Int. J. Environ. Res. Public Health 2010, 7 1714 Figure 3. Riverine forest destruction. At a first glance (see Figure 1), two factors seem to be slowing down the shift of the distribution limit toward the south: the presence of a permanent hydrographic network and the presence of protected areas. This is particularly the case in the west where the limit has remained relatively stable. Here, this can be explained by two types of factors: (i) macroclimatic, in particular the fact that this area belongs to the South Sudanian zone [37], with the presence of forest galleries, and the presence of the permanent Mouhoun River, which offers (ii) microclimatic conditions (relative hygrometry and host availability) in areas where the gallery has not been destroyed, allowing the survival of riverine tsetse such as G. palpalis gambiensis and G. tachinoides [10,22]. East of the Mouhoun River, the main rivers are the Nazinon and the Nakambe, and in this area where G. p. gambiensis is no longer present, the presence of protected areas and galleries allow the persistence of G. tachinoides. G. morsitans submorsitans is only present within the immediate vicinity of protected areas where it can feed on wildlife and cattle, but it has nearly disappeared from areas where the human population has become too high [16,17,38]. The same phenomenon occurred in Senegal on the Somone River (historical sleeping sickness focus), where tsetse disappeared due to intense vegetation degradation along this river after a drought which led people to migrate to this area [39]. Despite this global trend of a decrease in tsetse observed at the scale of Burkina Faso, it should not be forgotten that (i) tsetse do not need to be numerous to be vectors of trypanosomiasis, since small populations can sometimes be much more dangerous to humans than large populations [5], and (ii) opportunistic species of the G. palpalis group (G. palpalis gambiensis in the savannah, G. p. palpalis in the forest) can survive to very high human density levels, since they have been found to adapt to some of the largest towns on the continent, such as Dakar and Abidjan [17]. In addition, tsetse populations have been shown to increasingly transmit trypanosomes when they are submitted to environmental stress [40].