**3.1 Physicochemical parameters of the water points**

The averages of the eight [8] abiotic factors measured in our eight water points are shown in **Figure 2**. The highest water temperature was observed at Mbenguègne Boye (ME3) with 29.9°C. Thilla with its first site (TA1) recorded 29.8°C, while Ndellé Boye had the lowest temperature from its second NE2 water point with 28.5°C. Only the water points NE2 and TA2 recorded a speed greater than 0 (zero) with, respectively, 1 m/s and 3 m/s. The maximum conductivity was obtained at Ndellé Boye: 200.26 μS/cm for NE1 and 195.16 μS/cm for NE2. The lowest conductivity content was found at the first Menguègne Boye point (ME1, 132.06 μS/cm). Dissolved oxygen (mg/l) showed +/− significant variations from 38.3 mg/l to ME3 to 5.7 mg/l to TA2. At Menguègne Boye the maximum dissolved O2 content was observed at ME1

**75**

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

**Biom. B. fors. B. glob. B. sene. B. trun.**

**ME1** 2.16 (± 0.64) 1.04 (± 0.31) 0.5 (± 0.15) 0.11 (± 0.03) 27.54 (± 8.26) **ME2** 0.3 (± 0.09) 1.1 (± 0.33) 0.46 (± 0.13) 0.93 (± 0.27) 4.43 (± 1.32) **ME3** 0 0 0,13 (± 0,04) 0 2,86 (± 0,85) **ME4** 0.06 (± 0.01) 0.2 (± 0.06) 0.13 (± 0.041) 0 1.26 (± 0.37)

**NE2** 0 0.47 (± 0.14) 0 0.38 (± 0.11) 5.32 (± 1.59)

**TA2** 0 0.06 (± 0.01) 0 0 0.06 (± 0.01)

36.83 mg/l and ME3 (38.7 mg/l). Thilla had the lowest dissolved O2 content at point 2 (5.7 mg/l at TA2). The salt content obtained is very weak (1 ppt = 1–9 mg/l). We found in Ndellé (NE1) the largest salt measure (0.503 psu), followed by ME2 with 0.4 ppt. The lowest levels were found at ME1 (0.063 psu) and ME3 (0.066 psu). The pH showed its maximum values in Menguègne Boye—7.4 to ME1, 7.02 to ME2, and 6.92 at ME4—while Thilla recorded the lowest value at its first point (6.59). At Ndellé, we had the maximum nitrate content (0.95 mg/l) and the average phosphate levels: 0.46 mg/l at point 1 (NE1) and 0.56 mg/l at NE2. TA2 had the highest phosphate content (1.06 mg/l), and TA1 showed only 0.03 mg/l, while we found 0 mg/l at the third point of Menguégne Boye.

*Biom. = Biomphalaria, B. = Bulinus, fors. = forskalii, glob. = globosus, sene. = senegalensis, trun. = truncatus.*

*Density of intermediate host snail of human schistosomes (number of molluscs/scoop).*

Ndellé Boye **NE1** 0 0 0 0 0,69 (± 0,20)

Thilla **TA1** 2.38 (± 0.71) 0.05 (± 0.01) 0.076 (± 0.02) 0 1.38 (± 0.41)

The malacofauna intermediate host of human schistosomiasis consisted of *Biomphalaria pfeifferi*, *Bulinus forskalii*, *B. globosus*, *B. senegalensis*, and *B. truncatus*. The total population of snails recovered in the three villages (eight water points) was 2068 snails. The analysis of the malacological data showed that *B. truncatus* colonized all study sites (**Table 1**). Its dominance within this stand was observed in all water points. However, ME1 had the highest density of *B. truncatus*, while *B. globosus* (0.5 individuals/scoop) is absent at Ndellé (NE1, NE2) and TA1. *B. forskalii* was mainly present in Menguègne Boye with d = 1.04 at ME1 and d = 1.1 at ME2. *B. senegalensis* was found only at Mbenguègne Boye (ME1 and ME2) and at the second point of Ndellé (NE2, d = 0.38). *Biomphalaria pfeifferi* was the second most represented spe-

