Tapeworm

Cestoda
Scolex of Tenia solium
Scientific classification
Kingdom: Animalia Phylum: Platyhelminthes Class: Cestoda
Orders
Subclass Cestodaria Amphilinidea Gyrocotylidea Subclass Eucestoda Aporidea Caryophyllidea Cyclophyllidea Diphyllidea Lecanicephalidea Litobothridea Nippotaeniidea Proteocephalidea Pseudophyllidea Spathebothriidea Tetraphyllidea Trypanorhyncha

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. 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

Adult tapeworms share a basic body structure. All have a scolex, sometimes colloquially referred to as the "head," a "neck," and one or more proglottids, which are sometimes called "segments." These are the source of the name "tapeworm," because they look like a strip of tape. All cestodes have a nerve ring in the scolex with lateral trunks passing through the rest of the body.

Scolex

The Scolex or "head" of the worm attaches to the intestine of the definitive host. 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 that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex, though they may have other structures.

While the scolex is often the most distinctive part of an adult tapeworm, it is often unnoticed in a clinical setting as it is inside the patient. Thus, identifying eggs and proglottids in feces is important.

Neck

The Neck of a tapeworm is a relatively undifferentiated mass of cells that divide to form new proglottid "segments." This is where all growth in an adult tapeworm occurs.

Proglottids

The body is composed of successive units posterior to the scolex, the proglottids. The sum of the proglottids is called a strobila, which is thin, resembling a strip of tape, and is the source of the common name tapeworm. Like some other flatworms, cestodes use flame cells (protonephridia) for excretion, which are located in proglottids.

Mature or gravid proglottids are released from the mature tapeworm and leave the host in its feces.

Because each proglottid contains the male and female reproductive structures, they can reproduce independently. It has been suggested by some biologists that each should be considered a single organism, and that the tapeworm is actually a colony of proglottids.

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

• 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

Wikimedia Commons has media related to:

Cestoda

Wikispecies has information related to:

Cestoda

• Cysticercosis
• Schistosomiasis

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