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STRONG ASSOCIATION BETWEEN HOUSE CHARACTERISTICS AND MALARIA VECTORS IN SRI LANKA

ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
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

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With 200,000–400,000 laboratory-confirmed cases registered 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 program to reduce adult vector survival.

The introduction of DDT in 1946 is assumed to be responsible 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 insecticides was introduced, in which some areas with normally low transmission levels would be sprayed only if increased transmission 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 facilities. 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 included 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 program has taken up a very large part of the public health budget of Sri Lanka.² The stratification process might be further refined and the residual spraying program made more cost-effective if certain risk factors would be taken into account, 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 plastered 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 abundance of indoor-resting malaria vectors. To account for the large seasonal fluctuations in vector abundance, it was decided 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

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Study area. The study was done in seven consecutive villages 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 the most important vector of malaria, as elsewhere in Sri Lanka. Other important vectors are An. vagus, ranking second, An. peditaeniatus, third, An. subpictus, fourth, and An. varuna, fifth, based on an estimate considering species-specific abundance, circumsporozoite protein rate, and the human blood index.⁷

Entomologic collections. From mid-May 1996 to the end of December 1998, indoor-resting mosquito densities were estimated based on fortnightly collections. There were 56 sampling occasions of three days each during the study period. A new 10% random sample of all houses in each of the seven villages was selected for each sampling occasion. If a selected house could not be accessed, the nearest house was used for sampling. Before a spraysheet collection from a selected house, a member of the household was asked about the number of people who had slept there the previous night. A team of one entomologist and two assistants collected the mosquitoes from the house’s sampled room (in which individuals had slept the previous night). If more than one room in a house was used for sleeping, the room in which most people had slept the previous night was selected. After all exits had been covered, a white cotton sheet was placed on the floor, and a pyrethrum-based insecticide was sprayed in the room. Fifteen minutes later, the dead mosquitoes were collected from the sheet and stored in separate individual vials for each house. All mosquitoes collected were identified in the field with a microscope on the same day and recorded by species and gender, and by blood meal status for females. After each field collection, all sampled vials were re-examined under a microscope and identified by species at the University of Peradeniya.

House characteristics and location. Before the start of the entomologic sampling, the exact location of all houses in the seven villages was determined with a hand-held global positioning system receiver. An identification number was clearly marked on the door beam of the entrance to each house. The stream running through the area also was mapped using handheld global positioning system receivers, along with the location of houses, and this information was used to establish a geographic information system (GIS) using ArcInfo software. The GIS made it possible to calculate the distance from each house to the nearest stream point, and the information on the location of each house relative to the stream then was input into the detailed analysis. The average distance to the stream for each of the seven villages was obtained from the distance calculation for the individual houses and was used for the presentation of summarized information by village.

Shelters used during the night when protecting crops in the field were not included in the study. Uninhabited houses also were excluded, reducing the number of houses in the sample to 473. Houses built during the study period were not included.

The type of construction of each of the 473 houses was recorded in detail before the entomologic sampling. This included the construction material used for the roof and walls and whether the house was completely or incompletely constructed. A "good" house was defined as being complete, with brick walls and a roof of either tiles or corrugated iron or asbestos. A "poor" house did not have all these characteristics. Common building materials for a poor house were mud or cadjan for walls and cadjan or other plant material for the roof. Nine to 10 months after initiation of the study, all houses in the study area were revisited to generate a new profile of the house construction. Three houses whose construction profile had changed completely between the start of the study and the second survey were excluded from the analysis but included in the table summarizing the data obtained by village. The same two people evaluated all houses in both surveys.

Information on malaria control activities. During the study period, the routine residual spraying operations by the Anti-Malaria Campaign continued, and information on the date of spraying, the chemicals used, and the status of the spraying (complete/incomplete/not sprayed) was recorded for each house studied. Residual insecticide sprayings took place in May 1997, September–October 1997, and November–December 1998. In the analysis, two variables were used to represent the impact of the residual spraying program. The first showed whether a house was covered by the residual spraying program in the 30 days before the spraysheet collection. The second was a composite variable that made a distinction between houses that were fully covered during all three rounds of the residual spraying program and those that had incomplete coverage. The use of mosquito bed nets in the villages was very low, and none of the bed nets was impregnated with insecticide. Members of each household were questioned before the study on the use of pyrethrum coils or traditional fumigants to repel mosquitoes.

Data analysis. Data were analyzed with Epi-Info and SPSS 10.0 (SPSS, Chicago). Presence or absence of anopheline species in houses in relation to housing construction and location was modeled using logistic regression controlling for the main confounding variables.

RESULTS

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Of the 2,652 spraysheet collections from May 1996 to December 1998, 416 (15.7%) were positive for An. subpictus and 195 (7.4%) for An. culicifacies. An. varuna was collected in 17 samples (0.6%), An. vagus in 12 (0.4%), An. nigerrimus in two (0.1%), and An. annularis and An. jamesii in one sample each. In the further analysis, only An. culicifacies and An. subpictus were considered. Figure 1 shows the seasonal fluctuations in the collection of the two species during the study period.

View larger version (65K): [in this window] [in a new window]       FIGURE 1. Seasonal fluctuation in percentage of samples positive for Anopheles culicifacies (ancu) and An. subpictus (ansu). Y-axis gives percentage of samples positive.

DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
In this study, the occurrence of the two anopheline species inside houses was related to the type of housing construction, with poorly constructed houses having a 30% higher risk for harboring An. culicifacies and An. subpictus than those that were complete and built with permanent materials. A very strong association was found between the presence of An. culicifacies and the location of the house relative to the locally important breeding site. Generally, the findings from this study support the significance of housing construction in relation to the presence of indoor-resting anophelines found in an earlier study in Sri Lanka.³

It was expected that An. culicifacies and An. subpictus would be the most important indoor-resting anopheline mosquitoes. An earlier, more-detailed entomologic study in one of the seven villages (Meegaswewa) showed the same pattern, with only An. subpictus and An. culicifacies collected in significant numbers during indoor-resting catches.⁷ Anopheline species that were collected in large numbers with cattle bait catches and cattle-baited trap huts, such as An. vagus, were not found resting inside houses. In the present study, it was found that rooms in which more than two people had slept the night before the spraysheet collection had an increased risk of having An. culicifacies but not An. subpictus, suggesting that An. culicifacies entered the houses to feed on humans, whereas An. subpictus entered mainly to rest after having fed outside. This compares well with the low infectivity found in the An. subpictus analyzed from the study area.⁷ The significance of the entomologic findings presented in this article were supported by results from a case control study in the same seven villages in which people living within 750 meters of the stream were at much higher risk for malaria than those living farther away (van der Hoek W et al., unpublished data). Previous studies in the area have shown that the stream was the major breeding site for An. culicifacies, giving a biologic explanation for the very strong association found in this study between the presence of An. culicifacies and the location of the house relative to the locally important breeding site.⁶ The association was not that strong for An. subpictus, which is most likely explained by the fact that An. subpictus would, in addition to using breeding opportunities in the stream, readily exploit sun-exposed turbid water collections on cleared ground.⁸

The data suggest that houses that were covered by the residual spraying program in the 30 days before the spraysheet collection were protected against An. culicifacies but not against An. subpictus. This might be explained by the widespread resistance of An. subpictus to many commonly used insecticides.⁹

Since early this century, it has been known that An. culicifacies, the main vector of malaria in Sri Lanka, hides in cracks and crevices within houses. Wattle and daub houses with cadjan roofs provide many opportunities for this.¹⁰ The percentage of permanent houses using durable construction has increased over time. At the last census in 1981, 42% of the houses in Sri Lanka were permanent units constructed of durable materials such as cement, bricks, tiles, or asbestos sheets. The other 58% still used perishable materials alone or only some durable products.¹¹ Especially in rural areas, walls and floors of houses often still are constructed with mud, and roofs are made from perishable materials such as cadjan. Mosquito densities are higher in such temporary houses than in permanent houses, and this increases the risk of malaria for its inhabitants. A large number of non-government organizations implement reconstruction and housing activities throughout Sri Lanka, often as a part of rural development activities or refugee resettlement programs. Special consideration for the design, choice of materials, and siting of settlement areas could offer benefits with respect to malaria control. Similarly, government-supported housing programs have provided credit and financial and technical assistance since the establishment of the National Housing Department in 1952. As part of the policies and guidelines supporting new housing initiatives, special attention should be paid to the prevention of malaria. Clearly, overall housing construction is strongly connected to socioeconomic status, and fewer anopheline mosquitoes could be expected to be found indoors when poverty is reduced and houses improved, but this, of course, is a long-term strategy.

The findings here may be influenced by a bias in the spraysheet collections: Anopheline species that bite indoors but rest outdoors would not have been captured with the methods that were used. This could especially have influenced the collections of An. annularis and An. vagus but would still be of limited importance when compared with the overall collection of An. culicifacies and An. subpictus.^12,13 Another limitation of the study was that the use of traditional fumigants and mosquito coils was not followed continuously throughout the project period. A family’s change in fumigant and coil use practices during the study period may have influenced the overall number of indoor-resting mosquitoes collected but would not have been controlled for in the analysis.

Both the entomologic and epidemiologic studies in the area support a malaria control strategy for the malarious arid zone of Sri Lanka focusing on residential areas within 750 meters of streams and rivers and with special attention given to areas with the poorest type of house construction, such as refugee settlements, poor rural communities, and seasonal migrant settlements. Such focused control activities combined with a wider monitoring program to detect outbreaks farther from the waterways would provide a cost-effective alternative to the current control efforts. The findings of this study indicate that families living in houses with the poorest construction and close to the vector breeding sites should be the primary target for the provision of bed nets.

Received June 14, 2002. Accepted for publication October 10, 2002.

Acknowledgments: We are sad to report the untimely and tragic death of one of the co-authors, Maldeniya Piyaratne, a dedicated and hard-working young researcher and a good friend. We thank Mala Ranawake for her central role in database management and other essential support services. Also, we greatly appreciate the efforts of Ms. Ranawake and Lal Muttuwatta in producing geo-referenced maps of the study area. This study would not have been possible without the strong support of the government entomologic teams in Anuradhapura.

Financial support: This work was funded by grants to the International Water Management Institute by the government of Japan and the Danish International Development Agency.

Reprint requests: Wim van der Hoek, Bierstalpad 37, 1121 JK Landsmeer, Netherlands, Telephone: +31-20-4826312, Fax: +94-1-786854, E-mail: w.van-der-hoek{at}cgiar.org

REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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