**3.3 Relationship between intermediate hosts snails of schistosomes and** 

**Figure 3** shows that the presence of intermediate snail hosts was positively correlated with temperature, pH and dissolved oxygen, and its saturation. Salts (phosphate, salinity, and nitrates), conductivity, and velocity did not have any direct effect on the presence of snails. The PCA indicates that the pH, the dissolved O2, and its saturation (%) were strongly linked to the presence of the species *B. globosus, B. truncatus*, and *B. forskalii*, whereas they were moderately related to that of *Biomphalaria* as shown in the correlation matrix. Projections on PCA of salinity, nitrate, and phosphate levels of water are orthogonal to those of snails in general, indicating that there were no significant direct influences of these

cies (d = 2.16 at ME1 and d = 2.38 at TA1) after *B. truncatus*.

**measured abiotic factors of the biotope**

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

*ME= Mbenguègne Boye, NE= Ndellé Boye, TA= Thilla*

**Site Water** 

Menguègne Boye

**Table 1.**

**point**

**3.2 Malacological data**

**Figure 2.** *Physicochemical data (average) of the water points.*

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


*Biom. = Biomphalaria, B. = Bulinus, fors. = forskalii, glob. = globosus, sene. = senegalensis, trun. = truncatus. ME= Mbenguègne Boye, NE= Ndellé Boye, TA= Thilla*

#### **Table 1.**

*Invertebrates - Ecophysiology and Management*

**3.1 Physicochemical parameters of the water points**

The averages of the eight [8] abiotic factors measured in our eight water points are shown in **Figure 2**. The highest water temperature was observed at Mbenguègne Boye (ME3) with 29.9°C. Thilla with its first site (TA1) recorded 29.8°C, while Ndellé Boye had the lowest temperature from its second NE2 water point with 28.5°C. Only the water points NE2 and TA2 recorded a speed greater than 0 (zero) with, respectively, 1 m/s and 3 m/s. The maximum conductivity was obtained at Ndellé Boye: 200.26 μS/cm for NE1 and 195.16 μS/cm for NE2. The lowest conductivity content was found at the first Menguègne Boye point (ME1, 132.06 μS/cm). Dissolved oxygen (mg/l) showed +/− significant variations from 38.3 mg/l to ME3 to 5.7 mg/l to TA2. At Menguègne Boye the maximum dissolved O2 content was observed at ME1

**3. Results**

**74**

**Figure 2.**

*Physicochemical data (average) of the water points.*

*Density of intermediate host snail of human schistosomes (number of molluscs/scoop).*

36.83 mg/l and ME3 (38.7 mg/l). Thilla had the lowest dissolved O2 content at point 2 (5.7 mg/l at TA2). The salt content obtained is very weak (1 ppt = 1–9 mg/l). We found in Ndellé (NE1) the largest salt measure (0.503 psu), followed by ME2 with 0.4 ppt. The lowest levels were found at ME1 (0.063 psu) and ME3 (0.066 psu). The pH showed its maximum values in Menguègne Boye—7.4 to ME1, 7.02 to ME2, and 6.92 at ME4—while Thilla recorded the lowest value at its first point (6.59). At Ndellé, we had the maximum nitrate content (0.95 mg/l) and the average phosphate levels: 0.46 mg/l at point 1 (NE1) and 0.56 mg/l at NE2. TA2 had the highest phosphate content (1.06 mg/l), and TA1 showed only 0.03 mg/l, while we found 0 mg/l at the third point of Menguégne Boye.

#### **3.2 Malacological data**

The malacofauna intermediate host of human schistosomiasis consisted of *Biomphalaria pfeifferi*, *Bulinus forskalii*, *B. globosus*, *B. senegalensis*, and *B. truncatus*. The total population of snails recovered in the three villages (eight water points) was 2068 snails. The analysis of the malacological data showed that *B. truncatus* colonized all study sites (**Table 1**). Its dominance within this stand was observed in all water points. However, ME1 had the highest density of *B. truncatus*, while *B. globosus* (0.5 individuals/scoop) is absent at Ndellé (NE1, NE2) and TA1. *B. forskalii* was mainly present in Menguègne Boye with d = 1.04 at ME1 and d = 1.1 at ME2. *B. senegalensis* was found only at Mbenguègne Boye (ME1 and ME2) and at the second point of Ndellé (NE2, d = 0.38). *Biomphalaria pfeifferi* was the second most represented species (d = 2.16 at ME1 and d = 2.38 at TA1) after *B. truncatus*.

