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{{Automatic taxobox
| image = Botulus_microporus.jpg
| image_caption = ''Botulus microporus'', a giant digenean parasite from the intestine of a [[lancetfish]]
| taxon = Trematoda
| authority = [[Karl Rudolphi|Rudolphi]], 1808
| subdivision_ranks = Subclasses
| subdivision = {{Linked taxon list
|Aspidogastrea|
|Digenea|
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}}
'''Trematoda''' is a [[Class (biology)|class]] of [[flatworm]]s known as '''flukes''' or '''trematodes'''. They are [[obligate parasite|obligate]] internal [[Parasitism|parasites]]
==Etymology==
==Taxonomy and biodiversity==▼
Trematodes are commonly referred to as
The trematodes or flukes include 18,000<ref name="Cribb2001">{{cite book |last=Littlewood D T J |author2=Bray R. A. |title=Interrelationships of the Platyhelminthes |series=Systematics Association Special Volume |volume=60 |edition=1 |year=2000 |publisher=CRC |isbn=978-0-7484-0903-7|pages=168–185 |chapter=The Digenea}}</ref> to 24,000<ref name="PoulinMorand2004">{{cite book |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |author2=Serge Morand |title=Parasite Biodiversity |year=2005 |publisher=Smithsonian |isbn=978-1-58834-170-9 |pages=216}}</ref> species of [[obligate parasite]]s, divided into two subclasses — the [[Aspidogastrea]] and the [[Digenea]]. Aspidogastrea is the smaller subclass, comprising 61 species. These flukes mainly infect [[Bivalvia|bivalves]] and [[Osteichthyes|bony fishes]].<ref>https://www.biotaxa.org/Zootaxa/article/view/zootaxa.3918.3.2</ref> Digenea — which comprise the majority of trematodes — are found in certain [[Mollusca|mollusks]] and [[vertebrate]]s.▼
Flukes that cause disease in humans are often classified based on the [[organ system]] they infect. For example:▼
▲
==Trematodes of medical importance==
* [[Schistosoma|Blood flukes]] inhabit the blood in some stages of their life cycle. Blood flukes that cause disease in humans include ''[[Trichobilharzia regenti]]'', which causes [[swimmer's itch]], and seven species of genus ''[[Schistosoma]]'' which cause [[schistosomiasis]]: ''[[Schistosoma|S. guineensis]]'', ''[[Schistosoma haematobium|S.haematobium]]'', ''[[Schistosoma intercalatum|S. intercalatum]]'', ''[[Schistosoma japonicum|S. japonicum]]'', ''[[Schistosoma malayensis|S. malayensis]]'', ''[[Schistosoma mansoni|S. mansoni]]'', ''[[Schistosoma mekongi|S. mekongi]]''. As a [[Host_(biology)#Types_of_hosts|definitive host]], humans are infected when the [[cercaria]]e (the larval forms of trematodes) penetrate the skin. Any contact with water containing these cercariae can potentially result in infection. Adult blood flukes can live for years in human or animal [[Natural reservoir|reservoir hosts]].<ref name=":1">{{Cite journal |date=2014 |editor-last=Toledo |editor-first=Rafael |editor2-last=Fried |editor2-first=Bernard |title=Digenetic Trematodes |url=http://dx.doi.org/10.1007/978-1-4939-0915-5 |journal=Advances in Experimental Medicine and Biology |volume=766 |doi=10.1007/978-1-4939-0915-5 |isbn=978-1-4939-0914-8 |s2cid=28198614 |issn=0065-2598}}</ref> ''S.haematobium'' and ''S. japonicum'' are of particular importance, as these are [[carcinogenic parasite]]s. ''S.haematobium'', which infects the [[urinary bladder]], is among the most important causes of [[bladder cancer]] in humans.<ref>{{cite journal|last1=Antoni|first1=S.|last2=Ferlay|first2=J.|last3=Soerjomataram|first3=I.|last4=Znaor|first4=A.|last5=Jemal|first5=A.|last6=Bray|first6=F.