| Gastrodiscoides | |
|---|---|
 | |
| Longitudinal section of an adult | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Class: | Trematoda |
| Order: | Plagiorchiida |
| Family: | Paramphistomidae |
| Genus: | Gastrodiscoides Leiper, 1913 |
| Species: | G. hominis |
| Binomial name | |
| Gastrodiscoides hominis (Lewis and McConnell, 1876) | |
| Synonyms | |
Gastrodiscus hominisFischoeder | |
Gastrodiscoides isgenus ofzoonotictrematode under theclassTrematoda. It hasonly onespecies,Gastrodiscoides hominis. It is aparasite of a variety ofvertebrates, includinghumans. The first definitive specimen was described from a human subject in 1876.[1] It is prevalent inBangladesh,India,Burma,China,Kazakhstan,Philippines,Thailand,Vietnam, and theVolga Delta ofRussia,[2][3] with isolated cases fromAfrica, such asNigeria.[4] It is especially notable in theAssam,Bengal,Bihar,Madhya Pradesh,Odisha andUttar Pradesh states ofIndia.[5][2]
It is also known as thecolonic fluke, particularly when infecting other animals. Its naturalhabitat is thecolon ofpigs, and has also been found inrhesus monkeys,orang-utans,fish, fieldrats andNapu mouse-deer. In humans the habitat is on the wall of thecaecum.[6][7] Humans are considered an accidental host, as the parasite can survive without humans. It causes ahelminthic disease calledgastrodiscoidiasis.[6][7]
G. humanis is unique amonghelminths because it was first discovered and described from a human infection. The worm was discovered and described by twoBritishmedical doctors, Timothy Richard Lewis and James McConnell, in 1876, from the caecum of anindigenous Assamese man in India. Their description of theinternal structure was inaccurate and incomplete. They claimed that the parasite had onetestis and oneovary. They placed it in the genusAmphistomum, because of its obvious location of posterior sucker; the species was namedAmphistomum hominis, as it was found in human.[8] In 1902, F. Fischoeder recognised the affinity with other species and tentatively placed it in the genusGastrodiscus (Leuckart, 1877). However, the generic name was largely recognised as asynonym; it was then known asAmphistomum (Gastrodiscus) hominis. With a fresh look, J. W. W. Stephens re-described the parasite in 1906, and clearly noted the overlooked relatively small ovary and interpretation of the posterior testis as an ovary in the original description.[8]
A newhelminthologist at theLondon School of Tropical Medicine, Robert T. Leiper, re-examined the parasite in 1913. He noted the distinctive characters such as a tuberculated genital cone, the position of the genital orifice, a smooth ventral disc, and the testes in tandem position. These outstanding features prompted him to create an entirely new genus,Gastrodiscoides, for the specimen.[9] This taxonomic revision had criticism, as some of the descriptions were later found to be flawed, such as the position of testes; these criticisms prevented it from coming into general acceptance. It was later observed that the parasite was much more common to pigs and othermammals than in humans. The first report of infection of pigs was inCochinchina,Vietnam, in 1911. In 1913, it was further confirmed that the rate ofporcine infection was as high as 5%. Then a large number of living flukes was recovered from dead Napu mouse-deer at the Zoological Gardens of theZoological Society of London. The mouse-deer wasPrince of Wales's collection fromMalay. The shortcomings of Leiper's descriptions did not prevent the generic nameGastrodiscoides becoming more and more advocated in the early 1920s.[8] The currently accepted nomenclature was fortified by the Britishparasitologist J. J. C. Buckley, at theLondon School of Hygiene and Tropical Medicine (where he was then a Milner Research Fellow), whose descriptions were based on high incidence of theparasitism among thenative Assamese ethnic groups in Northeastern India. His first report in 1939,[1] followed by a body of evidences in support of Leiper's proposition, enabled him to vindicate the validity of a separate genus,Gastrodiscoides, hence thebinomial nameGastrodiscoides hominis.[10] His report was the pioneer description of the life cycle of the fluke and the prevalence of gastrodiscoidiasis.[1] In his survey of three villages in Assam, there was found a surprisingly high incidence, with over 40% of the population was infected. J. J. C. Buckley's report is the most useful to the modern classification ofG. hominis.[11]
It is typically an amphistome with the ventral sucker close to the posterior end. The body isbilaterally symmetrical and isacoelomate. It is dorsoventrally flatted, with a uniquepyramidal shape. The body is covered by ategument bearing numeroustubercles.[12] Thealimentary canal is incomplete, consisting of a pair of lateral pouches arising from the oral sucker and apharyngeal tube, which bifurcates into two gutcaeca. Thebladder is in the middle behind the ventral sucker. The genus ishermaphrodite, as both male and femalereproductive system are present.[6][8]
It is a large fluke,vase-shaped, and bright-pink in colour. In average it measures 5-8 mm long and 3-5 mm wide. The disc-shaped body is divisible into anterior conical and posterior discoidal regions. The anterior region is a conical projection bearing a prominent oral sucker. The posterior portion is relatively broad, up to 8 mm wide, discoidal, and ventrally excavated. It is an amphistome worm such that theventral sucker is close to the posterior end.[13] The body covering, called ategument, is smooth in appearance, but contains a fine structure in a series of concentric folds bearing numerous tightly packedtubercles. Ventral surface contains a specialised region of the tegument. Ciliated and non-ciliated papillae are arranged around the oral sucker.[14][15] The incompletealimentary canal consists of a pair of lateral pouches arising from the oral sucker and a slightly tortuouspharyngeal tube, which bifurcates into two gutcaeca. The largeexcretory bladder is in the middle, behind the ventral sucker. The species, beinghermaphrodite, has both male and femalereproductive systems, arranged in the posterior region. Thetestes lie in alongside the bifurcation of the caeca, and a common genital pore is on the cone just anterior to the bifurcation. The oval-shapedovary lies just posterior to the testes in the middle, and the loosely coileduterus opens to the genital pore. Vitelline glands are scattered around the caeca.[8]
Humans are now considered as the accidental host because humans are not the primary requirement for thelife cycle; pigs are recognised as the principaldefinitive host. Infection causes ahelminthic disease calledgastrodiscoidiasis.[6] It is adigenetic trematode with a complex life cycle involvingasexual reproduction in anintermediate host, presumably aquaticsnails, andsexual reproduction in the vertebrate host. As a hermaphrodite, eggs are produced byself-fertilisation and are released along the faeces of the host. Eggs measure ~146 by 66μm, arerhomboidal in shape, transparent, and green in colour. Each egg contains about 24 vitelline cells and a central unembryonatedovum. Eggs in a wet environment hatch intomiracidia in 9-14 days.
