Mechanism of invasion

In spite of the vertebrate intestine being a biologically hostile environment for parasites, cestodes have been very successful in developing a wide range of morphological, physiological and biochemical adaptations which enable them to become established and to reproduce there.

With rare exceptions (e.g. Archigetes, H. nand), a host becomes infected by ingesting an intermediate host containing a cestode larva which may be (i) free, (ii) encysted, (iii) encapsulated, or (iv) both encysted and encapsulated. The term encysted is used when a cyst is produced by the parasite, encapsulatedwhen it is produced by the host. In the latter case, the cyst may sometimes (e.g. Echinococcus) consist of two layers, the outer of which is derived from the host and the inner from the larva. The larval scolex is normally invaginated or withdrawn - a measure which presumably protects the scolex until it is stimulated to evaginate in the appropriate region of the host gut. To become established in the gut, therefore, a larva must (a) free itself from its surrounding membranes (i.e. excyst), (b) evaginate its scolex, (c) become ‘activated’ and (d) become attached to the intestinal mucosa. Excystment and evagination General account ROLE OF ENZYMES Although excystment and evagination are theoretically separate processes, in many cases the second follows so closely after the first that they cannot readily be separated.