Flagellates - Parasitic Protozoan

Several flagellate protozoa parasitise man and live in the blood stream or tissues of the reticulo-endothelial system. Most significant of these are the species of the genus Leishmania and Trypanosoma. They all require two hosts in their life cycle; a blood- sucking insect transmits the infective stage of the parasite to the vertebrate host. These protozoans are elongated slender or sometimes rounded possessing a single flagellum at the anterior end. In certain intracellular states the flagellum may be absent. The flagellum arises from a basal body or kinetosome at the floor of a flagellar pocket that occurs at the anterior or near the posterior extremity of the body. In close approximation to the basal body lies a spherical, rod or disc-shaped structure, the kinetoplast. The kinetoplast represents an enormous mitochondrion and contains DNA within it.

It has been suggested that the kinetoplast (or the mitochondria1 DNA) is a metabolic organelle which is essential for the survival of the parasite in the insect intermediate host but is not required for the life of trypanosomes in the vertebrate host's blood. Figure the fine structure of a trypanosome as seen under the electron microscope. Most blood forms are long and slender but as they become intracellular or are taken up by the insect vector, a modification in their form may occur. Accordingly, during the life cycle, a trypanosome may, exhibit several polymorphic forms.

                                                                                        Figure: Trypanosomid flagellates

a) The structure of trypanosoma brucei
b) Life cycle of trypanosoma brucei gambiense causal agent of African sleeping sickness.The flagellate is transmitted by the bite of the tsetse fly.
C) Swollen lymph glands where the parasite resides.