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 Printable version |
The role of irrigation design and water management parameters in the ecology of transmission and control of schistosomiasis in central Morocco |
Cahiers Agricultures. Volume 11, Number 1, 23-9, Janvier - Février 2002, Cultures irriguées et santé
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 Résumé
Article gratuit
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Author(s) : Hammou Laamrani, Eline Boelee |
Summary : In Morocco, the design of surface irrigation systems is basically the same throughout the country with primary, secondary and tertiary canals made of concrete cement. The quaternary canals are earth canals that convey water to the plots. Siphon boxes found at the canal off-take are typical structures made of upstream and downstream boxes connected by an underground pipeline. Water flow is controlled through a system of gate regulators.
A cross-section survey of the distribution of Bulinus truncatus, intermediate host of Schistosoma haematobium was carried out in the Tessaout Amont irrigation scheme affected by schistosomiasis. The study pointed out that the irrigation system design and management favour the occurrence and development of the snail population. This is partly due to the shape of the canals, which is telescope-like as the cross-section is decreasing from the head to the tail end. Water is stagnant in some of the canal structures between irrigation turns. The most suitable habitats for the intermediate hosts are siphon boxes locally named "puisards" or "regards". The highest frequency of occurrence of B. truncatus recorded in these habitats is mainly due to permanent stagnant water, with temperatures favourable to snail growth and reproduction throughout the year and ample availability of food that consists mainly of algae and detritus.
Furthermore, a one-year longitudinal survey was conducted in the same area. The results showed that the density of Bulinus truncatus decreased significantly when irrigation frequency and duration increased. Nevertheless, this negative correlation was more evident outside the irrigation season that starts in April/May and ends up in September/October. Subsequently, the same amount of water provided for irrigation can be served in several turns instead of one.
In a further study, the effect of environment modifications on snail breeding and density was tested in the Tessaout Amont scheme. Covering the "puisards" to create permanent darkness in siphons led to a significant reduction of snails. Cleaning with the removal of vegetation and mud was effective too, but re-colonisation occurred within a few months. Such environment-friendly methods need an intersectoral approach to identify community-based measures that are necessary to ensure the sustainability of any mitigating measure. The engineering methods to control snails can be cost-effective and sustainable if the community is involved in their design and implementation. |
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