Theses and Dissertations
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Item Activity and bionomic aspects of primary and secondary vectors for dengue transmission in Gampaha district, Sri Lanka(University of Kelaniya, Sri Lanka, 2022) Dalpadado, C.P.R.D.Sri Lanka had experienced periodic dengue epidemics every two to three years for the last two decades. In this scenario, understanding bionomics aspects, and the prevalence of dengue vector species in varied settings will help to develop more accurate and focused vector control approaches. Thus, the current study is intended to elucidate the bionomics of Aedes aegypti and Ae. albopictus, in urban (Negombo, Wattala, Kelaniya), suburban (Attanagalla, Gampaha, Minuwangoda) and rural (Dompe, Meerigama, Divulapitiya) areas of the Gampaha District, Sri Lanka from April 2017 to December 2019. The effectiveness of the insecticide space spraying depends on susceptibility status and the behaviour of the targeted species, hence a part of this study included testing susceptible levels of Ae. aegypti and Ae. albopictus against malathion and deltamethrin insecticides. The study also sought to establish area-specific threshold values for determining the risk of dengue transmission based on larval indices, and a forecasting model for predicting impending dengue outbreaks within the district. Total 19,835 possible breeding habitats were investigated at 13,563 premises where Aedes larvae were identified in 11.03% of premises and 1856 habitats were positive for larvae (Ae. aegypti; 9.8%; Ae. albopictus; 90.2 %). Results showed that Ae. aegypti prefers urban locations while Ae. albopictus is the predominant vector in all spatial settings. Ae. albopictus (54.5%; n= 999) dominated the adult collection, followed by Ae. aegypti (45.5%; n= 835), with a 1:4 male to female ratio. Ae. aegypti mosquitoes demonstrated endophilic resting behaviour, whereas Ae. albopictus were exophilic (Chi-square analysis between the two species; P < 0.001). Resting places of Aedes aegypti recorded as on cloth hangings (36.9%, n=308) and under furniture (40.4%) predominantly in bedrooms (40.4%) and living rooms (24.8%) while Ae. albopictus preferred to rest on outdoor vegetation Xxix 46%(n=460). The majority (43.7 %; n=801) of Aedes mosquitoes rest 1-2 m above ground, and 34.4 % (n=399) rested 1 m or less. The host-seeking cycle of Ae. albopictus was bimodal, with morning peak occurring between 05:00 and 11:00 and afternoon peak between 14:00 and 19:00. While that of Aedes aegypti was with a minor peak between 05:00 and 09:00 and a major peak between 13:00 and 19:00. Physicochemical parameters of mosquito breeding water changed significantly across breeding categories (Kruskal—Willi’s statistics, p < 0.001) showing the change of mosquito susceptibility status. There was a strong positive association between rainfall, larval vector density, and the likelihood of high dengue incidence. When Breteau Index for Ae. aegypti (BIA) exceeds 3.00 and relative humidity exceeds 80%, an early epidemic alert is triggered while BIA > 6.0, case incidence reached an epidemic level in urban areas. In suburban areas, when BI for Ae. albopictus (BIB) > 14.0 an early epidemic alert is triggered. At the BIB > 20, case incidence reached epidemic levels even in absence of Ae. aegypti. Case incidence exceeds the epidermic level in rural areas when the BI for Ae. albopictus is 10 with a one-month latency. In conclusion, area-based entomological thresholds and rational use of insecticides can be proposed for future control of rising dengue epidemics in the Gampaha district based on vector biology/bionomic related evidence through regular monitoring.Item School-based interventions to control dengue vector in the Gampaha district(University of Kelaniya, Sri Lanka, 2022) Radhika, N.M.L.This study was conducted to determine the effectiveness of school-based interventions to control dengue vectors in the Gampaha District, Sri Lanka. Sixty schools were selected randomly from the school registry in four selected educational zones (15 each) as interventional (Kelaniya and Gampaha) and control (Negombo and Minuwangoda). A baseline and three follow-up entomological surveys were conducted in all selected schools at 6-months intervals during 2016-2018. The intervention consisted an activity plan with entomological field surveys, larval control interventions followed by a structured awareness