Digenea
Encyclopedia
Digenea is a subclass within the Platyhelminthes consisting of parasitic flatworm
s with a syncytial
tegument
and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea
, it is now recognised that they are closest to the Aspidogastrea
and that the Monogenea are more closely allied with the Cestoda
. Around 6,000 species have been described to date.
; that is, a tegument where the junctions between cells are broken down and a single continuous cytoplasm surrounds the entire animal. A similar tegument is found in other members of the Neodermata; a group of platyhelminths comprising the Digenea, Aspidogastrea
, Monogenea
and Cestoda
. Digeneans possess a vermiform, unsegmented body-plan and have a solid parenchyma
with no body cavity, as in all platyhelminths.
There are typically two suckers, an anterior oral sucker surrounding the mouth
, and a ventral sucker sometimes termed the acetabulum, on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure.
Monostome is a term used to describe worms with one sucker (oral). Fluke
s with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes. Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker if somewhere other than posterior. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy.
. This is likely to be an adaptation to low abundance within hosts, allowing the life cycle to continue when only one individual successfully infects the final host. Fertilisation
is internal, with sperm
being transferred via the cirrus
to the Laurer's Canal
or genital aperture. A key group of digeneans which are dioecious
are the schistosomes
.
Asexual reproduction in the first larval stage is ubiquitous.
While the sexual formation of the digenean eggs
and asexual reproduction in the first larva
l stage (miracidium) is widely reported, the developmental biology
of the asexual stages remains a problem. Electron microscopic
studies have shown that the light microscopically
visible germ balls consist of mitotically
dividing cell
s which give rise to embryo
s and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes
is not proven, the exact definition remains doubtful.
is the general rule among the Digenea. Usually two testes are present, but some flukes can have more than 100. Also present are vasa efferentia, a vas deferens
, seminal vesicle
, ejaculatory duct
and a cirrus
(analogous to a penis) usually (but not always) enclosed in a cirrus sac. The cirrus may or may not be covered in proteinaceous spines. The exact conformation of these organs within the male terminal genitalia is taxonomically important at the familial and generic levels.
with an oviduct
, a seminal receptacle
, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlis’ gland, which is believed to lubricate the uterus for egg passage.
In addition, some digeneans possess a canal called Laurer's Canal
, which leads from the oviduct to the dorsal surface of the body. The function of this canal is debated, but it may be used for insemination in some species or for disposal of waste products from reproduction in other species.
Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down the uterus.
The uterus typically opens into a common genital atrium that also received the distal male copulatory organ (cirrus) before immediately opening onto the outer surface of the worm. The distal part of the uterus may be expanded into a metraterm, set off from the proximal uterus by a muscular sphincter, or it may be lined with spines, as in the Monorchiidae and some other families.
es, or with the excretory vesicle to form a uroproct. Digeneans are also capable of direct nutrient uptake through the tegument by pinocytosis
and phagocytosis by the syncitium. Most adult digeneans occur in the vertebrate alimentary canal or its associated organs
, where they most often graze on contents of the lumen (e.g., food ingested by the host, bile, mucus), but they may also feed across the mucosal wall (e.g., submucosa
, host blood). The blood flukes, such as schistosomes, spirorchiids and sanguinicolids, feed exclusively on blood. Asexual stages in mollusc intermediate hosts feed mostly by direct absorption, although the redia
stage found in some groups does have a mouth, pharynx and simple gut and may actively consume host tissue or even other parasites. Encysted metacercarial stages and free-living cercarial stages do not feed.
. From this nerves extend anteriorly and posteriorly. Sensory receptors
are, for the most part, lacking among the adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larva recognising and locating its host.
hosts in between. The three-host life cycle is probably the most common. In almost all species, the first host in the life cycle is a mollusc. This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently.
The alternation of sexual and asexual generations is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle.
