**4. Discussion**

## **4.1 Seasonal variations and environmental parameters driving the densities of snail hosts**

Ecosystem changes in the delta of the Senegal River has created favorable biotopes to the development of intermediate host snails of human schistosomes, which may have included particular the physicochemical conditions favorable to their abundance [3]. Permanent freshwater is associated with aquatic floating vegetation such as *Ceratophyllum sp* and *Ludwigia sp* that acts as beneficial snails habitat in several areas in the Delta of Senegal River. The presence of the intermediate host *Biomphalaria pfeifferi* was previously described in several studies in the study area [3, 6–8]. In this area, *Biomphalaria pfeifferi* was the second most abundant intermediate host of human schistosomiasis after *Bulinus truncatus* [3]. Our study highlights that *B. pfeifferi* abundance is lowest during a Dry1 season likely because of lower periphyton and water temperature than in the rainy season. Lower water temperature during Dry1 season can limit snail growth and reproduction. For example, *Biomphalaria glabrata*, a neotropical species that is another intermediate host for *S. mansoni*, generally avoids thermal extremes and

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

prefers temperatures from 27° to 32° C [9]. The greater density of *Biomphalaria* during periods of increasing temperature (19.9° to 32.5°C) during Dry2 and Rainy is increasing abundance of periphyton that acts as snail food, as well as periods of higher dissolved oxygen, temperature and aquatic vegetation that acts as snail habitat. In contrast, during this same period, negative environmental influences on snail abundance including pH and conductivity are declining. Aquatic vegetation provides both habitat and food resources to snail hosts and is itself positively associated to the snail abundance [6], while high conductivity and pH impact negatively snails' intermediate hosts of human schistosome [3]. Although 55.1% of total vegetation mass was collected in the rainy season, vegetation was only a significant predictor of snails in the Dry2 season (10.9%). *Biomphalaria* species prefer areas of clear water on sandy and gravel bottoms, with stagnant water or with a very light current, with sometimes abundant aquatic vegetation [9]. Snail abundance was most associated with oxygen during the Dry2 season, and *Biomphalaria pfeifferi* is known to be positively associated with dissolved oxygen [3], whereas pH in the range 5.0 to 7.5 is likely a weaker determinant of snails [9]. We suspect that snail density was not significantly correlated to water temperature because of every high temperature (32.5° C) we found during the rainy season. This driver could limit reproduction and other physiological functions or be lethal to the snail survival. *Biomphalaria pfeifferi* does best under warm stable conditions [10]. Thus, future studies should also consider non-linear relationships in their analyses. We found that the month of August was the most important for snail control because densities of host during the rainy season were fueled by ideal environmental conditions in freshwater (i.e: resource food availability that may increase host reproduction). During this period, rainwater transports organic wastes, fertilizer and pesticides from agricultural areas to adjacent waters. Such agrochemical runoff may affect the development aquatic vegetation and periphyton and pesticide pollution is a major driver in increasing the occurrence of host snails [11, 12]. Mesocosm studies support the assertion that fertilizer, herbicide, and insecticide, individually and as mixtures may be increasing the algae snails eat in the rainy season [13]. Moreover, such chemicals can decrease the densities of snail predators. Our findings may be context dependent to the Senegal River as seasonality may favor the dry season months in other contexts [14].