### **3.3 Relationship between intermediate hosts snails of schistosomes and measured abiotic factors of the biotope**

**Figure 3** shows that the presence of intermediate snail hosts was positively correlated with temperature, pH and dissolved oxygen, and its saturation. Salts (phosphate, salinity, and nitrates), conductivity, and velocity did not have any direct effect on the presence of snails. The PCA indicates that the pH, the dissolved O2, and its saturation (%) were strongly linked to the presence of the species *B. globosus, B. truncatus*, and *B. forskalii*, whereas they were moderately related to that of *Biomphalaria* as shown in the correlation matrix. Projections on PCA of salinity, nitrate, and phosphate levels of water are orthogonal to those of snails in general, indicating that there were no significant direct influences of these

**Figure 3.** *Relationship between the density of intermediate host snails and the physicochemical parameters of water.*

parameters on snail populations. The temperature is positively correlated with *Biomphalaria*. Conductivity, velocity, and salinity are more or less opposed to those of *B. globosus, B. truncatus*, and *B. forskalii*.

## **4. Discussion**

The temperatures measured ranged from a minimum of 28.5°C to Ndellé Boye to a maximum of 29.9°C to ME3. The lowest temperatures recorded at Ndellé could be explained by the fact that the freshwater ecosystem found in this environment is less exposed to light because of its highly developed eutrophication. It has been shown [11] that during the month of July, water temperatures on the other side of the Senegal River (Right River) vary between 26.1°C and 28.9°C. pH values vary almost by one unit (between 6.59 and 7.4). Its slight basicity accompanies the words of N'Diaye *et al*. [12] who said that the pH values measured in the waters of the Senegal River place the latter in the excellent to good level of surface water (6.5 < pH < 8.5). Phosphate and nitrate contents would not only come from agricultural land (leaching of fertilizers by rainwater) but also from household activities for which women use different detergents and antiseptics. Halstead et al. [13] shows the potential impact of agrochemicals on the transmission of schistosomiasis and therefore on snails. In a recent study in the same region, Diallo et al. [14] argue that washing clothes directly in the river alone probably justifies the presence of phosphates. They also argue that rains can be an important vector for the transport of domestic waste, urine, and excrement of animals to the river by the phenomenon of leaching. The strong eutrophication noted on the biotope particularly to Ndellé Boye would come from the important contents of nitrates and phosphates in the water. In addition, the presence of the Diama Dam and the association of

**77**

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

variation (47.4 and 67.1 μS/cm) than that observed in our study sites.

[16] confirm that they are favorable to the proliferation of this species.

The presence of biotopes favorable or not to the life of the snails is due to ecological transformations of the environments. Among these transformations, we can note the presence of vegetation that could determine the presence or absence of snails [22]. The physicochemical conditions that accompanied ecological changes could influence the distribution of snails. A high rate of water conduction has been noted at TA2; the rarity of snails observed at this point of water could be due to speed. Speed is a physical factor that opposes the residence of snails if it exceeds 0.3 m/s [19]. The values obtained for oxygen and pH are favorable for the habitat of gastropods. The study shows a positive correlation between temperature and *Biomphalaria*, whereas temperatures of 29.9°C have no influence on the *Bulinus*. In a study done in the lake of the Taabo Dam in Côte d'Ivoire, Gbocho et al. [16] show a positive correlation of temperature with *Biomphalaria* at a maximum T° of 31.5°, whereas this temperature acts negatively on the populations of *Bulinus*. The measured salinity has no significant influence on snails (except *Biomphalaria* and *B. truncatus* species). This could be due to its very low content. Diaw et al. [19] give the example of the effects of this parameter on the development of snails in the delta of the Senegal River where these gastropods have proliferated after the start of operation of the Diama anti-salt dam. Significant values of conductivity obtained at Ndellé

