Comparative susceptibility of mosquito populations in North Queensland, Australia to oral infection with dengue virus

Evolution and adaptation of dengue virus in response to high-temperature passaging in mosquito cells

The incidence of arboviral diseases like dengue, chikungunya, and yellow fever continues to rise in association with the expanding geographic ranges of their vectors, Aedes aegypti and Aedes albopictus. The distribution of these vectors is believed to be driven in part by climate change and increasing urbanization. Arboviruses navigate a wide range of temperatures as they transition from ectothermic vectors (from 15°C to 35°C) to humans (37°C) and back again, but the role that temperature plays in driving the evolution of arboviruses remains largely unknown. Here, we passaged replicate dengue serotype-2 virus populations 10 times at either 26°C (Low) or 37°C (High) in C6/36 Aedes albopictus cells to explore the differences in adaptation to these thermal environments. We then deep-sequenced the resulting passaged dengue virus populations and tested their replicative fitness in an all-cross temperature regime. We also assessed the ability of the passaged viruses to replicate in the insect vector. While viruses from both thermal regimes accumulated substitutions, only those reared in the 37°C treatments exhibited nonsynonymous changes, including several in the E, or envelope protein, and multiple non-structural genes. Passaging at the higher temperature also led to reduced replicative ability at 26°C in both cells and mosquitoes. One of the mutations in the E gene involved the loss of a glycosylation site previously shown to reduce infectivity in the vector. These findings suggest that viruses selected for growth at higher ambient temperatures may experience tradeoffs between thermostability and replication in the vector. Such associations might also have implications for the suitability of virus transmission under a changing climate.

Nutritional stress compromises mosquito fitness and antiviral immunity, while enhancing dengue virus infection susceptibility

Diet-induced nutritional stress can influence pathogen transmission potential in mosquitoes by impacting life history traits, infection susceptibility, and immunity. To investigate these effects, we manipulate mosquito diets at larval and adult stages, creating two nutritional levels (low and normal), and expose adults to dengue virus (DENV). We observe that egg number is reduced by nutritional stress at both stages and viral exposure separately and jointly, while the likelihood of laying eggs is exclusively influenced by adult nutritional stress. Adult nutritional stress alone shortens survival, while any pairwise combination between both-stage stress and viral exposure have a synergistic effect. Additionally, adult nutritional stress increases susceptibility to DENV infection, while larval nutritional stress likely has a similar effect operating via smaller body size. Furthermore, adult nutritional stress negatively impacts viral titers in infected mosquitoes; however, some survive and show increased titers over time. The immune response to DENV infection is overall suppressed by larval and adult nutritional stress, with specific genes related to Toll, JAK-STAT, and Imd immune signaling pathways, and antimicrobial peptides being downregulated. Our findings underscore the importance of nutritional stress in shaping mosquito traits, infection outcomes, and immune responses, all of which impact the vectorial capacity for DENV transmission.

Dengue and chikungunya virus loads in the mosquito Aedes aegypti are determined by distinct genetic architectures

Aedes aegypti is the primary vector of the arboviruses dengue (DENV) and chikungunya (CHIKV). These viruses exhibit key differences in their vector interactions, the latter moving more quicky through the mosquito and triggering fewer standard antiviral pathways. As the global footprint of CHIKV continues to expand, we seek to better understand the mosquito's natural response to CHIKV-both to compare it to DENV:vector coevolutionary history and to identify potential targets in the mosquito for genetic modification. We used a modified full-sibling design to estimate the contribution of mosquito genetic variation to viral loads of both DENV and CHIKV. Heritabilities were significant, but higher for DENV (40%) than CHIKV (18%). Interestingly, there was no genetic correlation between DENV and CHIKV loads between siblings. These data suggest Ae. aegypti mosquitoes respond to the two viruses using distinct genetic mechanisms. We also examined genome-wide patterns of gene expression between High and Low CHIKV families representing the phenotypic extremes of viral load. Using RNAseq, we identified only two loci that consistently differentiated High and Low families: a long non-coding RNA that has been identified in mosquito screens post-infection and a distant member of a family of Salivary Gland Specific (SGS) genes. Interestingly, the latter gene is also associated with horizontal gene transfer between mosquitoes and the endosymbiotic bacterium Wolbachia. This work is the first to link the SGS gene to a mosquito phenotype. Understanding the molecular details of how this gene contributes to viral control in mosquitoes may, therefore, also shed light on its role in Wolbachia.

