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

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/m(2) 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 < or =1m(2) in surface area and < or =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.

Spatiotemporal distribution of insecticide resistance in Anopheles culicifacies and Anopheles subpictus in Sri Lanka.

The malaria situation in Sri Lanka worsened during the 1990s with the emergence and spread of resistance to the drugs and insecticides used for control. Chloroquine resistance has increased rapidly over this period, but adverse changes in malaria transmission are more closely associated with insecticide use rather than drug resistance. Insecticide susceptibility tests were routinely carried out in key anopheline vectors across the country for more than a decade. These sentinel data were combined with data collected by other research programmes and used to map the spatial and temporal trends of insecticide resistance in the main vectors, Anopheles culicifacies and A. subpictus, and to examine the relationship between insecticide resistance, changes in national spraying regimens and malaria prevalence. Both species had widespread resistance to malathion, the insecticide of choice in the early 1990s. Both species were initially susceptible to the organophosphate and pyrethroid insecticides used operationally from 1993, but some resistance has now been selected. The levels of malathion and fenitrothion resistance in A. subpictus were higher in some ecological regions than others, which may be related to the distribution of sibling species, agricultural pesticide exposure and/or environmental factors. The study highlights that the emergence and spread of insecticide resistance is a constant threat and that active surveillance systems are vital in identifying key vectors and evidence of resistance.

Vectors and malaria transmission in a gem mining area in Sri Lanka.

Anopheline vectors of malaria were studied in the Kaluganga gem mining area in the dry zone of central Sri Lanka. Adult mosquitoes were collected using cattle-baited huts and nets, pyrethrum spray sheets, window exit traps and light traps and partial and full-night human landing collections over a period of 17 mo. The collections produced a total of 13,591 anophelines belonging to 14 species. Using ELISA for circumsporozoite proteins of Plasmodium vivax and P. falciparum, Anopheles culicifacies, An. subpictus and An. varuna were incriminated as malaria vectors. For these species, estimated entomological inoculation rates were 5.9, 5.7, and 6.7 infective bites per person per year, respectively. An. culicifacies and An. subpictus show endophagic behavior, while An. varuna were exophagic. Mosquito larval surveys indicated that the gem pits, when filled with water, contributed 60% of the larvae of the three vector species. Parasitological data were collected by two mobile malaria clinics and by hospitals. The incidence rates of fever per 1,000 person-years with P. vivax and P. falciparum were 122.8 and 26.1 respectively, with a significantly higher rate in males over 15 years of age than in females of that age. Changes in the environment due to gem mining may have caused the emergence of An. subpictus and An. varuna as significant malaria vectors. The conventional view of An. culicifacies being the main vector of malaria in Sri Lanka needs to be reconsidered when planning the vector control programs in this study area. Due to migration of malaria patients, there is a likelihood the disease could be spread to other parts of the island. Therefore, action should be taken to eliminate these pits as breeding sites.

Control of vectors and incidence of malaria in an irrigated settlement scheme in Sri Lanka by using the insect growth regulator pyriproxyfen.

An evaluation of pyriproxyfen as a larval control agent with the aim of reducing malaria vector populations and incidence of malaria was conducted in 12 villages in an irrigated settlement scheme in the dry zone of central Sri Lanka. In these villages, there are many pools in the beds of rivers, streams, and irrigation ditches during the dry season of the year. These are the major breeding places of the malaria vectors Anopheles culicifacies and An. subpictus. Collections of adult mosquitoes were carried out by using standard methods and parasitological data were collected by daily malaria clinics set up for the project and through the 2 government hospitals. All villages in the study area were under residual house spraying with lambdacyhalothrin water-dispersible powder. Using the 1st year's baseline data collection, the villages were stratified into 6 villages with high malaria incidence and 6 villages with low incidence. Within each group, 3 villages were randomly assigned for larval control by treating all the pools in the beds of rivers, streams, and irrigation ditches and agricultural wells with a granular formulation of the insect growth regulator pyriproxyfen at the rate of 0.01 mg active ingredient/liter. The field bioassays indicated that a single treatment of pyriproxyfen effectively inhibited the emergence of adult mosquitoes in the riverbed pools for a period of 190 days. The treatment caused significant reduction of the adult populations of An. culicifacies (78%) and An. subpictus (72%). Similarly, incidence of malaria was reduced in the treatment villages by about 70% (95% confidence interval 58-78%) compared with the controls. The conclusion is made that pyriproxyfen can be a very effective means of malaria control if all possible vector breeding places in the area can be located.

Laboratory and field comparisons of pyriproxyfen, polystyrene beads and other larvicidal methods against malaria vectors in Sri Lanka.

Hand-dug gem pits are important breeding sites for larvae of malaria vectors in Sri Lanka. Therefore, studies were carried out to help to select an effective, economic and convenient method that could be used to control malaria vector mosquito breeding in gem pits in a mining area. The effectiveness of four types of floating layers of polystyrene was compared in the laboratory and it was found that 2 mm expanded beads were the most effective for suffocating Anopheles larvae and pupae. The insect growth regulator, pyriproxyfen at dosages of 0.01 and 0.1 mg/l were tested in the laboratory and complete inhibition of emergence was found at both concentrations. A small-scale field trial was carried out for over a year to assess the efficacy of two concentrations of pyriproxyfen, 2 mm diameter expanded polystyrene beads, temephos, used engine oil and filling pits with soil. Pyriproxyfen only required re-application twice a year, whereas temephos or oil require 12 applications per year. Due to re-excavation by gem miners, polystyrene beads and filling of pits were not as permanent solutions as was expected. Calculations based on all available data showed that two annual treatments with pyriproxyfen at 0.01 mg/l would be the most cost-effective method with oil only slightly more expensive. However, the reduced required frequency for visiting every pit made the pyriproxyfen method the one of choice. The same low concentration of pyriproxyfen also effectively inhibited emergence of adults from river-bed pools.