Trematoda is a
class
Class or The Class may refer to:
Common uses not otherwise categorized
* Class (biology), a taxonomic rank
* Class (knowledge representation), a collection of individuals or objects
* Class (philosophy), an analytical concept used differentl ...
of
flatworm
The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegment ...
s known as flukes. They are
obligate {{wiktionary, obligate
As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as:
* Obligate aerobe, an organism that cannot survive without oxygen
* Obligate anaerobe, an organism that ...
internal
parasites
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
with a complex
life cycle
Life cycle, life-cycle, or lifecycle may refer to:
Science and academia
*Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring
*Life-cycle hypothesis, ...
requiring at least two
hosts
A host is a person responsible for guests at an event or for providing hospitality during it.
Host may also refer to:
Places
*Host, Pennsylvania, a village in Berks County
People
*Jim Host (born 1937), American businessman
*Michel Host ( ...
. The intermediate host, in which
asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
occurs, is usually a
snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
. The definitive host, where the flukes sexually reproduce, is a
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
. Infection by trematodes can cause disease in all five traditional vertebrate classes: mammals, birds, amphibians, reptiles, and fish.
Etymology
Trematodes are commonly referred to as flukes. This term can be traced back to the
Old English
Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
name for
flounder
Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries.
Taxonomy
The name "flounder" is used for several only distantly related species, thou ...
, and refers to the flattened,
rhomboid
Traditionally, in two-dimensional geometry, a rhomboid is a parallelogram in which adjacent sides are of unequal lengths and angles are non-right angled.
A parallelogram with sides of equal length (equilateral) is a rhombus but not a rhomboid.
...
al shape of the organisms.
Taxonomy
There are 18,000
to 24,000
known species of trematodes, divided into two subclasses — the
Aspidogastrea
The Aspidogastrea (Ancient Greek: ' “shield”, ' “stomach/pouch”) is a small group of Trematoda, flukes comprising about 80 species. It is a Subclass (biology), subclass of the Trematode, trematoda, and sister group to the Digenea. Specie ...
and the
Digenea
Digenea (Gr. ''Dis'' – double, ''Genos'' – race) is a class of trematodes in the Platyhelminthes phylum, consisting of parasitic flatworms (known as ''flukes'') with a syncytial tegument and, usually, two suckers, one ventral and one oral. ...
. Aspidogastrea is the smaller subclass, comprising 61 species. These flukes mainly infect
bivalves
Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
and
bony fishes
Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilage ...
.
[https://www.biotaxa.org/Zootaxa/article/view/zootaxa.3918.3.2] Digenea — which comprise the majority of trematodes — are found in certain
mollusks
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
and
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s.
Trematodes of medical importance
Flukes that cause disease in humans are often classified based on the
organ system
An organ system is a biological system consisting of a group of organs that work together to perform one or more functions. Each organ has a specialized role in a plant or animal body, and is made up of distinct tissues.
Plants
Plants have ...
they infect. For example:
*
Blood flukes inhabit the blood in some stages of their life cycle. Blood flukes that cause disease in humans include ''
Trichobilharzia regenti
''Trichobilharzia regenti'' is a neuropathogenic parasitic flatworm of birds which also causes cercarial dermatitis in humans. The species was originally described in 1998 in the Czech Republic and afterwards it was detected also in other Europe ...
'', which causes
swimmer's itch
Swimmer's itch, cercarial dermatitis or schistosome dermatitis is a short-term allergic contact dermatitis occurring in the skin of humans that have been infected by water-borne schistosomes, a type of flatworm. It is common in freshwater, brackis ...
, and seven species of genus ''
Schistosoma
''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed '' schistosomiasis'', which is considered by the World Health Organ ...
'' which cause
schistosomiasis
Schistosomiasis, also known as snail fever, bilharzia, and Katayama fever, is a disease caused by parasitic flatworms called schistosomes. The urinary tract or the intestines may be infected. Symptoms include abdominal pain, diarrhea, bloody s ...
: ''
S. guineensis'', ''
S.haematobium'', ''
S. intercalatum'', ''
S. japonicum'', ''
S. malayensis'', ''
S. mansoni'', ''
S. mekongi''. As a
definitive host
In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
, humans are infected when the
cercaria
A cercaria (plural cercariae) is the larval form of the trematode class of parasites. It develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long sw ...
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
reservoir hosts.
