Trematoda
 | This articleneeds additional citations forverification.(July 2023) |
| Trematoda | |
|---|---|
| |
| Botulus microporus,a giant digenean parasite from the intestine of alancetfish | |
Scientific classification
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Superclass: | Neodermata |
| Class: | Trematoda Rudolphi,1808 |
| Subclasses | |
Trematodais aclassofflatwormsknown asflukesortrematodes.They areobligateinternalparasiteswith a complexlife cyclerequiring at least twohosts.The intermediate host, in whichasexual reproductionoccurs, is usually asnail.The definitive host, where the flukes sexually reproduce, is avertebrate.Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.
Etymology
[edit]Trematodes are commonly referred to as flukes. This term can be traced back to theOld Englishname forflounder,and refers to the flattened,rhomboidalshape of the organisms.
Taxonomy
[edit]There are 18,000[1]to 24,000[2]known species of trematodes, divided into two subclasses — theAspidogastreaand theDigenea.Aspidogastrea is the smaller subclass, comprising 61 species. These flukes mainly infectbivalvesandbony fishes.[3]Digenea — which comprise the majority of trematodes — are found in certainmollusksandvertebrates.
Trematodes of medical importance
[edit]Flukes that cause disease in humans are often classified based on theorgan systemthey infect. For example:
- Blood flukesinhabit the blood in some stages of their life cycle. Blood flukes that cause disease in humans includeTrichobilharzia regenti,which causesswimmer's itch,and seven species of genusSchistosomawhich causeschistosomiasis:S. guineensis,S.haematobium,S. intercalatum,S. japonicum,S. malayensis,S. mansoni,S. mekongi.As adefinitive host,humans are infected when thecercariae(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 animalreservoir hosts.[4]S. haematobiumandS. japonicumare of particular importance, as these arecarcinogenic parasites.S. haematobium,which infects theurinary bladder,is among the most important causes ofbladder cancerin humans.[5][6]This organism is classified by theInternational Agency for Research on Cancer(IARC) as aGroup 1 (extensively proven) carcinogen.[7]S. japonicumis associated with the development ofliver cancer,and is classified as aGroup 2B (possibly carcinogenic to humans) carcinogen.[7]
- Liver flukesare commonly found withinbile ducts,liver, andgallbladderin certain mammalian and avian species. They includeClonorchis sinensis,Dicrocoelium dendriticum,Dicrocoelium hospes,Fasciola gigantica,Fasciola hepatica,Opisthorchis felineus,andOpisthorchis viverrini.ClonorchisandOpisthorchisare carcinogenic parasites that are strongly associated with the development ofcancer of the bile ducts.[8][9]
- Lung flukes:there are ten species of lung flukes that infect humans, causingparagonimiasis.[10]Of these, the most common cause of human paragonimiasis isParagonimus westermani,the oriental lung fluke.[11]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.[4]
- Intestinal flukes inhabit theepitheliumof thesmall intestine.These includeFasciolopsis buski(which causesfasciolopsiasis),Metagonimus miyatai,Metagonimus takahashii,Metagonimus yokogawai(which causemetagonimiasis), andHeterophyes heterophyesandHeterophyes nocens(which cause heterophyiasis).
Anatomy
[edit]
Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as 1 millimetre (0.039 in) are known. Their most distinctive external feature is the presence of twosuckers,one close to the mouth, and the other on the underside of the animal.[12]
The body surface of trematodes comprises a toughsyncytialtegument,which helps protect againstdigestive enzymesin those species that inhabit the gut of larger animals. It is also the surface of gas exchange; there are norespiratory organs.[12]
The mouth is located at the forward end of the animal, and opens into a muscular, pumpingpharynx.The pharynx connects, via a shortoesophagus,to one or two blind-endingcaeca,which occupy most of the length of the body. In some species, the caeca are themselves branched. As in other flatworms, there is noanus,and waste material must be egested through the mouth.[12]
Although the excretion ofnitrogenous wasteoccurs mostly through the tegument, trematodes do possess anexcretory system,which is instead mainly concerned withosmoregulation.This consists of two or moreprotonephridia,with those on each side of the body opening into a collecting duct. The two collecting ducts typically meet up at a singlebladder,opening to the exterior through one or two pores near the posterior end of the animal.[12]
Thebrainconsists of a pair ofgangliain the head region, from which two or three pairs ofnerve cordsrun 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 theAspidogastrea.Trematodes generally lack any specializedsense organs,although someectoparasiticspecies do possess one or two pairs of simpleocelli.[12]
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.[13]
Oral sucker and acetabulum: In some species of Trematoda, such as T. bragai, there is anacetabulum.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.[13]Together, the mouth, pharynx, and esophagus form the foregut in Trematodes.[14]
Reproductive system of blood flukes
[edit]Most trematodes arehermaphrodites,as are many internal parasites. Blood flukes (Schistosoma) are the only form of trematodes that aredioecious(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 thehepatopancreasof afreshwater 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 twotestes,though some species may have more. The testes are located posterior and dorsal to theventral sucker.Spermatogenesisproduces biflagellate sperm (sperm with two tails). Sperm is stored in theseminal vesicles,which are connected to the testes by thevas 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 oneovaryconnected to an elongateduterusby aciliatedoviduct.The uterus opens to the exterior at thegenital 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 — termedLaurer's canal— which is analogous to a vagina.Oocytesare released from the ovary into theoocapt(the dilated proximal end of the oviduct). Sperm cells travel from the seminal vesicles through the uterus to reach theootype(the dilated distal part of the oviduct), where fertilization occurs. The ootype is connected via a pair of ducts to a number ofvitelline ductsthatproduce yolk.After the egg is surrounded by yolk, its shell is formed from the secretions ofMehlis' 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 thegenital atrium.[15]
Life cycles
[edit]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 infectmolluscsas the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes aremonoeciousand alternately reproduce sexually and asexually. The two main exceptions to this are theAspidogastrea,which have noasexual reproduction,and theschistosomes,which aredioecious.
