THE EPIDEMIOLOGY OF HUMAN SCHISTOSOMIASIS IN THE SENEGAL RIVER BASIN

Extensive water development has taken place in the north of Senegal ov er the last decade, resulting in a large increase in the amount of fre sh water for irrigation. The objectives of the present study were to d etermine the prevalence and intensity of Schistosoma mansoni and S. ha ematobium in the Senegal river basin (SRB), and to ascertain the distr ibution of the snail species acting as intermediate hosts for both spe cies of schistosomes. The schistosomiasis survey started in January 19 94 and was completed in March 1995. Compared to studies before the con struction of the Diama dam, there was a significant increase in both t he prevalence and intensity of urinary and intestinal schistosomiasis in the human population in parts of the SRB. From the 9014 people who were registered from 180 villages and 4 towns (10 districts), 7750 wer e examined. S. mansoni was found in the lower valley (lower delta-Sene gal river, lower delta-Lampsar river, upper delta, and diere) but not in the middle valley. The mean prevalence ranged from 4.4% in the lowe r delta-Senegal River to 71.8% in the zone of Lac de Guiers, where pre valence and intensity of infection were higher on the eastern side of the lake (81.3% with a mean number of 2088 eggs/g of faeces) compared with the western side (50.3% with a mean 1111 eggs/g). S. haematobium was recorded throughout the area of study, ranging from a mean prevale nce of 0.37% in diere (lower valley) to 41.5% in the lower valley (Lam psar river), where the mean egg count was 313/10 mt of urine. Physical and chemical changes to the environment have favoured the spread and increase in the populations of freshwater snails. The only snail invol ved in the transmission of S. mansoni was Biomphalaria pfeifferi. Five species of bulinid snails were present-Bulinus globosus, Bu. umbilica tus, Bu. senegalensis, Bu. forskalii and Bu. truncatus-but only the fi rst 3 species were involved in the transmission of S. haematobium in t he lower and middle valleys.