On factors possibly restricting the distribution of Schistosoma intercalatum Fisher, 1934

Two hypotheses have been postulated explaining the limited distribution of Schistosoma intercalatum. The first hypothesis is correlated with physical factors and behaviour of cercariae. Histochemical and ultrastructural studies have shown that in response to increased temperature change the cercariae of S. intercalatum form aggregates, unlike other schistosome cercariae of man, which are non-infective to the definitive host. The aggregates are formed by the release of the adhesive post-acetabular gland secretion which causes the cercariae to stick together. It is suggested that if S. intercalatum spread from streams within tropical rain forest to pools and laybys of streams in the savannah, cercariae would be subjected to greater daily temperature changes thus triggering the release of post-acetabular gland secretion, thereby impairing invasion of the definitive host. The second hypothesis is based on the natural occurrence of hybridisation between S. intercalatum and Schistosoma haematobium. With some strains of these two species there are no genetical isolating mechanisms. It is suggested that if S. intercalatum extended into a savannah environment from tropical rain forest, hybridisation between S. intercalatum and S. haematobium would eventually occur. Experimental studies indicate that probably, as a result of introgressive hybridisation, a new strain of S. haematobium would eventually supersede the original S. intercalatum.