|title=Bladder Cancer incidence and mortality: A global overview and recent trends|journal=European Urology|date=2017|volume=71|issue=1|pages=96–108|doi=10.1016/j.eururo.2016.06.010|pmid=27370177}}</ref><ref>{{cite journal|date=April 2005|title=Association of parasitic infections and cancers|journal=Indian J Med Microbiol|volume=23|issue=2|pages=74–79|doi=10.1016/S0300-483X(01)00357-2|pmid=15928434|vauthors=Khurana S, Dubey ML, Malla N}}</ref> This organism is classified by the [[International Agency for Research on Cancer]] (IARC) as a [[IARC group 1|Group 1 (extensively proven) carcinogen]].<ref name=IARC>{{cite web|title=IARC Monographs on the Identification of Carcinogenic Hazards to Humans|url=https://monographs.iarc.who.int/list-of-classifications|access-date=4 December 2022}}</ref> ''S. japonicum'' is associated with the development of [[hepatocellular carcinoma|liver cancer]], and is classified as a [[IARC group 2B|Group 2B (possibly carcinogenic to humans) carcinogen]].<ref name=IARC/>▼
▲Flukes that cause disease in humans are often classified based on the [[organ system]] they infect. For example:
* [[Liver fluke]]s are commonly found within [[bile duct]]s, liver, and [[gallbladder]] in certain mammalian and avian species. They include ''[[Clonorchis sinensis]]'', ''[[Dicrocoelium dendriticum]]'', ''[[Dicrocoelium hospes]]'', ''[[Fasciola gigantica]]'', ''[[Fasciola hepatica]]'', ''[[Opisthorchis felineus]]'', and ''[[Opisthorchis viverrini]]''. ''Clonorchis'' and ''Opisthorchis'' are carcinogenic parasites that are strongly associated with the development of [[cholangiocarcinoma|cancer of the bile ducts]].<ref>{{cite journal | vauthors = Hughes T, O'Connor T, Techasen A, Namwat N, Loilome W, Andrews RH, Khuntikeo N, Yongvanit P, Sithithaworn P, Taylor-Robinson SD | title = Opisthorchiasis and cholangiocarcinoma in Southeast Asia: an unresolved problem | journal = [[International Journal of General Medicine]] | volume = 10 | pages = 227–237 | date = 2017 | pmid = 28848361 | pmc = 5557399 | doi = 10.2147/IJGM.S133292 }}</ref><ref name="kae">{{cite journal | vauthors = Kaewpitoon N, Kaewpitoon SJ, Pengsaa P, Sripa B | title = Opisthorchis viverrini: the carcinogenic human liver fluke | journal = World Journal of Gastroenterology | volume = 14 | issue = 5 | pages = 666–74 | date = February 2008 | pmid = 18205254 | pmc = 2683991 | doi = 10.3748/wjg.14.666 }}</ref>▼
▲* [[Schistosoma|Blood flukes]] inhabit the blood in some stages of their life cycle. Blood flukes that cause disease in humans include ''[[Trichobilharzia regenti]]'', which causes [[swimmer's itch]], and seven species of genus ''[[Schistosoma]]'' which cause [[schistosomiasis]]: ''[[Schistosoma|S. guineensis]]'', ''[[Schistosoma haematobium|S.haematobium]]'', ''[[Schistosoma intercalatum|S. intercalatum]]'', ''[[Schistosoma japonicum|S. japonicum]]'', ''[[Schistosoma malayensis|S. malayensis]]'', ''[[Schistosoma mansoni|S. mansoni]]'', ''[[Schistosoma mekongi|S. mekongi]]''. As a [[
▲* [[Liver fluke]]s are commonly found within [[bile duct]]s, liver, and [[gallbladder]] in certain mammalian and avian species. They include ''[[Clonorchis sinensis]]'', ''[[Dicrocoelium dendriticum]]'', ''[[Dicrocoelium hospes]]'', ''[[Fasciola gigantica]]'', ''[[Fasciola hepatica]]'', ''[[Opisthorchis felineus]]'', and ''[[Opisthorchis viverrini]]''. ''Clonorchis'' and ''Opisthorchis'' are carcinogenic parasites that are strongly associated with the development of [[cholangiocarcinoma|cancer of the bile ducts]].<ref>{{cite journal | vauthors = Hughes T, O'Connor T, Techasen A, Namwat N, Loilome W, Andrews RH, Khuntikeo N, Yongvanit P, Sithithaworn P, Taylor-Robinson SD | title = Opisthorchiasis and cholangiocarcinoma in Southeast Asia: an unresolved problem | journal = [[International Journal of General Medicine]] | volume = 10 | pages = 227–237 | date = 2017 | pmid = 28848361 | pmc = 5557399 | doi = 10.2147/IJGM.S133292 | doi-access = free }}</ref><ref name="kae">{{cite journal | vauthors = Kaewpitoon N, Kaewpitoon SJ, Pengsaa P, Sripa B | title = Opisthorchis viverrini: the carcinogenic human liver fluke | journal = World Journal of Gastroenterology | volume = 14 | issue = 5 | pages = 666–74 | date = February 2008 | pmid = 18205254 | pmc = 2683991 | doi = 10.3748/wjg.14.666 | doi-access = free }}</ref>
