Evaluation of environmental methods to control snails in an irrigation system in Central Morocco

The Moroccan Ministry of Public Health has launched a programme to eliminat e schistosomiasis. One of the components in this process is the control of Bulinus truncatus, the intermediate host snail of Schistosoma haematobium W e evaluated three environmentally safe measures to control B. truncatus in siphon boxes, the main breeding sires for these snails in the Tessaout Amon t irrigation system. The first method involved covering the siphon boxes to exclude light and reduce algal growth, the second consisted of increasing the frequency of emptying and cleaning the siphon boxes, and the third meth od increased water velocity to hinder the establishment of the intermediate hosts. The results showed that covering had a pronounced effect on snail a nd egg mass density, was accepted by the local community and prevented wate r contact. Cleaning the siphons three times during the irrigation season le d to a reduction in snail density although it was not statistically signifi cant and recolonization was rapid. Increasing water velocity by reducing th e dimensions of siphon boxes delayed recolonization, but such a control mea sure can be applied only in specific situations where it does not pose hydr aulic problems. The three interventions were selectively effective against B. truncatus, whereas other snails such as Physa acuta and Lymnaea peregra were hardly affected. Covering, the most promising control measure, could b e useful in the Moroccan schistosomiasis eradication programme. However, fu rther investigations are needed to assess its impact on water quality.