programme of dengue covering general epidemiology, clinical symptoms, prevention, vector control and waste management. Focus group discussions were conducted with stakeholders at 30 intervention schools to identify limitations in current control approaches. The level of awareness of dengue disease was assessed before and after the awareness programme using a structured questionnaire. Awareness levels were categorized as very poor (< 20%), low (21-40%) moderate (41-60%), good (61-80%) and excellent (> 80%). The change in awareness level was determined using General Linear Model. The effectiveness of the vector control interventions were assessed by the total number of dry, water-holding, and infested containers at follow-up survey compared to the baseline. Over, 46.31% (n=1016) of students had good level of awareness prior to the structured awareness programme and reached to the excellent level (41.84%; n=918) after the awareness programme. The improvement of the awareness level was identified as statistically significant according to the chi-square test of independence. At the base-line survey, Aedes albopictus was the predominant species in both control (60%; n=18) and intervention (63.33%; n=19) groups. Leaf axils were the leading breeding habitat followed by discarded plastic and metal containers in the control group. The number of discarded containers was reduced drastically in the interventional schools in the follow-up surveys. There was a statistically significant difference in dry (p<0.001), water-holding (p=0.027), and infested (p=0.006) containers after the last follow-up survey at interventional schools compared to the controls. This study warrants the need of sustainable participation of school community for dengue control. Therefore, strengthening Environmental Pioneer Programme will improve the capacity in dengue control interventions.Item Dynamics of aedes population in western province of Sri Lanka and a study on the feasibility of sterile insect technique (SIT) for controlling aedes albopictus(University of Kelaniya, Sri Lanka, 2022) Harishchandra, J.INTRODUCTION - Dengue is the most important vector-borne disease in Sri Lanka. Controlling dengue vectors is a challenge which needs multiple approaches. Sterile Insect Technique (SIT), which has been successfully applied in other countries, is a potential candidate for integrated control of dengue vectors in Sri Lanka. This study aimed to determine the dynamics of Aedes populations and to study the feasibility of SIT for controlling Aedes albopictus (Skuse) under laboratory and semi-field settings. METHODOLOGY - Dynamics of dengue vector populations were studied in two study areas in dengue high burdened districts, Colombo and Gampaha, in relation to climatic factors. Study area in the District of Gampaha where only a single dominant Aedes species reported was selected for the further studies. Size of the wild male population, survival and dispersal of Ae. albopictus were studied in Gampaha using Mark-Release-Recapture (MRR) method. A protocol for application of SIT for Ae. albopictus was developed including rearing, blood feeding, separation of male pupae, optimum radiation dose for sterilization and optimum release ratios of sterile males. RESULTS - The study revealed the presence of both Ae. aegypti and Ae. albopictus in Colombo and Ae. albopictus in Gampaha with a seasonal pattern. Significant correlations of entomological indices were found with rainfall, rainy days and relative humidity. Released males survived and dispersed up to 15 days and 200m respectively. Optimal radiation dose giving 99% induced sterility was 50Gy. Mating competitiveness quantified using Fried Competitiveness Index (FCI) of irradiated males against non-irradiated wild males resulted 0.5 which has met the international standards for competitiveness of Sri Lankan mosquitoes for application of SIT. Further, approximately 1245 male mosquitoes per hectare (ha) need to be released based on the density of the wild population. CONCLUSION - Densities of dengue vectors correlate with key climatic factors. Study area in the District of Gampaha where the presence of a single dominant Aedes species was selected for future SIT field trials. Developed protocols and study on behaviour of the selected mosquito species in the field will be useful for future application of SIT in integrated management of Ae. albopictus in Sri Lanka.