Anopheline breeding in river bed pools below major dams in Sri Lanka

Abstract

Anopheline mosquito larval surveys were carried out from September 2000 to August 2002 in Mahaweli and Kelani river beds, below five major dams in the wet and intermediate zones of Sri Lanka, to study the prevalence of anopheline species in these areas. In each study site, all permanent and semi-permanent pools were surveyed fortnightly by dipping at 6 dips/m² surface area of water. Larvae were collected in separate containers, staged and identified at their third and fourth stages. During each survey, the surface area and depth of pools were recorded and each reading was considered as an individual observation.

River bed pools below the dams contained stagnant clean water with a little or no aquatic vegetation. The majority of pools were ≤1 m² in surface area and ≤75 cm in depth. Anopheline mosquito breeding was seen throughout the year in each study site. The average percentage of pools positive for anopheline larvae, the number of larvae per 100 pools and 100 dips were 14.85%, 32.34 and 9.29, respectively. Thirteen anopheline species, including 10 potential vectors, namely, An. barbirostris, An. culicifacies, An. jamesii, An. maculatus, An. nigerrimus, An. peditaeniatus, An. subpictus, An. tessellatus, An. vagus and An. varuna were found breeding in the river bed pools.

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.