Variations morphologiques dans le groupe Bulinus forskalii (Gastropoda : Planorbidae) et compatibilité avec les schistosomes au Cameroun

Abstract

Bulinus camerunensis Mandahl-Barth, 1957, Bulinus forskalii (Ehrenberg, 1831) and Bulinus senegalensis Müller, 1781 are representatives of the Bulinus forskalii species group in Cameroon. The taxonomy in this group is of a high medical and veterinary importance, for these species are intermediate hosts for Trematodes. Nineteen populations among which twelve of Bulinus forskalii s.s. and seven of Bulinus senegalensis, distributed in five climatic zones from the forest region in typical equatorial climate to thorny steppes in classical Sahelian climate were studied based upon observations on the morphology, morphometry and cercarial shedding in order to assess populations’ interactions with the environmental ecosystems as well as their natural capacity to transmit schistosomes. An analysis of ecological factors enabled to confirm that Bulinus forskalii s.s. colonises a wide variety of habitats, from temporary pools to permanent streams as well as irrigation canals, ‘’mayos ‘’ and lakes, thus waterbodies with a slow or null velocity, generally with a moderate or high turbidity, meanwhile Bulinus senegalensis thrives essentially in temporary sites. The study of morphologic parameters revealed, for Bulinus forskalii s.s., a spatial repartition of morphotypes, all of them being in accordance with previous descriptions made on this species, except the sample from Marao in which shell height remained lower than the double of the maximal diameter (H < 2h). The whorl shape varied from evenly curved (76.47% in Sangmélima) to strongly shouldered in populations from the Sudanian and sudano-sahelian regions; shells from these regions showed a strong carene, permitting their clear distinction from Bulinus senegalensis whose shells do not display this character but presented a partial or total striation. The morphometric study, based on six shell parameters and ten allometric ratios, revealed an important interpopulational polymorphism marked by profound conchological intraspecific variations. Measures from the six shell parameters of Bulinus forskalii s.s. were highly correlated to shell height (r ≥ 0,66; p < 0,0001), proving that shell height constitutes the dominant character to which all the others are dependent; thus, a gradual decrease of shell height has been observed from Sangmélima to Kousséri, suggesting a morphocline that could probably be due to climate stratification. A proportion of 12.12% of similitude have been observed on different parameters between different samples, except samples from Marao and Sangmélima that did show similarities with other samples only based upon allometric ratios. It was also observed, between samples of Bulinus senegalensis populations, microgeographic morphometric variations without any apparent spatial structuring. The comparison of measurements of Bulinus forskalii s.s. samples from Dang, Marao and Yagoua to earlier published measurements on Bulinus camerunensis Page | xxiiPage | xxiii from Barombi Kotto and Debundscha showed 20.00% of similarity cases between both species. From a parasitological point of view, 1704 snails including 1064 Bulinus forskalii s.s. and 640 Bulinus senegalensis have been used for a natural cercarial shedding test; 5.17% of Bulinus forskalii s.s. shed Schistosomatidae furcocercaria while the shedding rate was rather low (1.41%) with Bulinus senegalensis. The present study revealed a great phenotypic polymorphism in Bulinus forskalii s.s and Bulinus senegalensis that are also known as intermediate hosts for human and animal schistosomes; furcocercaria, morphologically and ethologically similar to those of genus Schistosoma Weinland, 1858, were shed by some snails belonging to these two species. Further research is needed in order to (1) identify to the species level these furcocercaria and (2) reveal the genotypic diversity in the Bulinus forskalii group