cestode

条虫類

WordNet

1. tapeworms (同)class Cestoda

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/11/13 02:43:27」(JST)

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"Tapeworm" redirects here. For the band, see Tapeworm (band). For the early name for a computer "worm", see Computer worm.

This article is about the flatworm. For the medical condition, see Tapeworm infection.

Cestoda Temporal range: 270–0Ma PreЄ Є O S D C P T J K Pg N
Taenia saginata
Scientific classification
Kingdom:	Animalia
Phylum:	Platyhelminthes
Class:	Cestoda
Subclasses and orders
Cestodaria Amphilinidea Gyrocotylidea Eucestoda Aporidea Caryophyllidea Cyclophyllidea Diphyllidea Lecanicephalidea Litobothridea Nippotaeniidea Proteocephalidea Pseudophyllidea Spathebothriidea Tetraphyllidea Trypanorhyncha

Cestoda (Cestoidea) is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies of various animals as juveniles. Over a thousand species have been described, and all vertebrate species can be parasitised by at least one species of tapeworm. Several species parasitise humans after being consumed in underprepared meat such as pork (Taenia solium), beef (T. saginata), and fish (Diphyllobothrium spp.), or in food prepared in conditions of poor hygiene (Hymenolepis spp. or Echinococcus spp.).

T. saginata, the beef tapeworm, can grow up to 20 m (65 ft); the largest species, the whale tapeworm Polygonoporus giganticus, can grow to over 30 m (100 ft).

Tapeworm parasites of vertebrates have a long history: recognizable clusters of cestode eggs, one with a developing larva, have been discovered in fossil feces (coprolites) of a shark dating to the mid- to late Permian, some 270 mya.

Anatomy

Scolex

The worm's scolex ("head") attaches to the intestine of the definitive host. In some species, the scolex is dominated by bothria, or "sucking grooves" that function like suction cups. Other species have hooks and suckers that aid in attachment. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolex.

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.

Body systems

The main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number and complexity of the scolex. Smaller nerves emanate from the commissures to supply the general body muscular and sensory ending. The cirrus and vagina are innervated, and sensory endings around the genital pore are more plentiful than other areas. Sensory function includes both tactoreception and chemoreception. Some nerves are only temporary.

Proglottids

The body is composed of successive segments (proglottids). The sum of the proglottids is called a strobila, which is thin, and resembles a strip of tape. From this is derived the common name "tapeworm". Like some other flatworms, cestodes use flame cells (protonephridia), located in the proglottids, for excretion. Mature proglottids are released from the tapeworm's posterior end and leave the host in feces.

Because each proglottid contains the male and female reproductive structures, they can reproduce independently. Some biologists have suggested that each should not be considered a single organism, and that the tapeworm is actually a colony of proglottids.

The layout of proglottids comes in two forms, craspedote, meaning proglottids are overlapped by the previous proglottid, and acraspedote which indicates a non-overlapping conjoined proglottid.

Cestodes are unable to synthesise lipids and are entirely dependent on their host, although lipids are not used as an energy reserve, but for reproduction. Once anchored to the host's intestinal wall, the tapeworm absorbs nutrients through its skin as the food being digested by the host flows past it and it begins to grow a long tail, with each segment containing an independent digestive system and reproductive tract. Older segments are pushed toward the tip of the tail as new segments are produced by the neckpiece. By the time a segment has reached the end of the tail, only the reproductive tract is left. It then drops off, carrying the tapeworm eggs to the next host, since, by that point, the proglottid is, in essence, a sac of eggs.

Reproduction and life cycle of the tapeworms

True tapeworms are exclusively hermaphrodites; they have both male and female reproductive systems in their bodies. The reproductive system includes one or many testes, cirrus, vas deferens and seminal vesicle as male organs, and a single lobed or unlobed ovary with the connecting oviduct and uterus as female organs. There is a common external opening for both male and female reproductive systems, known as genital pore, which is situated at the surface opening of the cup-shaped atrium. Even though they are sexually hermaphroditic, self-fertilization is a rare phenomenon. In order to permit hybridization, cross-fertilization between two individuals is often practiced for reproduction. During copulation, the cirrus of one individual connects with that of the other through the genital pore, and then exchange their spermatozoa.

The life cycle of tapeworms is simple in the sense that there are no asexual phases as in other flatworms, but complicated in that at least one intermediate host is required as well as the definitive host. This life cycle pattern has been a crucial criterion for assessing evolution among Platyhelminthes. Many tapeworms have a two-phase life cycle with two types of host. The adult Taenia saginata lives in the gut of a primate such as a human, but more alarming are Taenia solium, which can form cysts in the human brain. Proglottids leave the body through the anus and fall onto the ground, where they may be eaten with grass by animals such as cows. This is known as the intermediate host. The juvenile form migrates and establishes as a cyst in the intermediate hosts body tissues such as muscles, rather than the gut. They cause more damage to this host than it does to its definitive host. The parasite completes its life cycle when the intermediate host passes on the parasite to the definitive host. This is usually done by the definitive host eating an infective intermediate host, such as possibly a human with a preference for raw meat—in whose gut the adult Taenia establishes itself.

Infection and treatment

Symptoms vary widely, as do treatment options, and these issues are discussed in detail in the individual articles on each worm. Praziquantel is an effective treatment for tapeworm infection, and is preferred over the older niclosamide. While tapeworm infections in first world countries have been largely eradicated, some US dieters have risked intentional infection for the purpose of weight-loss.

Footnotes

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 (tapeworms)

 This article incorporates public domain material from websites or documents of the Centers for Disease Control and Prevention.

Medline Plus - Taeniasis (tapeworm infection)

External links

• Knol.google.com
• How Disgusting are Tapeworms? Article from TalktotheVet.com
• Tapeworm

UpToDate Contents

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English Journal

• Invalidation of Diphyllobothrium hottai (Cestoda: Diphyllobothriidae) based on morphological and molecular phylogenetic analyses.

• Banzai-Umehara A1, Suzuki M1, Akiyama T1, Ooi HK2, Kawakami Y3.
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• Yamasaki H1, Kumazawa H2, Sekikawa Y3, Oda R3, Hongo I3, Tsuchida T4, Saito N5, Morishima Y6, Sugiyama H6.
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• Molecular identification of Spirometra spp. (Cestoda: Diphyllobothriidae) in some wild animals from Brazil.

• Almeida GG1, Coscarelli D2, Melo MN2, Melo AL2, Pinto HA2.
• Parasitology international.Parasitol Int.2016 Oct;65(5 Pt A):428-31. doi: 10.1016/j.parint.2016.05.014. Epub 2016 May 25.
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Japanese Journal

• アカエリカイツブリPodiceps grisegenaから得られた円葉類条虫

• 酪農学園大学紀要. 自然科学編 40(1), 7-9, 2015-10
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• Does cestode infection affect reproduction of the endangered golden venus chub, Hemigrammocypris neglectus?

• Limnology 16(3), 203-207, 2015-08
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• Epidemiological Study and Control Trial of Taeniid Cestode Infection in Farm Dogs in Qinghai Province, China

• Journal of Veterinary Medical Science 76(3), 395-400, 2014
• NAID 130003382387

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