**1. Introduction**

Schistosomiasis is among the most widespread human parasitic diseases with more than 200 million people infected worldwide, with the majority of these infections occurring in sub-Saharan Africa. The human schistosomiasis species present in Senegal are *Schistosoma haematobium* and *S. mansoni* which are transmitted by contact with freshwater snails as an intermediate host and caused urinary or intestinal schistosomiasis, respectively. The intermediate host for *S. mansoni* belongs to the genus *Biomphalaria pfeifferi*, while genus *Bulinus* harbors *S. haematobium* [1–3]. *Biomphalaria pfeifferi* is the only species involved in the transmission of *S. mansoni* in Senegal [4]. It was in the late 1980s that the first cases of intestinal schistosomiasis were diagnosed in the town of Richard-Toll, north-east of the lower Senegal River valley [5]. The transformation of the environment by human agricultural activities favors the creation of breeding sites for the development of snails that spread to

other sites [6, 7]. This situation has been aggravated by hydraulic developments, the construction of numerous small and large dams, as well as the multiplication of irrigation canals [8]. The modification of the practices of the populations bordering these developments is also concretized by intensification of human contact with infected water [9]. These factors contribute to the evolution of the incidence of these schistosome infections and their pathologies in the region [10].

In the present study, the importance of the physicochemical parameters of surface waters (pH, dissolved oxygen, conductivity, phosphates, salinity, nitrates, temperature, and flow velocity of water) on the spatial distribution of snail intermediate hosts of schistosomes in human beings has been studied.
