Life Under the Microscope: Trypanosoma brucei

First discovered by Sir David Bruce in 1894 and appropriately named after him, Trypanosoma brucei is the causative agent of African trypanosomiasis, also known as “sleeping sickness”. It is prevalent in tropical and subtropical regions of Africa. Trypanosoma brucei consists of a group of three organisms: T. brucei gambiense, T. brucei rhodesiense, and T. brucei brucei. While all of these organisms cause trypanosomiasis, T. brucei brucei only infects animals and is not pathogenic in humans. All of these organisms within the disease share the same vector, the tsetse fly.

tsetse

Tsetse fly, the vector of African trypanosomiasis

Trypanosoma brucei has several interesting characteristics that add to its pathogenicity. To begin with, the parasite’s outer coat has variant surface glycoproteins that help it evade the host’s immune system, causing chronic parasitic infections. In addition, Trypanosoma brucei is also able to cross the blood-brain barrier by affecting tight junctions within the brain.

The life cycle of T. brucei is separated into the trypomastigote (found within the human host) and epimastigote (found within the tsetse fly) stages. Whenever the tsetse fly takes a blood meal, it transmits metacyclic trypomastigotes into the human. While in the bloodstream, they are transformed into bloodstream trypomastigotes and are brought to other areas of the body, such as lymphatic or cerebrospinal fluid, and duplicate by binary fission. Another tsetse fly then takes a blood meal from the host and these bloodstream trypomastigotes are ingested. Within the fly’s midgut, the bloodstream trypomastigotes are transformed into procyclic trypomastigotes and multiply by binary fission. The procyclic trypomastigotes move out of the midgut and turn into epimastigotes. These epimastigotes multiply in the salivary gland and become metacyclic trypomastigotes. The course of one cycle takes approximately 20 days to be completed.

Morphologically, trypomastigotes and epimastigotes differ. Both variations contain kinetoplasts and basal bodies, however, their locations are different. Kinetoplasts are organelles that contain self-replicating DNA and are attached to the mitochondrion. Basal bodies are responsible for nucleating axoneme microtubules of the flagellum. In the epimastigote, the kinetoplast and basal body are anterior to the nucleus, with a long flagellum starting from the centre. In the trypomastigote, the kinetoplast and basal body are posterior to the nucleus, with the flagellum starting from the posterior end of the cell body.

parasite

Trypomastigote (right) and epimastigote (left) stages of Trypanosoma brucei