Schistosomiasis due to Schistosoma mansoni in Madagascar: Spread and focalpatterns.

Schistosoma mansoni and S. haematobium affect respectively 2 million and 50 0,000 persons in Madagascar. Over the past decade, S. mansoni has spread in the central Highlands of Madagascar, essentially throughout the mid-west a nd Antananarivo plain. To understand this recent change in the epidemiology of S, mansoni, we exam ined the relationship between its spatial distribution and several host fac tors, including labour migration, urbanization and water development projec ts. In the Highlands. the disease in distribution could be superimposed on the potential expansion areas of snail distribution defined in 1958. However, t he distribution is not homogeneous, as for example the road between Betafo and Mandoto (South West of Antananarivo). This focal pattern described in o ther African countries is unique To the central Highlands of Madagascar. Ri ce cultivation is the main economic activity and is associated with intense water contact The focal distribution may be related to an environmental ad aptation of host-parasite interaction depending on behavioural patterns, wa ter and soil chemistry and incompatibility between Biomphalaria pfeifferi a nd S. mansoni. It is also possible that these focal patterns precede homoge neous endemicity, as along the road Itasy-Tsiroanomandidy (west Antananariv o). Major water development carried out in this migration area led to a rap id endemization of the disease. In Befato-Mandoto, where soil management is more restricted schistosomiasis due to S, mansoni seems to have been estab lished in some foci where epidemiologic conditions are favourable (for exam ple, traditional irrigation canals). In contrast, the spread of S. mansoni in the Antananarivo plain closely fol lows the settlement of an infected rural population. Epidemiologic surveys conducted on school children in the Antananarivo suburbs where sanitary con ditions 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 ex posure of adult labour migrants has contributed to the widening of the ende mic area.