Observations and mechanistic modelling of the role of Aedes mosquitoes in dengue transmission in Singapore

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 182-209).Dengue is the most prevalent human arboviral disease in the world with an estimate of 390 million cases per year. The disease is mainly transmitted by two mosquitoes: Aedes aegypti and Ae. albopictus. Despite the continuous efforts to curb the spread of the disease in Singapore, there is an upsurge in dengue epidemics. In this thesis, the role of Aedes mosquitoes in shaping the spatiotemporal patterns of dengue transmission in Singapore is investigated. Field surveys are carried out to study the effect of monsoons on the disease vectors; the impact of urban housing on spatial patterns of dengue transmission is explored; and mechanistic models are developed to simulate the seasonality of dengue occurrence in the city. The role of monsoons in shaping the seasonal pattern of dengue is investigated. Singapore has no pronounced dry season, while dengue cases decrease every year after a very wet monsoon. A preliminary survey documents that Aedes mosquitoes breed in drains close to houses. A hypothesis that the very wet monsoon results in a strong reduction of outdoor breeding of Aedes is tested using field data. A one-year entomological survey confirms the hypothesis and shows that a monsoonal sequence of flushing-drying conditions in the drains shapes the seasonal abundance of Aedes in Singapore. This finding can be used to optimize vector control efforts and to better understand dengue transmission in the context of climate change. The influence of urban housing on the distribution of dengue at neighborhood and country scales is studied. Dengue incidence, vector abundance, and drainage network density are found to be higher in a subarea of low-rise housing in Geylang. Further, a holistic analysis at the country-scale confirms the role of urban housing in shaping spatial patterns of dengue. Dengue incidence is found to be proportional to the fraction of the area (or population) of low-rise housing. This finding suggests that affording public housing in agglomerations of high-rise buildings would have a positive impact on dengue control if this urban housing develops at the expense of low-rise areas. A mechanistic model is developed to investigate the role of drainage networks in the outdoor breeding of Aedes and transmission of dengue virus. The HYDRology, Entomology and DEngue Transmission Simulator (HYDREDETS) consists of three coupled dynamic models, with an explicit representation of the spatial domain. The model is calibrated using field observations to study rainfall-effects on flushing/drying of aquatic stages and re-inoculation by dormant eggs. Dengue vertical transmission and the role of human movements are incorporated in the model. HYDREDETS is capable of simulating patterns of dengue transmission in Singapore, however with reduced seasonality. This finding supports the hypothesis that dengue seasonality in Singapore can be attributed to a significant degree, to flushing-drying sequence in outdoor breeding habitats. Based on the conclusions of this thesis, spatiotemporal patterns of dengue in Singapore are shaped by climate and urban ecology which influence the ecology of Ae. aegypti and Ae. albopictus. HYDREDETS is a new modeling tool tailored to the study of vector-borne diseases related to urban hydrology. Dengue vector control can be optimized using these new tools and the improved understanding of the disease ecology.by Osama M. E. Seidahmed.Ph. D

A descriptive sketch for the rainfall flushing mechanism shows how intense rainstorms during the monsoon results in washing breeding of dengue vector from stagnant drains.

<p>A descriptive sketch for the rainfall flushing mechanism shows how intense rainstorms during the monsoon results in washing breeding of dengue vector from stagnant drains.</p

Seasonality of dengue in Singapore: boxplot of monthly percent of cases (1983–2015).

<p>Seasonality of dengue in Singapore: boxplot of monthly percent of cases (1983–2015).</p

The study area: Geylang neighborhood, Singapore.

<p>The figure shows locations of positive breeding sites for <i>Aedes aegypti</i>, <i>Ae albopictus</i> and <i>Culex spp</i>. Breeding drains of <i>Ae</i>. <i>aegypti</i> are given serial numbers that also used in <a href="http://www.plosntds.org/article/info:doi/10.1371/journal.pntd.0004842#pntd.0004842.g006" target="_blank">Fig 6</a>.</p

Spatial and temporal patterns of dengue transmission along a Red Sea coastline: a longitudinal entomological and serological survey in Port Sudan city.

