Symposium on Dengue - 2015
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Papers presented at the Symposium on Current Research Activities on Dengue, organized by the Molecular Medicine Unit, on 13th of November 2015
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Item Effect of γ-radiation on fertility and survival of Aedes albopictus (skuse) males in the laboratory for Sterile Insect Technique(Moleclar Medicine Unit, Faculty of Medicine, University of Kelaniya, Sri Lanka, 2015) Harishchandra, J.; Abeyewickreme, W.; Hapugoda, M.; Premaratne, R.G.; Gilles, J.R.S.BACKGROUND: It is believed that Aedes albopictus, one of the dengue vectors in Sri Lanka can be controlled using Sterile Insect Technique (SIT) when integrated with other conventional control methods. The objective of this study was to determine the effective dose of gamma radiation for producing sterile males of Ae. albopictus mosquitoes. METHODS: A batch of male pupae (n=32) aged 24-48 hours in F1 was irradiated using a gamma-ray irradiator (Gamma 220, Atomic Energy of Canada Ltd., Co60) with 25 Gy in duplicates. Following the same procedure, different doses (30 Gy, 40 Gy, 50 Gy, 60 Gy and 70 Gy) were given to each pupal batch in duplicates. Then they were transferred to laboratory cages (30 cm X 30 cm X 30 cm) for emergence and supplied with 10% sucrose solution. Adult emergence rate and male longevity were recorded. Virgin females from the same cohort (F1) were introduced into each cage for mating and fed blood starting 5 days after emergence. Females were then isolated in to individual tubes and hatching rate of individual egg batches was determined after two weeks of egg maturation in hatching solution containing 0.25 g BNB, 0.05g BY in 700 ml distilled water. Spermachecae of female mosquitoes were dissected and insemination rates were calculated after egg laying. RESULTS: Male mosquito pupae in F1 showed low mortality (0-3.12%) immediately after exposing to radiation (0-48 hours). After 21 days of the irradiation, probability of survival of male mosquitoes in F1 were 0.578, 0.494, 0.453, 0.313, 0.328, 0.381 and 0.219 at 0Gy, 25 Gy, 30 Gy, 40 Gy, 50 Gy, 60 Gy and 70 Gy respectively (Kaplan Meier survival analysis). Log Rank test indicated significant differences of survival of control males with males irradiated at 40Gy, 50 Gy, 60 Gy and 70 Gy. The survival of males irradiated at 25 Gy and 30 Gy did not differ from each other and from the control. The survival of males irradiated at 40 Gy, 50 Gy, 60 Gy were not significantly different among each other. The mean hatching rate (Mean+SE) of the F2 progeny of Ae. albopictus males (F1 progeny) were 10.89+2.76%, 7.36+1.75%, 3.09+0.71%, 0.79+0.27%, 0.66+0.23% and 0% when irradiated at 25 Gy, 30 Gy, 40 Gy, 50 Gy, 60 Gy and 70 Gy respectively. The control group showed a hatching rate of 64.26 + 7.12%. It was found that insemination rate of the irradiated males among different doses and controls were above 90% in F1 based on spermatheca dissection of blood-fed females (F1). CONCLUSION: 50 Gy is recommended as the most suitable γ radiation dose to produce 99% sterility in Ae. albopictus males which has 0.328 survival probability after 21 days of irradiation. This dose can be used to produce sterile males of Ae. albopictus for population suppression.Item Status of dengue vector breeding in pineapple plantations and environmental factors affecting their breeding(Moleclar Medicine Unit, Faculty of Medicine, University of Kelaniya, Sri Lanka, 2015) Dassanayaka, D.M.T.; Aryaprema, V.S.; Jayasooriya, H.T.R.; Abeyewickreme, W.BACKGROUND: Pineapple plantations are believed to have contributed to the transmission of dengue fever in pineapple growing areas in Sri Lanka. Hence, there is a need to introduce effective control methods to reduce dengue vector breeding in leaf axils of pineapple plants, a characteristic larval habitat of dengue vectors. The current study was designed primarily to obtain baseline information required for a much longer study planned to evaluate the use of pyriproxyfen and temephos in leaf axils of pineapple. METHOD: The study was carried out in five pineapple plantations in Medical Health Office