''S.haematobium'' and ''S. japonicum'' are of particular importance, as these are
carcinogenic parasite
Carcinogenic parasites are parasitic organisms that depend on other organisms (called hosts) for their survival, and cause cancer in such hosts. Three species of flukes (trematodes) are medically-proven carcinogenic parasites, namely the urinary ...
s. ''S.haematobium'', which infects the
urinary bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, is among the most important causes of
bladder cancer
Bladder cancer is any of several types of cancer arising from the tissues of the urinary bladder. Symptoms include blood in the urine, pain with urination, and low back pain. It is caused when epithelial cells that line the bladder become mali ...
in humans. This organism is classified by the
International Agency for Research on Cancer
The International Agency for Research on Cancer (IARC; french: Centre International de Recherche sur le Cancer, CIRC) is an intergovernmental agency forming part of the World Health Organization of the United Nations.
Its role is to conduct and ...
(IARC) as a
Group 1 (extensively proven) carcinogen.
''S. japonicum'' is associated with the development of
liver cancer
Liver cancer (also known as hepatic cancer, primary hepatic cancer, or primary hepatic malignancy) is cancer that starts in the liver. Liver cancer can be primary (starts in liver) or secondary (meaning cancer which has spread from elsewhere to th ...
, and is classified as a
Group 2B (possibly carcinogenic to humans) carcinogen.
[
* ]Liver fluke
Liver fluke is a collective name of a polyphyletic group of parasitic trematodes under the phylum Platyhelminthes.
They are principally parasites of the liver of various mammals, including humans. Capable of moving along the blood circulation, t ...
s are commonly found within bile duct
A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates.
Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. It ...
s, liver, and gallbladder
In vertebrates, the gallbladder, also known as the cholecyst, is a small hollow organ where bile is stored and concentrated before it is released into the small intestine. In humans, the pear-shaped gallbladder lies beneath the liver, although ...
in certain mammalian and avian species. They include ''Clonorchis sinensis
''Clonorchis sinensis'', the Chinese liver fluke, is a liver fluke belonging to the class Trematoda, phylum Platyhelminthes. It infects fish-eating mammals, including humans. In humans, it infects the common bile duct and gall bladder, feeding on ...
'', ''Dicrocoelium dendriticum
''Dicrocoelium dendriticum'', the lancet liver fluke, is a parasite fluke that tends to live in cattle or other grazing mammals.
History of discovery
Much of what is presently known about ''Dicrocoelium dendriticum'' is the result of the wor ...
'', ''Dicrocoelium hospes
''Dicrocoelium hospes'' is a species of flatworms belonging to the family Dicrocoeliidae.
The species may cause the disease dicrocoeliasis
''Dicrocoelium dendriticum'', the lancet liver fluke, is a parasite fluke that tends to live in cattl ...
'', ''Fasciola gigantica
''Fasciola gigantica'' is a parasitic flatworm of the class Trematoda, which causes tropical fascioliasis. It is regarded as one of the most important single platyhelminth infections of ruminants in Asia and Africa. Estimates of infection rates ...
'', ''Fasciola hepatica
''Fasciola hepatica'', also known as the common liver fluke or sheep liver fluke, is a parasitic trematode (fluke or flatworm, a type of helminth) of the class Trematoda, phylum Platyhelminthes. It infects the livers of various mammals, inc ...
'', ''Opisthorchis felineus
''Opisthorchis felineus'', or cat liver fluke is a trematode parasite that infects the liver in mammals. It was first discovered in 1884 in a cat's liver by Sebastiano Rivolta of Italy. In 1891, Russian parasitologist, Konstantin Nikolaevich V ...
'', and ''Opisthorchis viverrini
''Opisthorchis viverrini'', common name Southeast Asian liver fluke, is a food-borne trematode parasite from the family Opisthorchiidae that infects the bile duct. People are infected after eating raw or undercooked fish. Infection with the par ...
''. ''Clonorchis'' and ''Opisthorchis'' are carcinogenic parasites that are strongly associated with the development of cancer of the bile ducts.
* Lung flukes: there are ten species of lung flukes that infect humans, causing paragonimiasis
Paragonimiasis is a food-borne parasitic disease caused by several species of lung flukes belonging to genus ''Paragonimus''. Infection is acquired by eating crustaceans such as crabs and crayfishes which host the infective forms called metacerca ...
. Of these, the most common cause of human paragonimiasis is ''Paragonimus westermani
''Paragonimus westermani'' (Japanese lung fluke or oriental lung fluke) is the most common species of lung fluke that infects humans, causing paragonimiasis. Human infections are most common in eastern Asia and in South America. Paragonimiasis m ...