A typical digenean trematode life cycle is as follows. Eggs leave the vertebrate
host in faeces and use various strategies to infect the first intermediate host
, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a snail
) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid
worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium
, which must locate and penetrate the body wall
of the snail host.
After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst. The mother sporocyst undergoes a round of internal asexual reproduction
, giving rise to either rediae (sing. redia) or daughter sporocysts. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl. cercariae).
Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish
consumes a cercaria) or actively (the cercaria penetrates the fish).
The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host
. Two host life-cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost).
In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the metacercaria, which is usually encysted in a cyst
of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host
. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host’s gut in response to a variety of physical and chemical signals, such as gut pH
levels, digestive enzyme
s, temperature
, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.
ns that colonised the open mantle cavity of early molluscs.
It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by until the modern bewildering diversity of life cycle patterns developed.
There are some digenean families that are either basal to or incertae sedis
among the orders listed above:
to humans, but some of these species are important disease
s afflicting over 200 million people. The species that infect humans can be divided into groups, the schistosomes and the non-schistosomes.
of the definitive host. Humans become infected after free-swimming cercaria liberated from infected snails penetrate the skin. These dioecious worms are long and thin, ranging in size from 10 to 30 mm in length to 0.2 to 1.0 mm in diameter. Adult males are shorter and thicker than females, and have a long groove along one side of the body in which the female is clasped. Females reach sexual maturity after they have been united with a male. After mating the two remain locked together for the rest of their lives. They can live for several years and produce many thousands of eggs.
The four species of schistosomes that infect humans are members of the genus
Schistosoma
.
. These flukes generally cause mild pathology in humans, but more serious effects may also occur.
Flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s with a syncytial
Syncytium
In biology, a syncytium is a large cell-like structure; filled with cytoplasm and containing many nuclei. Most cells in eukaryotic organisms have a single nucleus; syncytia are specialized forms used by various organisms.The term may also refer to cells that are connected by specialized membrane...
tegument
Tegument
Tegument may refer to:* Integumentary system* Tegument * Viral tegument...
and, usually, two suckers, one ventral and one oral. Adults are particularly common in the digestive tract, but occur throughout the organ systems of all classes of vertebrates. Once thought to be related to the Monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
, it is now recognised that they are closest to the Aspidogastrea
Aspidogastrea
The Aspidogastrea is a small group of flukes comprising about 80 species. It is a subclass of the trematoda, and sister group to the Digenea. Species range in length from approximately one millimeter to several centimeters...
and that the Monogenea are more closely allied with the Cestoda
Cestoda
This article describes the flatworm. For the medical condition, see Tapeworm infection.Cestoda is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies...
. Around 6,000 species have been described to date.
Key features
Characteristic features of the digenea include a syncitial tegumentTegument
Tegument may refer to:* Integumentary system* Tegument * Viral tegument...
; that is, a tegument where the junctions between cells are broken down and a single continuous cytoplasm surrounds the entire animal. A similar tegument is found in other members of the Neodermata; a group of platyhelminths comprising the Digenea, Aspidogastrea
Aspidogastrea
The Aspidogastrea is a small group of flukes comprising about 80 species. It is a subclass of the trematoda, and sister group to the Digenea. Species range in length from approximately one millimeter to several centimeters...
, Monogenea
Monogenea
Monogenea are a group of largely ectoparasitic members of the flatworm phylum Platyhelminthes, class Monogenea.-Characteristics:Monogenea are very small parasitic flatworms mainly found on skin or gills of fish....
and Cestoda
Cestoda
This article describes the flatworm. For the medical condition, see Tapeworm infection.Cestoda is the name given to a class of parasitic flatworms, commonly called tapeworms, of the phylum Platyhelminthes. Its members live in the digestive tract of vertebrates as adults, and often in the bodies...
. Digeneans possess a vermiform, unsegmented body-plan and have a solid parenchyma
Parenchyma
Parenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
with no body cavity, as in all platyhelminths.
