The principal vector of T. b. rhodesiense is G. morsitans, which breeds along water courses, but then travels widely throughout the extensive shade cover provided by the forest belt. This open type of forest, commonly called miombo (mainly Brachystegia and Julbemardi spp.), is found in large areas of East Africa. Smaller wild animals inhabit it, especially the bushbuck that forms a reservoir of infection. Towards the margins of this forest belt, it breaks up into thickets separated by savannah grassland in which large numbers of wild animals are found. The tsetse fly ranges widely over these areas, feeding mainly on animals and using the thickets for cover and shade. It is, therefore, humans who travel through the forest fringing savannah in their occupational pursuit and the hunter and honey collector who become infected. Adult males are then the main victims in rhodesiense sleeping sickness.
T. b. rhodesiense infection is not a focal disease and because of its short clinical course, epidemics are uncommon. However, movements of people, such as the development of new settlements in forest areas, will expose a large number of people to infected flies all at the same time, so allowing an epidemic to start. The first signs that this is
happening are where women, and especially children, become infected.
Although these are the main patterns of the two diseases, sometimes a riverine tsetse fly becomes the vector of rhodesiense sleeping sickness.
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