Using Zoos as Sentinels for Re-Emerging Arboviruses: Vector Surveillance during an Outbreak of Epizootic Hemorrhagic Disease at the Minnesota Zoo

Vector-borne disease prevalence is increasing at a time when surveillance capacity in the United States is decreasing. One way to address this surveillance deficiency is to utilize established infrastructure, such as zoological parks, to investigate animal disease outbreaks and improve our epidemiological understanding of vector-borne pathogens. During fall 2020, an outbreak of epizootic hemorrhagic disease (EHD) at the Minnesota Zoo resulted in morbidity and seroconversion of several collection animals. In response to this outbreak, insect surveillance was conducted, and the collected insects were tested for the presence of epizootic hemorrhagic disease virus (EHDV) by RT-qPCR to better understand the local transmitting vector populations responsible for the outbreak. Six pools of Culicoides biting midges were positive for EHDV, including three pools of Culicoides sonorensis, two pools of Culicoides variipennis, and a pool of degraded C. variipennis complex midges. All three endemic serotypes of EHDV (1, 2, and 6) were detected in both animals and midge pools from the premises. Despite this outbreak, no EHDV cases had been reported in wild animals near the zoo. This highlights the importance and utility of using animal holding facilities, such as zoos, as sentinels to better understand the spatio-temporal dynamics of pathogen transmission.

Assessing Aedes aegypti candidate genes during viral infection and Wolbachia-mediated pathogen blocking

One approach to control dengue virus transmission is the symbiont Wolbachia, which limits viral infection in mosquitoes. Despite plans for its widespread use in Aedes aegypti, Wolbachia's mode of action remains poorly understood. Many studies suggest that the mechanism is likely multifaceted, involving aspects of immunity, cellular stress and nutritional competition. A previous study from our group used artificial selection to identify a new mosquito candidate gene related to viral blocking; alpha-mannosidase-2a (alpha-Mann-2a) with a predicted role in protein glycosylation. Protein glycosylation pathways tend to be involved in complex host-viral interactions; however, the function of alpha-mannosidases has not been described in mosquito-virus interactions. We examined alpha-Mann-2a expression in response to virus and Wolbachia infections and whether reduced gene expression, caused by RNA interference, affected viral loads. We show that dengue virus (DENV) infection affects the expression of alpha-Mann-2a in a tissue- and time-dependent manner, whereas Wolbachia infection had no effect. In the midgut, DENV prevalence increased following knockdown of alpha-Mann-2a expression in Wolbachia-free mosquitoes, suggesting that alpha-Mann-2a interferes with infection. Expression knockdown had the same effect on the togavirus chikungunya virus, indicating that alpha-Mann-2a may have broad antivirus effects in the midgut. Interestingly, we were unable to knockdown the expression in Wolbachia-infected mosquitoes. We also provide evidence that alpha-Mann-2a may affect the transcriptional level of another gene predicted to be involved in viral blocking and cell adhesion; cadherin87a. These data support the hypothesis that glycosylation and adhesion pathways may broadly be involved in viral infection in Ae. aegypti.

Infection, Dissemination, and Replication of Urban and Sylvatic Strains of Dengue Virus Type 2 (Flaviviridae: Flavivirus) in Australian Aedes aegypti (Diptera: Culicidae)

The dengue viruses (DENVs) occur throughout tropical and subtropical regions of the world where they infect 100s of millions of people annually. In Australia, the dengue receptive zone is confined to the northern state of Queensland where the principal vector Aedes aegypti (L.) is present. In the current study, two populations of Ae. aegypti from north Queensland were exposed to two urban outbreak strains and one sylvatic strain of dengue virus type 2 (DENV-2). The titer of virus required to infect 50% of mosquitoes was between 105 and 106 50% tissue culture infectious dose (TCID)50/ml and was influenced by the combination of the origin of Ae. aegypti population and virus strain. When exposed to infectious bloodmeal titers > 106 TCID50/ml, infection and dissemination rates were all > 50% and were significantly affected by the origin of the mosquito population but not by the strain of DENV-2. Replication of DENV-2 was also significantly affected by the mosquito population and the titer of the infectious bloodmeal that mosquitoes were exposed to. The results of this study are discussed in the context of DENV transmission dynamics in northern Australia and the relative fitness of the sylvatic virus strain in urban Ae. aegypti populations.