There are typically two suckers, an anterior oral sucker surrounding the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
, and a ventral sucker sometimes termed the acetabulum, on the ventral surface. The oral sucker surrounds the mouth, while the ventral sucker is a blind muscular organ with no connection to any internal structure.
Monostome is a term used to describe worms with one sucker (oral). Fluke
Fluke
Fluke may refer to:* Unlikely, usually positive chance occurrence, esp. a surprising piece of luck.* Fluke, a type of flatfish alternatively called a Summer flounder* Fluke, an alternative name for a whale tail...
s with an oral sucker and an acetabulum at the posterior end of the body are called Amphistomes. Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker if somewhere other than posterior. These terms are common in older literature, when they were thought to reflect systematic relationships within the groups. They have fallen out of use in modern digenean taxonomy.
Reproductive system
The vast majority of digeneans is hermaphroditicHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
. This is likely to be an adaptation to low abundance within hosts, allowing the life cycle to continue when only one individual successfully infects the final host. Fertilisation
Fertilisation
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...
is internal, with sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
being transferred via the cirrus
Cirrus
Cirrus may refer to:In science:*Cirrus cloud, a type of cloud*Cirrus , a German research rocket*Cirrus, a fleshy, downward extension of the upper lip in salamanders or fish*Cirrus, a thoracic limb of an adult barnacle...
to the Laurer's Canal
Laurer's canal
Laurer's canal is a part of the reproductive system of trematodes, analogous to the vagina. In the Digeneans it opens from the dorsal surface of the worm, and in the Aspidogastreans ends in a blind ended sac...
or genital aperture. A key group of digeneans which are dioecious
Dioecious
Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...
are the schistosomes
Schistosoma
A genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and are considered by the World Health Organization as the second most...
.
Asexual reproduction in the first larval stage is ubiquitous.
While the sexual formation of the digenean eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
and asexual reproduction in the first larva
Larva
A larva 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...
l stage (miracidium) is widely reported, the developmental biology
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
of the asexual stages remains a problem. Electron microscopic
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
studies have shown that the light microscopically
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
visible germ balls consist of mitotically
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
dividing cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s which give rise to embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s and to a line of new germ cells that become included in these embryonic stages. Since the absence of meiotic processes
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
is not proven, the exact definition remains doubtful.
Male organs
ProtandryHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
is the general rule among the Digenea. Usually two testes are present, but some flukes can have more than 100. Also present are vasa efferentia, a vas deferens
Vas deferens
The vas deferens , also called ductus deferens, , is part of the male anatomy of many vertebrates; they transport sperm from the epididymis in anticipation of ejaculation....
, seminal vesicle
Seminal vesicle
The seminal vesicles or vesicular glands are a pair of simple tubular glands posteroinferior to the urinary bladder of male mammals...
, ejaculatory duct
Ejaculatory duct
-Anatomy:The ejaculatory ducts are paired structures in male anatomy. Each ejaculatory duct is formed by the union of the vas deferens with the duct of the seminal vesicle. They pass through the prostate, and open into the urethra at the Colliculus seminalis...
and a cirrus
Cirrus
Cirrus may refer to:In science:*Cirrus cloud, a type of cloud*Cirrus , a German research rocket*Cirrus, a fleshy, downward extension of the upper lip in salamanders or fish*Cirrus, a thoracic limb of an adult barnacle...
(analogous to a penis) usually (but not always) enclosed in a cirrus sac. The cirrus may or may not be covered in proteinaceous spines. The exact conformation of these organs within the male terminal genitalia is taxonomically important at the familial and generic levels.
Female organs
Usually there is a single ovaryOvary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...
with an oviduct
Oviduct
In non-mammalian vertebrates, the passageway from the ovaries to the outside of the body is known as the oviduct. The eggs travel along the oviduct. These eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body...
, a seminal receptacle
Semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...