In water, eggs hatch intomiracidia, which then infect amollusc, in whichlarval development andfission occurs. The miracidium grows into the sporocyst stage. It is generally conceived that the unfertilised eggs are ingested by the snail, but there has been no direct observation. In an experimental infection of themolluscHelicorbis coenosus, miracidum develops intocercaria after 28-153 days ofingestion.[16] In the snail, mother and daughter rediae are found in the digestive gland, and are about 148-747/45-140 μm in size, sausage-shaped, and lack collar andlocomotory organs.
Infectivecercariae are produced and are released on water plants or directly infect other aquatic animals, such as fish.[17] Thecercariae released from the snail form metacercarial cysts on water plants. The complete life cycle is not yet observed in nature,[18] and the tiny snail,H. coenosus, remains the most commonly accepted vector, as it is coincidentally found in abundance in the pigsties. In some circumstances, fishes and other aquatic animals are found to be infected. It is hypothesised that the free cercaria in water bodies accidentally find and penetrate these animals as second intermediate host, where they encyst as metacercaria. These are directly infective tomammals upon consumption, while they get attached to vegetation, wherenight soil is used.
Humans ingest the metacercaria either by the infected fish or contaminated vegetable. The parasite travels through the digestive tract into theduodenum, then continues down to reach the caecum, where it self-fertilizes and lay eggs, continuing the cycle.Heavy infection in humans is suspected to causediarrhoea,fever, abdominal pain,colic,malnutrition,anaemia, and even death.[2][4]
Gastrodiscoidiasis is an infection that is usuallyasymptomatic and affects the small intestine in animals, such as pigs, to a very mild symptom, but when it occurs in humans it can cause serious health problems and evendeath. It is suspected to causediarrhoea,fever, abdominal pain,colic, and an increasedmucus production. In severe cases, where there are large amounts of eggs present, tissue reactions can occur in theheart or mesenteric lymphatics, and even death may occur if left neglected. Indeed, a number of mortality among native Assamese children is attributed to this infection.[2] In pigs, pathological symptoms include infiltration witheosinophils,lymphocytes, andplasma cells. The submucosa can showoedema and thickening, resulting in a subacuteinflammation of the caecum and mucoid diarrhoea.[6]
Human gastrodiscoidiasis isendemic in Assam, and to a lesser extent in the Philippines. The highest incidence so far recorded is among children inKamrup district of Assam, where the prevalence was as high as 41%.[1] First described from a native Assamese patient, it was initially believed to have a distribution restricted to NE India and the southeast Asia. Later investigations revealed that it is widespread, and is further spread by infected persons to other parts of the world, such asGuyana.[2] The level of infection in laboratory animals can be very high among Asian mammals.[13] Regions of high incidence can be attributed to low standard of sanitation, such as rural farms and villages where night soils are used. Infection in both humans and animals is most common through the ingestion of vegetation found in contaminated water. It is also assumed that transmission from infected fish that is under-cooked or eaten raw, as common among southeast Asian.[19] There is a unique case report of a seven-year-oldNigerian who showed symptoms ofmalnutrition andanaemia and was eventually diagnosed with infections ofG. hominis andAscaris lumbricoides. The child quickly recovered after proper medication.[4]
Diagnosis is made by examination of the fæces and the detection of eggs. Adult worms are easily identified from other helminths by their distinctive appearance. The eggs are readily distinguished from those of othertrematodes by their rhomboid shape and distinct green colour. Patients do not often directly show any symptoms, and if one appears, it indicates that the infection is already at a very high level. There is no prescribed treatment, but the traditional practice of soapenema has been very effective in removing the worms.[3] It works to flush the flukes from the colon which removes the parasite entirely, as it does not reproduce within the host. Some drugs that have been proven effective aretetrachloroethylene, at a dosage of 0.1 mg/kg on an empty stomach, and a more preferred drug,praziquantel, which eliminates the parasite with 3 doses at 25 mg/kg in one day.[6]Mebendazole was found to be efficient indeworming the parasite from a Nigerian girl who was shedding thousands of parasite eggs in stools even with a single dose of 500 mg.[4] Prevention of this disease is not difficult when simple sanitary measures are taken. Night soil should never be used as afertilizer because it could contain any number of parasites. Vegetables should be washed thoroughly, and meat properly cooked.[7]