| Digenea | |
|---|---|
 | |
| Helicometra sp. (Plagiorchiida:Opecoelidae) from the intestine of aFlame Cardinal fish | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Subphylum: | Rhabditophora |
| Superclass: | Neodermata |
| Class: | Trematoda |
| Subclass: | Digenea Carus, 1863 |
| Families | |
Digenea (Gr.Dis – double,Genos – race) is aclass oftrematodes in thePlatyhelminthesphylum, consisting ofparasiticflatworms (known asflukes) with asyncytialtegument and, usually, twosuckers, one ventral and one oral. Adults commonly live within the digestive tract, but occur throughout the organ systems of all classes ofvertebrates. Once thought to be related to theMonogenea, it is now recognised that they are closest to theAspidogastrea and that the Monogenea are more closely allied with theCestoda. Around 6,000 species have been described to date.
Characteristic features of the Digenea include a syncytial 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 theNeodermata; a group ofplatyhelminths comprising the Digenea,Aspidogastrea,Monogenea andCestoda. Digeneans possess avermiform, unsegmented body-plan and have a solidparenchyma with nobody cavity (coelom) as in all platyhelminths.
There are typically twosuckers, ananteriororal sucker surrounding themouth, and aventral sucker sometimes termed theacetabulum, 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.
Amonostome is a worm with one sucker (oral).Flukes with an oral sucker and an acetabulum at the posterior end of the body are calledAmphistomes.Distomes are flukes with an oral sucker and a ventral sucker, but the ventral sucker is 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.
The vast majority of digeneans arehermaphrodites. 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, withsperm being transferred via thecirrus to theLaurer's Canal orgenital aperture. A key group of digeneans which aredioecious are theschistosomes.Asexual reproduction in the first larval stage is ubiquitous.
While the sexual formation of the digeneaneggs and asexual reproduction in the firstlarval stage (miracidium) is widely reported, thedevelopmental biology of the asexual stages remains a problem.Electron microscopic studies have shown that thelight microscopically visible germ balls consist ofmitotically dividingcells which give rise toembryos and to a line of newgerm cells that become included in these embryonic stages. Since the absence ofmeiotic processes is not proven, the exact definition remains doubtful.
Protandry is the general rule among the Digenea. Usually twotestes are present, but some flukes can have more than 100. Also present arevasa efferentia, avas 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.
Usually there is a singleovary with anoviduct, aseminal receptacle, a pair of vitelline glands (involved inyolk and egg-shell production) with ducts, the ootype (a chamber where eggs are formed), a complex collection of glands cells calledMehlis’ gland, which is believed to lubricate the uterus for egg passage.
In addition, some digeneans possess a canal calledLaurer'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 theMonorchiidae and some other families.
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 orcyclocoel. In others thecaeca may fuse with the body wall posteriorly to make one or moreanuses, or with the excretory vesicle to form auroproct. Digeneans are also capable of direct nutrient uptake through the tegument bypinocytosis andphagocytosis by thesyncitium. Most adult digeneans occur in the vertebratealimentary canal or its associatedorgans, 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 inmollusc intermediate hosts feed mostly by direct absorption, although theredia stage found in some groups does have a mouth, pharynx and simple gut and may actively consume host tissue or even other parasites. Encystedmetacercarial stages and free-livingcercarial stages do not feed.
Pairedganglia at the anterior end of the body serve as thebrain. From thisnerves extend anteriorly and posteriorly.Sensory receptors are, for the most part, lacking among the adults, although they do havetangoreceptor 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.
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 orparatenic 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 amollusc.[2] 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 thevertebrate host infaeces and use various strategies to infect the firstintermediate host, in which sexual reproduction does not occur. Digeneans may infect the first intermediate host (usually asnail) by either passive or active means. The eggs of some digeneans, for example, are (passively) eaten by snails (or, rarely, by anannelid worm),[2] in which they proceed to hatch. Alternatively, eggs may hatch in water to release an actively swimming, ciliated larva, themiracidium, 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-likemother sporocyst. The mother sporocyst undergoes a round of internalasexual reproduction, giving rise to eitherrediae (sing. redia) ordaughter 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, thecercaria (pl. cercariae).
Free-swimming cercariae leave the snail host and move through the aquatic ormarine 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., afish 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 thedefinitive host. Two-host life cycles can be primary (there never was a third host) as in theBivesiculidae, 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, themetacercaria, which is usually encysted in acyst of host and parasite origin, or encapsulated in a layer of tissue derived from the host only. This stage is infective to thedefinitive 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 gutpH levels, digestiveenzymes,temperature, etc. Once excysted, adult digeneans migrate to more or less specific sites in the definitive host and the life cycle repeats.
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 rhabdocoelturbellarians 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 the same process until the modern bewildering diversity of life cycle patterns developed.
Digenea includes about 80 families.[3] They are listed below, organised by order.
Digenea
Only about 12 of the 6,000 known species areinfectious to humans, but some of these species are importantdiseases afflicting over 200 million people. The species that infect humans can be divided into groups, the schistosomes and the non-schistosomes.
The Schistosomes occur in thecirculatory system 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 thegenusSchistosoma.
| Scientific Name | First Intermediate Host | Endemic Area |
|---|---|---|
| Schistosoma mansoni | Biomphalaria spp. | Africa,South America,Caribbean,Middle East |
| Schistosoma haematobium | Bulinus spp. | Africa,Middle East |
| Schistosoma japonicum | Oncomelania spp. | China,East Asia,Philippines |
| Schistosoma intercalatum | Bulinus spp. | Africa |
The 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 gastrointestinal tract, where they shed eggs along with host feces.Paragonimus westermani, which colonizes the lungs, can also pass its eggs insaliva. 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 sp. | Plants | Asia,India |
| Heterophyes heterophyes | Pirinella[citation needed] | Mullet,Tilapia | Asia,Eastern Europe,Egypt,Middle East |
| Metagonimus yokogawaii | Semisulcospira sp.[citation needed] | Carp,Trout | Siberia |
| Gastrodiscoides hominis | Helicorbis sp.[citation needed] | Plants | India,Vietnam,Philippines |
| Clonorchis sinensis | Bulinus sp. | Fish | East Asia,North America |
| Fasciola hepatica | Galba truncatula | Plants | Worldwide |
| Paragonimus westermani | Oncomelania sp. | Crabs,crayfish | Asia |
Authority control databases: National |
|---|