The Adult Digenean Fluke

The basic body form of the adult trematode takes a number of different forms,some of which are describe below;

Amphistome

 

These have largefleshy bodies, witha prominent suckerat the posterior ofthe body(e.g.Gastrodiscoides hominis )

Distome

 

These are the mostcommon type, with the mouth surrounded by the oral sucker and a ventral sucker, present anywhere on the ventral surface except the extreme posterior (e.g. Fasciola hepatica )

Echinostome

Similar to the distomes, except that the oral sucker is surrounded by a prominent collar, equiped with spines (e.g.Echinostoma sp.)

Monostome

   In these there is either only one sucker present (usually only the oral sucker), or there are two suckers, but one very reduced, or in some cases no suckers)(e.g. Notocotylus attenuatus )

Schistosome 

 

 

    Elongate trematodes, with separate sexes, the male generally larger, holding the female within a groove formed by a folding of the male body (the gynaecophoric canal). Found within the circulatory system. (e.g.Schistosoma mansoni) There are other forms as well, for example the 'Holostome' type, where the body of the trematode is divided into two distinct regions, the anterior of which may hold an additional adhesive organ, (e.g. Diplostomum sp.), and the 'Gasterostome', where the gut is a very simple, sac like, structure, attached to a mouth situated near the centre of the body (reminiscent of the arrangement of some of the free living platyhelminthes).

The Basic Lifecycle of the Major Groups of the Digeneans

   Most digeneans are hermaphroditic (the major exception being the schistosomes, and one other group). In the majority of these parasites self fertilization may occur, but cross fertilization between different individuals is more generally the rule. The sperm enter the female system, either via the Laurers canal or more commonly through the common genital atrium, which opens into the uterus.

The Digenean Trematode Egg

  The formation of the digenean egg follows that described for the platyhelminthes as a group. Briefly, as the egg enters the öotype of the fluke it becomes surrounded  by a predetermined number of vitelline cells, the number of which will be specific for different parasites, which form the food reserve of the egg. These vitelline cells produce globules of a mixture of proteins and phenols, which are extruded to theouter surface of the developing egg.

  Here the phenols oxidise to form quinone, which then coalesces with the protein, reacting to form scleratin, a hard inert yellowish substance, making up the egg shell. As the eggs of different species may vary in thickness, their colours may vary from yellow, to a dark brown.The digenean egg is usually operculate, in common with other platyhelminthes.

  Exceptions to this may occur however, the most important being with the schistosomes. Here the eggs are non-operculate, and are ornamented with spines, the appearance of which are characteristic for different species of schistosome.

  The eggs hatch of operculate eggs involves the release of the opercular cap. This takes place under a variety of conditions, modified according to the particular species of trematode. For example some trematode lifecycles involve the ingestion of the egg before hatching (e.g. Dicrocoelium dendriticum, the lancet fluke), whilst others such as those of Fasciola hepatica, (the liver fluke), hatch in water.

   For the eggs that hatch in the external environment, a number of factors may be important,for example light, temperature and changes in osmotic pressure. Again the exact details of these environmental requirements will be optimised for the particular conditions which will maximise the chances of completion of the parasite lifecycle. In all cases the egg hatches to release the miracidium.