In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimminglarvalforms(Miracidia)that are infective to the intermediate host, in whichasexual reproductionoccurs.
A species that exemplifies the remarkable life history of the trematodes is thebirdfluke,Leucochloridium paradoxum.The definitive hosts, in which theparasitereproduces, are various woodlandbirds,while the hosts in which theparasitemultiplies (intermediate host) are variousspeciesofsnail.The adult parasite in the bird's gut produces eggs and these eventually end up on the ground in the bird's feces. Some eggs may be swallowed by a snail and hatch into larvae (miracidia). These larvae grow and take on a sac-like appearance. This stage is known as thesporocystand it forms a central body in the snail's digestive gland that extends into a brood sac in the snail's head, muscular foot and eye-stalks. It is in the central body of the sporocyst where the parasite replicates itself, producing many tiny embryos (redia). These embryos move to the brood sac and mature intocercaria.
Life cycle adaptations
[edit]Trematodes have a large variation of forms throughout their life cycles. Individual trematode parasites life cycles may vary from this list. They have five larval stages along with the cystic and fully matured adult phases.
- Trematodes are released from the definitive host as eggs, which have evolved to withstand the harsh environment
- Released from the egg which hatches into themiracidium.This infects the first intermediate host in one of two ways, either active or passive transmission. The first host is normally a mollusk. a) Active transmission has adapted for dispersal in space as a free swimming ciliated miracidium with adaptations for recognizing and penetrating the first intermediate host. b) Passive transmission has adapted for dispersal in time and infects the first intermediate host contained within the egg.
- Thesporocystforms inside the snail first intermediate host and feeds throughdiffusionacross thetegument.
- Therediaealso forms inside the snail first intermediate host and feeds through a developedpharynx.Either the rediae or the sporocyst develops into the cercariae throughpolyembryonyin the snail.
- Thecercariaeare adapted for dispersal in space and exhibit a large variety in morphology. They are adapted to recognize and penetrate the second intermediate host, and contain behavioral and physiological adaptations not present in earlier life stages.
- Themetacercariaeare an adapted cystic form dormant in the secondary intermediate host.
- 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 will develop from the miracidium. They also have an anterior spin which helps them drill into the snail. The miracidium develops into the sporocyst, which is a sac-like structure, and in this sac the larvae begin to develop. The cells multiply. The rediae and cercariae develop from the larvae which are then released and encyst as metacercariae, for instance on aquatic plants. Humans as well as larger sea creatures get infected when they eat these plants.
When they infect humans, it can take 3–4 months for the metacercariae to mature into adult flukes and lay eggs.
Example: liver flukes
[edit]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 infect humans when they eat raw or undercooked fish.
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.
Eusociality
[edit]One species of tremtaoda,Haplorchis pumilio,has evolvedeusocialityinvolving 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 will might also invade and replace them, a specialized caste of sterile soldier trematodes protects the colony.[16]
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 hostin vitro.Interestingly,H. pumiliosoldiers 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".[16]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!"[17]
Infections
[edit]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.[14]
Human trematode infections are most common in Asia, Africa and Latin America. However, trematodes can be found anywhere whereuntreated human wasteis used asfertilizer.Humans can be infected by trematodes by immersion in or ingestion of contaminated water, or by consuming raw or undercooked contaminated animals or plants.[18]
Treatment
[edit]Albendazolecan be used to treatclonorchiasisandopisthorchiasis.Triclabendazoleis often used to treatfasciolosis,[19]and may also be useful in the treatment ofparagonimiasis[20]anddicrocoeliasis.[21]Praziquantelis effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis,echinostomiasis,fasciolopsiasis,fasciolosis,gastrodiscoidiasis,heterophyiasis,metagonimiasis,opisthorchiasis, paragonimiasis, andschistosomiasis).[22]
References
[edit]- ^Littlewood D T J; Bray R. A. (2000). "The Digenea".Interrelationships of the Platyhelminthes.Systematics Association Special Volume. Vol. 60 (1 ed.). CRC. pp. 168–185.ISBN978-0-7484-0903-7.
