**2. Methods**

#### **2.1 Study sites**

We sampled intermediate hosts biweekly at four water access points across three villages in Senegal, West Africa: Kaban (KA: 16° 3.338 N - 16°24.133 O) Minguegne (ME 1: 16°01.055' N - 16°21.397' O and ME2:16°01.090' N - 16°21.369 O) and

*Seasonal Variations of Densities of* Biomphalaria pfeifferi*, the Intermediate Host… DOI: http://dx.doi.org/10.5772/intechopen.99217*

Ndiawdoune (NW: 16°4.075' N - 16°23.635' O). Kaban village is bordered by the Senegal river, Minguegne is bordered to the Ngalam outlet Senegal River in the "Trois marigots" zone and Ndiawdoune is bordered by the Lampsar River (**Figure 1**).

### **2.2 Environmental factors driving host snail abundance**

We recorded dissolved oxygen (DO), pH, water conductivity and water temperature using a YSI Professional Plus handheld multiparameter meter. We also recorded periphyton fluorescence using an Aquapen AP 100-C handheld flourometer. Periphyton was collected from a study site during snail sampling (see below) by wading into the water and cutting a stem of *Typha* spp. vegetation at the water surface and again at a depth of 10 cm below the water. The 10 cm section of Typha was taken to the lab and its surface scrubbed using a toothbrush. We then washed all algae off the Typha and brush using deionized water into a 50 mL falcon tube and filled all samples to a standardized volume of 50 mL. Chlorphyll a was then quantified by filling a 1 mL cuvette and recording Ft values using the Aquapen. We recorded the surface area of *Typha* that was sampled and used it to standardize the periphyton flouresence values based on sampling area.

#### **2.3 Snail sampling**

We used the snail sampling method described in [6]. At each water access point, we conducted 10 1-m sweeps with a 2.5-mm mesh aquatic dipnet at random sampling points. Any aquatic plants in the dipnet were placed into a wash pale with water, shaken vigorously to remove snails, and examined for any attached snails before weighing the vegetation mass using a spring scale. We recorded the number of *Biomphalaria pfeiferii* snails (**Figure 2**) captured per sweep.

#### **2.4 Statistical analysis**

We conducted our data analysis using R software version string *R version 4.0.5 (2021-2103-31)*. We applied a forest plot in the *Forest model* package to a linear model to assess the variation of snail's density across the seasons. In forest plots, a line of no effect (at 0) marks the point where there is no clear difference between the variables. If the 95% confidence intervals (CIs) do not cross the line of no effect, then the result is significant (p-value <0.05). To elucidate actions between

#### **Figure 2.**

Biomphalaria pfeifferi *snail species of the current study. This is the only known intermediate host of*  Schistosoma mansoni, *the parasite of the intestinal schistosomiasis in Senegal.*

environmental parameters and hosts density, we applied Pearson's correlation, which gives a measure of the strength of a linear association between two variables. We also determined the *p-value* or probability that we would have found the current result if the correlation coefficient *r* were in fact zero (null hypothesis). If this probability is lower than the conventional 5% (*p* < 0.05) the correlation coefficient is called statistically significant. We used linear model in *ggplot2* package that utilizes a *lm* function to assess the significance of linear predictor-response relationships between hosts and environmental drivers.
