intermediate host.
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 toxins 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 harm their host by stealing vital nutrients, causing malnutrition, and anemia, and multiple infections can cause intestinal blockages.
Taenia solium (pork tapeworm) and T. saginata (beef tapeworm) are the most common tapeworms of human beings. Their larval stages involve pigs and cattle respectively as the intermediate hosts. A person can become infected by these parasites by consuming raw or undercooked meat (mainly pork or beef) that has been infected by their larvae (cysticercus). The beef tapeworm is longer than the pork tapeworm and lack rostellum as well as hooks on the scolex. Symptoms generally include abdominal pain, diarrhea, nausea, and other gastrointestinal symptoms. Sometimes, the parasite may migrate to the appendix, pancreas, or bile duct causing severe abdominal pain.
A dangerous complication of the parasite T. solium, cysticercosis, may occur if due to reverse peristalsis 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 (Merck 2005).
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 Taenia saginata and Taenia solium, but can also include weakness and fatigue (Clark 2002).
The hydatid worm, Echinococcus granulosus, is a parasite of dogs, cats, and so forth; and its intermediate 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 into a cyst in the liver, lung, and in a few cases in the brain, heart, bone marrow, kidney, spleen, muscles, and so forth. A hydatid cyst may grow up to football size in 12 to 20 years in man, necessitating surgery.
Another common tapeworm of human beings is the dwarf tapeworm, Hymenolepis nana. The same host serves for the development of larvae (cysticercus) and the adult without requiring an intermediate host (monogenetic condition). However, certain rat fleas and beetles may work as an intermediate host to facilitate the propagation of the parasite. In the monogenetic condition, onchophore larvae are ingested in contaminated food; the 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.
Treatment
Centers for Disease Control and Prevention (CDC) guidelines for treatment typically involves a prescription drug called praziquantel. Praziquantel is generally well tolerated. Other effective drugs are mepacrine (Atebrin), diclorophen, and yamesan. Sometimes more than one treatment is necessary (CDC 2004).
As both primary and secondary hosts are infected via dietary means, personal hygiene and community level sanitation is a priority, as well as the sanitation of domestic and pet animals. At the same time, careful inspection of meat at slaughter houses, avoiding of poorly quality, raw, or undercooked pork and beef, and proper sanitary control of sewage disposal have to be practiced (Mayo Clinic 2006).
See also
ReferencesISBN links support NWE through referral fees
- Campbell, N. A., J. B. Reece, and L. G. Mitchell. 1999. Biology. Benjamin Cummings. ISBN 0805330445.
- Centers for Disease Control, Division of Parasitic Diseases (CDC). 2004. Hymenolepis infection. Parasitic Disease Information Fact Sheet. Retrieved November 10, 2007.
- Clark, G. N. 2002. Tapeworm diseases. Gale Encyclopedia of Medicine reprinted in Healthline.com. Retrieved November 10, 2007.
- Kimball, J. W. 2006. Tapeworms (cestoda). Kimball's Biology Pages.
- Mayo Clinic. 2006. Tapeworm infection. Mayo Clinic. Retrieved November 10, 2007.
- Merck. 2005. Tapeworm infection. The Merck Manuals of Medication Information, Second Home Edition. Online Version. Retrieved November 10, 2007.
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