Ultrastructural characteristics of host-parasite interactions in intracellular protozoan parasites

The ultrastructural characteristics of intracellular parasitic protozoa, with particular reference to the Apicomplexa, are reviewed in an attempt to illustrate the variety of reactions which develop in the host-parasite relationship. The basic features of the host-parasite interaction are the entry of the protozoon into the host cell, the growth of the parasite within the host cell and the changes related to contact between host and parasite plasma membrane. Electron microscope studies have revealed the complex sequences of the events associated with the entry of parasite protozoa, which, in the Apicomplexa, start with morphological changes in rhoptries and micronemes. Successively, the host membrane invaginates and a parasitophorous vacuole develops. The entry of a parasitic protozoon into a host-cell is usually accompanied by a membrane/ membrane interaction and by the involvement of the parasite's contractile system. Our knowledge of specific mechanisms involved in the interaction between host cells and intracellular protozoa is still incomplete, owing to the variety of survival strategies employed by these organisms.