Density of intermediate host snail of human schistosomes (number of molluscs/scoop).
The objective of the study was to evaluate the influence of physicochemical parameters of water on the spatial distribution of snail intermediate hosts of human schistosomes in the Senegal River Delta. Eight water points in three endemic villages for schistosomiasis were selected for biweekly monitoring of snail numbers and physicochemical parameters of water at the beginning of the rainy season. The results show that the spatial distribution of snail populations is a function of certain parameters. The pH, the dissolved oxygen and its saturation, and the temperature have a positive influence on the Bulinus and Biomphalaria, while the conductivity, the speed of flow, and the salts (phosphates, salinity, and nitrates) seem to act negatively on the populations of these snails.
- physicochemical parameters of water
- intermediate hosts
- spatial distribution
Schistosomiasis is among the most widespread human parasitic diseases with more than 200 million people infected worldwide, with the majority of these infections occurring in sub-Saharan Africa. The human schistosomiasis species present in Senegal are
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 intermediate hosts of schistosomes in human beings has been studied.
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
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 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
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
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
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.
3.1 Physicochemical parameters of the water points
The averages of the eight  abiotic factors measured in our eight water points are shown inFigure 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 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
|Site||Water point||Biom.||B. fors.||B. glob.||B. sene.||B. trun.|
|Menguègne Boye||2.16 (± 0.64)||1.04 (± 0.31)||0.5 (± 0.15)||0.11 (± 0.03)||27.54 (± 8.26)|
|0.3 (± 0.09)||1.1 (± 0.33)||0.46 (± 0.13)||0.93 (± 0.27)||4.43 (± 1.32)|
|0||0||0,13 (± 0,04)||0||2,86 (± 0,85)|
|0.06 (± 0.01)||0.2 (± 0.06)||0.13 (± 0.041)||0||1.26 (± 0.37)|
|Ndellé Boye||0||0||0||0||0,69 (± 0,20)|
|0||0.47 (± 0.14)||0||0.38 (± 0.11)||5.32 (± 1.59)|
|Thilla||2.38 (± 0.71)||0.05 (± 0.01)||0.076 (± 0.02)||0||1.38 (± 0.41)|
|0||0.06 (± 0.01)||0||0||0.06 (± 0.01)|
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
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  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
The simultaneous presence of both intermediate host snail species is indicative of the existence of both schistosomiases in the study area. Diaw et al.  note the presence of
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 . 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 . The values obtained for oxygen and pH are favorable for the habitat of gastropods. The study shows a positive correlation between temperature and
The study of the influence of physicochemical parameters on the spatial distribution of intermediate snail hosts in human schistosomes in the Senegal River delta provided insights on the diversity of snail and their density and, secondly, the role of these parameters in the distribution of snails. The presence of snail intermediate hosts of human schistosomes would be conditioned by the temperature, oxygen, and pH with which they are positively correlated. An increase in salt (phosphates, nitrates, and salinity), conductivity, and velocity would lead to a lower density of snails.
Isabwe A, Ruberanziza E, Mupfasoni D, Ruxin J, Clerinx J, White PT. Potential for transmission of schistosomiasis in Kayonza. Rwanda Medical Journal. 2012; 6:8
Sturrock RF. The parasites and their life cycles. Human Schistosomiasis Rwanda Medical Journal. 1993; 69:14-19
Sy I, Diawara L, Ngabo D, Barbier D, Dreyfuss G, Georges P. Bilharzioses au Sénégal oriental prévalence chez les enfants de la région de Bandafassi. La Medicina Tropical. 2008; 68:267
