title: Environmental drivers of Dengue, Chikungunya and Zika transmission and their mosquito vector, Aedes aegypti, in two coastal hotspots in Ecuador
Creators: name: Ortega López, Leonardo Daniel
Publisher: name: University of Glasgow
Year: 2021
Subjects: subject: Aedes aegypti, dengue, Zika, chikungunya, vector ecology, epidemiology.

description: In the Americas, arbovirus transmission is concentrated within urban settings in tropical zones, where high human population densities and environmental conditions enhance the survival and reproduction of Aedes aegypti. Since its re-emergence in South America in the 70’s, dengue virus has been expanding and increasing in urban settings where it is now endemic. Additionally, the recent arrival of new arboviruses into the region, such as chikungunya (2013) and Zika virus (2015), have triggered major epidemics leading significant public health and economic impacts. These pathogens are linked in sharing a common mosquito vector in Ae. aegypti. Given the absence of effective licenced vaccines, vector control is thus the primary strategy for reducing the transmission of all of these pathogens. Effective vector control and public health preparedness require detailed understanding of vector ecology and human exposure risk within foci of transmission. Both vector populations and viral dynamics are highly dependent on environmental conditions, but the nature of environmental impacts likely depends on local ecological context. Ecuador bears an important burden of arboviral transmission in South America. Most transmission is concentrated in coastal cities where dengue is endemic and rising, and major outbreaks of chikungunya and Zika have recently occurred. However, there has been limited investigation of vector ecology in these rapidly expanding urban settings, and its association with seasonal patterns of arboviral transmission. To address this gap, this study aimed to assess the environmental drivers of Ae. aegypti ecology, infection rates and arboviral transmission within two major urban hotspots in Coastal Ecuador. This was accomplished through a series of field studies of vector ecology, laboratory analyses of arboviruses, and modelling investigations designed to identify environmental determinants of human exposure and infection incidence. The first chapter reviews what is known about the most important Aedes-borne viruses and their vectors in South America, with particular focus on Ecuador, vector control, and the global and regional disease burden. The second chapter presents results from a field study carried out in a urban neighbourhood of Quinindé, Ecuador, that evaluated a novel trapping method, the Mosquito Electrocuting Trap (MET), for direct estimation of human exposure to Ae. aegypti bites. The third chapter describes results of a 6-month field study aimed to characterize the environmental determinants of Ae. aegypti abundance and distribution, behaviour and arboviral infection rates within two cantons in Coastal Ecuador at the tail end of the 2016-17 Zika outbreak. The fourth chapter presents an analysis of seasonal and annual variation in dengue, chikungunya and Zika virus within these 2 cantons, and associations with climatic and entomological variables. The fifth chapter discusses the key results of each of the chapters and the implications of the findings towards an effective vector control in Ecuador and beyond the country. It was found that the MET was effective for measuring Ae. aegypti host-seeking behaviour, and generated representative estimates of their biting rate and activity time relative to the standard BG-sentinel trap (BGS). Analysis of Ae. aegypti ecology indicated that its abundance varied significantly between cantons, neighbourhoods within cantons associated with urbanization gradient, temporal timing of collection and past rainfall. Additionally, there was significant variation in Ae. aegypti resting behaviour (resting in or outside houses) between cantons. This demonstrates the existence of heterogeneity in Ae. aegypti population dynamics and behaviour between and within the study sites, which highlights the importance of localized surveillance to guide vector control. Likewise, arboviral incidence of dengue and chikungunya (as reported to the health system) also differed between the two study sites, being dengue 1.5 and chikungunya 2.5 times higher in Portoviejo than in Quinindé during the peaking week. The seasonal pattern of disease incidence varied among the three arboviruses, with a difference of 5 weeks between each of their peak of incidence. Intra-annual incidence was also found to be linked with climatic and entomological variables, with dengue and chikungunya incidence being positively associated with temperature and rainfall, while Zika incidence negatively associated with such climatic variables. Outdoor Aedes collections with Prokopack aspirators and BGS were positively related to dengue incidence, while indoor Prokopack aspirations were negatively associated with this variable. The interannual incidence of dengue differed between years analysed (2013-2018) with 2015 being the year with highest dengue incidence. Such findings highlight the importance of conducting focalized epidemiological surveillance on each site, but also differentiating between arboviruses, rather than assuming they all will follow dengue trends. Findings from this work have provided new entomological and epidemiological information to the study sites and despite the short period of study, fine spatial scale heterogeneity was detected in arbovirus transmission dynamics.