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"Malaria causes human mortality, morbidity and economic loss, especially in tropical rural communities. The disease is transmitted by Anopheles mosquitoes whose larval stages
breed in watery habitats such as those found in irrigation systems. Mosquitoes that transmit other diseases, as well as nuisance mosquitoes, may also breed in such habitats. A previous study in 1994 in the Upper Yan Oya watershed in the north-central dry zone of Sri Lanka indicated the high malariogenic potential of a small irrigation reservoir that forms part of a cascade irrigation system in the dry zone of Sri Lanka. The present work followed up on this finding, and investigated mosquito breeding in nine small irrigation reservoirs (known locally as tanks) in the same watershed during 1995 - 1997. The objectives were to determine a) whether important malaria-vector mosquitoes breed in the tanks, b) tank characteristics that may enhance mosquito breeding, and c)rehabilitation and management measures that help reduce mosquito breeding opportunities in the tanks.
"The investigation showed that the major Anopheles vector of malaria in Sri Lanka occurred infrequently in the tanks. However, important secondary vectors and others that are involved in malaria transmission did occur frequently. Thus tanks certainly contribute to the malaria risk in Sri Lanka. Additionally, they also generate Aedes and Culex mosquitoes that constitute a biting nuisance. Tanks varied considerably in characteristics such as the extent of the water margin, the vegetation cover of the margin and free water area, the degree of pooling and the extent of seepage. These characteristics could be expected to have impacts on mosquito breeding depending on the preferences of individual species. Not surprisingly, tanks also varied in their attractiveness as breeding habitats for different mosquito species.
"All three major tank-related habitats (tank margins, tank-bed pools and seepage pools) provided breeding opportunities for different mosquitoes. Habitat characteristics such as water and light conditions, vegetation, and potential predators of mosquito larvae were determinants of mosquito occurrence. Based on detailed analyses, we provide a simplified schematic that serves as a guide to the species likely to occur in three major habitat types, under different sets of habitat conditions.
"Tanks provide opportunities for mosquito breeding as a result of uneven spatial siltation (which creates shallow water pools), the presence of marginal, emergent and floating vegetation (which provides refuges), and seepage across the bund (which creates new breeding habitats). Selective desiltation to remove depressions, seepage proofing of tanks and the management of vegetation would reduce these opportunities. A further issue is the use of the tank bed for activities such as brick building and livestock wallowing during drier periods: these result in the creation of new tank-bed habitats that are exploited by mosquitoes. Thus, both rehabilitation and continuing management are necessary to maintain tanks in a condition in which they pose the minimum risk of generating disease-causing or nuisance mosquitoes that affect the lives and livelihoods of poor rural communities." |
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