* [[Paragonimus|Lung flukes]]: there are ten species of lung flukes that infect humans, causing [[paragonimiasis]].<ref>{{Cite journal |last1=Cong |first1=Cung-Van |last2=Anh |first2=Tran-Thi Tuan |last3=Ly |first3=Tran-Thi |last4=Duc |first4=Nguyen Minh |date=2022 |title=Paragonimiasis diagnosed by CT-guided transthoracic lung biopsy: Literature review and case report |journal=Radiology Case Reports |volume=17 |issue=5 |pages=1591–1597 |doi=10.1016/j.radcr.2022.02.046 |issn=1930-0433 |pmc=8927937 |pmid=35309377}}</ref> Of these, the most common cause of human paragonimiasis is ''[[Paragonimus westermani]]'', the oriental lung fluke.<ref name="cdc">{{cite web |date=2010-10-13 |title=Paragonimiasis |url=http://www.dpd.cdc.gov/dpdx/HTML/Frames/M-R/Paragonimiasis/body_Paragonimiasis_page1.htm |url-status=dead |archive-url=https://web.archive.org/web/20131216074345/http://www.dpd.cdc.gov/dpdx/HTML/Frames/M-R/Paragonimiasis/body_Paragonimiasis_page1.htm |archive-date=2013-12-16 |access-date=2012-09-06 |publisher=Center for Global Health, U.S. Centers for Disease Control and Prevention (CDC)}}</ref> Lung flukes require three different hosts in order to complete their life cycle. The first intermediate host is a snail, the second intermediate host is a crab or crayfish, and the definitive host for lung flukes is an animal or human host.<ref name=":1" />
* Intestinal flukes inhabit the [[intestinal epithelium|epithelium]] of the [[small intestine]]. These include ''[[Fasciolopsis buski]]'' (which causes [[fasciolopsiasis]]), ''[[Metagonimus miyatai]]'', ''[[Metagonimus takahashii]]'', ''[[Metagonimus yokogawai]]'' (which cause [[metagonimiasis]]), and ''[[Heterophyes heterophyes]]'' and ''[[Heterophyes nocens]]'' (which cause heterophyiasis).
==
[[File:TrematodesFig9 EncBrit1911.png|thumb|
Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although
The body surface of trematodes comprises a tough [[syncitium|syncytial]] [[Tegument (Helminth)|tegument]], which helps protect against [[digestive enzymes]] in those
The mouth is located at the forward end of the animal, and opens into a muscular, pumping [[pharynx]]. The
Although the excretion of [[nitrogenous waste]] occurs mostly through the
The [[brain]] consists of a pair of [[ganglion|ganglia]] in the head region, from which two or three pairs of [[Ventral nerve cord|nerve cord]]s run down the length of the body. The nerve cords running along the ventral surface are always the largest, while the dorsal cords are present only in the [[Aspidogastrea]]. Trematodes generally lack any specialized [[sense organs]], although some [[ectoparasite|ectoparasitic]]
=== Reproductive system ===▼
Most trematodes are [[hermaphrodite]]s, as are many internal parasites. Blood flukes (''Schistosoma'') are the only form of trematodes that are [[Dioecy|dioecious]] (have both a male and female sex). Blood flukes are unique in the way that they can undergo both asexual and sexual reproduction. Asexual reproduction occurs in the [[hepatopancreas]] of a [[freshwater snail]], which serves as an intermediate host. Sexual reproduction occurs later in the life cycle, in the definitive (vertebrate) host.
