The effects of irrigated agriculture on the transmission of urinary schistosomiasis in the Middle and Upper Valleys of the Senegal River basin

The importance of the increase in irrigated land on the perimeters of the M iddle and Upper Valleys of the Senegal River basin, on the prevalence and i ntensity of urinary schistosomiasis, was investigated. Surveys were conduct ed, in May-June 1997, to determine the prevalence and intensity of Schistos oma haematobium infection among 1445 children aged 7-14 years: 1011 in 10 v illages near Matam, and 434 in four villages near Bakel. Macrohaematuria wa s present in seven of the study villages (four near Matam and three near Ba kel), whereas microhaematuria was present in all the villages, with prevale nces of 10%-73%. A second survey, conducted, in June 1999, on 755 children from nine of the study villages near Matam, demonstrated significant increa ses in the prevalences of both micro- and macro-haematuria in three of the villages, all of which were adjacent to the Senegal River and practising ir rigated agriculture. None of the other study villages re-surveyed was irrig ating any of its agricultural land. A longitudinal survey was also carried out, between May 1997 and November 1 998, on about 10% of the population (2272 subjects) of Nguidjilone, north o f Matam; selective treatment with praziquantel (40 mg/kg) was given in May 1997, and mass treatment in May 1998. The data analysed were those relating to the 125 individuals who provided samples at each survey. Very severe in fections (> 1000 eggs/10 ml urine) were seen in five subjects in May 1997. One year later (i.e. 1 year after the selective treatment), the prevalence of urinary schistosomiasis had increased in every age-group. Although preva lence had decreased slightly by November 1998 (6 months after the mass trea tment), the intensity of the infections seen had increased in every age-gro up. At the end of the dry season (May-June 1997), Bulinus truncatus infected wi th schistosome cercariae were recovered from the Senegal River. However, im mediately after the next rainy season (November 1997), no snails were found at any collection site on the river.