Irrigation ecology of schistosomiasis : environmental control options in Morocco

The concept of irrigation ecology is introduced to study the transmission and the control of urinary schistosomiasis in Moroccan irrigation systems. By distinguishing a biological, a human and an irrigation environment, crucial interactions are identified in the overlap of these three environments. In the semi-arid Haouz plain in Central Morocco, schistosomiasis was introduced after the construction of the Tessaout Amont irrigation system in the early 1970s. The typical design of this canal irrigation system, with elevated semi-circular conduits as secondary and tertiary canals, is based on upstream control and the water is distributed in rotation. Inverted siphons, consisting of two square boxes connected by an underground pipe, have been constructed to convey the water under roads or tracks. The boxes contain stagnant water and provide excellent breeding sites for Bulinus truncatus , the intermediate snail host of schistosomiasis.

A cross-sectional survey showed that especially inverted siphons on tertiary canals harbour high densities of B.truncatus . A length profile study along one secondary canal and four of its tertiaries showed that conditions near the tail end of canals, especially in the downstream siphon boxes, are most favourable to the intermediate snail host. The transmission of schistosomiasis in Tessaout Amont is concentrated at these siphons as, in the absence of other sources, water from the boxes is used for all kinds of agricultural and domestic purposes, inducing frequent water contact.

Three environmental control options have been studied. Regular emptying and cleaning of siphon boxes had a limited effect on densities of Bulinus truncatus snails and eggs. Creating a dark environment by covering siphon boxes with iron plates proved to be much more effective in reducing B.truncatus populations. Some of the covers were equipped with moveable lids to leave the water accessible to the villagers. The third control option concerned measures to increase the water flow velocity in siphons. Combining flow velocities with the duration of the flow results in a mean annual flow velocity. According to literature, above a critical value of 0.042 m/s, no B.truncatus snails are to be found in siphon boxes. In siphons with a lower mean annual flow velocity, this critical value can be obtained by reducing the inner dimensions of the siphon boxes, thus increasing the water flow velocity. However, in experiments with such smaller siphon boxes, the siphons were quickly repopulated with B.truncatus . Better results might be achieved by redefining the critical value. However, the small diameter siphons generate higher energy losses. Consequently, such siphons can only be applied in a layout where access to the fields is guaranteed through simple bridges over the drains, which significantly reduces the number of required siphons.