The simultaneous presence of both intermediate host snail species is indicative of the existence of both schistosomiases in the study area. Diaw et al. [18] note the presence of *Biomphalaria pfeifferi, Bulinus globosus, B. truncatus, B. senegalensis*, and *B. forskalii* at Mbodiène bordered by Lampsar, which was a major focus of schistosomiasis. It also shows the existence of these organisms in the delta with an increase in populations of *Biomphalaria* and a wider distribution. Five [5] species of snail intermediate hosts of human schistosomes in Senegal were found in our study sites. *B. senegalensis*, an intermediate host of *S. haematobium*, is very common and abundant in the regions of Saint-Louis, Tambacounda, Kaolack, and Fatick [19, 20]. We thus find its presence in Menguègne Boye (d = 0.11 for ME1 and 0.93 for ME2). *B. globosus* and *B. truncatus*, intermediate hosts of *S. haematobium*, are very commonly found in the Senegal River delta. *Biomphalaria pfeifferi*, the main intermediate host of *S. mansoni*, was not found in Ndellé Boye although its presence has been previously mentioned [21] in Lampsar. These authors indicate that in the delta, this species was the most abundant in the early 1990s. However, the presence of this snail in Thilla and Menguègne Boye with respective densities of 2.38 and 2.16 was quite poor, which has been confirmed by the work of Ndir [20] who supports a reduced presence in the delta but with a range tending to extend toward the southern region (Louga region) since the impoundment of the dams. The dominance of *B. truncatus* in our study sites is very remarkable. This could be explained by the fact that the sites Menguègne Boye, Ndellé Boye, and Thilla are permanent watercourses. Gbocho et al.

dikes reduce the oxidation and flow velocity of the valley water [15]. These facts, combined with waste dumps around the villages and cities and agrochemicals from irrigated fields, accelerated eutrophication of water in the Senegal River Valley. The very low salt levels indicate that the watercourses of our study sites are freshwater ecosystems. The electrical conductivity which is a measure of the ability of an aqueous solution to conduct electrical current shows significant variations (132.06 and 200.26 μS/cm). The high levels obtained to Ndellé (200, 26 μS/cm at NE1 and 195.16 μS/cm to NE2) are due to the strong contributions of organic matter in the water, resulting in their greater mineralization. By comparing the values of the conductivity measured at the WHO reference water level, which is 300 μs/cm [16], we deduce that the water from these effluents of the Senegal River is of good quality. However, a study by Tfeila et al. [17] on the Senegal River indicates a much smaller

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

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

dikes reduce the oxidation and flow velocity of the valley water [15]. These facts, combined with waste dumps around the villages and cities and agrochemicals from irrigated fields, accelerated eutrophication of water in the Senegal River Valley. The very low salt levels indicate that the watercourses of our study sites are freshwater ecosystems. The electrical conductivity which is a measure of the ability of an aqueous solution to conduct electrical current shows significant variations (132.06 and 200.26 μS/cm). The high levels obtained to Ndellé (200, 26 μS/cm at NE1 and 195.16 μS/cm to NE2) are due to the strong contributions of organic matter in the water, resulting in their greater mineralization. By comparing the values of the conductivity measured at the WHO reference water level, which is 300 μs/cm [16], we deduce that the water from these effluents of the Senegal River is of good quality. However, a study by Tfeila et al. [17] on the Senegal River indicates a much smaller variation (47.4 and 67.1 μS/cm) than that observed in our study sites.

The simultaneous presence of both intermediate host snail species is indicative of the existence of both schistosomiases in the study area. Diaw et al. [18] note the presence of *Biomphalaria pfeifferi, Bulinus globosus, B. truncatus, B. senegalensis*, and *B. forskalii* at Mbodiène bordered by Lampsar, which was a major focus of schistosomiasis. It also shows the existence of these organisms in the delta with an increase in populations of *Biomphalaria* and a wider distribution. Five [5] species of snail intermediate hosts of human schistosomes in Senegal were found in our study sites. *B. senegalensis*, an intermediate host of *S. haematobium*, is very common and abundant in the regions of Saint-Louis, Tambacounda, Kaolack, and Fatick [19, 20]. We thus find its presence in Menguègne Boye (d = 0.11 for ME1 and 0.93 for ME2). *B. globosus* and *B. truncatus*, intermediate hosts of *S. haematobium*, are very commonly found in the Senegal River delta. *Biomphalaria pfeifferi*, the main intermediate host of *S. mansoni*, was not found in Ndellé Boye although its presence has been previously mentioned [21] in Lampsar. These authors indicate that in the delta, this species was the most abundant in the early 1990s. However, the presence of this snail in Thilla and Menguègne Boye with respective densities of 2.38 and 2.16 was quite poor, which has been confirmed by the work of Ndir [20] who supports a reduced presence in the delta but with a range tending to extend toward the southern region (Louga region) since the impoundment of the dams. The dominance of *B. truncatus* in our study sites is very remarkable. This could be explained by the fact that the sites Menguègne Boye, Ndellé Boye, and Thilla are permanent watercourses. Gbocho et al. [16] confirm that they are favorable to the proliferation of this species.