'', the oriental lung fluke. 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.
* Intestinal flukes inhabit the epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
of the small intestine
The small intestine or small bowel is an organ in the gastrointestinal tract where most of the absorption of nutrients from food takes place. It lies between the stomach and large intestine, and receives bile and pancreatic juice through the p ...
. These include ''Fasciolopsis buski
''Fasciolopsis'' () is a genus of trematodes. They are also known as giant intestinal flukes.
Only one species is recognised: ''Fasciolopsis buski''. It is a notable parasite of medical importance in humans and veterinary importance in pigs. I ...
'' (which causes fasciolopsiasis
Fasciolopsiasis results from an infection by the trematode ''Fasciolopsis buski'', the largest intestinal fluke of humans (up to 7.5 cm in length).
Signs and symptoms
Most infections are light, almost asymptomatic. In heavy infections, sympto ...
), '' Metagonimus miyatai'', '' Metagonimus takahashii'', ''Metagonimus yokogawai
''Metagonimus yokogawai'' is a species of a trematode, or fluke worm, in the family Heterophyidae.
It is a human parasite causing metagonimiasis. It is among a few species of '' Metagonimus'' that cause metagonimiasis diseases (others being '' ...
'' (which cause metagonimiasis
Metagonimiasis is a disease caused by an intestinal trematode, most commonly ''Metagonimus yokagawai'', but sometimes by ''M. takashii'' or ''M. miyatai''. The metagonimiasis-causing flukes are one of two minute flukes called the heterophyids. Met ...
), and '' Heterophyes heterophyes'' and '' Heterophyes nocens'' (which cause heterophyiasis).
Anatomy
Trematodes are flattened oval or worm-like animals, usually no more than a few centimeters in length, although species as small as are known. Their most distinctive external feature is the presence of two suckers, one close to the mouth, and the other on the underside of the animal.
The body surface of trematodes comprises a tough syncytial
A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
tegument, which helps protect against digestive enzymes
Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of anim ...
in those species that inhabit the gut of larger animals. It is also the surface of gas exchange; there are no respiratory organs
The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies gre ...
.[
The mouth is located at the forward end of the animal, and opens into a muscular, pumping ]pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
. The pharynx connects, via a short oesophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
, to one or two blind-ending caeca, which occupy most of the length of the body. In some species, the caeca are themselves branched. As in other flatworms, there is no anus
The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...
, and waste material must be egested through the mouth.[
Although the excretion of ]nitrogenous waste
Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, w ...
occurs mostly through the tegument, trematodes do possess an excretory system
The excretory system is a passive biological system that removes excess, unnecessary materials from the body fluids of an organism, so as to help maintain internal chemical homeostasis and prevent damage to the body. The dual function of excreto ...
, which is instead mainly concerned with osmoregulation
Osmoregulation is the active regulation of the osmotic pressure of an organism's body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism's water content; that is, it maintains the fluid balance and the concentration o ...
. This consists of two or more protonephridia
The nephridium (plural ''nephridia'') is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body. Neph ...
, with those on each side of the body opening into a collecting duct. The two collecting ducts typically meet up at a single bladder
The urinary bladder, or simply bladder, is a hollow organ in humans and other vertebrates that stores urine from the kidneys before disposal by urination. In humans the bladder is a distensible organ that sits on the pelvic floor. Urine enters ...
, opening to the exterior through one or two pores near the posterior end of the animal.[
The ]brain
A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...
consists of a pair of ganglia
A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...
in the head region, from which two or three pairs of nerve 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 the Aspidogastrea
The Aspidogastrea (Ancient Greek: ' “shield”, ' “stomach/pouch”) is a small group of Trematoda, flukes comprising about 80 species. It is a Subclass (biology), subclass of the Trematode, trematoda, and sister group to the Digenea. Specie ...
. Trematodes generally lack any specialized sense organs
A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system rec ...
, although some ectoparasitic
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
species do possess one or two pairs of simple ocelli
A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
.[
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.]
Oral sucker and acetabulum: In some species of Trematoda, such as T. bragai, there is an acetabulum
The acetabulum (), also called the cotyloid cavity, is a concave surface of the pelvis. The head of the femur meets with the pelvis at the acetabulum, forming the hip joint.
Structure
There are three bones of the ''os coxae'' (hip bone) that c ...
. 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. Together, the mouth, pharynx, and esophagus form the foregut in Trematodes.
Reproductive system of blood flukes
Most trematodes are hermaphrodite
In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes.
Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...
s. Blood flukes (''Schistosoma'') are the only form of trematodes that are dioecious
Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...
(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
The hepatopancreas, digestive gland or midgut gland is an organ of the digestive tract of arthropods and molluscs. It provides the functions which in mammals are provided separately by the liver and pancreas, including the production of digestive ...
of a freshwater snail
Freshwater snails are gastropod mollusks which live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs ...
, 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 testes
A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...
, though some species may have more. The testes are located posterior and dorsal to the ventral sucker. Spermatogenesis
Spermatogenesis is the process by which haploid spermatozoa develop from germ cells in the seminiferous tubules of the testis. This process starts with the mitotic division of the stem cells located close to the basement membrane of the tubule ...
produces biflagellate sperm (sperm with two tails). Sperm is stored in the seminal vesicles
The seminal vesicles (also called vesicular glands, or seminal glands) are a pair of two convoluted tubular glands that lie behind the urinary bladder of some male mammals. They secrete fluid that partly composes the semen.
The vesicles are 5 ...
, which are connected to the testes by the vas deferens
The vas deferens or ductus deferens is part of the male reproductive system of many vertebrates. The ducts transport sperm from the epididymis to the ejaculatory ducts in anticipation of ejaculation. The vas deferens is a partially coiled tube ...
. 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
The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
connected to an elongated uterus
The uterus (from Latin ''uterus'', plural ''uteri'') or womb () is the organ in the reproductive system of most female mammals, including humans that accommodates the embryonic and fetal development of one or more embryos until birth. The uter ...
by a ciliated
The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
oviduct
The oviduct in mammals, is the passageway from an ovary. In human females this is more usually known as the Fallopian tube or uterine tube. The eggs travel along the oviduct. These eggs will either be fertilized by spermatozoa to become a zygote, o ...
. 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
An oocyte (, ), oöcyte, or ovocyte is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in a female fetus in the ovary during female gametogenesis. The female ...
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 produce yolk. After the egg is surrounded by yolk, its shell is formed from the secretions of '' 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''.
Life cycles
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
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are recognized. The number of fossil sp ...
as the first host in the life cycle, and most have a complex life cycle involving other hosts. Most trematodes are monoecious
Monoecy (; adj. monoecious ) is a sexual system in seed plants where separate male and female cones or flowers are present on the same plant. It is a monomorphic sexual system alongside gynomonoecy, andromonoecy and trimonoecy.
Monoecy is conne ...
and alternately reproduce sexually and asexually. The two main exceptions to this are the Aspidogastrea
The Aspidogastrea (Ancient Greek: ' “shield”, ' “stomach/pouch”) is a small group of Trematoda, flukes comprising about 80 species. It is a Subclass (biology), subclass of the Trematode, trematoda, and sister group to the Digenea. Specie ...
, which have no asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
, and the schistosomes
''Schistosoma'' is a genus of trematodes, commonly known as blood flukes. They are parasitic flatworms responsible for a highly significant group of infections in humans termed ''schistosomiasis'', which is considered by the World Health Organi ...
, which are dioecious
Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...
.
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
A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle.
The ...
l forms (Miracidia) that are infective to the intermediate host, in which asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...
occurs.
A species that exemplifies the remarkable life history of the trematodes is the bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
fluke, ''Leucochloridium paradoxum
''Leucochloridium paradoxum'', the green-banded broodsac, is a parasitic flatworm (or helminth). Its intermediate hosts are land snails, usually of the genus ''Succinea''. The pulsating, green broodsacs fill the eye stalks of the snail, thereby ...
''. The definitive hosts, in which the parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
reproduces, are various woodland bird
Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s, while the hosts in which the parasite
Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
multiplies (intermediate host) are various species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of snail
A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
. 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
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing sec ...
). These larvae grow and take on a sac-like appearance. This stage is known as the sporocyst 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 into cercaria
A cercaria (plural cercariae) is the larval form of the trematode class of parasites. It develops within the germinal cells of the sporocyst or redia. A cercaria has a tapering head with large penetration glands. It may or may not have a long sw ...
.
Life cycle adaptations
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 the miracidium
The Miracidium is the second stage in the life cycle of trematodes. When trematode eggs are laid and come into contact with fresh water, they hatch and release miracidium. In this phase, miracidia are ciliated and free-swimming. This stage is comp ...
. 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.
#The sporocyst forms inside the snail first intermediate host and feeds through diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
across the tegument.
#The rediae also forms inside the snail first intermediate host and feeds through a developed pharynx
The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its struc ...