- ^Poulin, Robert;Serge Morand (2005).Parasite Biodiversity.Smithsonian. p. 216.ISBN978-1-58834-170-9.
- ^"A Checklist of the Aspidogastrea (Platyhelminthes: Trematoda) of the World".www.biotaxa.org.Retrieved11 June2024.
- ^abToledo, Rafael; Fried, Bernard, eds. (2014).Digenetic Trematodes.Advances in Experimental Medicine and Biology. Vol. 766.doi:10.1007/978-1-4939-0915-5.ISBN978-1-4939-0914-8.ISSN0065-2598.S2CID28198614.
- ^Antoni, S.; Ferlay, J.; Soerjomataram, I.; Znaor, A.; Jemal, A.; Bray, F. (2017). "Bladder Cancer incidence and mortality: A global overview and recent trends".European Urology.71(1): 96–108.doi:10.1016/j.eururo.2016.06.010.PMID27370177.
- ^Khurana S, Dubey ML, Malla N (April 2005). "Association of parasitic infections and cancers".Indian J Med Microbiol.23(2): 74–79.doi:10.1016/S0300-483X(01)00357-2.PMID15928434.
- ^ab"IARC Monographs on the Identification of Carcinogenic Hazards to Humans".Retrieved4 December2022.
- ^Hughes T, O'Connor T, Techasen A, Namwat N, Loilome W, Andrews RH, Khuntikeo N, Yongvanit P, Sithithaworn P, Taylor-Robinson SD (2017)."Opisthorchiasis and cholangiocarcinoma in Southeast Asia: an unresolved problem".International Journal of General Medicine.10:227–237.doi:10.2147/IJGM.S133292.PMC5557399.PMID28848361.
- ^Kaewpitoon N, Kaewpitoon SJ, Pengsaa P, Sripa B (February 2008)."Opisthorchis viverrini: the carcinogenic human liver fluke".World Journal of Gastroenterology.14(5): 666–74.doi:10.3748/wjg.14.666.PMC2683991.PMID18205254.
- ^Cong, Cung-Van; Anh, Tran-Thi Tuan; Ly, Tran-Thi; Duc, Nguyen Minh (2022)."Paragonimiasis diagnosed by CT-guided transthoracic lung biopsy: Literature review and case report".Radiology Case Reports.17(5): 1591–1597.doi:10.1016/j.radcr.2022.02.046.ISSN1930-0433.PMC8927937.PMID35309377.
- ^"Paragonimiasis".Center for Global Health, U.S. Centers for Disease Control and Prevention (CDC). 13 October 2010. Archived fromthe originalon 16 December 2013.Retrieved6 September2012.
- ^abcdeBarnes, Robert D. (1982).Invertebrate Zoology.Philadelphia, PA: Holt-Saunders International. pp. 230–235.ISBN0-03-056747-5.
- ^abD’ávila, Sthefane (18 March 2010)."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".Acta Zoologica.91(2): 139–149.doi:10.1111/j.1463-6395.2008.00393.x.
- ^abBogitsh, Burton J.; Carter, Clint E.; Oeltmann, Thomas N. (28 May 2018).Human Parasitology.Academic Press.ISBN978-0-12-813713-0.
- ^Peoples, Robert C.; Fried, Bernard (2014)."Form and function in the Digenea".In Toledo, Rafael; Fried, Bernard (eds.).Digenetic Trematodes.Springer. pp. 14–15.ISBN978-1-4939-0915-5.
- ^abMetz, Daniel C. G.; Hechinger, Ryan F. (30 July 2024). "The physical soldier caste of an invasive, human-infecting flatworm is morphologically extreme and obligately sterile".Proceedings of the National Academy of Sciences.121(31): e2400953121.Bibcode:2024PNAS..12100953M.doi:10.1073/pnas.2400953121.ISSN0027-8424.PMC11295071.PMID39042696.
- ^Richards, Miriam H. (10 September 2024). "Social evolution and reproductive castes in trematode parasites".Proceedings of the National Academy of Sciences.121(37): e2414228121.doi:10.1073/pnas.2414228121.ISSN0027-8424.PMC11406270.PMID39226370.
- ^"Foodborne trematode infections".www.who.int.Retrieved25 April2022.
- ^"Triclabendazole: MedlinePlus Drug Information".
- ^"CDC - Paragonimiasis - Resources for Health Professionals".3 December 2021.
- ^Massoud, A.; Morsy, T. A.; Haridy, F. M. (2003)."Treatment of Egyptian dicrocoeliasis in man and animals with Mirazid".Journal of the Egyptian Society of Parasitology.33(2): 437–442.PMID14964658.
- ^"Praziquantel".The American Society of Health-System Pharmacists.Archivedfrom the original on 20 December 2016.Retrieved8 December2016.
