Anopheline breeding in river bed pools below major dams in Sri Lanka
Introduction
In Sri Lanka, 22 anopheline species have been recorded (Amerasinghe, 1992). Of these, 13 have been shown to support the sporogonic cycle of human malaria in the laboratory, and are thus considered as “potential vectors” of malaria in the country (Herath et al., 1983, Subramanium, 1985, Konradsen et al., 2000). However, An. culicifacies is still considered the principal vector of malaria while An. annularis, An. subpictus and An. tessellatus have been recorded as vectors of local importance (Mendis et al., 1990, Amerasinghe et al., 1992, Ramasamy et al., 1992).
In Sri Lanka, An. culicifacies primarily breeds in stream and river bed pools and margins. It has also been reported in a variety of secondary breeding places, such as irrigation canals, wells, tanks, temporary water pools, burrow pits, brick fields, quarries, puddles, abandoned gem pits and hoof prints of cattle (Wickramasinghe, 1981; Abhayawardana, 1995). Among the many rivers traversing the country, the Mahaweli and Kelani rivers are of major importance for malaria transmission in Kandy and Nuwara Eliya districts which are located in the wet and intermediate zones of Sri Lanka. The Mahaweli and Kelani rivers have been dammed to form upstream reservoirs for hydroelectric power generation and irrigation of the dry zone areas. Water in each reservoir is diverted at the dam site to a power plant through an underground tunnel or a canal, and the used water by the power plant is discharged to the river at the site of the power station which is situated a few kilometers below the dam. Diversion of water at the dam site has resulted in formation of rock pools in the river beds between the dam and the power plant. As the area experiences a considerable amount of rain being in the wet and intermediate zones, pool formation is perennial due to rain water collection even though spillage over the dam is minimal. These pools have been identified as major breeding places of anopheline species, including An. culicifacies, the principal vector of malaria in Sri Lanka, during previous malaria outbreaks (Kusumawathie, 1995, Kusumawathie and Wijesundera, 1992, Wijesundera, 1988).
Information on anopheline breeding is important in forecasting malaria outbreaks and instituting cost-effective malaria control measures in river basins below the dams. This study was carried out to ascertain the diversity of anopheline breeding in river bed pools below the major dams of the Mahaweli and Kelani rivers of Sri Lanka.
Section snippets
The study area
This study was carried out in the Kandy and Nuwara Eliya Districts of Sri Lanka. The area is generally non-malarious but is subject to malaria epidemics/outbreaks seasonally and/or periodically, mainly in association with its river systems. Ten major dams/barrages, namely, Castlereigh, Kotmale, Mousakelle, Nilambe, Polgolla, Randenigala, Rantembe, Victoria, Laxapana and Norton bridge, across the Mahaweli and Kelani rivers, are situated in the study area. Of these, five river beds below the
Results
River bed pools below the dams contained stagnant clean water with a little or no aquatic vegetation. The majority of the pools were less than 1 m2 in surface area (82%) and 75 cm in depth (98%).
In the study sites, percentages of pools positive for anopheline larvae, number of larvae per 100 pools and 100 dips varied from 4.97% to 20.12%, 11.16 to 50.53, and 3.69 to 12.56, respectively, with maximum anopheline larval density in Laxapana (Table 1).
Anopheline mosquito breeding was found throughout
Discussion
Malaria in Sri Lanka is closely associated with the river systems in the country as An. culicifacies, the principal vector of malaria in Sri Lanka, primarily breeds in the river bed pools and river margins (Abhayawardana, 1995; van der Hoek et al., 1998). During periods of unusual rainfall including periods of drought, the rivers, specially in the wet and intermediate zones of Sri Lanka, are subjected to pooling resulting in increased vector breeding along the river beds. Malaria
Acknowledgements
Financial support by National Research Council, Sri Lanka (Grant No. 99/09) is gratefully acknowledged. Assistance of Mr. M.M. Raafi, Entomological Assistant, in field investigations is much appreciated.
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