The male reproductive system usually includes two [[Testicle|testes]], though some species may have more. The testes are located posterior and dorsal to the [[Acetabulum (morphology)#Trematode|ventral sucker]]. [[Spermatogenesis]] produces biflagellate sperm (sperm with two tails). Sperm is stored in the [[seminal vesicles]], which are connected to the testes by the [[vas deferens]]. The male reproductive system varies considerably in structure between species; this can be very useful in species identification.
The female reproductive system consists of one [[ovary]] connected to an elongated [[uterus]] by a [[Cilium|ciliated]] [[oviduct]]. The uterus opens to the exterior at the ''genital pore'' (the common external opening of the male and female reproductive systems). The location of the ovary varies among different species, making the female reproductive system useful in species identification. At the base of the oviduct is a copulatory duct — termed ''[[Laurer's canal]]'' — which is analogous to a vagina. [[Oocyte]]s are released from the ovary into the ''oocapt'' (the dilated proximal end of the oviduct). Sperm cells travel from the seminal vesicles through the uterus to reach the ''ootype'' (the dilated distal part of the oviduct), where fertilization occurs. The ootype is connected via a pair of ducts to a number of ''vitelline ducts'' that [[Vitellogenesis|produce yolk]]. After the egg is surrounded by yolk, its shell is formed from the secretions of ''[[Mehlis gland|Mehlis' glands]]'', the ducts of which also open into the ootype. From the ootype, the fertilized egg then travels back into the uterus, and is ultimately released from the ''genital atrium''.<ref name=Peoples>{{cite book|last1=Peoples|first1=Robert C.|last2=Fried|first2=Bernard|editor1-last=Toledo|editor1-first=Rafael|editor2-last=Fried|editor2-first=Bernard|title=Digenetic Trematodes|year=2014|publisher=Springer|isbn=978-1-4939-0915-5|pages=14–15|chapter-url=https://books.google.com/books?id=PPW_AwAAQBAJ&pg=PA14|chapter=Form and function in the Digenea}}</ref>
▲'''Body wall musculature:''' Formed of three different muscle layers: circular, longitudinal, and diagonal. The outermost layer is formed by the circular muscle fibers, directly behind that are the longitudinal muscle fibers. The inner layer is formed by the diagonal muscle fibers. Together these muscle fibers form the segmented body wall of trematodes.<ref name=":0">{{Cite journal |last=D’ávila |first=Sthefane |date=18 March 2010 |title=Gross anatomy of the musculature and a new description of the reproductive system of Tanaisia bragai and Tanaisia inopina (Trematoda: Eucotylidae) analysed by confocal laser scanning microscopy |url=https://onlinelibrary.wiley.com/doi/full/10.1111/j.1463-6395.2008.00393.x |journal= Acta Zoologica|volume=91| issue = 2|pages=139–149 |doi=10.1111/j.1463-6395.2008.00393.x }}</ref>
▲'''Oral sucker and acetabulum:''' In some species of Trematoda, such as T. bragai, there is an [[Acetabulum (morphology)|acetabulum]]. This saucer-shaped organ is attached to the oral sucker in some Trematodes and other parasitic worms. This allows for parasitic worms to attach to their host by penetrating the host’s tissue with spines lining the acetabulum organ. In trematodes, the oral sucker is linked to the [[pharynx]] via a canal composed of meridional, equatorial, and radial muscle fibers.<ref name=":0" /> Together, the mouth, pharynx, and [[esophagus]] form the foregut in Trematodes.<ref name=":2">{{Cite book |last1=Bogitsh |first1=Burton J. |url=https://books.google.com/books?id=UexdDwAAQBAJ&dq=Human+parasitology++Burton+Bogitsh+2005&pg=PP1 |title=Human Parasitology |last2=Carter |first2=Clint E. |last3=Oeltmann |first3=Thomas N. |date=2018-05-28 |publisher=Academic Press |isbn=978-0-12-813713-0 |language=en}}</ref>
Trematodes have a very complex life cycle and depending on what taxa they belong to, their life cycles can be completed with as little as one host compared to the typical three hosts.
