Evaluation of focal mollusciciding in the Rahad Irrigation Scheme, Sudan.

Since the establishment of the Rahad Irrigation Scheme in 1979, the prevalence of schistosomiasis has increased to high levels in some villages despite the efforts of the Blue Nile Health Project to prevent this. Although a programme of focal snail control has been implemented since 1979, when the first intermediate host snails were detected, infected snails are abundant in human water contact near villages where transmission of both S. mansoni and S. haematobium takes place. The objective of the present study was to assess the feasibility and effectiveness of the focal snail control operations in irrigation canals, in terms of reductions in snail density and repopulation rates of treated sites. The efficacy of focal mollusciciding was evaluated by (1) evaluation of the routine molluscicide operation done by the Blue Nile Health Project, (2) experimental application of various dosages of the molluscicide and (3) study of snail repopulation rates. Efficiency was measured by mortality among caged snails placed in the target stretch and from pre- and post-treatment counts of relative snail densities. The short-time application of niclosamide can be very effective in reducing the density of the intermediate host snails in the target stretch of canal. An efficient exposure seems to be approximately 3 ppm (a.i.) for 30-40 min when water flow is slow, while 1 ppm is efficient when water velocity is very slow, i.e. less than 0.04 m S-1, and 0.6 ppm is efficient when the water is virtually stagnant. Turbidity seems not to impair the effect of the treatment and vegetation does not reduce the efficiency of niclosamide, provided the current speed is low and the chemical is applied to the entire width of the canal. However, the results also clearly demonstrate some of the shortcomings of the standard procedure of niclosamide application. A high water velocity and discharge will rapidly wash the chemical through the target canal stretch and the snails will be exposed only for a short period if the routine procedure is followed. In addition, the chemical will not mix sufficiently with water within standards of aquatic macrophytes. Recolonization of treated canal stretches is fast, especially during the irrigation period when water velocities are high and juvenile snails are abundant. Snail density may attain pretreatment levels three weeks after an effective application. Since the routine procedure is most effective at low current speeds, it is suggested that water velocity could be controlled during the application in coordination with the Irrigation Department.