area Meerigama in the Gampaha District from December 2014 to June 2015. Ninety plants from the five plantations were selected randomly and the volume of water in each plant and number of leaf axils positive with immature stages of mosquitoes was recorded twice a month. The water contents of leaf axils positive for mosquito larvae were transported to the laboratory and larvae were identified using standard keys. Chemical parameters: pH and temperature were measured in situ in the same samples and a few selected chemical parameters (concentrations of nitrates, phosphates, potassium and total dissolved solids) were measured in the laboratory using standard methods. Climatic data were obtained from the Meteorological Department of Sri Lanka. RESULTS: Eight mosquito species belonging to five genera were identified in leaf axil water of pineapple. Four species of Aedes including Aedes albopictus, the secondary vector of dengue, were identified while the primary vector Aedes aegypti was not recorded during the entire study period. The water volume per plant varied from 2.34 – 25.27 ml and it showed a significant positive correlation with the vector density (r=0.54, p=0.001). The number of rainy days in each month (r=0.76, p=0.000) and the monthly mean rainfall (r=0.53, p=0.001) showed significantly positive correlations with monthly vector density. Although monthly mean pH value showed no correlation with the monthly mean water temperature correlated negatively with the vector density (r= -0.5, p=0.004). None of the chemical factors except the concentration of nitrate (r=-0.45, p=0.016) in leaf water were associated with the vector density. CONCLUSIONS: Ae. albopictus was the most abundant species in all months except in January. The study revealed that the water volume in leaf axils, number of rainy days per month, monthly mean rainfall, water temperature and nitrate concentration of axil water seemed to affect the Ae. albopictus breeding, which are the important data useful in evaluating the use of pyriproxyfen and temephos in leaf axils of pineapple in the future. However, a continuation of the study for another six months to observe seasonal variation within a year is recommended.Item Vector prevalence and insecticide resistance status of Aedes sp. in dengue high and low risk areas in the Colombo District(Moleclar Medicine Unit, Faculty of Medicine, University of Kelaniya, Sri Lanka, 2015) Janaki, M.D.S.; Aryapreme, V.S.; Jayasooriya, H.T.R.; Abeyewickreme, W.BACKGROUND: In areas where dengue transmission is high, space spraying against adult Aedes sp. is practiced to reduce the density of infected mosquitoes and thereby reduce transmission within a very short time. Repeated use of insecticides in this manner, in dengue high risk areas, lead to the development of insecticide resistance in vector populations. The aim of the present study was to determine the prevalence and status of insecticide resistance of Aedes vectors in dengue high risk and low risk areas in the Colombo District. METHODS: The study was carried out in the area of Regional Director of Health Services, Colombo (RDHS area Colombo) and the Colombo Municipal Council area (CMC area), from December 2014 to June 2015. One each of comparatively high risk and low risk MOH area were selected from each study site based on the reported number of dengue cases during last five years. MOH areas Kolonnawa and D3 (Borella) were selected as high risk areas while Padukka and D5(Wellawatta) were selected as low risk areas respectively from the RDHS area Colombo and the CMC area. Ovitrap survey method was used to determine the vector prevalence and to collect Aedes eggs for bio assay tests. Larval and adult bio assay tests were conducted according to WHO guidelines. RESULTS: Both Ae. aegypti and Ae. albopictus were recorded from all MOH areas, except Padukka where there was only Ae. albopictus present. The vector prevalence in terms of Ovitrap Index and Egg Density Index was higher in Padukka (p=0.001 and p=0.029 respectively) than those in Kolonnawa in the RDHS area Colombo. In the CMC area, there was no such significant difference in the low risk and the high risk areas. Vector