, a pair of vitelline glands (involved in yolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells called Mehlis’ gland, which is believed to lubricate the uterus for egg passage.
In addition, some digeneans possess a canal called Laurer's Canal
Laurer's canal
Laurer's canal is a part of the reproductive system of trematodes, analogous to the vagina. In the Digeneans it opens from the dorsal surface of the worm, and in the Aspidogastreans ends in a blind ended sac...
, which leads from the oviduct to the dorsal surface of the body. The function of this canal is debated, but it may be used for insemination in some species or for disposal of waste products from reproduction in other species.
Most trematodes possess an ovicapt, an enlarged portion of the oviduct where it joins the ovary. It probably controls the release of ova and spaces out their descent down the uterus.
The uterus typically opens into a common genital atrium that also received the distal male copulatory organ (cirrus) before immediately opening onto the outer surface of the worm. The distal part of the uterus may be expanded into a metraterm, set off from the proximal uterus by a muscular sphincter, or it may be lined with spines, as in the Monorchiidae and some other families.
Digestive system
As adults, most digeneans possess a terminal or subterminal mouth, a muscular pharynx that provides the force for ingesting food, and a forked, blind digestive system consisting of two tubular sacs called caeca (sing. caecum). In some species the two gut caeca join posteriorly to make a ring-shaped gut or cyclocoel. In others the caeca may fuse with the body wall posteriorly to make one or more anusAnus
The anus 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, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
es, or with the excretory vesicle to form a uroproct. Digeneans are also capable of direct nutrient uptake through the tegument by pinocytosis
Pinocytosis
In cellular biology, pinocytosis is a form of endocytosis in which small particles are brought into the cell—forming an invagination, and then suspended within small vesicles that subsequently fuse with lysosomes to hydrolyze, or to break down, the particles...
and phagocytosis by the syncitium. Most adult digeneans occur in the vertebrate alimentary canal or its associated organs
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
, where they most often graze on contents of the lumen (e.g., food ingested by the host, bile, mucus), but they may also feed across the mucosal wall (e.g., submucosa
Submucosa
In the gastrointestinal tract, the submucosa is the layer of dense irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle .-Contents:Blood vessels, lymphatic vessels, and nerves will run through...
, host blood). The blood flukes, such as schistosomes, spirorchiids and sanguinicolids, feed exclusively on blood. Asexual stages in mollusc intermediate hosts feed mostly by direct absorption, although the redia
Redia
Redia or redia can be:-*A synonym for the plant genus Cleidion of the family Euphorbiaceae*A stage in the development of a trematode*A dipstick for urinalysis, determining levels of glucose, albumin and erythrocytes...
stage found in some groups does have a mouth, pharynx and simple gut and may actively consume host tissue or even other parasites. Encysted metacercarial stages and free-living cercarial stages do not feed.
Nervous system
Paired ganglia at the anterior end of the body serve as the brainBrain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
. From this nerves extend anteriorly and posteriorly. Sensory receptors
Sense
Senses are physiological capacities of organisms that provide inputs for perception. The senses and their operation, classification, and theory are overlapping topics studied by a variety of fields, most notably neuroscience, cognitive psychology , and philosophy of perception...
are, for the most part, lacking among the adults, although they do have tangoreceptor cells. Larval stages have many kinds of sensory receptors, including light receptors and chemoreceptors. Chemoreception plays an important role in the free-living miracidial larva recognising and locating its host.
Life cycles
There is a bewildering array of variation on the complex digenean life cycle, and plasticity in this trait is probably a key to the group's success. In general, the life cycles may have two, three, or four obligate (necessary) hosts, sometimes with transport or paratenicParatenic
In parasitology, the term paratenic describes a host which is not needed for the development of the parasite, but nonetheless serves to maintain the life cycle of the parasite. Alaria americana may serve as an example: the so-called mesocercarial stages of this parasite reside in tadpoles, which...
hosts in between. The three-host life cycle is probably the most common. In almost all species, the first host in the life cycle is a mollusc. This has led to the inference that the ancestral digenean was a mollusc parasite and that vertebrate hosts were added subsequently.
