| Echinostoma | |
|---|---|
 | |
| Two specimens ofEchinostoma revolutum | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Class: | Trematoda |
| Order: | Plagiorchiida |
| Family: | Echinostomatidae |
| Subfamily: | Echinostomatinae |
| Genus: | Echinostoma Rudolphi, 1809[1] |
Echinostoma is agenus oftrematodes (flukes), which can infect both humans and otheranimals. These intestinalflukes have a three-host life cycle withsnails or other aquatic organisms asintermediate hosts,[2] and a variety of animals, including humans, as theirdefinitive hosts.
Echinostoma infect thegastrointestinal tract of humans, and can cause a disease known as echinostomiasis. The parasites are spread when humans or animals eat infected raw or undercooked food, such asbivalve molluscs orfish.[3]
There has been debate about the number of species in this group, with estimates as high as 120 unique species ofEchinostoma,[4] however, currently 16 species have been accepted as valid species with another 10 to be validated[5]Echinostoma are difficult toclassify based on morphology alone. Many species look alike and can be consideredcryptic species (different lineages are considered to be the same species, due to high morphological similarity between them).[6] Manyspecies ofEchinostoma have been re-classified several times. For example, the species now known asEchinostoma caproni, was previously known by a variety of names includingE. liei,E. paraensei andE. togoensis.[4]
Methods for classifyingEchinostoma species, such as theEchinostoma revolutum group, were devised by Kanev.[7] TheEchinostoma species in this group are now classified according to their sharedmorphological and biological characteristics, such as the presence of 37 collar spines.[7]
Molecular methods, such as sequencingmitochondrial DNA andribosomal DNA, are also used to distinguish between species ofEchinostoma as an alternative to morphological classification methods.[8]
Phylogeny of most knownEchinostoma species based on themitochondrial nad1 sequences:[5]
| Echinostoma |
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Further confirmed species areE. lindoenseSandground & Bonne, 1940 andE. luisreyiMaldonado, Vieira & Lanfredi, 2003, these are not included in the phylogeny because of a lack of genetic material.[5] Unconfirmed species areE. acuticauda,E. barbosai,E. chloephagae,E. echinatum,E. jurini,E. nudicaudatum,E. parvocirrus,E. pinnicaudatum,E. ralli, andE. rodriguesi.[5]
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Echinostoma are internaldigeneantrematode parasites which infect the intestines andbile duct[4] of theirhosts.
The length and width of adultEchinostoma varies between species, but they tend to be approximately 2-10mm × 1-2mm in size.[9]
AdultEchinostoma have twosuckers: an anterior oral sucker and a ventral sucker.[4] They also have a characteristic head collar with spines surrounding their oralsucker.[10] The number of collar spines varies betweenEchinostoma species, but there are usually between 27 and 51.[4] These spines can be arranged in one or two circles around thesucker, and their arrangement may be a characteristic feature of anEchinostoma species.[4]
Echinostoma have a digestive system consisting of apharynx,oesophagus and an excretory pore.[4]
Echinostoma arehermaphrodites,[10] and have both male and female reproductive organs. Thetestes are found in the posterior part of the fluke's body, in the area furthest from themouth.[4] Theovary is also found in this location, close to the testes.[4]
Theeggs (ova) ofEchinostoma are operculate[4] and vary in size, but are typically in the range of 80–135 × 55–80 μm.[11]
ThegenusEchinostoma has a globaldistribution. These parasites are particularly common in South East Asia, in countries such asSouth Korea and thePhilippines.[12] However, they are also found in some European countries,[7] and species such asEchinostoma trivolvis are found inNorth America.[13]
Echinostoma have threehosts in theirlife cycle: a firstintermediate host, a second intermediate host and adefinitive host.Snail species such asLymnaea spp. are common intermediate hosts forEchinostoma,[4] althoughfish and otherbivalve molluscs can also be intermediate hosts for these parasites.[14][15]
Echinostoma species have low specificity for their definitivehosts, and caninfect a variety of differentspecies ofanimal, includingamphibians,[16]aquatic birds,mammals and humans.[14] A definitive host which is infected withEchinostoma will shed unembryonatedEchinostomaeggs in their faeces. When the eggs are in contact withfresh water they may become embryonated, and will then hatch and releasemiracidia.[3] The miracidia stage ofEchinostoma is free-swimming, and actively penetrates the first intermediate snail host, which then becomes infected.[3]
In the first intermediate host, the miracidium undergoesasexual reproduction[17] for several weeks, which includessporocyst formation, a few generations ofrediae and the production ofcercariae.[3] The cercariae are released from the snail host into water and are also free-swimming. The cercariae penetrate a second intermediate host, or they remain in the first intermediate host, where they form metacercariae.[14] Definitive hosts become infected by eating secondary hosts which are infected with metacercariae.[14] Once the metacercariae have been eaten, they excyst in the intestine of the definitive host[14] where the parasite then develops into an adult.
Echinostoma arehermaphrodites. A single adult individual has both male and female reproductive organs, and is capable of self-fertilization.[17]Sexual reproduction of adultEchinostoma in the definitive host leads to the production of unembryonated eggs.[3] The life cycle ofEchinostoma is temperature dependent, and occurs quicker at higher temperatures.[4]Echinostoma eggs can survive for about 5 months and still have the ability to hatch and develop into the next life cycle stage.[18]
Infection of humans with members of the family Echinostomatidae, includingEchinostoma, can lead to a disease called echinostomiasis.E. revolutum,E. echinatum,E. malaynum andE. hortense are particularly common causes ofEchinostoma infections in humans.[4] Humans can become infected withEchinostoma by eating infected raw or undercookedfood, particularlyfish,clams and snails.[3] Infection with these parasites tends to be common in regions where cultural dishes require the use of raw or undercooked food that may be infected withEchinostoma.[19] A mild infection may not have anysymptoms.[20] If symptoms are present they can include abdominal pain,diarrhoea, tiredness and weight loss.[3]
Echinostomiasis isendemic in South East Asia and the Far East, in countries includingChina, Korea,Taiwan, Philippines,Malaysia,Indonesia andIndia.[21] Echinostomiasis has also been reported inJapan,Singapore,Romania,Hungary andItaly.[21] Theprevalence of echinostomiasis varies between countries[21] but there tend to be foci of infection in areas where raw or undercooked hosts ofEchinostoma, such as snails or fish, are widely consumed.[20]
Echinostoma are not highly pathogenic.[20] Symptoms of greater severity tend to be seen in an echinostomiasis infection where there is a higher number of flukes.[20] The flukes cause damage to the intestinal mucosa, which leads toulceration andinflammation.[20]
AnEchinostoma infection can be diagnosed by observing the parasite eggs in thefaeces of an infected individual, under amicroscope. Methods such as theKato-Katz procedure can be used to do this.[3] The eggs typically have a yellow-brown appearance, and are ellipsoid in shape.[20] To confirm which species is causing the infection adult worms must be recovered from the infected individual, such as with anthelmintic treatment.[20]
Echinostomiasis can be treated with theanthelmintic drugpraziquantel, as for other intestinal trematode infections.[3] Side effects of anthelmintic drug treatment may includenausea, abdominal pain,headaches ordizziness.[3][22]
Echinostomiasis can be controlled at the same time as otherfood-borne parasite infections, using existing control programmes.[21] Interrupting the parasite's lifecycle by efficient diagnosis and subsequent treatment of infected individuals, and preventing reinfection, may help to control this disease.[23] As echinostomiasis is acquired through the consumption of raw or undercooked infected food, cooking food thoroughly will prevent infection.
