421 Am. J. Trop. Med. Hyg., 60(3), 1999, pp. 421–429 Copyright  1999 by The American Society of Tropical Medicine and Hygiene MALARIA VECTORS IN A TRADITIONAL DRY ZONE VILLAGE IN SRI LANKA PRIYANIE H. AMERASINGHE, FELIX P. AMERASINGHE, FLEMMING KONRADSEN, KAPILA T. FONSEKA, AND ROBERT A. WIRTZ Department of Zoology, University of Peradeniya, Peradeniya, Sri Lanka; International Irrigation Management Institute, Colombo, Sri Lanka; Division of Parasitic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia Abstract. Malaria transmission by anopheline mosquitoes was studied in a traditional tank-irrigation–based rice- producing village in the malaria-endemic low country dry zone of northcentral Sri Lanka during the period August 1994–February 1997. Adult mosquitoes were collected from human and bovid bait catches, bovid-baited trap huts, indoor catches, and pit traps. Mosquito head-thoraces were tested for the presence of Plasmodium falciparum and P. vivax, and blood-engorged abdomens for the presence of human blood by ELISAs. House surveys were done at two- day intervals to record cases of blood film–confirmed malaria among the villagers. A total of 7,823 female anophelines representing 14 species were collected. Trends in anopheline abundance were significantly correlated with rainfall of the preceding month in An. annularis, An. barbirostris, An. subpictus, An. vagus, and An. varuna, but were not significant in An. culicifacies and An. peditaeniatus. Malaria parasite infections were seen in seven mosquito species, with 75% of the positive mosquitoes containing P. falciparum and 25% P. vivax. Polymorph PV247 was recorded from a vector (i.e., An. varuna) for the first time in Sri Lanka. Computations of mean number of infective vector (MIV) rates using abundance, circumsporozoite (CS) protein rate, and human blood index (HBI) showed the highest rate in An. culicifacies. A malaria outbreak occurred from October 1994 to January 1995 in which 45.5% of village residents experienced at least a single disease episode. Thereafter, malaria incidence remained low. Anopheles culi- cifacies abundance lagged by one month correlated positively with monthly malaria incidence during the outbreak period, and although this species ranked fifth in terms of abundance, infection was associated with a high MIV rate due to a high CS protein rate and HBI. Abundance trends in other species did not correlate significantly with malaria. It was concluded that An. culicifacies was epidemiologically the most important vector in the study area. Anopheles culicifacies Giles (species B of Green and Miles 1 ) was long regarded as the only malaria vector in Sri Lanka following its incrimination in the 1920s. 2,3 However, recent field studies have produced evidence of the involve- ment of several other anopheline species in malaria trans- mission, based on ELISAs to detect the circumsporozoite protein (CS) of Plasmodium vivax and P. falciparum, the only malaria parasite species that are currently known to be present in Sri Lanka. 4–8 This has led to some confusion re- garding the true field vector status of these different anoph- eline species that have been incriminated by ELISA. The overall relative contribution of different local anopheline species to transmission has been quantified, 7 but there have been no studies that link temporal trends in vector population and transmission dynamics to trends in human malaria to clarify which species are of real significance in initiating and maintaining transmission. One of the factors that compli- cates such an approach is the highly unstable nature of ma- laria in Sri Lanka, where the case load has fluctuated from 150,000 to 400,000 cases per year during the present decade (Anti-Malaria Campaign of Sri Lanka, unpublished data), and foci of the disease shift spatially virtually from year to year. Studies on epidemiologic entomology relating to malaria in Sri Lanka have been concentrated in rural habitats asso- ciated with modern irrigation development projects such as the Mahaweli Project, 4,5,8 or in village habitats in the south- ern part of the island. 6,7 Vector bionomics and transmission parameters have not been studied in traditional tank-based villages that still form a substantial small-scale irrigation net- work in the rice-producing low country dry zone of Sri Lan- ka. Recently, a multidisciplinary study on the ecology and socioeconomic impact of malaria was initiated in such an area located within the Huruluwewa watershed in the North- central Province of the island. Studies relating to the eco- nomic costs of malaria and the population dynamics of im- mature stages of anopheline mosquitoes have already been published. 9–11 The present paper seeks to clarify the role of vector anophelines in malaria transmission in a typical vil- lage within the watershed. MATERIALS AND METHODS Study area. The study was done in the village of Maha- meegaswewa situated in the 41,950 hectares watershed area of the Huruluwewa Reservoir (capacity = 7,500 hectares) in northcentral Sri Lanka. Details of the layout and climate of the study area have been presented previously. 9 Briefly, the village (area = 163 hectares) consisted of 58 homes, a pri- mary school, Buddhist temple, rice fields, and stabilized chenas (i.e., cleared scrub-forest areas where nonirrigated subsidiary crops were cultivated). Irrigation water for rice cultivation was obtained from two small reservoirs (known locally as tanks) located within the village area. The Yan Oya stream defined the western boundary of the village. In total, 309 people occupied the village at the start of the study. Mahameegaswewa is one of several purana (ancient) villages located within the degraded forest of the watershed, each with its own cascade of small tanks, which are consid- ered to be remnants of a tank-based rice irrigation system dating back 1,000–2,000 years. The study area was situated within the low country dry zone of Sri Lanka, with an annual rainfall of 600–1,000 mm. The patterns of rainfall recorded at Mahameegaswewa vil- lage, and temperature and humidity recorded at the nearest meteorologic station at Mahailluppallama (30 km distance) during the study period are presented in Figure 1. Scientific and ethical review of the research protocol was carried out within the International Irrigation Management Institute. Malaria prophylaxis and mosquito-proof tents for