▲== Life cycles ==
▲{{main|Trematode lifecycle stages}}
▲Trematodes have a very complex life cycle and depending on what taxa they belong to, their life cycles can be completed with as little as one host compared to the typical three hosts. When there is one host, this is normally a specific species of snail of the family Lymnaeidae. Almost all trematodes infect [[molluscs]] as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are [[monoecious]] and alternately reproduce sexually and asexually. The two main exceptions to this are the [[Aspidogastrea]], which have no [[asexual reproduction]], and the [[Schistosomatidae|schistosomes]], which are [[dioecious]].
In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimming [[larva]]l forms [[Trematode Miracidium|(Miracidia)]] that are infective to the intermediate host, in which [[asexual reproduction]] occurs.
A
===Life cycle adaptations===
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#The adult is the fully developed form which infects the definitive host.
The first stage is the miracidium that is triangular in shape and covered by a ciliated ectoderm which is the outermost layer of the three germ layers. The epidermis and epidemic tissues of the parasite
When they infect humans, it
====
Liver flukes, one of the different species, are responsible for causing liver fluke disease which is also known as fasciolosis. They are hermaphroditic internal parasites. They are caused by the migration of a large number of immature flukes through the liver passageway or by adult flukes that migrate to the bile ducts. Liver flukes infect all grazing animals and
Like other flukes, the liver flukes need intermediate hosts and as a result, the transmission from animals to humans happens in three phases. The first phase is the infection of the snail (the first intermediate host) via feces. They complete their gestation and hatch as cercariae. They leave their snail hosts and infect fish who are their second intermediate host. Lastly, larger animals ingest the metacercariae in raw and undercooked fish. In humans or grazing animals, the metacercariae complete their life cycle and become full grown liver flukes.
Human infections are most common in Asia, Africa and Latin America. However, trematodes can be found anywhere where untreated human waste is used as [[fertilizer]]. Humans can be infected by trematodes either by consuming undercooked contaminated fish or plants, or by ingesting contaminated water. The metacercariae of trematodes attach to fish and plants. In addition, humans can be infected by certain species through swimming in contaminated water. Humans cannot get infected by trematodes directly, trematodes require an intermediate host to do so.<ref>{{Cite web |title=Foodborne trematode infections |url=https://www.who.int/health-topics/foodborne-trematode-infections |access-date=2022-04-25 |website=www.who.int |language=en}}</ref>▼
=== Eusociality ===
One species of tremtaoda, [[Haplorchis pumilio|''Haplorchis pumilio'']], has evolved [[eusociality]] involving a colony of them creating a class of sterile soldiers. One fluke invades a host and establishes a colony of dozens to thousands of clones that work together to take it over. Since rival trematode species might also invade and replace them, a specialized caste of sterile soldier trematodes protects the colony.<ref name="s770">{{cite journal |last1=Metz |first1=Daniel C. G. |last2=Hechinger |first2=Ryan F. |date=2024-07-30 |title=The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile |journal=Proceedings of the National Academy of Sciences |volume=121 |issue=31 |pages=e2400953121 |doi=10.1073/pnas.2400953121 |issn=0027-8424 |pmc=11295071 |pmid=39042696|pmc-embargo-date=January 23, 2025 |bibcode=2024PNAS..12100953M }}</ref>
Soldiers are smaller, more mobile, and develop along a different pathway than sexually mature reproductives. One big difference is their mouthparts (pharynx), which are five times as big as those of the reproductives. They make up nearly a quarter of the volume of the soldier. These soldiers don’t have a germinal mass, can’t metamorphose to be reproductive, and are, therefore, obligately sterile.
