**Abstract**

Schistosomiasis is becoming more persistent because of the widespread distribution of intermediate host snails in several regions of Africa, including Senegal. The intermediate snail host of the human intestinal schistosome is *Biomphalaria pfeifferi* and is permanently present in northern Senegal because of the presence of the abundant freshwater habitat throughout the year. Here, we observed the seasonal variation in *B. pfeifferi* abundance in the Saint-louis region at the North of Senegal in West Africa. We performed snail and environmental parameter sampling across two different seasons described for Senegal: a dry season that runs roughly from mid-October to mid-June and a rainy season that spans approximately from late June to early October. We also split the dry season into two categories representing periods of time when water temperatures were either decreasing (dry1) or increasing (dry2). We used regression analyses to model snail density across the seasons and investigated which environmental variables influenced snail abundance. Results suggested that snails were more abundant and peaked during the rainy season, which lowest abundances during the dry season when temperatures were declining. The above seasonal variations of snail density were positively linked to the environmental drivers including periphyton (food resource for snails), aquatic vegetation abundance, water temperature and dissolved oxygen and negatively to both pH and water conductivity. Our findings may be useful for snail control efforts by targeting specific periods and/or site conditions when snail abundances are greatest.

**Keywords:** schistosomiasis, intermediate hosts, snails, seasonal variation, *Biomphalaria pfeifferi*, *Schistosoma mansoni*

#### **1. Introduction**

To completely understand many infectious disease systems, it will become increasingly important to understand how seasonality affects multiple processes, including (but not limited to): host behavior, reproduction, survival in the environment [1]. Seasonal variations in temperature, rainfall and resource availability are ubiquitous and can exert strong pressures on host population dynamics.

Infectious diseases provide some of the best-studied examples of the role of seasonality in shaping population fluctuations [1]. Predicting disease dynamics requires an understanding of the traits of hosts across seasons.

Schistosomiasis remains a significant health burden in many areas of the world [2]. Intermediate hosts snails of human schistosomes release parasites that cause human schistosomiasis, are strongly driven by environmental factors [3–5]. The transformation of ecosystems in the river delta Senegal has created favorable biotopes to the development of intermediate host snails of human schistosomes [3], and schistosomiasis rates have increased from historic levels. Thus, limiting or controlling snail populations is an important step in disease control. Yet, consideration of the relative influence of seasonality on the environmental factors that influence snail host populations remains needed.

In this study, we investigated how snail host abundance varies at water access sites in North Senegal, West Africa, a location where human schistosomiasis prevalence is among the highest globally. We conducted our study in Senegal River at four sites used for water access in three villages. We performed biweekly monitoring of *Biomphalaria pfeifferi* across two different seasons in Senegal: a dry season that runs roughly from mid-October to mid-June and a rainy season spanning approximately from late June to early October. Further, we sampled two periods of water temperature T (°C): Dry1 (periods of decreasing temperature (29.4 ≥ T (Dry1) ≥ 15.4) and Dry2 (periods of increasing temperature (15.5 < T (Dry2) ≤ 32.5)). We conducted a total of *n = 21* visits to survey density of hosts and the environmental parameters in the field (as described below) for one year from October 2019 to October 2020.