The alternation of sexual and asexual generations is an important feature of digeneans. This phenomenon involves the presence of several discrete generations in one life-cycle.
A typical digenean trematode life cycle is as follows. Eggs leave the vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
host in faeces and use various strategies to infect the first intermediate host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually a snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by an annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worm), in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, the miracidium
Miracidium
Trematodes are small parasitic flatworms that use vertebrates as their definitive host, and molluscs as their intermediate host. In order to accomplish this, they have several varied lifecyle stages....
, which must locate and penetrate the body wall
Body
With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...
of the snail host.
After post-ingestion hatching or penetration of the snail, the miracidium metamorphoses into a simple, sac-like mother sporocyst. The mother sporocyst undergoes a round of internal asexual reproduction
Asexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
, giving rise to either rediae (sing. redia) or daughter sporocysts. The second generation is thus the daughter parthenita sequence. These in turn undergo further asexual reproduction, ultimately yielding large numbers of the second free-living stage, the cercaria (pl. cercariae).
Free-swimming cercariae leave the snail host and move through the aquatic or marine environment, often using a whip-like tail, though a tremendous diversity of tail morphology is seen. Cercariae are infective to the second host in the life cycle, and infection may occur passively (e.g., a fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
consumes a cercaria) or actively (the cercaria penetrates the fish).
The life cycles of some digeneans include only two hosts, the second being a vertebrate. In these groups, sexual maturity occurs after the cercaria penetrates the second host, which is in this case also the definitive host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
. Two host life-cycles can be primary (there never was a third host) as in the Bivesiculidae, or secondary (there was at one time in evolutionary history a third host but it has been lost).
In three-host life cycles, cercariae develop in the second intermediate host into a resting stage, the metacercaria, which is usually encysted in a cyst
Cyst
A cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...
of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to the definitive host
Parasitic life cycles
Parasite life cycles can take a variety of forms, all involving the exploitation of one or more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex or indirect life cycles, while those that infect a single species have direct life...
. Transmission occurs when the definitive host preys upon an infected second intermediate host. Metacercariae excyst in the definitive host’s gut in response to a variety of physical and chemical signals, such as gut pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
levels, digestive enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s, temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.
Evolution
The evolutionary origins of the Digenea have been debated for some time, but there appears general agreement that the proto-digenean was a parasite of a mollusc, possibly of the mantle cavity. Evidence for this comes from the ubiquity of molluscs as first intermediate hosts for digeneans, and the fact that most aspidogastreans (the sister group to the Digenea) also have mollusc associations. It is thought that the early trematodes (the collective name for digeneans and aspidogastreans) likely evolved from rhabdocoel turbellariaTurbellaria
The Turbellaria are one of the traditional sub-divisions of the phylum Platyhelminthes , and include all the sub-groups that are not exclusively parasitic. There are about 4,500 species, which range from to in length...
ns that colonised the open mantle cavity of early molluscs.
It is likely that more complex life cycles evolved through a process of terminal addition, whereby digeneans survived predation of their mollusc host, probably by a fish. Other hosts were added by until the modern bewildering diversity of life cycle patterns developed.
