**2. Material and method**

#### **2.1 Study sites**

The choice of prospecting sites was guided by their human and animal associations and the presence of human schistosomiasis transmission. The selected villages are bordered by creeks and tributaries of the river in which local people carry out domestic or work activities related to water (**Figure 1**).

In this study, the three villages that have been selected were co-exposed to *S. haematobium* and *S. mansoni* infection. The village of Menguègne Boye (ME) (16°017,315 N, −16°356,659 O) has four (04) water points (ME1, ME2, ME3, ME4), the village of Ndellé Boye (NE) (16°168,725 N, −16°289,124 O) has two (02) water points (NE1, NE2), and the village of Thilla (TA) (16°054,994 N, −16°331,894 O) also has two (02) water points (TA1 and TA2). NE1 and NE2 are separated by a dam and communicate with each other through transverse pipes.

The study covers the period from 03 July to 02 August 2017 (beginning of the rainy season), due to a survey every 15 days from 9 am to 12 am. Three surveys

**73**

*Intermediate Host Snails of Human Schistosomes in the Senegal River Delta: Spatial…*

at the different water points constituting zones of human-water contact, zones of transmission of the disease, were carried out. In parallel with the malacological investigations, in situ measurements of the physicochemical factors were

The evaluation of the physicochemical parameters was carried out to a depth not exceeding 0.5 meters. Conductivity (μS/cm), salinity (psu), dissolved oxygen (mg/l) and its saturation (%), and nitrate concentration (mg/l) were measured with the YSI 600 multiparameter digital probe recorder (HANNA instruments). The plunging probes were covered by a scraper. Phosphate measurement (mg/l) was performed by using the Phosphate High Range (PHR) model HI 96717 C (HANNA instruments). Briefly, 10 ml of water was first mixed in a tube with the reagent Phosphate HR Reagent B (HANNA instruments). Then the tube was placed in the device for 3 minutes to obtain the value. Hydric potential (pH) was obtained using an ESEE pH-meter (HANNA instruments) by immersion of a probe. The flow velocity (m/s) and the water temperature (°C) were measured by a flowatch (JDC electronic) equipped with a submerged propeller at a depth of

The presence of snails was looked for at each water point using a wire wick (2.5 mm) by diversifying the areas surveyed. The areas of prospect were the *Typha* area, floating vegetation, and mud. Depth was measured before each scoop with the dip net. The number of scoops was a function of the surface of the water point and varied between 10 and 15. After washing the vegetation in the basin, the water was filtered using a metal screen (2.5 mm) only passing water and fine debris. The snails were harvested with tongs. All snails from the same waterhole were grouped in one or more pots if necessary and brought to the laboratory for identification.

The identification is based on the Mandahl-Barth key based on the morphology of the shell. Snails not identifiable to the eye were observed with a binocular magnifying glass. The latter method was mainly of interest to snails of the genus *Bulinus* because the species of the genus *Biomphalaria pfeifferi* is easily recognizable by the discoid shape of its shell. The density (d) of snails was expressed in (average)

The R Studio, Excel software, and XLSTAT extension were used for the analysis of the results. Results are presented +/− SD averages. In order to establish a relationship between the different physical (conductivity, flow rate, water temperature) and chemical (dissolved oxygen, saturation, water salinity, pH, nitrate contents, and phosphates) parameters and the density of snails, a statistical principal component analysis (PCA) was applied to all variables. With XLSAT, the realization of principal component analysis allowed to analyze a table of observations/quantita-

*DOI: http://dx.doi.org/10.5772/intechopen.85842*

**2.2 Evaluation of physicochemical parameters**

conducted.

0.5 meters.

**2.3 Malacological study**

*2.3.2 Identification of snails*

numbers of snails per scoop [11].

tive variables or a correlation or covariance matrix.

