This study is the first malacological survey conducted in the Niakhar study area. B. umbilicatus and B. senegalensis were the only snail species encountered in the 17 ponds sampled in this study. Previous malacological surveys done in the other localities of Fatick and Kaolack regions in the Sine-Saloum ecological zone reported the presence of B. senegalensis, B. forskalii, B. globosus and B. truncatus [11]. The presence of B. umbilicatus is reported for the first time in the Niakhar study area situated in the ecological area of Sine-Saloum. The fact that this species was not reported by previous studies conducted in other localities of the Sine-Saloum zone [11] could be due to its low geographical distribution. Indeed, in Niakhar, B. umbilicatus was only encountered in the village of Ngangarlam in one pond out of the 17 surveyed from 2012 to 2013.
In 2012, the rains were late and low quantities were recorded in the area compared to 2013. Thus the water in 2013 remained longer in the largest ponds at Sass njafaj, Puday and Diohin. This persistence of water in the ponds allowed the snails to undergo more breeding cycles before the ponds dried up. This could explain the differences in snail abundance observed at the end of the rains between 2012 and 2013. The fact that B. umbilicatus displayed a single peak at the end of the rainy season, contrary to B. senegalensis, suggests that this snail has a longer reproductive cycle. Indeed, the rapid draining of ponds in December prevents this species from reaching its second peak. B. senegalensis lay quickly when the pond fill in July-August, allowing them to reach the first peak in September whose progeny will give the second peak in November at the end of the rains. In general, maximum snail numbers were observed at the end of the rainy season. The fluctuations in snail abundance correlated to the drying up of ponds. The period and the number of population peaks varied between species and from one year to another. Similar observations were made in temporary ponds in the region of Tambacounda in Senegal. In these temporary habitats, the period of great egg-laying after re-emergence of the snails is situated in July-August and September where a great number of eggs are observed. These are times of great reproduction, especially from August to September and October. The population thus consists of snails of all ages [7]. In these types of habitats, the ecological conditions are also hostile to the development of many snail species. The duration of the rainy season and the quantities of water, are important elements governing the abundance and density of B. umbilicatus and B. senegalensis. These snails adapt to their environment by establishing a short cycle, allowing them to achieve high population density and to fulfill their role as intermediate hosts for S. haematobium [11].
In the current study, both species were found to be infested with schistosome cercariae. B. senegalensis was found to be infested by schistosome cercariae in four different ponds with rates varying from 0.05 % to 1.11 %. This snail is known to be involved in the transmission of S. haematobium in the middle valley around Podor and Matam [18] and also in the transmission of S. bovis in the Senegambia [19]. In Senegal, B. senegalensis is involved in the transmission of S. haematobium and not with Schistosoma bovis and Schistosoma curassoni [9, 20, 21]. According to these previous studies and the fact that it is the only freshwater snail in the ponds of Niakhar where it was found to be infested by Schistosoma spp. cercariae, it is clear that B. senegalensis plays a role in the transmission of S. haematobium in Niakhar.
B. umbilicatus is involved in the transmission of S. haematobium and S. curasonni in Senegal, and is well-adapted to these temporary ponds [7, 22]. In Niakhar, in the village of Ngangarlam, B. umbilicatus was also found to be infested by Schistosoma spp. in one pond in 2012 and 2013 with high infestation rates. Cercaria species, however, were not differentiated. Nevertheless, a high prevalence of 65.5 % of urogenital schistosomiasis was reported for the people in this particular village of Ngangarlam close to the B. umbilicatus site and this pond is rarely frequented by livestock (Senghor, personal communication). In addition, in other localities in the region of Tambacounda with similar ecology, B. umbilicatus was the only snail infested with Schistosoma spp. in a pond system in villages endemic for urogenital schistosomiasis [7]. Considering the epidemiology of urogenital schistosomiasis in this village, the fact that S. bovis and S. curasonni are not present in the Fatick region and that the water points are rarely or not frequented by livestock [23], we suspect that the cercariae produced by B. umbilicatus are those of S. haematobium. However, this result requires the use of molecular biology techniques to better characterize the cercariae produced by B. umbilicatus in order to better define its role in the transmission of S. haematobium. The same suggestion is necessary for the snail intermediate hosts in general and particularly for the snails of the genus Bulinus that can shelter in addition to S. haematobium, animal schistosomes, such as S. bovis, S. curassoni and others [17, 24].
The current study also showed that B. umbilicatus and B. senegalensis can withstand 7 to 8 months of drought and that it is the larger adult specimens that are able to persist. Similar observations were reported in the laboratory and in the field by previous studies in Senegal [7, 25]. This phenomenon of drought resistance is well known among African snails, especially the genus Bulinus [4, 19], but little information is available concerning the ability of these snails to maintain schistosome larvae during this period.
In the present study, for the first time, B. umbilicatus was found to be infested in July 2012 and 2013, after a dry period of 7 months and also, 21 and 25 days after the first rains respectively. The same finding was reported for Bulinus nasutus in Tanzania after five months of drought and also, 21 days after the first rains [26]. This phenomenon is also of great importance in the epidemiology of urogenital schistosomiasis in the Niakhar study area and other seasonal transmission foci in Senegal because it can increase the risk and the period of transmission, positioned by previous studies between September and November [7, 11]. The fact that B. umbilicatus was found to be infested with Schistosoma spp. in Tambacounda [7] and in Niakhar from July to November with a high prevalence proves that even though this snail is well-adapted to these rain-temporary ecosystems and despite its limited geographical distribution, it plays a significant role in the epidemiology of urogenital schistosomiasis in Senegalese areas with only temporary ponds. Thus, extensive studies of this Bulinus are necessary because it is able to withstand drought and to maintain Schistosoma spp. larvae from one season to another. In addition, B. umbilicatus is involved in the transmission of animal schistosomiasis in Senegal [22].
Snail control strategies in these temporary habitats in Niakhar but also in other seasonal foci in Senegal, must take into account these phenomena. This study recommends an adaptation of snail control strategies to the cycles of ponds and snails. Two annual treatments of the pond with Bayluscide at the end of the rainy season in November before aestivation and upon onset of rains in July would be more beneficial. This would have the advantage to be focused in the puddles of residual waters or newly created waters and kill almost all snails. This would reduce snail density, but also decrease the risk of transmission by eliminating individual snails that have maintained the parasite during the drought.