Dengue virus dominates lipid metabolism modulations in Wolbachia-coinfected Aedes aegypti

Competition between viruses and Wolbachia for host lipids is a proposed mechanism of Wolbachia-mediated virus blocking in insects. Yet, the metabolomic interaction between virus and symbiont within the mosquito has not been clearly defined. We compare the lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of the Wolbachia wMel strain and dengue virus serotype 3 (DENV3). We found metabolic signatures of infection-induced intracellular events but little evidence to support direct competition between Wolbachia and virus for host lipids. Lipid profiles of dual-infected mosquitoes resemble those of DENV3 mono-infected mosquitoes, suggesting virus-driven modulation dominates over that of Wolbachia. Interestingly, knockdown of key metabolic enzymes suggests cardiolipins are host factors for DENV3 and Wolbachia replication. These findings define the Wolbachia-DENV3 metabolic interaction as indirectly antagonistic, rather than directly competitive, and reveal new research avenues with respect to mosquito × virus interactions at the molecular level.

Koh, Islam, Ye et al. describe lipid profiles of Aedes aegypti mosquitoes bearing mono- or dual-infections of Wolbachia (wMel) and dengue virus serotype 3 (DENV3), finding that virus modulation dominates the dual-infection lipid profile and that cardiolipins support DENV3 and Wolbachia replication. This study suggests that direct competition for lipids do not underlie Wolbachia-mediated virus blocking.

Artificial Selection Finds New Hypotheses for the Mechanism of Wolbachia-Mediated Dengue Blocking in Mosquitoes

Wolbachia is an intracellular bacterium that blocks virus replication in insects and has been introduced into the mosquito, Aedes aegypti for the biocontrol of arboviruses including dengue, Zika, and chikungunya. Despite ongoing research, the mechanism of Wolbachia-mediated virus blocking remains unclear. We recently used experimental evolution to reveal that Wolbachia-mediated dengue blocking could be selected upon in the A. aegypti host and showed evidence that strong levels of blocking could be maintained by natural selection. In this study, we investigate the genetic variation associated with blocking and use these analyses to generate testable hypotheses surrounding the mechanism of Wolbachia-mediated dengue blocking. From our results, we hypothesize that Wolbachia may block virus replication by increasing the regeneration rate of mosquito cells via the Notch signaling pathway. We also propose that Wolbachia modulates the host’s transcriptional pausing pathway either to prime the host’s anti-viral response or to directly inhibit viral replication.

Australian Aedes aegypti mosquitoes are susceptible to infection with a highly divergent and sylvatic strain of dengue virus type 2 but are unlikely to transmit it

Background

Humans are the primary hosts of dengue viruses (DENV). However, sylvatic cycles of transmission can occur among non-human primates and human encroachment into forested regions can be a source of emergence of new strains such as the highly divergent and sylvatic strain of DENV2, QML22, recovered from a dengue fever patient returning to Australia from Borneo. The objective of the present study was to evaluate the vector competence of Australian Aedes aegypti mosquitoes for this virus.

Methods

Four- to five-day-old mosquitoes from two strains of Ae. aegypti from Queensland, Australia, were fed a meal of sheep blood containing 10⁸ 50% cell culture infectious dose per ml (CCID₅₀/ml) of either QML22 or an epidemic strain of DENV serotype 2 (QML16) isolated from a dengue fever patient in Australia in 2015. Mosquitoes were maintained at 28 °C, 75% relative humidity and sampled 7, 10 and 14 days post-infection (dpi). Live virions in mosquito bodies (abdomen/thorax), legs and wings and saliva expectorates from individual mosquitoes were quantified using a cell culture enzyme-linked immunosorbent assay (CCELISA) to determine infection, dissemination and transmission rates.

Results

The infection and dissemination rates of the sylvatic DENV2 strain, QML22, were significantly lower than that for QML16. While the titres of virus in the bodies of mosquitoes infected with either of these viruses were similar, titres in legs and wings were significantly lower in mosquitoes infected with QML22 at most time points although they reached similar levels by 14 dpi. QML16 was detected in 16% (n = 25) and 28% (n = 25) of saliva expectorates at 10 and 14 dpi, respectively. In contrast, no virus was detected in the saliva expectorates of QML22 infected mosquitoes.