Important families
The Digenea includes at least 50 families. Below is a list of the more commonly encountered ones.| Acanthocolpidae | Derogenidae | Haplosplanchnidae | Paramphistomidae |
| Accacoeliidae | Dicrocoeliidae | Hemiuridae | Paragonimidae |
| Allocreadiidae | Didymozoidae | Heronomidae | Philophthalmidae |
| Angiodictyidae | Diplostomidae | Heterophyidae Heterophyidae Heterophyidae is a family of intestinal flukes of the class trematoda.Tegument covered by spines. Oral sucker not armed or armed by cyrcumoral spines. Pharynx presented. Genital synus presented. Ventral and genital suckers ussualy not combined. Cirrus and bursa absent. Two testes located in... |
Plagiorchiidae |
| Apocreadiidae | Echinostomatidae | Lecithasteridae | Sanguinicolidae |
| Atractotrematidae | Enenteridae | Lepocreadiidae | Schistosomatidae Schistosomatidae Schistosomatidae is a family of digenetic trematodes with complex parasitic life cycles. Immature developmental stages of schistosomes are found in molluscs and adults occur in vertebrates. The best studied group, the blood flukes of the genus Schistosoma, infect and cause disease in humans... |
| Azygiidae | Fasciolidae Fasciolidae Fasciolidae is a family of trematodes and includes several veterinary and medical important parasites. Family Fasciolidae is divided into 5 genera. Flukes of the family are localized in liver, gall bladder, and the intestine... |
Microphallidae | Sclerodistomidae |
| Bivesiculidae | Faustulidae | Monorchiidae | Strigeidae |
| Bucephalidae Bucephalidae Members of the family Bucephalidae are trematode flatworms with no oral sucker, using instead a rhynchus as an adhesive organ at their anterior end. Many are common parasites of freshwater fish.... |
Fellodstomidae | Nasitrematidae | Syncoeliidae |
| Campulidae | Gorgoderidae | Notocotylidae | Tandanicolidae |
| Cryptogonimidae | Gyliauchenidae | Opecoelidae | Transversotrematidae |
| Cyclocoelidae Cyclocoelidae The Cyclocoelidae are a family of parasitic flatworms. They are placed in the monotypic suborder Cyclocoelata of the order Echinostomida.... |
Haploporidae | Opisthorchiidae Opisthorchiidae Opisthorchiidae is a family of digeneans, parasitic flatworms.The most medically important species in the family Opisthorchiidae are Clonorchis sinensis, Opisthorchis viverrini and Opisthorchis felineus, that are causing disease clonorchiasis.... |
Zoogonidae Zoogonidae The Zoogonidae are a family of parasitic flatworms. They are the namesake family of the suborder Zoogonata in the order Plagiorchiida.Selected genera are:* Brevicreadium* Deretrema Linton, 1910* Diphtherostomum Stossich, 1904... |
There are some digenean families that are either basal to or incertae sedis
Incertae sedis
, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...
among the orders listed above:
- Acanthocollaritrematidae
- Echinoporidae
- Gekkonotrematidae
- Gyliauchenidae
- Jubilariidae
- Meristocotylidae
- Mesotretidae
Human digenean infections
Only about 12 of the 6,000 known species are infectiousInfection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
to humans, but some of these species are important disease
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
s afflicting over 200 million people. The species that infect humans can be divided into groups, the schistosomes and the non-schistosomes.
Schistosomes
The Schistosomes occur in the circulatory systemCirculatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
of the definitive host. Humans become infected after free-swimming cercaria liberated from infected snails penetrate the skin. These dioecious worms are long and thin, ranging in size from 10 to 30 mm in length to 0.2 to 1.0 mm in diameter. Adult males are shorter and thicker than females, and have a long groove along one side of the body in which the female is clasped. Females reach sexual maturity after they have been united with a male. After mating the two remain locked together for the rest of their lives. They can live for several years and produce many thousands of eggs.
The four species of schistosomes that infect humans are members of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Schistosoma
Schistosoma
A genus of trematodes, Schistosoma, commonly known as blood-flukes and bilharzia, includes flatworms which are responsible for a highly significant parasitic infection of humans by causing the disease schistosomiasis, and are considered by the World Health Organization as the second most...
