Trematoda: Difference between revisions

{{shortShort description|Class of parasitic flatworms}}

'''Trematoda''' is a [[Class (biology)|class]] of [[flatworm]]s known as '''flukes''' or '''trematodes'''. They are [[obligate parasite|obligate]] internal [[Parasitism|parasites]] with a complex [[biological life cycle|life cycle]] requiring at least two [[Host_Host (biology)|hosts]]. The intermediate host, in which [[asexual reproduction]] occurs, is usually a [[snail]]. The definitive host, where the flukes sexually reproduce, is a [[vertebrate]]. Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.

There are 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> known species of trematodes, 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 name=biotaxa>{{Cite web | url=https://www.biotaxa.org/Zootaxa/article/view/zootaxa.3918.3.2 {{bare| URLtitle=A Checklist of the Aspidogastrea (Platyhelminthes: Trematoda) of the World inline| access-date=April2024-06-11 2023| website=www.biotaxa.org}}</ref> Digenea — which comprise the majority of trematodes — are found in certain [[Mollusca|mollusks]] and [[vertebrate]]s.

* [[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_Host (biology)#Types_of_hostsTypes 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 journalbook |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 |journalseries=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/>

[[File:TrematodesFig9 EncBrit1911.png|thumb|VariedVarious trematodes, from 1911 Encyclopædia Britannica]]

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).&nbsp;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_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.

{{mainMain|Trematode lifecycle stages}}

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 arewill going to evolvedevelop 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 then get infected when they eat these plants.

When they infect humans, it takescan take 3–4 months for the metacercariae to mature into adult flukes and lay eggs.

===Life= cycle ofExample: liver flukes ====

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 areinfect passed from human to humanhumans 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, humanslarger 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.