Cestoda
Cestoda Temporal range:
| |
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Taenia saginata | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Platyhelminthes |
Subphylum: | Rhabditophora |
Superclass: | Neodermata |
Class: | Cestoda |
Subclasses | |
Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of many similar units known as proglottids—essentially packages of eggs which are regularly shed into the environment to infect other organisms. Species of the other subclass, Cestodaria, are mainly fish infecting parasites.
All cestodes are
The adult tapeworm has a scolex (head), a short neck, and a strobila (segmented body) formed of proglottids. Tapeworms anchor themselves to the inside of the intestine of their host using their scolex, which typically has hooks, suckers, or both. They have no mouth, but absorb nutrients directly from the host's gut. The neck continually produces proglottids, each one containing a reproductive tract; mature proglottids are full of eggs, and fall off to leave the host, either passively in the feces or actively moving. All tapeworms are hermaphrodites, with each individual having both male and female reproductive organs.
Humans are subject to infection by several species of tapeworms if they eat undercooked meat such as pork (Taenia solium), beef (T. saginata), and fish (Diphyllobothrium), or if they live in, or eat food prepared in, conditions of poor hygiene (Hymenolepis or Echinococcus species). The unproven concept of using tapeworms as a slimming aid has been touted since around 1900.
Diversity and habitat
All 6,000 species of Cestoda are
Anatomy
Cestodes have no gut or mouth[6] and absorb nutrients from the host's alimentary tract through their specialised neodermal cuticle, or tegument,[7] through which gas exchange also takes place.[2] The tegument also protects the parasite from the host's digestive enzymes[8] and allows it to transfer molecules back to the host.[7]
The body form of adult eucestodes is simple, with a scolex, or grasping head, adapted for attachment to the
The Cestodaria have 10 larval hooks while Eucestoda have 6 larval hooks.[9]
Scolex
The scolex, which attaches to the intestine of the definitive host, is often minute in comparison with the proglottids. It is typically a four-sided knob, armed with suckers or hooks or both.[2] In some species, the scolex is dominated by bothria, or "sucking grooves" that function like suction cups. Cyclophyllid cestodes can be identified by the presence of four suckers on their scolices.[10] Other species have ruffled or leaflike scolices, and there may be other structures to aid attachment.[2]
In the larval stage the scolex is similarly shaped and is known as the protoscoleces.[11]
Body systems
Circular and longitudinal muscles lie under the neodermis, beneath which further longitudinal, dorso-ventral and transverse muscles surround the central parenchyma. Protonephridial cells drain into the parenchyma. There are four longitudinal collection canals, two dorso-lateral and two ventro-lateral, running along the length of the worm, with a transverse canal linking the ventral ones at the posterior of each segment. When the proglottids begin to detach, these canals open to the exterior through the terminal segment.[2]
The main nerve centre of a cestode is a cerebral ganglion in its scolex. Nerves emanate from the ganglion to supply the general body muscular and sensory endings, with two lateral nerve cords running the length of the strobila.[2] The cirrus and vagina are innervated, and sensory endings around the genital pore are more plentiful than in other areas. Sensory function includes both tactoreception (touch) and chemoreception (smell or taste).[8]
Proglottids
Once anchored to the host's intestinal wall, tapeworms absorb nutrients through their surface as their food flows past them.[12] Cestodes are unable to synthesise lipids, which they use for reproduction, and are therefore entirely dependent on their hosts.[13]
The tapeworm body is composed of a series of segments called proglottids. These are produced from the neck by mitotic growth, which is followed by transverse constriction. The segments become larger and more mature as they are displaced backwards by newer segments.[2] Each proglottid contains an independent reproductive tract, and like some other flatworms, cestodes excrete waste through flame cells (protonephridia) located in the 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". Proglottids are continually being produced by the neck region of the scolex, as long as the scolex is attached and alive.[14]
