Introduction:

The phylum Platyhelminthes includes six groups which comprise the free living forms and those which are of zoological, medical and economic significance. The medically significant groups are the Trematodes and cestodes. Trematode as well termed as flukes which cause different clinical infections in humans that take place globally.

The parasites are so named due to their conspicuous suckers, the organs of attachment (that is, trematos signifies 'pierced with holes'). All the flukes which cause infections in humans belong to the group of the digenetic Trematodes.

The class Trematoda is categorized into three subclasses:

Subclass 1: Aspidogastrea: They encompass big ventral adhesive organ categorized by longitudinal and transverse septa into the sucking discs. They are parasites of fishes, turtles and molluscs.

Subclass 2: Didymozoidea: These are tissue-dwelling parasites of the fish. They are mainly elongated, dioecious having sexual dimorphism. No complete life-cycle is acknowledged.

Subclass 3: Digenea: This includes parasites of medical and economic significance to man and thus will be dealt with more widely.

The Adult Digenean Fluke:

The basic body form of the adult Trematode acquires a number of various forms, some of which are described below:

a) Amphistome: These have big fleshy bodies having a prominent sucker at the posterior of the body (example: Gastrodiscoides hominis)

b) Distome: These are the most common kind having the mouth surrounded by the oral sucker and a ventral sucker, present anywhere on the ventral surface apart from the extreme posterior (example: Fasciola hepatica )

c) Echinostome: Identical to the distomes, apart from that the oral sucker is surrounded through a prominent collar, equipped by spines (example: Echinostoma sp.)

d) Monostome: In these there is either just one sucker present (generally the oral sucker), or there are two suckers, however one very reduced, or in certain cases no suckers) (example: Notocotylus attenuatus)

Basic Lifecycle of the Major Groups of Digenean:

Most of the Digenean are hermaphroditic (the main exemption being the schistosome, and one other group). In the majority of such parasites self fertilization might take place, however cross fertilization among various individuals is more usually the rule. The sperm enter the female system, either through the Laurers canal or more generally via the common genital atrium that opens into the uterus.

The Digenean Trematode Egg:

The Digenean egg formation obeys illustrated for the platyhelminthes as a group. In brief, as the egg enters the öotype of the fluke it becomes bounded by a predetermined number of vitelline cells, the number of which will be precise for various parasites, which make the food reserve of the egg. Such vitelline cells generate globules of a mixture of phenols and proteins, which are extruded to the outer surface of the developing egg. Here the phenols oxidize to make quinone, which then coalesces by the protein, reacting to form scleratin, a hard inert yellowish substance, building up the egg shell. As the eggs of different species might differ in thickness, their colors might differ from yellow, to a dark brown. The Digenean egg is generally operculate, in common by other platyhelminthes.

Exceptions to this might take place though, the most significant being with the schistosomes. Here the eggs are non-operculate and are ornamented by spines, the appearance of which are distinctive for various species of schistosome.

The eggs hatch of operculate eggs comprises the discharge of the opercular cap. This occurs beneath a diversity of conditions, modified according to the specific species of Trematode. For illustration some Trematode life-cycles entail the ingestion of the egg before hatching (example: Dicrocoelium dendriticum, the lancet fluke), whilst others such as those of the Fasciola hepatica, (that is, the liver fluke), hatch in water. For the eggs which hatch in the external environment, a number of factors might be significant, for illustration: light, temperature and modifications in osmotic pressure. Again the precise details of such environmental requirements will be optimized for the specific conditions that will maximize the chances of completion of the parasite life-cycle.

In whole cases, the egg hatches to discharge the miracidium. 

The Larval Digeneans:

a) The miracidium:

The ciliated first-phase larva of a Trematode which emerges from the egg and should penetrate to the tissues of a suitable intermediate host snail if it is to continue its life-cycle; followed by the growth into a mother sporocyst and by the production of a number of offspring of the succeeding larval generations.

b) The sporocyst:

The sporocyst builds up in the molluscan host as a hollow fluid filled germinal sac, to which protrude germinal masses. At the conical anterior of the sporocyst body a birth pore is positioned, from which ensuing generations of larvae emerge. The germinal masses build up internally into either daughter sporocyst, that is basically similar as their parent sporocyst, or into a second larval phase, the redia.

c) The redia:

Intramolluscan growth phase of a digenetic Trematode, following the primary sporocyst phase that forms after penetration of the snail tissues through the miracidium. Rediae are generated from cells in the sporocyst, are discharged from the latter and build up in the tissues of the host snail as elongated, saclike, muscular organisms having a mouth and gut. The redia might generate one or a number of additional generations in the snail; however they ultimately generate the final development phase, the cercaria.

d) The cercaria:

The free-swimming Trematode larva which comes out from its host snail; it might penetrate the skin of a final host (as in the Schistosoma of humans), encyst on vegetation (as in Fasciola), in or on fish (as in Clonorchis), or penetrate and encyst in different arthropod hosts. Body and tail are greatly differed in form and specialized function is adapted to the specific life cycle demands of each and every species.

e) The mesocercaria:

The mesocercaria is a definite prolonged phase in the adult generation of the strigeate Trematodes that closely looks like the cercarial body, from which it builds up in the second intermediate host and which doesn't possess metacercarial characteristics; it builds up in turn into the metacercaria in the other host.

f) The metacercaria:

The postcercarial encysted phase in the life history of a fluke, prior to transfer of the definitive host. A few cercariae joins themselves to grass or other vegetation, form metacercaria, and later are ingested through herbivores, as in the Fasciola and similar forms; others encyst in the muscles of fish, as in the Clonorchis, or in crayfish, as in the Paragonimus.

g) The Larval Digeneans - the Juvenile Adult Stages:

On ingestion the metacercaria (or cercaria) should converted into the adult form. The exact details of this procedure will differ considerably, based on how the definitive host was infected. For illustration, in a few species the adult flukes are found in the alimentary tract. In such cases the metacercarial cyst wall is broken down to discharge what is essentially a young fluke that just has to migrate a short distance to reach their chosen site in the hosts body. In other groups though the adult forms are positioned in other sites in the body. In such cases the liberated young fluke should penetrate the gut wall, or in the case of the schistosomes penetrate the host's skin. Then they should experience a migration via the host's body. This is generally through the circulatory system, however again the exact details of the migratory path will differ considerably.

Characteristics of digenetic Trematodes: 

1) Digenetic Trematodes are unsegmented leaf-shaped worms which are flattened dorsoventrally.

2) They tolerate two suckers, one surrounding the mouth (that is, oral sucker) and the other on the ventral surface of the body (that is, ventral sucker). These serve up as the organs of attachment.

3) The sexes of the parasites are not separating (that is, monoecious). An exception is schistosome that is diecious (or unisexual).

4) The alimentary canal is not complete, and the anus is not present.

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