Entomological aspects, physico-chemical characteristics and socio-economic factors related to the breeding habitats of aedes aegypti and aedes albopictus in a dengue high risk area in the colombo district

Abstract

Dengue has emerged as an important public health problem in Sri Lanka. The entomologicalaspects, physico-chemical characteristics and the socio-economic factors that induce breeding habitats of Aedes aegypti and Aedes albopictus, vectors of dengue, were studiedin the Narahenpita Public Health Inspector's area of the Colombo Municipal Council fromAugustto November 2007. A preliminary survey in 50 premises to accrue basic information on possible Aedes breedinghabitats in the study area was followed by a general survey, which encompassed a total of 352 premises including normal residences, apartments, shanties, work stations, commercialsites, schools, government and private institutions. All premises were examined bothindoors and outdoors for possible Aedes breeding sites. The mosquito larvae encountered were collected and identified to species in the laboratory. The selected physico-chemical parametersin Aedes positive containers were measured using standard instruments and socioeconomic data were gathered by a validated questionnaire. All entomological, physicochemicaland socio-economic data were statistically analyzed. A total of 174 water holding receptacles including 29 with Aedes breeding were observed in the preliminary study. In the general survey, dengue vector breeding was observedin 81 premises. 702 water holding containers that could be considered as "potential" Aedesbreeding habitats were encountered, where coconut shells were the most abundant receptacle.Of the 127 Aedes "positive" water containers and plastic containers were the majorbreeding habitats of both Aedes species in the study area. Fish tanks, glass bottles, clay pots,bird baths and natural containers (bamboo stumps and leaf axils) were among the most attractivecontainers for vector breeding. The majority of potential Aedes breeding containers were found in normal residences (47.24%) followed by work stations (14.17%) and commercialsites (14.17%). For Ae. aegypti and Ae. albopictus, there were no significant differenceseither between the number of containers positive or in the mean number of larvae per positive container. Both vector species were predominant in the study site. The larval abundanceindices (Premises Index = 23.01%, Breteau Index = 36.08) indicated that the area is highrisk for the transmission of dengue, dengue haemorrhagic fever and chikungunya. The mean temperature of Aedes breeding water was 30.1 :t 0.8 °C and the pH values weremore or less neutral (mean 6.65 :t 0.47). The mean dissolved oxygen concentrations were 4.09 :t 1.02 mg/I. The breeding sites had a wide range of conductivity (50.2- 1451 IlS/cm),turbidity (6.94- 519 NTU) and salinity (0.1- 0.8 ppt). There was a significant positive relationship between the total Aedes larval density and the dissolved oxygen (DO). Ae. albopictuslarval density was positively and significantly related to DO, but negatively related topH. The frequency of examining Aedes breeding sites, the materials used to construct the wall,floor and the roof of houses, the level of education and income of the householders and the frequency of garbage collection by the local authorities were the most important socioeconomic factors which may induce Aedes larval breeding. There was a significant association between the frequency of examining for breeding sites and the presence or absenceof Aedes positive containers. The recommendations formulated based on these findings will be helpful in controllingdengue vector breeding, and thus preventing disease transmission in such urban areas.