The presence of biotopes favorable or not to the life of the snails is due to ecological transformations of the environments. Among these transformations, we can note the presence of vegetation that could determine the presence or absence of snails [22]. The physicochemical conditions that accompanied ecological changes could influence the distribution of snails. A high rate of water conduction has been noted at TA2; the rarity of snails observed at this point of water could be due to speed. Speed is a physical factor that opposes the residence of snails if it exceeds 0.3 m/s [19]. The values obtained for oxygen and pH are favorable for the habitat of gastropods. The study shows a positive correlation between temperature and *Biomphalaria*, whereas temperatures of 29.9°C have no influence on the *Bulinus*. In a study done in the lake of the Taabo Dam in Côte d'Ivoire, Gbocho et al. [16] show a positive correlation of temperature with *Biomphalaria* at a maximum T° of 31.5°, whereas this temperature acts negatively on the populations of *Bulinus*. The measured salinity has no significant influence on snails (except *Biomphalaria* and *B. truncatus* species). This could be due to its very low content. Diaw et al. [19] give the example of the effects of this parameter on the development of snails in the delta of the Senegal River where these gastropods have proliferated after the start of operation of the Diama anti-salt dam. Significant values of conductivity obtained at Ndellé

*Invertebrates - Ecophysiology and Management*

of *B. globosus, B. truncatus*, and *B. forskalii*.

**4. Discussion**

**Figure 3.**

parameters on snail populations. The temperature is positively correlated with *Biomphalaria*. Conductivity, velocity, and salinity are more or less opposed to those

*Relationship between the density of intermediate host snails and the physicochemical parameters of water.*

The temperatures measured ranged from a minimum of 28.5°C to Ndellé Boye to a maximum of 29.9°C to ME3. The lowest temperatures recorded at Ndellé could be explained by the fact that the freshwater ecosystem found in this environment is less exposed to light because of its highly developed eutrophication. It has been shown [11] that during the month of July, water temperatures on the other side of the Senegal River (Right River) vary between 26.1°C and 28.9°C. pH values vary almost by one unit (between 6.59 and 7.4). Its slight basicity accompanies the words of N'Diaye *et al*. [12] who said that the pH values measured in the waters of the Senegal River place the latter in the excellent to good level of surface water (6.5 < pH < 8.5). Phosphate and nitrate contents would not only come from agricultural land (leaching of fertilizers by rainwater) but also from household activities for which women use different detergents and antiseptics. Halstead et al. [13] shows the potential impact of agrochemicals on the transmission of schistosomiasis and therefore on snails. In a recent study in the same region, Diallo et al. [14] argue that washing clothes directly in the river alone probably justifies the presence of phosphates. They also argue that rains can be an important vector for the transport of domestic waste, urine, and excrement of animals to the river by the phenomenon of leaching. The strong eutrophication noted on the biotope particularly to Ndellé Boye would come from the important contents of nitrates and phosphates in the water. In addition, the presence of the Diama Dam and the association of

**76**

(200, 26 μS/cm at NE1 and 195.16 μS/cm to NE2) generally have a negative effect on the density of snails. Which could confirm the remarks of Gbocho et al. [16] who argue that conductivity levels (74 and 77.4 μS/cm) that they observed do not correlate significantly with intermediate host populations because of its low values. Our study demonstrates that physicochemical parameters such as conductivity, dissolved oxygen and its saturation, pH, and water flow velocity have an important role on the spatial distribution of snail intermediate hosts of human schistosomiasis.
