Cestoda

For the musical collaboration named Tapeworm, see Tapeworm (band).

Tapeworm is also an older name for a type of computer virus.

In biology, Cestoda is the class of parasitic flatworms, called Cestoda or tapeworms, that live in the digestive tract of vertebrates as adults and often in the bodies of various animals as juveniles.

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Overview

Adult worms absorb food predigested by the host, so the worms have no need for a digestive tract or a mouth. Large tapeworms are made almost entirely of reproductive structures with a small "head" for attachment. Symptoms vary widely, depending on the species causing the infestation.

Symptoms may include upper abdominal discomfort, diarrhea, and loss of appetite. However, infestations are usually asymptomatic. Worm segments or eggs may be found in the stool of an infected person. Tapeworms can grow 15 to 30 feet (10 metres) in length.^[1] The largest tapeworms grow up to 59 feet (18 metres)[1]. Most tapeworms enter humans through infected food, the same way they enter pets. Tapeworms harm their host by stealing vital nutrients, causing malnutrition and, if left untreated, can cause intestinal blockages.

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 and livestock. Two important tapeworms are the pork tapeworm, Taenia solium, and the beef tapeworms, T. saginata. Different types of tapeworms have radically different larval stages (see their specific articles).

Taenia solium and T. saginata are the most common tapeworms. A person can become infected by these parasites by eating raw or undercooked meat that has been infected. 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.

Cysticercosis, a dangerous complication of the parasite Taenia solium, may occur when the larvae develop outside the intestinal tract. This parasite 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 Taenia solium, but can also include weakness and fatigue.

In many cases the tape worm will cause the arm to turn a blue/green in coloration due to the lack of protein in the blood. in this case the worm must be immediately removed before it increases in size and the appendage needs to be removed.

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.

In popular culture

An episode of House M.D. (a primetime television drama) features a woman who has been infected with the pork tapeworm, Taenia solium. In this case the tapeworm has lodged one of its spawn into the woman's brain. To prove that there is a tapeworm infection, one of House's junior colleagues points out that an x-ray of the thigh muscles should be taken, as this is a "popular" spot for worm larvae to be found. In a separate episode, a teenage girl with CIPA has a tapeworm extracted from her stomach, which relieves her problem of Vitamin B12 deficiency. Since CIPA is a genetic disorder that prevents the sensation of pain, heat, and cold, the girl remained unsedated and conscious during the operation of removing the 25 foot tapeworm.

In the book Filth by Irvine Welsh of Trainspotting fame, the lead character is infected by a tapeworm. At a certain point in the book, the narrative starts to be interrupted by a tube-like structure that appears on top of the text, and at first is only made of the word "eat" being repeated amidst the zeros that fill the empty space within the tube. It is later revealed that this is actually the thoughts of the tapeworm growing inside Bruce's intestines. At first, the tapeworm only encourages Bruce to eat. Later, after becoming self-aware (and naming itself "The Self"), the tapeworm starts to ask basic existential questions and names Bruce as "The Host". It also stumbles upon the existence of other worms (collectively named by the initial worm as "The Other"). The tapeworm's monologue, towards the end of the story, is used to explain the way Bruce turned into the person presented in the book; explaining the events of his past, producing monologue about Bruce's true feelings, and completing the cycle of his life with him.

Comic creator Jhonen Vasquez often uses the pseudonym "Mr. Scolex" when he is credited in his books or for his various voice works in his cartoon, Invader Zim. He also makes fun of his own imaginary tape worm.

In the ITV programme Primeval, a giant cestoid infects dodos, and also attacks and kills a person. This cestoid feeds by destroying the internal organs and central nervous system of their hosts, and after passing on the eggs that are released in saliva, which are passed onto other hosts by biting, the cestoid and the host both die.

In an episode of the show Mr. Meaty, Parker is infected by a tapeworm which eats any food that he puts in front of his mouth in the blink of an eye. Parker and Josh bait the tapeworm with food and pull it out with a fishing rod. They end up selling the tapeworm to a fisherman.

See also

• Diphyllobothrium (Fish tapeworm)
• Cysticercosis
• Schistosomiasis

Notes

References

• 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
• Medline Plus - Taeniasis (tapeworm infection)
• University of South Carolina - School of Medicine - CESTODES (TAPE WORMS)
• This article contains material from the CDC (Center for Disease Control) website which, as a US government publication, is in the public domain.