WHO Report on Global Surveillance of Epidemic-prone Infectious Diseases - African trypanosomiasis
African sleeping sickness is transmitted primarily by bites from infected tsetse flies. Transmission is also possible through contamination with infected blood or through the placenta (congenital). There are seven different species of tsetse fly which can transmit the disease, and all live uniquely in sub-Saharan Africa. Tsetse flies have a life span on average of between one and six months. They live in warm, shady, humid areas. Once infected with trypanosoma, they remain infective for life.
Gambiense sleeping sickness occurs mainly in lowland rain forests of West and Central Africa. It is spread primarily by peri-domestic tsetse flies, living in areas surrounding human habitats such as cultivated land, and near small rivers or pools of water, frequented by people. Thus, there is close contact between people and tsetse flies as people go about their daily activities. Gambiense sleeping sickness is a chronic disease with a long latency period and people can be infective for many years without knowing. Studies have indicated that a small number of tsetse flies can maintain endemic transmission cycles at relatively high levels (3). In light of the above, it is not surprising that it is very difficult to stop transmission of gambiense sleeping sickness completely in a given locality, and in many villages, sleeping sickness recurs periodically.
Rhodesiense sleeping sickness is much more virulent than gambiense and infected people usually die within a matter of months. Tsetse flies that carry the disease live primarily in the savannah woodlands of eastern and southern Africa. Humans are affected when they go into the savannah for activities such as gathering wood, gathering honey, hunting, fishing, keeping cattle or cultivating land.
An important feature of African trypanosomiasis is its focal nature. It tends to occur in circumscribed zones. Observed prevalence rates vary greatly from one geographical area to another, and even between one village and another within the same area. Thus, it is important to understand the ecology and resulting transmission patterns in each locality.
(3)Lyons M, African Trypanosomiasis. In: Kiple, ed, The Cambridge History of Human Disease, Cambridge University Press, 1993.