Larval crowding enhances dengue virus loads in Aedes aegypti, a relationship that might increase transmission in urban environments.

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.

Jamestown Canyon virus is transmissible by Aedes aegypti and is only moderately blocked by Wolbachia co-infection.

Jamestown Canyon virus (JCV), a negative-sense arbovirus, is increasingly common in the upper Midwest of the USA. Transmitted by a range of mosquito genera, JCV's primary amplifying host is white-tailed deer. Aedes aegypti is responsible for transmitting various positive-sense viruses globally including dengue (DENV), Zika, chikungunya, and Yellow Fever. Ae. aegypti's distribution, once confined to the tropics, is expanding, in part due to climate change. Wolbachia, an insect endosymbiont, limits the replication of co-infecting viruses inside insects. The release and spread of the symbiont into Ae. aegypti populations have been effective in reducing transmission of DENV to humans, although the mechanism of Wolbachia-mediated viral blocking is still poorly understood. Here we explored JCV infection potential in Ae. aegypti, the nature of the vector's immune response, and interactions with Wolbachia infection. We show that Ae. aegypti is highly competent for JCV, which grows to high loads and rapidly reaches the saliva after an infectious blood meal. The mosquito immune system responds with strong induction of RNAi and JAK/STAT. Neither the direct effect of viral infection nor the energetic investment in immunity appears to affect mosquito longevity. Wolbachia infection blocked JCV only in the early stages of infection. Wolbachia-induced immunity was small compared to that of JCV, suggesting innate immune priming does not likely explain blocking. We propose two models to explain why Wolbachia's blocking of negative-sense viruses like JCV may be less than that of positive-sense viruses, relating to the slowdown of host protein synthesis and the triggering of interferon-like factors like Vago. In conclusion, we highlight the risk for increased human disease with the predicted future overlap of Ae. aegypti and JCV ranges. We suggest that with moderate Wolbachia-mediated blocking and distinct biology, negative-sense viruses represent a fruitful comparator model to other viruses for understanding blocking mechanisms in mosquitoes.

Wolbachia as translational science: controlling mosquito-borne pathogens.

In this review we examine how exploiting the Wolbachia-mosquito relationship has become an increasingly popular strategy for controlling arbovirus transmission. Field deployments of Wolbachia-infected mosquitoes have led to significant decreases in dengue virus incidence via high levels of mosquito population suppression and replacement, emphasizing the success of Wolbachia approaches. Here, we examine how improved knowledge of Wolbachia-host interactions has provided key insight into the mechanisms of the essential phenotypes of pathogen blocking and cytoplasmic incompatibility. And we discuss recent studies demonstrating that extrinsic factors, such as ambient temperature, can modulate Wolbachia density and maternal transmission. Finally, we assess the prospects of using Wolbachia to control other vectors and agricultural pest species.

The impact of artificial selection for Wolbachia-mediated dengue virus blocking on phage WO.

Wolbachia is currently at the forefront of global efforts to control arbovirus transmission from the vector Aedes aegypti. The use of Wolbachia relies on two phenotypes-cytoplasmic incompatibility (CI), conferred by cifA and cifB genes in prophage WO, and Wolbachia-mediated pathogen blocking (WMPB). These traits allow for local, self-sustaining reductions in transmission of dengue (DENV) following release of Wolbachia-infected A. aegypti. Here, aided by previous artificial selection experiment that generated Low and High pathogen blocking lines, we examined the potential link between WMPB and phage WO. We found no evidence that Wolbachia or phage WO relative densities predict DENV blocking strength across selected lines. However, selection resulted in reduced phage WO relative density for the Low WMPB line. The Low blocking line was previously shown to have reduced fitness as a result of selection. Through subsequent genomic analyses, we demonstrate that SNP variation underpinning selection for low blocking led to elevated frequency of potential deleterious SNPs on chromosome 1. The key region on chromosome 1 contains genes relating to cell cycle regulation, oxidative stress, transcriptional pausing, among others, that may have cascading effects on Wolbachia intracellular environment. We hypothesize that reduction in phage WO may be driven by changes in the loci directly under selection for blocking, or by the accumulation of predicted deleterious alleles in linkage disequilibrium with blocking loci resulting from hitchhiking. For the Low line with fewer phage WO, we also detected reduced expression of cifA and cifB CI genes, with patterns of expression varying between somatic and reproductive tissues. In conclusion, we propose that artificial selection for WMPB trait had corresponding impacts on phage WO densities, and also the transcription of CI-causing genes. Future studies may include a more detailed analysis of the regions the A. aegypti chromosome 1's ability to affect WMPB and other Wolbachia-associated intrinsic factors such as phage WO.

