Temporal range: 270–0Ma
|Subclasses and orders|
Cestoda (Cestoidea) is the name given to a class of parasitic flatworms, of the phylum Platyhelminthes. Biologists informally refer to them as Cestodes. The best-known species are commonly called tapeworms. Cestodes are all parasitic and their life histories vary, but typically they live in the digestive tracts of vertebrates as adults, and often in the bodies of other species of animals as juveniles. Over a thousand species have been described, and all vertebrate species may be parasitised by at least one species of tapeworm.
Humans are subject to parasitism by several species of tapeworms if they eat underprepared meat such as pork (Taenia solium), beef (T. saginata), and fish (Diphyllobothrium spp.), or if they live in, or eat food prepared in, conditions of poor hygiene (Hymenolepis spp. or Echinococcus species).
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). On the other hand, species using small vertebrates as hosts tend to be small. For example, vole and lemming tapeworms are only 13–240 mm (0.51–9.45 in) in length, and those parasitizing shrews only 0.8–60 mm (0.031–2.362 in).
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.
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 host. Therefore identifying eggs and proglottids in feces is the simplest way to diagnose an infection.
The main nerve centre of a cestode is a cerebral ganglion in its scolex. Motor and sensory innervation depends on the number of nerves in and complexity of the scolex. Smaller nerves emanate from the ganglion 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 (touch) and chemoreception (smell or taste). Some nerves are only temporary.
The body is composed of successive segments called 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 end segment and leave the host in feces or migrate as independent motile proglottids.
The layout of proglottids comes in two forms: craspedote, meaning that any given proglottid os overlapped by the previous proglottid, and acraspedote, indicating the proglottids are non-overlapping.
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 over and around it. Soon it begins to grow a tail composed of a series of segments, 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 worm's tail, only the reproductive tract is left. The segment then separates, carrying the tapeworm eggs out of the definitive host as what is basically a sack of eggs.
Reproduction and life cycle of the tapeworms
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True tapeworms are exclusively hermaphrodites; they have both male and female reproductive systems in their bodies. The reproductive system includes one or more testes, cirri, vas deferens and seminal vesicles 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 the 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 cirri of one individual connects with that of the other through the genital pore, and then spermatozoa are exchanged.
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 an animal such as a cow. If the tapeworm is compatible with the eating animal. this animal becomes an intermediate host. The juvenile form of the worm enters through the mouth but then migrates and establishes as a cyst in the intermediate host's body tissues such as muscles, rather than the gut. This can cause more damage to the intermediate 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 a suitably infected intermediate host, e.g., a human eating raw or undercooked meat.
Infection and treatment
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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. Cestodes can also be treated with certain kinds of antibiotics. While accidental tapeworm infections in first world countries are quite rare, some US dieters have risked intentional infection for the purpose of weight loss.
The taxonomy of the Cestoda has been clarified with molecular data. The Gyrocotylidea is a sister group to all other Cestoda (Nephroposticophora): the Amphilinidea forms the sister group to the Eucestoda. The Caryophyllidea are the sister group to Spathebothriidea + remaining Eucestoda. The Haplobothriidea are the sister group to Diphyllobothriidae. The Diphyllidea and Trypanorhyncha may be sister groups but this is not definite.
At the more derived groups the taxonomy appears to be as follows
The Tetraphyllidea appear to be paraphyletic. The relations between Nippotaeniidea, Mesocestoididae, Tetrabothriidea and Cyclophyllidea require further clarification.
The taxonomy of the Eucestoda has been also clarified. The current taxonomy is
The Tetraphyllidea, Lecanicephalidea, Proteocephalidea, Nippotaeniidea, Tetrabothriidea and Cyclophyllidea are considered to be the 'higher' tapeworms.
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||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (February 2008)|
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Medline Plus - Taeniasis (tapeworm infection)
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