Campbell G, Noble LR, Rollinson D, Southgate VR, Webster JP, Jones CS. Low genetic diversity in a snail intermediate host ( Biomphalaria pfeifferiKross, 1848) and schistosomiasis transmission in the Senegal River basin. Molecular Ecology. 2010; 19:241-256
Talla I, Kongs A, Verle P, Belot J, Sarr S, Coll AM. Outbreak of intestinal schistosomiasis in the Senegal River basin. Annales de la Société Belge de Médecine Tropicale. 1990; 70:173-180
Symoens JJ, Burgis M, Gaudet JJ, others. Ecologie et Utilisation des Eaux Continentales Africaines. UNEP; 1981
Steinmann P, Keiser J, Bos R, Tanner M, Utzinger J. Schistosomiasis and water resources development: Systematic review, meta-analysis, and estimates of people at risk. The Lancet Infectious Diseases. 2006; 6:411-425
Traoré M. Importance des aménagements hydrauliques dans la transmission des schistosomoses. In: Chippaux J.P. Lutte Contre les Schistosomes en Afrique de l’Ouest; 15-18 Février 2000; Niamey-Cermes. Paris: éditions IRD. 2000. p. 292
Helmut K, Andréa G, others. Microgeographical patterns of schistosomiasis and water contact behavior; examples from Africa and Brazil. 1998. pp. 37-50
Cecchi P, Baldé S, Yapi YG. Mollusques hôtes intermédiaires de bilharzioses dans les petits barrages. L’eau en partage. Les petits barrages de Côte d’Ivoire. 2007. pp. 175-189
Mandahl-Barth G. Les hôtes intermédiaires de Schistosoma: Biomphalariaet Bulinusafricains. Danemarks Akvarium, Charlottenlund, Danemark,Palais des Nations, Genève, 16, pp. 1103-1163 et 17, pp. 1-65. 1959
N’Diaye AD, Salem KMM. Contribution a l’étude de la qualité physico-chimique de l’eau de la rive droite du fleuve Sénégal. Larhyss Journal. Janvier 2013; 12:71-83. ISSN: 1112-3680
Halstead NT, Hoover CM, Arakala A, Civitello DJ, De Leo GA, Gambhir M, et al. Agrochemicals increase risk of human schistosomiasis by supporting higher densities of intermediate hosts. Nature Communications. 2018; 9:837. DOI: 10.1038/s41467-018-03189
Diallo AD, Namr KI, N’Diaye AD, Garmes H, Kankou M, Wane O. L’intérêt des méthodes d’analyses statistiques dans la gestion du suivi de la qualité physico-chimique de l’eau de la rive droite du fleuve Sénégal. Larhyss Journal. 2014; 17:101-114. ISSN: 1112-3680
Manikowski S, Strapasson A. Sustainability assessment of large irrigation dams in Senegal: A cost-benefit analysis for the Senegal River valley. Frontiers in Environmental Science. 2016; 4:18
Gbocho FY, Diakité RN, Akotto FO, N’Goran EK. Dynamique des populations de mollusques hôtes intermédiaires de Schistosoma haematobiumet Schistosoma mansonidans le lac du barrage de Taabo (sud Côte d’Ivoire). Journal of Animal & Plant Sciences. 2015; 25:3939-3953
Tfeila MM, MOSA KO, Souabi S, Aboulhassan MA, Taleb A, Bouezmarni M. Suivi de la qualité physicochimique de l’eau du fleuve Sénégal: Cas du captage du Beni Nadji alimentant en eau potable les wilayas de Nouakchott (monitoring of water physico-chemical quality of the Senegal River: The case of capture of Beni Nadji supplying drinking water of the Wilaya of Nouakchott). Journal of Materials and Environmental Science. 2016; 7(1):148-160
Diaw OT, Vassiliades G, Seye M, Sarr Y. Rôle épidémiologie des mollusques du genre Bulinus dans la transmission des schistosomiases animales et humaines au Sénégal. Revue Sénégalaise des Recherches Agricoles et Halieutiques. 1988; 1(1):74-78
Diaw OT, Ndir O, Toupane MG. Guide de surveillance malacologique et de lutte contre les mollusques hôtes intermédiaires des bilharzioses. Ministère de la santé: Service national des grandes endémies; 1999. p. 60
Ndir O. Situation des schistosomes au Sénégal. Lutte contre les schistosomes en Afrique de l’Ouest. Paris: Editeur scientifique; 2000. pp. 225-236
Ernould JC. Epidémiologie des schistosomoses humaines dans le delta du fleuve Sénégal. Phénomène récent de compétition entre Schistosoma haematobiumSambon, 1907 et S. mansoni(Bilharz, 1852) [thèse de doctorat]. Université Paris XII; 1996. p. 589
Ndione RA. Etude de l’influence des paramètres environnementaux sur la dynamique des populations de mollusques hôtes intermédiaires des schistosomes humains dans la vallée du fleuve Sénégal (cas de la commune de Richard-Toll). Mémoire de Master 2 en Biologie Animale: Ecologie et gestion des écosystèmes. Université Cheikh Anta Diop de Dakar; 2015. p. 30