Introduction:

The Digeneans are a group of specialized endoparasitic platyhelminthes. A general characteristic is that all encompass complex lifecycles, comprising one or more intermediate hosts, the first of which is forever a mollusc, which is generally aquatic. As adults, they are mainly found in most vertebrates groups, comprising fish, amphibians, birds, reptiles and mammals, acting as definitive hosts, where they might be highly pathogenic. They might be situated in most of the internal organs of these definitive hosts, comprising the lungs, bladder and blood stream, even though the greater part is found in the gastrointestinal tract, or closely related organs like the bile duct and liver. They show a flattened leaf-like body, structurally identical to most of the free living turbellarians. The Digeneans are categorized beneath based on their positions in the definitive hosts:

• Blood flukes
• Liver flukes
• Pancreatic flukes
• Lung flukes
• Intestinal flukes

Blood flukes (Schistosoma species):

Schistosomiasis or bilharzia is a tropical parasitic disease caused due to blood-dwelling fluke worms of the genus Schistosoma. The main schistosomes which infect human beings comprise S haematobium (transmitted through Bulinus snails and causing urinary schistosomiasis in the Africa and the Arabian Peninsula), S mansoni (transmitted through Biomphalaria snails and causing intestinal and hepatic schistosomiasis in the Africa, the Arabian Peninsula and South America), and S japonicum (transmitted through the amphibious snail Oncomelania and causing intestinal and hepatosplenic schistosomiasis in the China, Indonesia and Philippines).

S. intercalatum and S. mekongi are just of local significance. S. japonicum is a zoonotic parasite which infects a broad range of animals, comprising cattle, pigs, dogs and rodents. S. mansoni as well infects primates and rodents; however human beings are the major host. A dozen other schistosome species are animal parasites, a few of which occasionally infect the humans.

a) Morphology:

The male adult measure up to 15 mm in length and females up to 10 mm. The schistosomes remain in the copula all through their life span, the uxorious male surrounding the female having his gynaecophoric canal. The male is in reality flat however the sides roll up forming the groove. The cuticle of the male is covered by the minute papillae. The female just possess these at the anterior and posterior end as the middle part being covered through the male body. Oral and ventral suckers are present by the ventral one being lager serving to hold the worms in place, preventing them from being taken away by the circulatory current.

The ova of S. mansoni is around 114-175 µm long by 45-68 µm wide.  They are light yellowish brown, elongate and encompass a lateral spine. The shell is acid fast when stained having modified Ziehl-Neelsen Stain.

b) Life-Cycles and Transmission of Schistosomes:

Once the eggs are laid via the adult female worms, the majority of them first pass via the veins of the blood vessel in which the worm is living and then to the lumen of the intestine and are passed in the faeces (that is, S. mansoni and S. japonicum) or into the lumen of the bladder and are then passed in the urine (S. haematobium). Those eggs which reach fresh water hatch, discharging a miracidium which, to build up further should infect a specific snail species in 24 hours. The eggs of each and every species are markedly distinct however each produce virtually similar miracidium. A single miracidium can multiply in the snail to generate nearly 100,000 cercariae.

Asexual multiplication occurs in the snail and outcomes in the release of cercariae (that is, minute in size having forked tails, 200mm long) into the water around 3 to 6 weeks later. Cercariae actively swim around and when they have positioned, or come into contact by a definitive host, they actively penetrate the skin. They can stay active looking for the host for 24 to 48 hours after which when they do not find out a host they will die. 

The head of the cercariae migrates to the liver and builds up into either adult male or female worms (that is, flukes), where they pair up and then migrate to their area of the venous blood system (that species specific sites). The females leave the males and shifts to smaller venules closer to the lumen of the intestine or bladder to lay her eggs (around 6 weeks after infection). The greater part of adult worms live from 2 to 4 years, however some can live considerably longer.

Lung flukes (Paragonimus species):

Paragonimiasis is a disease caused through a parasite, as well termed as a fluke, which generally infects the lungs and other body sites of the humans and other mammals.

The parasites most generally related with paragonimiasis are Paragonimus westermani and Paragonimus kellicotti however there are other Paragonimus species which can as well cause disease in the humans.

The disease can present as pulmonary (lung) and extra pulmonary (exterior of the lung, like in the brain or abdomen).

How does infection with Paragonimus species take place?

Humans most generally become infected through eating raw, undercooked, salted or pickled fresh-water crabs or crayfish which have the parasite inside a cyst. One study found that around 17 % of harvested crabs have the infectious cysts.

Once in the intestine, the parasites come out of the cyst, penetrate the intestine, go to the abdomen and then into the lungs. On reaching their destination, the parasites mature and then begin making eggs. The eggs in the lungs are either coughed up or swallowed and excreted in the feces. However this parasite has other life-cycle forms, it is merely infectious to humans in the form which is found in crayfish and crabs.

How is paragonimiasis diagnosed?

The eggs of this parasite are featured and might be seen beneath a microscope in stool, lung fluid, sputum, cerebrospinal fluid (that is, the fluid covering the brain and spinal cord) and tissue specimens from the infected patients. Though, the eggs might not be present till two to three months after infection.

