Temporal range: 270–0Ma
|Subclasses and orders|
Cestoda (Cestoidea) is the name given to a class of parasitic flatworms, of the phylum Platyhelminthes. Informally biologists refer to them as Cestodes. The most notorious species commonly are called tapeworms, for example Taenia solium, the "pork tapeworm". 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 patient. Thus, identifying eggs and proglottids in feces is important.
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 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 (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
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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 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 host's 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
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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. Cestoda can also be treated with some kinds of antibiotics. While tapeworm infections in first world countries have been largely eradicated, 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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