Conclusions

Australia urban/peri-urban Ae. aegypti species are susceptible to infection by the sylvatic and highly divergent DENV 2 QML22 but replication of QML22 is attenuated relative to the contemporary strain, QML16. A salivary gland infection or escape barrier may be acting to prevent infection of saliva and would prevent onward transmission of this highly divergent virus in Australia.

A Role for the Insulin Receptor in the Suppression of Dengue Virus and Zika Virus in Wolbachia-Infected Mosquito Cells

Wolbachia-infected mosquitoes are refractory to super-infection with arthropod-borne pathogens, but the role of host cell signaling proteins in pathogen-blocking mechanisms remains to be elucidated. Here, we use an antibody microarray approach to provide a comprehensive picture of the signaling response of Aedes aegypti-derived cells to Wolbachia. This approach identifies the host cell insulin receptor as being downregulated by the bacterium. Furthermore, siRNA-mediated knockdown and treatment with a small-molecule inhibitor of the insulin receptor kinase concur to assign a crucial role for this enzyme in the replication of dengue and Zika viruses in cultured mosquito cells. Finally, we show that the production of Zika virus in Wolbachia-free live mosquitoes is impaired by treatment with the selective inhibitor mimicking Wolbachia infection. This study identifies Wolbachia-mediated downregulation of insulin receptor kinase activity as a mechanism contributing to the blocking of super-infection by arboviruses.

Intra-host growth kinetics of dengue virus in the mosquito Aedes aegypti

Dengue virus (DENV) transmission by mosquitoes is a time-dependent process that begins with the consumption of an infectious blood-meal. DENV infection then proceeds stepwise through the mosquito from the midgut to the carcass, and ultimately to the salivary glands, where it is secreted into saliva and then transmitted anew on a subsequent bite. We examined viral kinetics in tissues of the Aedes aegypti mosquito over a finely graded time course, and as per previous studies, found that initial viral dose and serotype strain diversity control infectivity. We also found that a threshold level of virus is required to establish body-wide infections and that replication kinetics in the early and intermediate tissues do not predict those of the salivary glands. Our findings have implications for mosquito GMO design, modeling the contribution of transmission to vector competence and the role of mosquito kinetics in the overall DENV epidemiological landscape.

DENV infection in the mosquito is a complex and dynamic process. Following ingestion of an infected blood meal, DENV enters the mosquito midgut epithelial cells, where it replicates. Subsequently, the virus disseminates and infects other tissues, including hemocytes, fat body and reproductive organs, ultimately reaching the salivary glands. The kinetics of infection are influenced by genetic variation in the virus. Comparisons between strains within single serotypes, have revealed variation in infection rates in mosquitoes. To explore the role of infectious dose, serotype and tissue in viral infection kinetics we sampled DENV loads in populations of infected mosquitoes over numerous, sequential time-points. We reveal that the kinetics of DENV infection in the midgut, carcass and salivary glands of the mosquito Aedes aegypti are strikingly different among the strains selected for this study, and that these differences are also driven by the initial infectious dose.

Selection on Aedes aegypti alters Wolbachia-mediated dengue virus blocking and fitness

The dengue, Zika and chikungunya viruses are transmitted by the mosquito Aedes aegypti and pose a substantial threat to global public health. Current vaccines and mosquito control strategies have limited efficacy, so novel interventions are needed^1,2. Wolbachia are bacteria that inhabit insect cells and have been found to reduce viral infection-a phenotype that is referred to as viral 'blocking'³. Although not naturally found in A. aegypti⁴, Wolbachia were stably introduced into this mosquito in 2011^4,5 and were shown to reduce the transmission potential of dengue, Zika and chikungunya^6,7. Subsequent field trials showed Wolbachia's ability to spread through A. aegypti populations and reduce the local incidence of dengue fever⁸. Despite these successes, the evolutionary stability of viral blocking is unknown. Here, we utilized artificial selection to reveal genetic variation in the mosquito that affects Wolbachia-mediated dengue blocking. We found that mosquitoes exhibiting weaker blocking also have reduced fitness, suggesting the potential for natural selection to maintain blocking. We also identified A. aegypti genes that affect blocking strength, shedding light on a possible mechanism for the trait. These results will inform the use of Wolbachia as biocontrol agents against mosquito-borne viruses and direct further research into measuring and improving their efficacy.

