The main risk factor of the Dengue and dengue haemorrhagic fever in Sri Lanla is the prevalence of vector mosquito species, Aedes aegypti and Ae. albopictus. Vector population enhances by indoor and outdoor water accumulated receptacles facilitating mosquito breeding. Therefore, it is warranted to understand the breeding preference of the dengue vector mosquitoes and the population dynamics to prevent dengue outbreak in high risk areas.
Weather factors such as monthly rainfall, number of rainy days, rainfall per wet days, temperature and humidity were significantly correlated with the monthly mean number of Aedes eggs and dengue vector mosquito species abundance. Ovitrap collection from March to July gave a peak in April. This was positively correlated with monthly total rainfall, number of rainy days, rainfall per wet day and wind speed. Breeding habitats of Aedes mosquitoes were rapidly created with water collecting receptacles in rainy season. Further, the range of monthly temperature between maximum and minimum values and the range of monthly humidity between day and night values were low during this period.
There is a slightly lower abundance of both species of mosquitoes during the dry season compare to the rainy season (Pinheiro and Tadei, 2002). Mosquito larval development is highly dependant on temperature. High moisture content helps to break resistivity of mosquito eggs and induce egg laying of female.
Ovitrap indices based on ovitrap surveillance within six PHI divisions indicated that Hunupitiya was recorded as higher values whereas Kelaniya and Peliyagoda followed it. This study observed that Aedes vectors deposited their eggs in outdoor containers (total eggs = 154/ovitrap) which exposed to sunlight (≥ 40 % of the day) than in indoor containers (total eggs = 57/ovitrap) throughout the study period. Maximum daily water temperatures in each surveyed containers typically 27°C – 30°C in outdoor sites and 25°C – 28°C in indoor sites. Larval development is highly temperature-dependent (Reuda et al, 1990). Female may have a selective advantage if they are able to detect containers with warmer water where their offspring develop faster. Immature Aedes vectors were more abundant in shaded containers with low water temperature (≤ 29°C), indicating that females oviposit more frequently in containers shielded from full sunlight (Barrera et al, 2006). Due to environmental differences between indoor sites and outdoor sites, outdoor containers in Kelaniya MOH area received commonly sun exposure up to 50% of the day, in contrast indoor sites received sun light up to 10% of the day for most indoor containers.
Results revealed that Ae. albopictus was higher abundant in both indoor and outdoor. There were many artificial and natural large oviposition sites. During the study period, Hunupitiya and Peliyagoda PHI divisions were under construction sites and there were many artificial containers (65% of containers) and open concrete slab areas (10% of division). Females laid more eggs in cement containers compare to plastic or metal containers when all were similar in size. (Wong et al, 2011). Kelaniya PHI division is bounded from its eastern margin with the Kelani river, hence undergo flooded in many months in the year. There also many abandoned and vacant lands implying to increase ovitrap indices and Aedes vector mosquito abundance in Kelaniya PHI division. Rest of other three PHI divisions were recorded with lower ovitrap indices and Aedes vector mosquito abundance with the lowest value recorded in Wedamulla.
The wider prevention and control of dengue is currently reliant on vector control methods. These include environmental, biological, and chemical vector control strategies and methodologies. It is therefore important to elaborate on some of the potential factors that drive dengue activity, as well as the global strategic direction to address this growth. Effective vector control is the mainstay of dengue prevention and control.
This study encountered 68.96% of total outdoor breeding sites mainly consist of rain water accumulated discarded, polythene and plastic (42.65%), coconut shells (33.08%) and tires (7.35%) as suitable receptacles for dengue mosquitoes oviposition. Among them, discarded used tires with retained rain water are the main outdoor breeding habitat for Ae. aegypti in this study. The present study denotes that 68% of surrounding was sparsely covered with vegetation while another 60% was categorized under built environment. Napier, 2001 stated that the housing pattern and the distance between houses have a significant influence on the dengue outbreak and its distribution trend. Therefore, high human population density and short distance between houses could lead to more efficient transmission of the virus and thus, increased exposure to infection. Simultaneously, the present study encountered that 66% of houses were almost interconnected or located in very close vicinity to each other in a crowded areas with very high human population. However, transmission of the disease is normally limited by the flight distance of Aedes mosquitoes in which Ae. aegypti could range from a few meters to more than 50 m and less than 500 m in a closed urban environment (Morlan & Hayes, 1958). General expectation is that there is a negative correlation between dengue incidence rate and level of education or the level of awareness of dengue. However, present study revealed that though communities may perform well in terms of knowledge of the disease, they may also do less in attitude and practice against dengue transmission. In the present study, most of the dengue positive cases were found from middle class families and those engaged in any kind of employment. Therefore, it can be presumed that patient number has been increased due to lack of time in spending at home in attending on dengue patient in early stages. This statement was advocated by having only 13% indoor breeding places with reference to outdoor breeding places (87%) supporting to the fact that due the lack of time they may have not concerned on their outdoor surrounding environment that have already provided suitable breeding grounds for the vector mosquitoes. Hence, all these factors have interconnected with dengue situation in this study area.