STRONG ASSOCIATION BETWEEN HOUSE CHARACTERISTICS AND MALARIA VECTORS IN SRI LANKA FLEMMING KONRADSEN, PRIYANIE AMERASINGHE, WIM VAN DER HOEK, FELIX AMERASINGHE, DEVIKA PERERA, AND MALDENIYA PIYARATNE Department of International Health, University of Copenhagen, Copenhagen, Denmark; International Water Management Institute, Colombo, Sri Lanka; Department of Zoology, Faculty of Science, University of Peradeniya, Sri Lanka; Anti-Malaria Campaign, Anuradhapura, Sri Lanka Abstract. The objective of this study was to determine whether house characteristics could be used to further refine the residual insecticide-spraying program in Sri Lanka. Indoor-resting mosquito densities were estimated in 473 houses based on fortnightly collections over a two-and-a-half-year period. The type of house construction and the exact location of all houses were determined. In a multivariate analysis, distance of less than 750 meters between a house and the main vector-breeding site was strongly associated with the presence of Anopheles culicifacies in the house (odds ratio [OR] 4.8, 95% confidence interval [CI] 3.4–6.8) and to a lesser extent with the presence of An. subpictus (OR 1.4, 95% CI 1.1–1.7). Poor housing construction also was an independent risk factor (OR for An. culicifacies 1.3, 95% CI 1.0–1.9; OR for An. subpictus 1.3, 95% CI 1.0–1.6). It is recommended that a malaria control strategy focus on residential areas within 750 meters of streams and rivers, with special attention given to areas with the poorest type of house construction. INTRODUCTION With 200,000–400,000 laboratory-confirmed cases regis- tered annually during the 1990s, malaria remains a significant public health problem in Sri Lanka. Since malaria cases were first recorded on a routine basis in Sri Lanka, the country has experienced significant outbreaks of the disease, with the most devastating epidemic (more than 5 million cases and 80,000 deaths) occurring in 1934–1935. Important outbreaks have been recorded more recently, although mortality linked to the disease has remained very low for the past five decades. The backbone of the malaria control strategy in Sri Lanka has been an extensive in-house residual insecticide-spraying pro- gram to reduce adult vector survival. The introduction of DDT in 1946 is assumed to be respon- sible for the dramatic reduction in the number of registered malaria cases from more than 1.3 million in 1947 to only about 37,000 in 1954. 1 From the 1970s until 1994, about a million houses were sprayed each year, first with DDT and later with malathion. In 1993 a strategy of selective spraying of insecti- cides was introduced, in which some areas with normally low transmission levels would be sprayed only if increased trans- mission was recorded or suspected, while other areas with high transmission would be sprayed three times a year. The stratification used in the new spraying program is based on the number of malaria cases reported by government facili- ties. The spraying operations also are intensified in areas with a large number of recorded Plasmodium falciparum cases. At the same time, fenitrothion and lambda-cyhalothrin were in- cluded in the spraying program to supplement malathion. This stratified operation significantly reduced the amount of insecticide used and the number of houses covered. Ever since it was implemented, the residual spraying pro- gram has taken up a very large part of the public health budget of Sri Lanka. 2 The stratification process might be fur- ther refined and the residual spraying program made more cost-effective if certain risk factors would be taken into ac- count, especially characteristics of the houses to be sprayed and a further specification of high-risk areas. A study by Gamage-Mendis et al. 3 in southeastern Sri Lanka found a strong association between malaria incidence and the type of house construction, independent of the house’s location. The risk of getting malaria was greater for inhabitants of the poorest type of houses, characterized by incomplete construction with thatched roofs and walls made of mud or cadjan (woven coconut palm leaves), compared with better-constructed houses with complete brick and plas- tered walls and tiled roofs. In the study, a significantly higher number of indoor resting mosquitoes was found in the poorly constructed houses than in the better-constructed ones. In a later study in the same area, the risk of malaria was found to be 2.5-fold higher for people living in poorly constructed houses than for those living in houses of good construction. 4 In our previous study conducted in one village in the north- central province of Sri Lanka, the location of the house was a risk factor, with people living close to a vector-generating stream at greater risk for malaria than those living farther from breeding sites. 5 Type of housing construction did not come out as an important factor, but the village under study was very homogeneous and provided little exposure contrast, with people living under almost-similar housing conditions. This article presents the findings of a detailed investigation in the same study area, assessing the importance of housing construction and house location in determining the abun- dance of indoor-resting malaria vectors. To account for the large seasonal fluctuations in vector abundance, it was de- cided to do a more-frequent and longer-term entomologic sampling than done in previous studies. The objective of the study was to find out whether housing construction and house location are important independent determinants of indoor- resting malaria vectors and could be used to further refine the current residual spraying program in Sri Lanka. MATERIALS AND METHODS Study area. The study was done in seven consecutive vil- lages in the Huruluwewa watershed in the Anuradhapura District of the north-central dry zone of Sri Lanka. The seven villages were selected because they overlapped with the area used as the focus of other related studies, making possible the use of existing baseline information. Previous studies showed that the stream running through the study area was, by far, the most important breeding site for Anopheles culicifacies. 6 Research in the area had clearly identified An. culicifacies as Am. J. Trop. Med. Hyg., 68(2), 2003, pp. 177–181 Copyright © 2003 by The American Society of Tropical Medicine and Hygiene 177