Using genetic variation in Aedes aegypti to identify candidate anti-dengue virus genes

BACKGROUND

Transcriptomic profiling has generated extensive lists of genes that respond to viral infection in mosquitoes. These gene lists contain two types of genes; (1) those that are responsible for the insect's natural antiviral defense mechanisms, including some known innate immunity genes, and (2) genes whose change in expression may occur simply as a result of infection. As genetic modification tools for mosquitoes continue to improve, the opportunities to make refractory insects via allelic replacement or delivery of small RNAs that alter gene expression are expanding. Therefore, the ability to identify which genes in transcriptional profiles may have immune function has increasing value. Arboviruses encounter a range of mosquito tissues and physiologies as they traverse from the midgut to the salivary glands. While the midgut is well-studied as the primary tissue barrier, antiviral genes expressed in the subsequent tissues of the carcass offer additional candidates for second stage intervention in the mosquito body.

METHODS

Mosquito lines collected recently from field populations exhibit natural genetic variation for dengue virus susceptibility. We sought to use a modified full-sib breeding design to identify mosquito families that varied in their dengue viral load in their bodies post infection.

RESULTS

By delivering virus intrathoracically, we bypassed the midgut and focused on whole body responses in order to evaluate carcass-associated refractoriness. We tested 25 candidate genes selected for their appearance in multiple published transcriptional profiles and were able to identify 12 whose expression varied with susceptibility in the genetic families.

CONCLUSIONS

This method, using natural genetic variation, offers a simple means to screen and reduce candidate gene lists prior to carrying out more labor-intensive functional studies. The extracted RNA from the females across the families represents a storable resource that can be used to screen subsequent candidate genes in the future. The aspect of vector competence being assessed could be varied by focusing on different tissues or time points post infection.

Sustained Wolbachia-mediated blocking of dengue virus isolates following serial passage in Aedes aegypti cell culture

Wolbachia is an intracellular endosymbiont of insects that inhibits the replication of a range of pathogens in its arthropod hosts. The release of Wolbachia into wild populations of mosquitoes is an innovative biocontrol effort to suppress the transmission of arthropod-borne viruses (arboviruses) to humans, most notably dengue virus. The success of the Wolbachia-based approach hinges upon the stable persistence of the ‘pathogen blocking’ effect, whose mechanistic basis is poorly understood. Evidence suggests that Wolbachia may affect viral replication via a combination of competition for host resources and activation of host immunity. The evolution of resistance against Wolbachia and pathogen blocking in the mosquito or the virus could reduce the public health impact of the symbiont releases. Here, we investigate if dengue 3 virus (DENV-3) is capable of accumulating adaptive mutations that improve its replicative capacity during serial passage in Wolbachia wMel-infected cells. During the passaging regime, viral isolates in Wolbachia-infected cells exhibited greater variation in viral loads compared to controls. The viral loads of these isolates declined rapidly during passaging due to the blocking effects of Wolbachia carriage, with several being lost all together and the remainder recovering to low but stable levels. We attempted to sequence the genomes of the surviving passaged isolates but, given their low abundance, were unable to obtain sufficient depth of coverage for evolutionary analysis. In contrast, viral loads in Wolbachia-free control cells were consistently high during passaging. The surviving isolates passaged in the presence of Wolbachia exhibited a reduced ability to replicate even in Wolbachia-free cells. These experiments demonstrate the challenge for dengue in evolving resistance to Wolbachia-mediated blocking.

