[Intestinal schistosomiasis from Schistosoma mansoni in Madagascar: extent and center of the endemic].

Schistosoma mansoni and S. haematobium affect respectively 2 million and 500,000 persons in Madagascar. Over the past decade, S. mansoni has spread in the central Highlands of Madagascar, essentially throughout the mid-west and Antananarivo plain. To understand this recent change in the epidemiology of S. mansoni, we examined the relationship between its spatial distribution and several host factors, including labour migration, urbanization and water development projects. In the Highlands, the disease in distribution could be superimposed on the potential expansion areas of snail distribution defined in 1958. However, the distribution is not homogeneous, as for example the road between Betafo and Mandoto (South West of Antananarivo). This focal pattern described in other African countries is unique to the central Highlands of Madagascar. Rice cultivation is the main economic activity and is associated with intense water contact. The focal distribution may be related to an environmental adaptation of host-parasite interaction depending on behavioural patterns, water and soil chemistry and incompatibility between Biomphalaria pfeifferi and S. mansoni. It is also possible that these focal patterns precede homogeneous endemicity, as along the road Itasy-Tsiroanomandidy (west Antananarivo). Major water development carried out in this migration area led to a rapid endemization of the disease. In Befato-Mandoto, where soil management is more restricted, schistosomiasis due to S. mansoni seems to have been established in some foci where epidemiologic conditions are favourable (for example, traditional irrigation canals). In contrast, the spread of S. mansoni in the Antananarivo plain closely follows the settlement of an infected rural population. Epidemiologic surveys conducted on school children in the Antananarivo suburbs, where sanitary conditions are poor, showed a prevalence of 25%. Human migration linked to development projects and urbanization seems to be the principal factor associated with the spread of schistosomiasis in the mid-west area and Antananarivo plain. In the Highlands, the preferential exposure of adult labour migrants has contributed to the widening of the endemic area.