Soldiers are readily distinguished from the immature and mature reproductive worms. Soldiers are more aggressive than reproductives, attacking heterospecific trematodes that infect their host ''in vitro''. Interestingly, ''H. pumilio'' soldiers do not attack conspecifics from other colonies.
The soldiers are not evenly distributed throughout the host body. They’re found in the highest numbers in the basal visceral mass, where competing trematodes tend to multiply during the early phase of infection. This strategic positioning allows them to effectively defend against invaders, similar to how soldier distribution patterns are seen in other animals with defensive castes.
They "appear to be an obligately sterile physical caste, akin to that of the most advanced social insects".<ref name="s770" /> Reflecting on their use for understanding the evolution of animal social castes, one review commented, "trematodes are a lineage for sociobiologists to keep a careful watch on!"<ref name="o307">{{cite journal |last=Richards |first=Miriam H. |date=2024-09-10 |title=Social evolution and reproductive castes in trematode parasites |journal=Proceedings of the National Academy of Sciences |volume=121 |issue=37 |pages=e2414228121 |doi=10.1073/pnas.2414228121 |pmid=39226370 |pmc=11406270 |pmc-embargo-date=March 3, 2025 |issn=0027-8424}}</ref>
==
Trematodes can cause disease in many types of vertebrates, including mammals, birds, reptiles, and fish. Cattle and sheep can become infected by eating contaminated food. These infections lead to a reduction in milk or meat production, which can be of significant economic importance to the livestock industry.<ref name=":2" />
▲Trematodes are commonly referred to as ''flukes''. This term can be traced back to the [[Old English]] name for [[flounder]], and refers to the flattened, [[rhomboid]]al shape of the [[worm]]s.
▲Human trematode infections are most common in Asia, Africa and Latin America. However, trematodes can be found anywhere where [[Night soil|untreated human waste]] is used as [[fertilizer]]. Humans can be infected by trematodes
=== Treatment ===
[[Albendazole]] can be used to treat [[clonorchiasis]] and [[opisthorchiasis]]. [[Triclabendazole]] is often used to treat [[fasciolosis]],<ref>{{cite web | url=https://medlineplus.gov/druginfo/meds/a619048.html | title=Triclabendazole: MedlinePlus Drug Information }}</ref> and may also be useful in the treatment of [[paragonimiasis]]<ref>{{cite web | url=https://www.cdc.gov/parasites/paragonimus/health_professionals/index.html | title=CDC - Paragonimiasis - Resources for Health Professionals | date=3 December 2021 }}</ref> and [[Dicrocoelium dendriticum|dicrocoeliasis]].<ref>{{cite journal | url=https://pubmed.ncbi.nlm.nih.gov/14964658/ | pmid=14964658 | year=2003 | last1=Massoud | first1=A. | last2=Morsy | first2=T. A. | last3=Haridy | first3=F. M. | title=Treatment of Egyptian dicrocoeliasis in man and animals with Mirazid | journal=Journal of the Egyptian Society of Parasitology | volume=33 | issue=2 | pages=437–442 }}</ref> [[Praziquantel]] is effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis, [[Echinostoma|echinostomiasis]], [[fasciolopsiasis]], fasciolosis, [[Gastrodiscoides|gastrodiscoidiasis]], [[Heterophyes heterophyes|heterophyiasis]], [[metagonimiasis]], opisthorchiasis, paragonimiasis, and [[schistosomiasis]]).<ref name=AHFS2016>{{cite web|title=Praziquantel|url=https://www.drugs.com/monograph/praziquantel.html|publisher=The American Society of Health-System Pharmacists|access-date=8 December 2016|url-status=live|archive-url=https://web.archive.org/web/20161220223422/https://www.drugs.com/monograph/praziquantel.html|archive-date=20 December 2016}}</ref>
==References==
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{{Fish disease topics}}
{{Taxonbar|from=Q207547}}
{{Authority control}}
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