Mature proglottids are essentially bags of eggs, each of which is infective to the proper intermediate host. They are released and leave the host in feces, or migrate outwards as independent motile proglottids.[14] The number of proglottids forming the tapeworm ranges from three to four thousand. Their layout comes in two forms: craspedote, meaning any given proglottid is overlapped by the previous proglottid, or acraspedote, indicating the proglottids do not overlap.[15]
Reproduction
Cestodes are exclusively
Life cycle
Cestodes are parasites of vertebrates, with each species infecting a single definitive host or group of closely related host species. All but
Cestodes produce large numbers of eggs, but each one has a low probability of finding a host. To increase their chances, different species have adopted various strategies of egg release. In the Pseudophyllidea, many eggs are released in the brief period when their aquatic intermediate hosts are abundant (semelparity). In contrast, in the terrestrial Cyclophyllidea, proglottids are released steadily over a period of years, or as long as their host lives (iteroparity). Another strategy is to have very long-lived larvae; for example, in Echinococcus, the hydatid larvae can survive for ten years or more in humans and other vertebrate hosts, giving the tapeworm an exceptionally long time window in which to find another host.[23]
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, its definitive host. Proglottids leave the body through the anus and fall to the ground, where they may be eaten with grass by a grazing animal such as a cow. This animal then becomes an intermediate host, the oncosphere boring through the gut wall and migrating to another part of the body such as the muscle. Here it encysts, forming a cysticercus. The parasite completes its life cycle when the intermediate host passes on the parasite to the definitive host, usually when the definitive host eats contaminated parts of the intermediate host, for example a human eating raw or undercooked meat.[2] Another two-phase life cycle is exhibited by Anoplocephala perfoliata, the definitive host being an equine and the intermediate host an oribatid mite.[24]
Diphyllobothrium exhibits a more complex, three-phase life cycle. If the eggs are laid in water, they develop into free-swimming oncosphere larvae. After ingestion by a suitable freshwater crustacean such as a copepod, the first intermediate host, they develop into procercoid larvae. When the copepod is eaten by a suitable second intermediate host, typically a minnow or other small freshwater fish, the procercoid larvae migrate into the fish's flesh where they develop into plerocercoid larvae. These are the infective stages for the mammalian definitive host. If the small fish is eaten by a predatory fish, its muscles too can become infected.[2]
Schistocephalus solidus is another three-phase example. The intermediate hosts are copepods and small fish, and the definitive hosts are waterbirds. This species has been used to demonstrate that cross-fertilisation produces a higher infective success rate than self-fertilisation.[25]
Host immunity
Hosts can become immune to infection by a cestode if the lining, the mucosa, of the gut is damaged. This exposes the host's immune system to cestode antigens, enabling the host to mount an antibody defence. Host antibodies can kill or limit cestode infection by damaging their digestive enzymes, which reduces their ability to feed and therefore to grow and to reproduce; by binding to their bodies; and by neutralising toxins that they produce. When cestodes feed passively in the gut, they do not provoke an antibody reaction.[26]
Evolution and phylogeny
Fossil history
Parasite fossils are rare, but recognizable clusters of cestode eggs, some with an operculum (lid) indicating that they had not erupted, one with a developing larva, have been discovered in fossil shark coprolites dating to the Permian, some 270 million years ago.[1][27]
The fossil Rugosusivitta, which was found in China at base of the Cambrian deposits in Yunnan [28] just above the Ediacaran-Cambrian border, has great similarities to present day Cestodians. If correct, this would be the earliest example of a Platyzoan and also one of the earliest bilaterian body-fossils and might thus provide an insight to the living mode of Cestodians before they became specialized parasites.
