Trypanosoma Brucei

Trypanosoma brucei is a single-celled parasite found across sub-Saharan Africa that, unlike most protozoan parasites, lives entirely outside the cells of its host, moving through the blood plasma and other body fluids. It causes African trypanosomiasis, commonly known as sleeping sickness, in humans, and a related disease called nagana in cattle and horses, and it is spread by the bite of the tsetse fly. The parasite is divided into several subspecies and is notable for repeatedly changing the proteins on its outer surface, a strategy called antigenic variation that lets it evade the host immune system and sustain a long-running infection; in advanced cases it can cross into the brain, complicating treatment. It was first identified by Scottish pathologist David Bruce in 1894.