prevalence and the number of reported dengue cases showed no significant correlation in any of the MOH area. Aedes larvae in all four MOH areas have acquired resistance to diagnostic dosage of temephos (0.012 mg/l). Ae. aegypti in Kolonnawa, Ae. albopictus in Padukka and Ae. aegypti in D3 (Borella) showed possible resistance to 0.025% Deltamethrin while, Ae. aegypti in D5 (Wellawatta) has already acquired resistance. Ae. albopictus in D5 (Wellawatta) was still susceptible to the same insecticide. CONCLUSION: The vector prevalence does not significantly correlate with disease incidence, but further correlations with patients in localities within MOH and the vector densities will reveal reliable information. Despite the comparative status of high and low risk, all MOH areas have acquired resistance to insecticides, specifically to temephos and deltamethrin. Study reveals the emerging trends in resistance development of Aedes sp. in the Colombo District which has to be taken into account when designing further studies on resistance monitoring with a view to design effective control strategies.Item Emerging spatio-temporal trends of dengue incidence in Colombo and Kandy Districts, Sri Lanka(Moleclar Medicine Unit, Faculty of Medicine, University of Kelaniya, Sri Lanka, 2015) Udayanga, N.W.B.A.L.; Gunathilaka, P.A.D.H.N.; Iqbal, M.C.M.; Kusumawathie, P.H.D.; Abeyewickreme, W.BACKGROUND: Dengue has become a prominent challenge to the health sector in Sri Lanka. Prior identification/prediction of possible risk areas with high susceptibility to dengue, could be of major importance in ensuring the effective regulation of dengue epidemics. Therefore, the current study attempts to investigate the spatio-temporal trends in dengue incidence and priority risk factors that characterize dengue epidemics, aiming to stratify the Colombo and Kandy Districts into high, moderate and low risk areas via development of dengue risk maps by devising a statistical and geo-informatics based approach. METHODOLOGY: Socio-economic data of dengue patients and healthy volunteers in selected high risk MOH areas of Colombo (5) and Kandy (5) Districts were obtained through an interviewer administrated questionnaire. Previously reported dengue cases from 2009 to 2014 in each MOH were acquired. The spatio-temporal trends in dengue incidence of studied MOHs, were investigated at the Grama Niladhari Division (GND) level by using Arc GIS (version 10.2). The impacts of gender and age of the community on the disease prevalence were investigated by using normal Chi square test and Paired-Chi square test in SPSS (version 23). RESULTS: At present, 95% and 81% of the overall socio-economic surveillance has been completed in Kandy and Colombo Districts, respectively. Spatial maps on disease outbreaks have been developed for Akurana, Doluwa, Gampola, Gangawatakorale, Kandy Municipal Council (KMC), Kundasale and Poojapitiya MOH areas. According to results of the Paired-Chi square test, the emergence of dengue outbreaks indicated a significantly declining trend in recorded dengue cases in most of the GNDs within the studied MOHs, during the recent years. The Percentage Infected Male:Female Ratio (PIMFR) remained significantly altered throughout the period of study (p=0.001<0.05 at 95% of significance). Within the studied MOHs, males tend to indicate relatively high susceptibility for dengue infection than females (with 58.6: 41.5 of average PIMFR). The age group of 11 – 20 and 21 – 30 years were identified as with highest vulnerability to dengue, while the age > 61 years indicated the least vulnerability in all the MOHs. According to the Paired-Chi square test, the vulnerability of age groups was found to shift significantly throughout the study period [>Χ2 (7, 0.95) = 14.067]. CONCLUSIONS: The KMC and Gampola MOHs indicate relatively high susceptibility to dengue within the district of Kandy. Within all the studied MOHs, males tend to indicate relatively high susceptibility to dengue. Age groups of 11 - 20 and 21 – 30 denote relatively higher vulnerability to dengue, while age group of > 61 emerge as the least vulnerable group for the infection of dengue in the studied MOHs.