Natural infection of Bulinus senegalensis by Schistosoma haematobium in a temporary pool focus in Niger: characterization by cercarial emergence patterns.

The transmission in West Africa of Schisto&&a haematobium by Bulinus truncatus and B. globosus is well known (BROWN, 1980). On the contrary, B. senegalensis, widely distributed in the sub-desert and sahelian zone and associated principally with a temporary environment (BETTERTON et al., 1983), has been found naturally infected in Senegal and The Gambia only (VERCRUYSSE et al., 1985). In Niger, snail-schistosome infection experiments have proved that B. senegalensis is an excellent potential vector of S. haematobium originating from the sahelian zone (VERA et al., 1990). The purpose of this study was to establish and determine the natural role of B. senegalensis in the transmission of urinary schistosomiasis. The survey was carried out in 1988 in temporary rain-fed pools near the village of Bomberi, in the western part of Niger, 120 km north-east of Niamey in the south sahelian zone. A parasitological survey of schoolchildren in that village showed a prevalence of urinary schistosomiasis of 83% (77/92). A fortnightly hand collection of snails (lasting 20 min) was conducted from the time when the ponds filled until their drying up. The schistosomes were characterized by determining the emergence rhythms of cercariae under standardized laboratory conditions by means of a chronocercariometric apparatus (PAGÈS & THÉRON, 1990). The survey showed that B. senegalensis was the only snail species present in the ponds near the village which were used by human populations. That species was also the only one present in ponds located within a range of 10 km around the village. The populations of B. senegalensis developed after the filling of the ponds, in late June, and survived until their drying up between October and December. Only 3 of 600 snails (0.5%) collected in October 1988 released cercariae, characterized by peak emergence between 1200 h and 1400 h. Studies in our laboratory on several strains from different localities in Niger have shown that the mean shedding time (MST) was between 1200 h and 1400 h for S. haematobium and between 0800 h and 1000 h for S. bovis and S. curassoni (MOUCHET et al., 1990). Our results agree with those obtained by previous workers. For S. haematobium, the MST was between 1200 h and 1400 h, whether under natural outdoor conditions in South Africa (PITCHFORD & DU TOIT, 1976) or under experimental conditions identical to ours in Niger (PAGÈS & THÉRON, 1990). Different African strains of S. bovis were characterized by an MST between 0800 h and 1000 h under similar experimental conditions (MOUAHID et al., 1987; PAGÈS & THÉRON, 1990). These results indicate that the chronobiology of cercarial emergence can be used as a reliabie character for distinguishing between S. haematobium on one hand and S. bovis andS. curassoni on the other hand. Furthermore, in Niger, S. curassoni is present only in the eastern part of the country (MOUCHET et al., 1989). These results consequently allow us to affirm the natural infection of B. senegalensis by S. haematobium in the focus of