<b>Larval crowding enhances dengue virus loads in </b><b><i>Aedes aegypti,</i></b><b> a relationship that might increase transmission in urban environments</b>

Background. Climate change and urbanization will alter the global distribution of disease vectors, changing the disease burden in yet unpredictable ways. Aedes aegypti is a mosquito responsible for transmitting dengue, Zika, chikungunya, and yellow fever viruses that breeds in containers associated with urban environments. We sought to understand how ambient temperature and larval densities in the immature aquatic phases determine adult life history traits and dengue virus loads post-infection. We predicted that larval crowding and high temperatures would both lead to smaller mosquitoes that might struggle to invest in an immune response and, hence, would exhibit high viral loads. Methods. We first examined larval densities from urban and rural areas via a meta-analysis. We then used these data to inform a laboratory-based 2x2 design examining the interacting effects of temperature (21 vs. 26°C) and density (0.2 vs. 0.4 larvae/mL) on adult life history and dengue virus loads. Results. We found that urban areas had an ~8-fold increase in larval densities compared to more rural sites. In the lab, we found that crowding had more impact on mosquito traits than temperature. Crowding led to slower development, smaller mosquitoes, less survival, lower fecundity, and higher viral loads, as predicted. The higher temperature led to faster development, reduced fecundity, and lower viral loads. The virus-reducing effect of higher temperature rearing was, however, overwhelmed by the impact of larval crowding when both factors were present. Conclusions. These data reveal complex interactions between the environmental effects experienced by immature mosquitoes and adult traits. They especially highlight the importance of crowding with respect to adult viral loads. Together, these data suggest that urban environments might enhance dengue virus loads and, therefore, possibly transmission, a concerning result given the increasing rates of urbanization globally.研究背景 气候变化与城市化将改变病媒生物的全球分布格局，进而以尚未明确的方式改变疾病负担。埃及伊蚊（Aedes aegypti）是一种可传播登革热、寨卡、基孔肯雅热及黄热病毒的蚊种，其幼虫孳生于与城市环境相关的各类容器中。本研究旨在明确幼虫水生发育期的环境温度与幼虫密度，如何影响成虫的生活史特征以及感染登革病毒后的病毒载量。我们提出假说：幼虫拥挤与高温均会导致成蚊体型变小，使其难以投入足够资源进行免疫应答，因此病毒载量会更高。 研究方法 首先通过荟萃分析梳理城乡区域的幼虫密度数据；随后基于该数据设计实验室2×2析因实验，探究温度（21℃与26℃）与幼虫密度（0.2与0.4条幼虫/毫升）的交互作用对成虫生活史特征及登革病毒载量的影响。 研究结果 相较于农村样点，城市区域的幼虫密度提升了约8倍。实验室实验显示，幼虫拥挤对蚊类特征的影响强于温度。如假说所述，幼虫拥挤会延长发育时长、减小成蚊体型、降低存活率与繁殖力，并提升病毒载量；高温则会加快发育速度、降低繁殖力并减少病毒载量。但当温度与密度两个因素同时存在时，高温饲养对病毒载量的抑制作用会被幼虫拥挤的影响所抵消。 研究结论 本研究数据揭示了伊蚊幼期所处环境效应与成虫性状间的复杂交互作用，尤其凸显了幼虫拥挤对成虫病毒载量的重要影响。综合来看，城市环境可能会提升登革病毒载量，进而可能促进病毒传播；鉴于全球城市化速率不断加快，该结果具有重要的公共卫生警示意义。