BackgroundDengue is an emerging health problem in several coastlines along the Red Sea. The objective of the present work is to elucidate spatial and temporal patterns of dengue transmission in Port Sudan.Methods/findingsA longitudinal study with three cross-sectional surveys was carried out in upper, middle and lower class neighborhoods, from November 2008 to October 2009. Monthly, entomological surveys were followed by serological surveys in dengue vector-positive houses. Meteorological records were obtained from two weather stations in the city during the same time. Overall, 2825 houses were inspected. Aedes aegypti represented 65% (35,714/54,944) and 68% (2526/3715) of the collected larvae and pupae, respectively. Out of 4640 drinking water containers, 2297 were positive for Ae. aegypti. Clay-pots "Zeirr" followed by plastic barrels were key productive containers for pupae of dengue vector, 63% (n = 3959) and 26% (n = 1651), respectively. A total of 791 blood samples were tested using PanBio Capture/Indirect IgM ELISA. Overall, the sero-prevalence rate of dengue ranged between 3%-8% (41/791), compared to an incidence of 29-40 new cases per 10,000 (193/54886) in the same examined population. Lower and middle class neighborhoods had higher entomological indices compared with upper class ones (pConclusionDengue incidence is heterogeneously distributed across the neighborhoods of Port Sudan and exhibits a bi-cyclic intra-annual pattern. Hence, it should be feasible to carry out timely vector control measures to prevent or reduce dengue transmission

Impacts of Agricultural Practices on Insecticide Resistance in the Malaria Vector Anopheles arabiensis in Khartoum State, Sudan.

BACKGROUND Agricultural pesticides may play a profound role in selection of resistance in field populations of mosquito vectors. The objective of this study is to investigate possible links between agricultural pesticide use and development of resistance to insecticides by the major malaria vector Anopheles arabiensis in northern Sudan. METHODOLOGY/PRINCIPAL FINDINGS Entomological surveys were conducted during two agricultural seasons in six urban and peri-urban sites in Khartoum state. Agro-sociological data were collected from 240 farmers subjected to semi-structured questionnaires based on knowledge attitude and practice (KAP) surveys. Susceptibility status of An. arabiensis (n=6000) was assessed in all sites and during each season using WHO bioassay tests to DDT, deltamethrin, permethrin, Malathion and bendiocarb. KAP analysis revealed that pesticide application was common practice among both urban and peri-urban farmers, with organophosphates and carbamates most commonly used. Selection for resistance is likely to be greater in peri-urban sites where farmers apply pesticide more frequently and are less likely to dispose of surpluses correctly. Though variable among insecticides and seasons, broad-spectrum mortality was slightly, but significantly higher in urban than peri-urban sites and most marked for bendiocarb, to which susceptibility was lowest. Anopheles arabiensis from all sites showed evidence of resistance or suspected resistance, especially pyrethroids. However, low-moderate frequencies of the L1014F kdr allele in all sites, which was very strongly associated with DDT, permethrin and deltamethrin survivorship (OR=6.14-14.67) suggests that resistance could increase rapidly. CONCLUSIONS Ubiquitous multiple-resistance coupled with presence of a clear mechanism for DDT and pyrethroids (kdr L1014F) in populations of An. arabiensis from Khartoum-Sudan suggests careful insecticide management is essential to prolong efficacy. Our findings are consistent with agricultural insecticide use as a source of selection for resistance and argue for coordination between the integrated vector control program and the Ministry of Agriculture to permit successful implementation of rational resistance management strategies

Cross-sectional community-based study of the socio-demographic factors associated with the prevalence of dengue in the eastern part of Sudan in 2011 Infectious Disease epidemiology

BACKGROUND: Dengue is caused by an arthropod-borne flavivirus. Infection can be either primary or secondary based on serology, with each stage of the disease characterized by specific serological conversion and antibody formation. Further study is needed to fully identify the factors associated with and predisposing to dengue infection. The objective of this study was to identify socio-demographic factors associated with the prevalence of dengue serotypes in Kassala State in the eastern part of Sudan in 2011. METHODS: This was a cross-sectional community-based study with 530 participants who were randomly selected through multi-stage cluster sampling. Dengue serotype prevalence was determined using capture Enzyme-linked immunosorbent assay (ELISA). ELISA IgG. A multivariate logistic regression model was designed to measure the strength of associations between socio-demographic factors and dengue serotype prevalence. All participants who tested negative for dengue were used as the statistical reference group. RESULTS: From this study, the prevalence of dengue in Kassala was estimated to be 9.4 % (95 % CI: 7.1-12.3). Lack of knowledge about dengue fever disease (OR 2.8, 95 % CI: 1.24-6.53) and a household density of more than 3 people per room (OR 2.1, 95 % CI: 1.06-4.09) were the most important factors associated with dengue infection among the study population. CONCLUSIONS: Community-oriented interventions are needed to modify existing social behaviors to reduce the risk of dengue in the eastern part of Sudan. Additional studies are also required in this field

IgM Seroprevalence rates compared to incidence rates in the study areas during the period November 08–Oct 09.

***<p> <b>: p<0.001.</b></p

Distribution of Dengue IgM Seropositive cases among age groups and gender.

<p>Distribution of Dengue IgM Seropositive cases among age groups and gender.</p

Port Sudan map.

<p>Sketch map of Port Sudan city shows approximate location of the study neighborhoods.</p