. Either the rediae or the sporocyst develops into the cercariae through polyembryony Polyembryony is the phenomenon of two or more embryos developing from a single fertilized egg. Due to the embryos resulting from the same egg, the embryos are identical to one another, but are genetically diverse from the parents. The genetic differ ...
in the snail.
#The cercariae
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
are 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.
#The metacercariae
Trematodes are parasitic flatworms of the class ''Trematoda'', specifically parasitic flukes with two suckers: one ventral and the other oral. Trematodes are covered by a tegument, that protects the organism from the environment by providing secr ...
are 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 are going to evolve 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 on aquatic plants. Humans as well as larger sea creatures then get infected when they eat these plants.
When they infect humans, it takes 3–4 months for the metacercariae to mature into adult flukes and lay eggs.
Life cycle of 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 are passed from human to human 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, humans ingest the metacercariae in raw and undercooked fish. In humans, the metacercariae complete their life cycle and become full grown liver flukes.
Infections
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.
Human trematode infections are most common in Asia, Africa and Latin America. However, trematodes can be found anywhere where untreated human waste is used as fertilizer
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
. Humans can be infected by trematodes by immersion in or ingestion of contaminated water, or by consuming raw or undercooked contaminated animals or plants.
Albendazole
Albendazole (also known as albendazolum) is a broad-spectrum anthelmintic and antiprotozoal agent of the benzimidazole type. It is used for the treatment of a variety of intestinal parasite infections, including ascariasis, pinworm infection, h ...
can be used to treat clonorchiasis
Clonorchiasis is an infectious disease caused by the Chinese liver fluke (''Clonorchis sinensis'') and two related species.
Clonorchiasis is a known risk factor for the development of cholangiocarcinoma, a neoplasm of the biliary system.
Sympt ...
and opisthorchiasis
Opisthorchiasis is a parasitic disease caused by certain species of genus '' Opisthorchis'' (specifically, '' Opisthorchis viverrini'' and '' Opisthorchis felineus''). Chronic infection may lead to cholangiocarcinoma, a cancer of the bile ducts.
...
. Triclabendazole
Triclabendazole, sold under the brand name Egaten among others, is a medication used to treat fascioliasis and paragonimiasis. It is very effective for both conditions. Treatment in hospital may be required. It is taken by mouth with typically ...
is often used to treat fasciolosis
Fasciolosis is a parasitic worm infection caused by the common liver fluke ''Fasciola hepatica'' as well as by ''Fasciola gigantica''. The disease is a plant-borne trematode zoonosis, and is classified as a neglected tropical disease (NTD). It ...
, and may also be useful in the treatment of paragonimiasis
Paragonimiasis is a food-borne parasitic disease caused by several species of lung flukes belonging to genus ''Paragonimus''. Infection is acquired by eating crustaceans such as crabs and crayfishes which host the infective forms called metacerca ...
and dicrocoeliasis
''Dicrocoelium dendriticum'', the lancet liver fluke, is a parasite fluke that tends to live in cattle or other grazing mammals.
History of discovery
Much of what is presently known about ''Dicrocoelium dendriticum'' is the result of the wor ...
. Praziquantel
Praziquantel (PZQ), sold under the brandname Biltricide among others, is a medication used to treat a number of types of helminthiasis, parasitic worm infections in mammals, birds, amphibians, reptiles, and fish. In humans specifically, it is us ...
is effective in the treatment of all diseases caused by flukes (clonorchiasis, dicrocoeliasis, echinostomiasis, fasciolopsiasis
Fasciolopsiasis results from an infection by the trematode ''Fasciolopsis buski'', the largest intestinal fluke of humans (up to 7.5 cm in length).
Signs and symptoms
Most infections are light, almost asymptomatic. In heavy infections, sympto ...
, fasciolosis, gastrodiscoidiasis, heterophyiasis, metagonimiasis
Metagonimiasis is a disease caused by an intestinal trematode, most commonly ''Metagonimus yokagawai'', but sometimes by ''M. takashii'' or ''M. miyatai''. The metagonimiasis-causing flukes are one of two minute flukes called the heterophyids. Met ...
, opisthorchiasis, paragonimiasis, and schistosomiasis
Schistosomiasis, also known as snail fever, bilharzia, and Katayama fever, is a disease caused by parasitic flatworms called schistosomes. The urinary tract or the intestines may be infected. Symptoms include abdominal pain, diarrhea, bloody s ...
).
References
External links
Earthlife
BioLib
ZipcodeZoo
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Protostome classes
Taxa named by Karl Rudolphi