Recent studies on Schistosoma intercalatum: Taxonomic status, puzzling distribution and transmission foci revisited

Schistosoma intercalatum, which causes human rectal schistosomiasis in Afri ca, still presents a great interest for its imprecise taxonomic status and its puzzling distribution in Africa. Two geographically isolated strains of S. intercalatum are recognized, the Lower Guinea strain and the Congo stra in, which differ from each other in a number of morphological, biological a nd biochemical characteristics. Recent molecular data using RAPD markers in dicate high divergence between the two strains, with values of Nei and Li s imilarity indice allowing recognition of two genetically distinct taxa: exp eriments on pre- and post-isolating mechanisms are in progress in order to re-evaluate the taxonomic status of this polytypic species. With regard to its geographical distribution, S. intercalatum is characterized by the exis tence of two stable endemic areas (localized in Lower Guinea and North East of Democratic Republic of Congo) which correspond to the historical areas of species discovery, and the emergence during the last 15 years of new foc i of the Lower Guinea strain outside previously known endemic areas. The ab sence of local adaptation of the Lower Guinea strain to its intermediate ho st, supported by experimental studies, may help to facilitate the spread of this strain. Nevertheless, the present restricted distribution of this spe cies remains puzzling, because its potential snail hosts (bulinids) are wid ely distributed throughout much of Africa. Recent experimental and epidemio logical studies suggest that interspecific sexual interactions between huma n schistosomes could have a role in limiting the distribution of S. interca latum: the competitive sexual processes acting among human schistosomes sho w that S. haematobium and S. mansoni are always competitively dominant over S. intercalatum. These epidemiological observations lead the authors to di stinguish three kinds of transmission foci for S. intercalatum.