.
| Scientific Name | First Intermediate Host | Endemic Area |
|---|---|---|
| Schistosoma mansoni Schistosoma mansoni Schistosoma mansoni is a significant parasite of humans, a trematode that is one of the major agents of the disease schistosomiasis. The schistosomiasis caused by Schistosoma mansoni is intestinal schistosomiasis.... |
Biomphalaria Biomphalaria Biomphalaria is a genus of air-breathing freshwater snails, aquatic pulmonate gastropod mollusks in the family Planorbidae, the ram's horn snails and their allies.Biomphalaria is the type genus of the tribe Biomphalariini.... spp. |
Africa Africa Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area... , South America South America South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east... , Caribbean Caribbean The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north... , Middle East Middle East The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East... |
| Schistosoma haematobium Schistosoma haematobium Schistosoma haematobium is an important digenetic trematode, and is found in the Middle East, India, Portugal and Africa. It is a major agent of schistosomiasis; more specifically, it is associated with urinary schistosomiasis.... |
Bulinus Bulinus Bulinus is a genus of small tropical freshwater snails, aquatic gastropod mollusks in the family Planorbidae, the ramshorn snails and their allies.... spp. |
Africa Africa Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area... , Middle East Middle East The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East... |
| Schistosoma japonicum Schistosoma japonicum Schistosoma japonicum is an important parasite and one of the major infectious agents of schistosomiasis.This parasite has a very wide host range, infecting at least 31 species of wild mammals, including 9 carnivores, 16 rodents, one primate , two insectivores and three artiodactyls and therefore... |
Oncomelania Oncomelania Oncomelania is a genus of very small tropical freshwater snails, aquatic gastropod mollusks in the family Pomatiopsidae.These Oncomelania snails are distantly related to the marine periwinkle, and more closely related to the small marine snails of the family Rissoidae.-Species:There are two species... spp. |
China China Chinese civilization may refer to:* China for more general discussion of the country.* Chinese culture* Greater China, the transnational community of ethnic Chinese.* History of China* Sinosphere, the area historically affected by Chinese culture... , East Asia East Asia East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms... , Philippines Philippines The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam... |
| Schistosoma intercalatum Schistosoma intercalatum Schistosoma intercalatum is an important parasite. It is one of the major agents of schistosomiasis.A freshwater snail Bulinus forskalii is an intermediate host for Schistosoma intercalatum.... |
Bulinus Bulinus Bulinus is a genus of small tropical freshwater snails, aquatic gastropod mollusks in the family Planorbidae, the ramshorn snails and their allies.... spp. |
Africa Africa Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area... |
Non-schistosomes
There seven major species of non-schistosomes that infect humans are listed below. People become infected after ingesting metacercarial cysts on plants or in undercooked animal flesh. Most species inhabit the human gastrontestinal tract, where they shed eggs along with host feces. Paragonimus westermani, which colonizes the lungs, can also pass its eggs in salivaSaliva
Saliva , referred to in various contexts as spit, spittle, drivel, drool, or slobber, is the watery substance produced in the mouths of humans and most other animals. Saliva is a component of oral fluid. In mammals, saliva is produced in and secreted from the three pairs of major salivary glands,...
. These flukes generally cause mild pathology in humans, but more serious effects may also occur.