**2.4 Statistical analyses**

*2.3.1 Snail collection*

**Figure 1.** *Map of the study sites.*

*Intermediate Host Snails of Human Schistosomes in the Senegal River Delta: Spatial… DOI: http://dx.doi.org/10.5772/intechopen.85842*

at the different water points constituting zones of human-water contact, zones of transmission of the disease, were carried out. In parallel with the malacological investigations, in situ measurements of the physicochemical factors were conducted.

#### **2.2 Evaluation of physicochemical parameters**

The evaluation of the physicochemical parameters was carried out to a depth not exceeding 0.5 meters. Conductivity (μS/cm), salinity (psu), dissolved oxygen (mg/l) and its saturation (%), and nitrate concentration (mg/l) were measured with the YSI 600 multiparameter digital probe recorder (HANNA instruments). The plunging probes were covered by a scraper. Phosphate measurement (mg/l) was performed by using the Phosphate High Range (PHR) model HI 96717 C (HANNA instruments). Briefly, 10 ml of water was first mixed in a tube with the reagent Phosphate HR Reagent B (HANNA instruments). Then the tube was placed in the device for 3 minutes to obtain the value. Hydric potential (pH) was obtained using an ESEE pH-meter (HANNA instruments) by immersion of a probe. The flow velocity (m/s) and the water temperature (°C) were measured by a flowatch (JDC electronic) equipped with a submerged propeller at a depth of 0.5 meters.

#### **2.3 Malacological study**

#### *2.3.1 Snail collection*

*Invertebrates - Ecophysiology and Management*

**2. Material and method**

**2.1 Study sites**

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

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 inter-

The choice of prospecting sites was guided by their human and animal associations and the presence of human schistosomiasis transmission. The selected villages are bordered by creeks and tributaries of the river in which local people carry out

In this study, the three villages that have been selected were co-exposed to *S. haematobium* and *S. mansoni* infection. The village of Menguègne Boye (ME) (16°017,315 N, −16°356,659 O) has four (04) water points (ME1, ME2, ME3, ME4), the village of Ndellé Boye (NE) (16°168,725 N, −16°289,124 O) has two (02) water points (NE1, NE2), and the village of Thilla (TA) (16°054,994 N, −16°331,894 O) also has two (02) water points (TA1 and TA2). NE1 and NE2 are separated by a dam

The study covers the period from 03 July to 02 August 2017 (beginning of the rainy season), due to a survey every 15 days from 9 am to 12 am. Three surveys

schistosome infections and their pathologies in the region [10].

mediate hosts of schistosomes in human beings has been studied.

domestic or work activities related to water (**Figure 1**).

and communicate with each other through transverse pipes.

**72**

**Figure 1.**

*Map of the study sites.*

The presence of snails was looked for at each water point using a wire wick (2.5 mm) by diversifying the areas surveyed. The areas of prospect were the *Typha* area, floating vegetation, and mud. Depth was measured before each scoop with the dip net. The number of scoops was a function of the surface of the water point and varied between 10 and 15. After washing the vegetation in the basin, the water was filtered using a metal screen (2.5 mm) only passing water and fine debris. The snails were harvested with tongs. All snails from the same waterhole were grouped in one or more pots if necessary and brought to the laboratory for identification.

#### *2.3.2 Identification of snails*

The identification is based on the Mandahl-Barth key based on the morphology of the shell. Snails not identifiable to the eye were observed with a binocular magnifying glass. The latter method was mainly of interest to snails of the genus *Bulinus* because the species of the genus *Biomphalaria pfeifferi* is easily recognizable by the discoid shape of its shell. The density (d) of snails was expressed in (average) numbers of snails per scoop [11].

#### **2.4 Statistical analyses**

The R Studio, Excel software, and XLSTAT extension were used for the analysis of the results. Results are presented +/− SD averages. In order to establish a relationship between the different physical (conductivity, flow rate, water temperature) and chemical (dissolved oxygen, saturation, water salinity, pH, nitrate contents, and phosphates) parameters and the density of snails, a statistical principal component analysis (PCA) was applied to all variables. With XLSAT, the realization of principal component analysis allowed to analyze a table of observations/quantitative variables or a correlation or covariance matrix.