External
The position of the Cestoda within the Platyhelminthes and other
Platytrochozoa |
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580 mya |
Internal
The evolutionary history of the Cestoda has been studied using
Cestoda |
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The
Interactions with humans
Infection and treatment
Like other species of mammal, humans can become infected with tapeworms. There may be few or no symptoms, and the first indication of the infection may be the presence of one or more proglottids in the stools. The proglottids appear as flat, rectangular, whitish objects about the size of a grain of rice, which may change size or move about. Bodily symptoms which are sometimes present include abdominal pain, nausea, diarrhea, increased appetite and weight loss.[36]
There are several classes of
History and culture
In
Tapeworms have occasionally appeared
There are unproven claims that, around 1900, tapeworm eggs were marketed to the public as slimming tablets.[43] A full-page coloured image, purportedly from a women's magazine of that period, reads "Fat: the enemy ... that is banished! How? With sanitized tape worms. Jar packed. No ill effects!"[35] When television presenter Michael Mosley deliberately infected himself with tapeworms he gained weight due to increased appetite.[44] Dieters still sometimes risk intentional infection, evidenced by a 2013 warning on American television.[45]
Notes
- ^ Tapeworms are not formed of fixed body segments as are the annelids, arthropods or chordates.[citation needed]
References
- ^ a b "Tapeworm Eggs Discovered in 270-Million-Year-Old Fossil Shark faeces", ScienceDaily, 30 January 2013
- ^ ISBN 978-81-315-0104-7.)
{{cite book}}
: CS1 maint: multiple names: authors list (link - ^ "The Persistent Parasites". Time. 8 April 1957. Archived from the original on June 27, 2008.
- ^ Hargis, William J. (1985). "Parasitology and pathology of marine organisms of the world ocean". NOAA Tech. Rep. National Oceanic and Atmospheric Administration.
- JSTOR 3546556. Archived from the original(PDF) on 2016-03-04. Retrieved 2015-08-29.
- ^ ISBN 978-0-323-14010-2.
- ^ ISSN 0008-4301.
- ^ ISBN 978-1-61731-756-9.
- ^ "Helminth Parasites". parasite.org.au. Retrieved 2018-07-27.
- ^ "Flatworm". Encyclopædia Britannica Ultimate Reference Suite. Chicago: Encyclopædia Britannica, 2010.
- ISBN 9780415308557.
- ^ "The Common Tapeworm (Dipylidium caninum)". Mar Vista Animal Medical Center. 6 May 2012. Archived from the original on 29 October 2013. Retrieved 26 November 2013.
- S2CID 39258998.
- ^ ISBN 9780321929150.
- ^ "Cestodes". Scribd. Retrieved 24 May 2018.
- ISBN 0-12-170755-5
- ISBN 0-8138-0718-2
- PMID 14635918.
- ^ This article incorporates public domain material from websites or documents of the Centers for Disease Control and Prevention.
- ISBN 978-1-60535-375-3.
- ^ "Tapeworm infection". Mayo Clinic. Retrieved 23 July 2018.
- PMID 3294640.
- PMID 3282055.
- ^ "Tapeworms in Horses". Merck Veterinary Manual. Retrieved 21 May 2018.
- PMID 12487354.
- ^ Cheng, Thomas C. (1973). General Parasitology. Academic Press. pp. 535–536.
- PMID 23383033.
- S2CID 235047029.
- PMID 24732282.
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- ^ PMID 17950292.
- ^ S2CID 6288037.
- ^ PMID 22406529.
- PMID 9488348.
- ^ ISBN 978-0-7011-8180-2.
- ^ a b c "Tapeworms". NHS Choices. 19 December 2017. Retrieved 20 May 2018.
- ISBN 978-9241547109.
- ^ Holden-Dye, Lindy; Walker, Robert J. "Anthelmintic drugs". WormBook. Retrieved 23 May 2018.
- ISBN 9780199759712.
- ^ PMID 15145374.
- National Public Radio. Retrieved 15 June 2018.
- ^ Hartmann, Graham (April 21, 2021). "System of a Down Had a Massive Fight Over That 'Tapeworm' Lyric While Making 'Toxicity'". Loudwire. Retrieved October 12, 2021.
- ^ "'Eat! Eat! Eat!' Those notorious tapeworm diet pills". The Quack Doctor. 2015-01-23. Retrieved 2018-07-26.
- ^ Morgan, James (2014). "TV doctor infests himself with worms". BBC News. Retrieved 2018-07-26.
- Today. 16 August 2013.
Further reading
- 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)