[Drought and malaria decrease in the Niayes area of Senegal]

The Niayes is geographically characterized by an alternating succession of sand dunes and wet depressions, 20 km wide, which are behind the offshore bars from Saint-Louis to Dakar. Since 1970, the area has been affected by drought. The rainfall which was around 700 mm per year before 1960 has fallen below 500 mm during the last 30 years. In 1991 it was only 350 mm and in 1992, 260 mm. The vegetation has become impoverished in the depressions which are now more and more cultivated for vegetables. Entomological, parasitological and clinical studies on malaria were carried out in 1991 and 1992 in three villages and the results were compared with data gathered from 1967 to 1968 with comparable methods. The main points concerning the evolution of malaria are listed as follows. An. funestus, which was previously the predominant malaria vector, has almost disappeared. An. gambiae has regressed ahead of its sibling species An. arabiensis, a less competent vector. These changes in vector populations have led to lower malaria transmission and consequently a decrease of the endemicity. The children parasitic index, which was between 40 to 80% according to the localities in 1967, fell to 10% or less in 1991 and 1992. Finally, the incidence of clinical cases, calculated on a cohort of 100 children was only 4% in 1992, which is very low for an African endemic country. We conclude that drought and human activities have modified the environment such that malaria endemicity has seriously decreased. However, the risk of epidemic remains with the eventual occurrence of heavy rains. Therefore a surveillance of the area is needed to prevent a future epidemic.