| 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 | |
Trematoda is aclass offlatworms known asflukes ortrematodes.[1] They areobligate internalparasites with a complexlife cycle requiring at least twohosts. The intermediate host, in whichasexual reproduction occurs, aremollusks and usually asnail. The definitive host, where the flukessexually reproduce, is avertebrate.[1] Infection by trematodes can cause disease in all five traditionalvertebrate classes:mammals,birds,amphibians,reptiles, andfish.[2]
Trematodes are commonly referred to as flukes. This term can be traced back to theOld English name forflounder, and refers to the flattened,rhomboidal shape of the organisms.[3] The etymology of trematode stems from theGreek wordtrēmatṓdēs, which means "pierced with holes", and refers to the worm'ssucker, which pierces a hole in the host while the worm is attached and feeding.[4]
There are 18,000[5] to 24,000[6] known species of trematodes, divided into two subclasses — theAspidogastrea and theDigenea. Aspidogastrea is the smaller subclass, comprising 61 species. These flukes mainly infectbivalves andbony fishes.[7] Digenea — which comprise the majority of trematodes — are found in certainmollusks andvertebrates.
Flukes that cause disease in humans are often classified based on theorgan system they infect. For example:
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.[16]
The body surface of trematodes comprises a toughsyncytialtegument, which helps protect againstdigestive enzymes in those species that inhabit the gut of larger animals. It is also the surface of gas exchange; there are norespiratory organs.[16]
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.[16]
Although the excretion ofnitrogenous waste occurs 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.[16]
Thebrain consists of a pair ofganglia in the head region, from which two or three pairs ofnerve cords 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 theAspidogastrea. Trematodes generally lack any specializedsense organs, although someectoparasitic species do possess one or two pairs of simpleocelli.[16]
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.[17]
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.[17] Together, the mouth, pharynx, and esophagus form the foregut in Trematodes.[18]
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 thehepatopancreas of 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.Spermatogenesis produces 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 oneovary connected to an elongateduterus by 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.Oocytes are 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 ducts thatproduce 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.[19]
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 ofsnail of the family Lymnaeidae. Almost all trematodes infectmolluscs as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes aremonoecious and alternately reproduce sexually and asexually. The two main exceptions to this are theAspidogastrea, which have noasexual reproduction, and theschistosomes, which aredioecious.[2]
In the definitive host, in which sexual reproduction occurs, eggs are commonly shed along with host feces. Eggs shed in water release free-swimminglarval forms(Miracidia) that are infective to the intermediate host, in whichasexual reproduction occurs.[1]
A species that exemplifies the remarkable life history of the trematodes is thebird fluke,Leucochloridium paradoxum. The definitive hosts, in which theparasite reproduces, are various woodlandbirds, while the hosts in which theparasite multiplies (intermediate host) are variousspecies of snail. 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 thesporocyst and 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.[2]
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.
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.
Liver flukes, one of the different species, are responsible for causing liver fluke disease which is also known asfasciolosis. 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.
One species of tremtaoda,Haplorchis pumilio, has evolvedeusociality 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.[20]
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. 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".[20] 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!"[21]
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.[18]
Human trematode infections are most common in Asia, Africa and Latin America. However, trematodes can be found anywhere whereuntreated human waste is 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.[22]
Albendazole can be used to treatclonorchiasis andopisthorchiasis.Triclabendazole is often used to treatfasciolosis,[23] and may also be useful in the treatment ofparagonimiasis[24] anddicrocoeliasis.[25]Praziquantel is effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis,echinostomiasis,fasciolopsiasis, fasciolosis,gastrodiscoidiasis,heterophyiasis,metagonimiasis, opisthorchiasis, paragonimiasis, andschistosomiasis).[26]
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