A blood test is as well available which recognizes the antibodies the body generates if infected by Paragonimus. Early in the illness, blood tests might show a raise in a certain kind of cell, termed as an eosinophil that is often related by the parasitic infections.

How is paragonimiasis treated?

There is a prescription drug available termed as praziquantel which is efficient for treatment of this infection in the lungs. The usual treatment course is two days. The other prescription drug, bithionol, is as well efficient.

Liver fluke (Fasciola hepatica and F. gigantica):

Fascioliasis is a zoonotic disease caused via infection by F. hepatica. It is a main disease of livestock which is related with significant economic losses due to the mortality; liver condemnation; lessen production of milk, meat and wool; and expenses for anthelmintics. The disease consists of a cosmopolitan distribution having cases reported from Scandinavia to New Zealand and southern Argentina to Mexico. As well of significance is the West Africa species of Fasciola (F. gigantica). The two share identical morphology, life-cycle and pathogenicity. They belong to the family 'Fasciolidae' having the given main features:

a) They are big having flattened leaf-like forms.

b) They have ramifying and complex digestive and reproductive systems.

c) Nearly all members of the family live in the liver and the bile duct. Though, Fasciolopsis buski lives in the intestine.

d) Cercariae are gymnocephalous.

e) Metacercariae encysts on the vegetation therefore establishing a two-host cycle.

 Morphology of the Adult:

a) They are leaf-like having oral cone and shoulder at the anterior end.

b) The intestinal caeca, testes as well as ovary are branched.

c) Tight and relatively small uterus is opposite to the ovary at the anterior end.

d) Vitellaria are wide and are laterally distributed.

Life cycle:

In the liver of affected final hosts, adult fluke in the bile ducts produce eggs which flow in the bile to the intestines and are then passed to the sheep faeces. The eggs hatch in wet regions on pasture when mean daily temperatures are above 10°C. Larvae (miracidia) invade the intermediate hosts that are lymnaeid snails: most generally Lymnaea tomentosa in Australia and New Zealand. Once inside the snail, larvae build up and multiply as sporocysts, rediae and cercariae.

The tadpole-like cercariae leave the snails, swimming till they encyst on the vegetation, making metacercariae: the infective phase of liver fluke. Grazing animals ingest the metacercariae thatr discharge immature flukes in the small intestine. The young flukes go through the intestinal wall, make their way to the liver and then migrate via the liver tissue for 6 to 7 weeks before entering the bile ducts to become adults. Egg production begins 8 to 10 weeks after infection. Adult fluke can live for some years and produce above 20,000 eggs per day.

Intestinal flukes (Fasciolopsis. buski, Heterophyes heterophyes, Metagonimus yokogawai):

F. buski is the most general intestinal nematode which causes infections in the humans. It is broadly distributed in Asia and the Indian subcontinent, particularly in the regions where humans raise pigs and use freshwater plants. The trematodes M. yokogawai and H. heterophyes are less-common causes of the human infection.

Life cycle:

F. buski, generally known as the giant intestinal fluke, is found in the duodenum and jejunum of pigs and humans and is the biggest intestinal fluke to parasitize humans. Humans are infected through eating freshwater aquatic plants like water caltrops, water bamboo and water chestnuts that can harbour the metacercariae.

In the intestine, the metacercariae excyst, join to the duodenum or jejunum, build up and grow into the adult worms. They lay unembryonated eggs that are excreted in the faeces.

In water, inside the egg, a ciliated miracidium builds up, comes out and penetrates an appropriate snail host. Within the snail, after some phases of asexual multiplication, large numbers of cercariae are generated. The latter come out from the snail and encyst on the surface of aquatic plants to metacercariae. Ingestion of such plants causes infection in humans and the cycle is repeated.

Morphology:

Eggs of the Fasciolopsis buski are generally ellipsoidal, operculated and measure 130 to 150 µm long by 60-90 µm wide.  The eggs are unembry onated when passed in the feces. The eggs of F. buski can be hard to differentiate from Fasciola hepatica; however the abopercular end of the latter often consists of a roughened or irregular region.

The adults of F. buski measure 20 to 75 mm long and encompass poorly-developed oral and ventral suckers. Adults exist in in the intestine of the mammalian host.

Pancreatic flukes (Eurytrema pancreaticum, E. coelomaticum, E. ovis):

These are broadly distributed in Korea, China, Japan, South America, Hong Kong and so on. E. pacreaticum is a general parasite of pancreatic (or hardly ever bile) ducts of herbivorous mammals, that is, sheep, cattle, goats, monkeys and camels.

Life cycle:

The adult flukes survive in the pancreatic passages of the herbivores. Eggs are passed in the faeces and ingested through land snail that is, the first intermediate host (snail). The cercariae build up into infective metacercariae only when ingested through grasshoppers, the second intermediate host. The life-cycle is completed when the infected insects are eaten through grazing herbivores. The metacercariae excyst and migrate to the pancreatic passage, where they build up into adults. Humans become infected if they accidentally swallow infected grasshoppers.

Morphology:

The parasite (10~18 x 5~9 mm in size) is flat, broad and oval to fusiform. The suckers are big; the oral sucker is bigger than the ventral sucker. The eggs (50~80 x 35~40) are embryonated in the uterus.

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