| Scientific Name | First Intermediate Host | Mode of Human Infection | Endemic Area |
|---|---|---|---|
| Fasciolopsis buski | Segmentina Segmentina Segmentina is a genus of small, air-breathing, freshwater snails, aquatic gastropod mollusks or micromollusks in the family Planorbidae, the ramshorn snails.... sp. |
Plants | Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... , India India India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world... |
| Heterophyes heterophyes Heterophyes heterophyes Heterophyes heterophyes is a human parasite.-Geography:Common in north Africa, Asia Minor, Korea, China, Japan, Taiwan, and the Philippines.-Morphology:... |
Pirinella | Mullet Mullet (fish) The mullets or grey mullets are a family and order of ray-finned fish found worldwide in coastal temperate and tropical waters, and in some species in fresh water. Mullets have served as an important source of food in Mediterranean Europe since Roman times... , Tilapia Tilapia Tilapia , is the common name for nearly a hundred species of cichlid fish from the tilapiine cichlid tribe. Tilapia inhabit a variety of fresh water habitats, including shallow streams, ponds, rivers and lakes. Historically, they have been of major importance in artisan fishing in Africa and the... |
Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... , Eastern Europe Eastern Europe Eastern Europe is the eastern part of Europe. The term has widely disparate geopolitical, geographical, cultural and socioeconomic readings, which makes it highly context-dependent and even volatile, and there are "almost as many definitions of Eastern Europe as there are scholars of the region"... , Egypt Egypt Egypt , officially the Arab Republic of Egypt, Arabic: , is a country mainly in North Africa, with the Sinai Peninsula forming a land bridge in Southwest Asia. Egypt is thus a transcontinental country, and a major power in Africa, the Mediterranean Basin, the Middle East and the Muslim world... , Middle East Middle East The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East... |
| Metagonimus yokogawaii | Semisulcospira Semisulcospira Semisulcospira is a genus of freshwater snails with an operculum, an aquatic gastropod mollusks in the family Semisulcospiridae.Species in the genus Semisulcospira are viviparous.- Species :Species withn the genus Semisulcospira include:... sp. |
Carp Carp Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain... , Trout Trout Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water... |
Siberia Siberia Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th... |
| Gastrodiscoides hominis | Helicorbis sp. | Plants | India India India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world... , Vietnam Vietnam Vietnam – sometimes spelled Viet Nam , officially the Socialist Republic of Vietnam – is the easternmost country on the Indochina Peninsula in Southeast Asia. It is bordered by China to the north, Laos to the northwest, Cambodia to the southwest, and the South China Sea –... , Philippines Philippines The Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam... |
| Clonorchis sinensis Clonorchis sinensis Clonorchis sinensis, the Chinese liver fluke, is a human liver fluke in the class Trematoda, Phylum Platyhelminthes. This parasite lives in the liver of humans, and is found mainly in the common bile duct and gall bladder, feeding on bile... |
Bulinus Bulinus Bulinus is a genus of small tropical freshwater snails, aquatic gastropod mollusks in the family Planorbidae, the ramshorn snails and their allies.... sp. |
Fish | East Asia East Asia East Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms... , North America North America North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas... |
| Fasciola hepatica Fasciola hepatica Fasciola hepatica, also known as the common liver fluke or sheep liver fluke, is a parasitic flatworm of the class Trematoda, phylum Platyhelminthes that infects the livers of various mammals, including humans. The disease caused by the fluke is called fascioliasis . F... |
Galba truncatula Galba truncatula Galba truncatula is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Lymnaeidae, the pond snails.Until recently, this species was commonly known as Lymnaea truncatula.-Description:... |
Plants | Central America Central America Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent... , North America North America North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas... , South America South America South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east... |
| Paragonimus westermani Paragonimus westermani Paragonimus westermani is a lung fluke and is most prominent in Asia and South America. It was discovered from two Bengal tigers that died in zoos in Europe in 1878... |
Oncomelania Oncomelania Oncomelania is a genus of very small tropical freshwater snails, aquatic gastropod mollusks in the family Pomatiopsidae.These Oncomelania snails are distantly related to the marine periwinkle, and more closely related to the small marine snails of the family Rissoidae.-Species:There are two species... sp. |
Crab Crab True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax... s, crayfish Crayfish Crayfish, crawfish, or crawdads – members of the superfamilies Astacoidea and Parastacoidea – are freshwater crustaceans resembling small lobsters, to which they are related... |
Asia Asia Asia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population... |
Further reading
- Olson P. D., Cribb T. H., Tkach V. V., Bray R. A. & Littlewood D. T. J. (2003). "Phylogeny and classification of the Digenea (Platyhelminthes: Trematoda)". International Journal for Parasitology 33(7): 733-755. doi:10.1016/S0020-7519(03)00049-3.