Microbes increase thermal sensitivity in the mosquito Aedes aegypti, with the potential to change disease distributions.

The mosquito Aedes aegypti is the primary vector of many disease-causing viruses, including dengue (DENV), Zika, chikungunya, and yellow fever. As consequences of climate change, we expect an increase in both global mean temperatures and extreme climatic events. When temperatures fluctuate, mosquito vectors will be increasingly exposed to temperatures beyond their upper thermal limits. Here, we examine how DENV infection alters Ae. aegypti thermotolerance by using a high-throughput physiological 'knockdown' assay modeled on studies in Drosophila. Such laboratory measures of thermal tolerance have previously been shown to accurately predict an insect's distribution in the field. We show that DENV infection increases thermal sensitivity, an effect that may ultimately limit the geographic range of the virus. We also show that the endosymbiotic bacterium Wolbachia pipientis, which is currently being released globally as a biological control agent, has a similar impact on thermal sensitivity in Ae. aegypti. Surprisingly, in the coinfected state, Wolbachia did not provide protection against DENV-associated effects on thermal tolerance, nor were the effects of the two infections additive. The latter suggests that the microbes may act by similar means, potentially through activation of shared immune pathways or energetic tradeoffs. Models predicting future ranges of both virus transmission and Wolbachia's efficacy following field release may wish to consider the effects these microbes have on host survival.

Pathogen blocking in Wolbachia-infected Aedes aegypti is not affected by Zika and dengue virus co-infection.

BACKGROUND: Wolbachia's ability to restrict arbovirus transmission makes it a promising tool to combat mosquito-transmitted diseases. Wolbachia-infected Aedes aegypti are currently being released in locations such as Brazil, which regularly experience concurrent outbreaks of different arboviruses. A. aegypti can become co-infected with, and transmit multiple arboviruses with one bite, which can complicate patient diagnosis and treatment. METHODOLOGY/PRINCIPLE FINDINGS: Using experimental oral infection of A. aegypti and then RT-qPCR, we examined ZIKV/DENV-1 and ZIKV/DENV-3 co-infection in Wolbachia-infected A. aegypti and observed that Wolbachia-infected mosquitoes experienced lower prevalence of infection and viral load than wildtype mosquitoes, even with an extra infecting virus. Critically, ZIKV/DENV co-infection had no significant impact on Wolbachia's ability to reduce viral transmission. Wolbachia infection also strongly altered expression levels of key immune genes Defensin C and Transferrin 1, in a virus-dependent manner. CONCLUSIONS/SIGNIFICANCE: Our results suggest that pathogen interference in Wolbachia-infected A. aegypti is not adversely affected by ZIKV/DENV co-infection, which suggests that Wolbachia-infected A. aegypti will likely prove suitable for controlling mosquito-borne diseases in environments with complex patterns of arbovirus transmission.

Exploiting Intimate Relationships: Controlling Mosquito-Transmitted Disease with Wolbachia.

Mosquito-transmitted diseases impose a growing burden on human health, and current control strategies have proven insufficient to stem the tide. The bacterium Wolbachia is a novel and promising form of control for mosquito-transmitted disease. It manipulates host biology, restricts infection with dengue and other pathogens, and alters host reproduction to promote rapid spread in the field. In this review, we examine how the intimate and diverse relationships formed between Wolbachia and their mosquito hosts can be exploited for disease control purposes. We consider these relationships in the context of recent developments, including successful field trials with Wolbachia-infected mosquitoes to combat dengue, and new Wolbachia infections in key malaria vectors, which have enhanced the disease control prospects of this unique bacterium.