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Chapter: Medical Microbiology: An Introduction to Infectious Diseases: Flagellates

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African Trypanosoma - Parasitology

The trypanosomes that produce these diseases are morphologically and serologically identical.

African Trypanosoma

PARASITOLOGY

The trypanosomes that produce these diseases are morphologically and serologically identical. Accordingly, they are considered varieties of a single species, Trypanosomabrucei. The three subspecies, known as T. brucei gambiense, T. brucei rhodesiense, and T. brucei brucei, can be distinguished by their biologic characteristics, zymodeme types,mitochondrial morphology, and DNA hybridization patterns. All undergo similar develop-mental changes in the course of their passage between their insect and mammalian host. On ingestion by the tsetse fly (Glossina spp.), and after a period of multiplication in the midgut, they migrate to the insect’s salivary glands and assume the epimastigote form. After a period of weeks they are transformed into metacyclic trypomastigotes, rendering them infectious to mammals. When the fly again takes a meal, the parasites are inoculated with the fly’s saliva. In the mammalian host, they acquire a highly variable surface glyco-protein (VSG), multiply extracellularly, and eventually invade the bloodstream. During the initial stages of parasitemia, some trypomastigotes elongate to become graceful, slen-der organisms 30μm or more in length and divide every 5 to 10 hours. For reasons appar-ently independent of the host’s immune response, multiplication eventually slows. Some forms lose their flagella and assume a short, stumpy appearance. The latter forms have a more developed mitochondria and are thought to be particularly infective to the insect host. Near the end of the episode of parasitemia, both morphologic types may be seen in a single blood specimen. Individual strains of T. brucei can change the antigenic character of their glycoprotein coat in a sequential and, at times, predictable fashion. A single strain is capable of producing dozens, perhaps hundreds, of these variable antigen types, each of which is encoded in its own structural gene. The genetic repertoire seems to be strain spe-cific. Expression of individual genes appears to be controlled by the sequential duplica-tion and subsequent transfer of each gene (expression-linked copy) to one or more areas of the genome responsible for gene expression.


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