Trypanosoma brucei is particularily important as a model of human disease as it is used extensively in research laboratories around the world. It is not supposed to be infective to humans, however, it is always handled as if it is.
All species of trypanosoma, so-called trypanosomes, belong to the large group of protozoa known as flagellates. All are characterized by the possession, at least at some point in their life-cycle, of a flagellum and therefore are, again during life-cycle stages when the flagella are present, motile. A schematic drawing of the trypanosome is shown here:
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| A-the anterior flagellum. B-the underlying complex cytoskeleton. C-the nucleus. D-the mitochondrion. E-the kinetoplast (mitochondrial genome). F-glycosome (a unique organelle where glycolysis occurs). G-the flagellar pocket. H-the basal body. I-Golgi apparatus. J-endoplasmic reticulum. K-the undulating membrane. L-attachment of the flagella to the undulating membrane. M-attachment of the flagella directly to the cell body. |
The distribution of trypanosomiasis in Africa is absolutely dependent on the distribution of the tsetse fly. Exceptions to this are T. vivax, which can be transmitted either by other flies, including the common stable fly and T. evansi and T. equiperdum which are trasmitted by tabanid flies and by sexual intercourse, respectively.T. vivax has spread outside of Africa to Indonesia and the West Indies.
The life-cycle of T. brucei is shown diagrammatically below. The procyclic is the stage which reproduces within the tsetse fly. Procyclics can be grown in vitro and have been very carefully studied as to their morphology, antigenicity, metabolic requirements and mode of regulation of gene expression. The same is not true for the bloodstream stages, the stage which is actually responsible for disease in the human host. The constraint here is the development of a high yielding system for growing these stages in vivo, although recently a system for in vitro cultivation for the study of the genetics of differentiating trypanosomes has been described.
A life-cycle illustrating the morphology of the developmental stages of T. brucei is shown here:
