The Senegalese climate, with its short rain season from July to October, provokes a very rapid pullulation of many arthropodes, particularly mosquitoes, which persist in great amount only till the arrival of cold trade-winds from the North. This climatic rhythm explains the two epidemic explosions of mosquitoe borne viroses which we have observed: yellow fever and chikungunya, as well as their not less rapid diseappearance. This pattern of brusk appearance at the end of the rain season and disappearance with the cold season has been observed for all yellow fever epidemics reported in this country since 1778.
On the contrary, ornithodores are much less influenced by external conditions; this fact seems to explain the appearance of virus harboured by these vecturs during all dry and cold season. In this last case, other influences certainly play a role: biology of the host, intervention of the vectors.
In spite of these persisting uncertainties, the contrast between the two epidemiological schemes is striking and correspond to a situation which very likely exists in a great part of Western Africa savannahs.