The mesocercaria

   The mesocercaria is essentially a resting stage within the parasitic life cycle, employing a second intermediate host in a parasite lifecycle utilising four hosts. The mesocercaria is a definite prolonged stage in the adult generation of strigeate trematodes, which closely resembles the cercarial body, from which it develops in the second intermediate host, and which does not possess metacercarial features; it develops in turn into the metacercaria in another host.

   In parasites having this larval stage the mesocercaria are capable of infecting and surviving within a very wide range of paratenic hosts which may ingest the second intermediate host, thus in effect increasing the number of hosts which the parasite may use in its lifecycle. For example, amphibians infected with mesocercaria of Alaria may themselves infect a wide variety of other amphibians, reptiles, birdsand mammals if they are ingested by these animals.


This is a much more common "resting" larval stage of the trematode parasitic lifecycle, formed either in a final intermediate host (when a mesocercaria, or more commonly a cercaria enters its body), or on a solid substrate in the external environment.

  The final intermediate host may be a fish (e.g. Opisthorchis sinensis), an arthropod (e.g.Dicrocoelium dendtriticum, employing an ant second intermediate host, and Paragonimus westermani employing a crustacean), or another mollusc, as with some of the echinostomes. As stated above, some trematodes however do not have second intermediate hosts, but either encyst as metacercariae on solid substrate's, such as aquatic vegetation or on shells of aquatic organisms, which will in turn be ingested by the parasites definitive host, or in some groups such as the schistosomes, as already described, the cercariae directly penetrate the skin of, and infect, the parasites definitive host.

  Although generally the metacercariae are inactive encysted forms, the metacercaria of some species do remain free and action. In most other metacercariae however, encystment does occur. The structure of the cyst wall itself varies considerably, though generally it is a complex mixture of tanned proteins, lipids and polysaccharides.

   Within the cyst wall the morphology of the larva usually closely resembles that of the cercarial body, although as described above, in some groups sexual maturation may occur either fully or partially. To continue further the metacercaria must be ingested, either along with the body of the intermediate host it inhabits by a carnivorous definitive host, or along with the vegetation it has encysted on by a herbivorous or omnivorous host.

The Larval Digeneans - the Juvenile Adult Stages

    On ingestion the metacercaria (or cercaria) must transform into the adult form. The precise details of this process will vary considerably, depending on how the definitive host was infected. For example, in some species the adult flukes are found within the alimentary tract. In these cases the metacercarial cyst wall is broken down to release what is essentially a young fluke, which only has to migrate short distance to reach their preferred site within the hosts body.

  In other groups however the adult forms are located in other sites within the body. In these cases the liberated young fluke must penetrate the gut wall, or in the case of the schistosomes penetrate the hosts skin. Then they must undergo a migration through the hosts body. This is usually via the circulatory system, but again the precise details of the migratory path will vary considerably.

Features of digenetic trematodes

• Digenetic trematodes are unsegmented leaf-shaped worms that are flattened
dorsoventrally.
• They bear 2 suckers, one surrounding the mouth (oral sucker) and another on the
ventral surface of the body (ventral sucker). These serve as the organs of
attachment.
• The sexes of the parasites are not separate (monoecious). An exception is
schistosomes, which are diecious (unisexual).
• The alimentary canal is incomplete, and no anus is present.
• The excretory system is bilaterally symmetrical. It consists of flame cells and
collecting tubes. These flame cells provide the basis for the identification of the
species.
• The reproductive system consists of male and female reproductive organs and is
complete in each fluke.
• The flukes are oviparous. They lay operculated eggs. An exception is schistosome
eggs, which are not operculated.
• All have complicated life cycles, with alternating asexual and sexual
developments in different hosts.