Schistosoma intercalatum: an endangered species in Cameroon? Louis-Albert Tchuem Tchuente ´ 1 , Vaughan R. Southgate 2 , Joseph Jourdane 3 , Bonnie L. Webster 2 and Jozef Vercruysse 4 1 Laboratory of General Biology, Faculty of Sciences, University of Yaounde ´ I, P.O. Box 812 Yaounde ´ , Cameroon; and Centre for Schistosomiasis and Parasitology, P.O. Box 7244 Yaounde ´ , Cameroon 2 Biomedical Parasitology Division, Wolfson Wellcome Biomedical Laboratories, Department of Zoology, The Natural History Museum, Cromwell Road, South Kensington, London SW7 5BD, UK 3 Centre de Biologie et d’Ecologie Tropicale et Me ´ diterrane ´ enne (UMR CNRS 5555), Universite ´ de Perpignan, Avenue de Villeneuve, 66860 Perpignan Cedex, France 4 Ghent University, Faculty of Veterinary Medicine, Department of Virology, Parasitology and Immunology, Salisburylaan 133, B9820 Merelbeke, Belgium Despite the extremely wide distribution of both inter- mediate (snail) and definitive hosts, the species Schisto- soma intercalatum has a highly restricted distribution in parts of West Africa (Cameroon, Equatorial Guinea, Gabon, Nigeria, Democratic Republic of Congo and Sa ˜o Tome ´ ). Recent studies showed that the epidemiology of this species is very dynamic, including invasions and extinctions. This article reviews the epidemiology of S. intercalatum in Cameroon, where the species is endangered, and analyses its overall status in Africa. The possible reasons for the limited distribution of S. intercalatum are discussed. The species Schistosoma intercalatum is characterized by the existence of two geographically isolated strains, the Lower Guinea strain and the Zaire strain, which differ from each other in several morphological [1], biological [1,2], biochemical [3,4] and molecular [5] characteristics. More recently, laboratory hybridization between the two strains of S. intercalatum was carried out [6] and con- firmed that the intraspecific cross was only viable to the F 3 adult generation, clearly demonstrating the existence of a post-zygotic isolating mechanism. Three mitochondrial genes – cytochrome oxidase subunit 1 (cox 1), NADH dehydrogenase subunit 6 (nad 6) and the small ribosomal RNA gene (rrnS) – have been studied [7] and the phylo- genetic analysis of the mitochondrial data reveals a clear dichotomy between the two strains of S. intercalatum, supporting other biological, biochemical and molecular biological data indicating that these two strains should be considered as distinct species. The Lower Guinea strain is transmitted by Bulinus forskalii, and is reported from the Lower Guinea forest region (Cameroon, Gabon, Equatorial Guinea, Nigeria and Sa ˜o Tome ´), whereas the Zaire strain is transmitted by Bulinus globosus and is only known to be present in the Democratic Republic of Congo (DRC) (Fig. 1a). The Lower Guinea area and the northeast of the DRC are considered as the historical areas of species discovery, from where the parasite strains might have spread. Previous studies [8] suggested that the Lower Guinea strain, in particular, has spread geographically over the past 15 years, either within the historically endemic countries or into new countries (Equatorial Guinea, Sa ˜o Tome ´ and Nigeria). Moreover, there have been sporadic reports of S. inter- calatum infections from other countries such as the Central African Republic, Chad, Angola, Burkina Fasso, Congo, Uganda, Mali and Senegal, but without subsequent confirmation of active transmission foci, which might suggest that the parasite has either disappeared from these countries or was incorrectly diagnosed [9] (Fig. 1a). For example, these data could relate to mixed infections of Schistosoma mansoni and Schistosoma haematobium resulting in some S. mansoni male X S. haematobium female pairings, and producing S. intercalatum-like eggs (terminal-spined eggs) in human fecal samples. It is difficult to be sure of the true origin of such terminal- spined eggs in faeces; additional studies are needed to avoid misidentifications. Nevertheless, the present restricted distribution of S. intercalatum in Africa is puzzling because its potential snail hosts are very widely distributed throughout much of Africa, and display (at least with regard to B. forskalii)a good compatibility with the parasite, irrespective of the geographical origin of the snail [10,11]. The biology of snail hosts and their susceptibility to infection are thus not responsible for the restricted distribution of S. inter- calatum; other factors must be playing a part, such as, hybridization with S. haematobium, heterologous immunity and mating behaviour with S. mansoni. The epidemiology of S. intercalatum is apparently somewhat dynamic, possibly including invasions and extinctions. Therefore, the question arises as to whether the present distribution of S. intercalatum results either from an extinction of the species from some foci, or from a recent appearance of the species in several new foci. Alternatively, both extinctions and invasions could be occurring simultaneously in different areas. Corresponding author: Louis-Albert Tchuem Tchuente ´ (tchuemtchuente@ camnet.cm). Opinion TRENDS in Parasitology Vol.19 No.9 September 2003 389 http://parasites.trends.com 1471-4922/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S1471-4922(03)00193-4