Aedes aegypti vector competence studies: A review

Aedes aegypti is the primary transmitter of the four viruses that have had the greatest impact on human health, the viruses causing yellow fever, dengue fever, chikungunya, and Zika fever. Because this mosquito is easy to rear in the laboratory and these viruses grow in laboratory tissue culture cells, many studies have been performed testing the relative competence of different populations of the mosquito to transmit many different strains of viruses. We review here this large literature including studies on the effect of the mosquito microbiota on competence. Because of the heterogeneity of both mosquito populations and virus strains used, as well as methods measuring potential to transmit, it is very difficult to perform detailed meta-analysis of the studies. However, a few conclusions can be drawn: (1) almost no population of Ae. aegypti is 100% naturally refractory to virus infection. Complete susceptibility to infection has been observed for Zika (ZIKV), dengue (DENV) and chikungunya (CHIKV), but not yellow fever viruses (YFV); (2) the dose of virus used is directly correlated to the rate of infection; (3) Brazilian populations of mosquito are particularly susceptible to DENV-2 infections; (4) the Asian lineage of ZIKV is less infective to Ae. aegypti populations from the American continent than is the African ZIKV lineage; (5) virus adaptation to different species of mosquitoes has been demonstrated with CHIKV; (6) co-infection with more than one virus sometimes causes displacement while in other cases has little effect; (7) the microbiota in the mosquito also has important effects on level of susceptibility to arboviral infection; (8) resistance to virus infection due to the microbiota may be direct (e.g., bacteria producing antiviral proteins) or indirect in activating the mosquito host innate immune system; (9) non-pathogenic insect specific viruses (ISVs) are also common in mosquitoes including genome insertions. These too have been shown to have an impact on the susceptibility of mosquitoes to pathogenic viruses. One clear conclusion is that it would be a great advance in this type of research to implement standardized procedures in order to obtain comparable and reproducible results.

Potential of Aedes aegypti populations in Madeira Island to transmit dengue and chikungunya viruses

BACKGROUND

Aedes spp. mosquitoes mainly transmit the arboviruses dengue virus (DENV) and chikungunya virus (CHIKV) in urban areas, causing a severe public health problem. In 2012-2013, a major dengue outbreak occurred on Madeira Island where the mosquito Aedes aegypti was the only vector. Up to now, the competence of Ae. aegypti populations from Madeira to transmit DENV or CHIKV remains unknown. This study aimed to assess experimentally the ability of Ae. aegypti populations from Madeira to transmit these viruses.

RESULTS

By orally exposing mosquitoes to CHIKV (NC/2011-568) and DENV-2 (Bangkok), the vector competence of two field-collected Ae. aegypti populations, i.e. Funchal and Paúl do Mar, was evaluated. We found that both populations were similarly infected and ensured the dissemination and transmission of CHIKV at the same rates. With DENV-2, viral dissemination was significantly higher in the Funchal population compared to Paúl do Mar. We found no significant differences in transmission rates between populations.

CONCLUSIONS

To our knowledge, this study has demonstrated for the first time the ability of temperate European Ae. aegypti populations from Madeira to transmit DENV and CHIKV. As our results suggest, there is a potential risk for the local transmission of DENV and CHIKV if introduced to Madeira or continental Europe where Aedes albopictus is present. Our results highlight the need for continuing vector surveillance and control on Madeira Island to future-proof the Island against mosquito-borne epidemics.

Characterization of a Western Pacific Zika Virus Strain in Australian Aedes aegypti

Zika virus (ZIKV) is a globally emerging arbovirus responsible for widespread epidemics in the western Pacific, the Americas, and Asia. The virus predominately circulates in urban transmission cycles between Aedes aegypti and humans. Australia is considered at risk to outbreaks of ZIKV due to the presence of A. aegypti populations in northern areas of the state of Queensland. Furthermore, close proximity to epidemic regions has led to almost 50% of imported cases reported since 2012 originating in the Pacific region. We conducted the first vector competence experiments with A. aegypti from three Australian populations for a western Pacific strain of ZIKV. When exposed to bloodmeals containing between 10⁵ and 10⁸ tissue culture infectious dose (TCID)₅₀/mL of virus, infection, dissemination, and transmission, rates were <10%. In comparison to using frozen virus stock, exposing mosquitoes to freshly cultured virus also did not increase infection or transmission rates. It was only when bloodmeal titers exceeded 10⁸ TCID₅₀/mL that infection rates approached 50% and transmission rates increased to >20%. However, this concentration of virus is considerably higher than levels previously reported in blood samples from viremic humans. The Australian A. aegypti tested appear to express a midgut barrier to ZIKV infection, as 50% of mosquitoes that became infected developed a disseminated infection, and 50% of those mosquitoes transmitted the virus. Overall, these results suggest that while Australian A. aegypti strains are able to transmit the western Pacific ZIKV strain, they are relatively inefficient vectors of the virus.

Family level variation in Wolbachia-mediated dengue virus blocking in Aedes aegypti

BACKGROUND

The mosquito vector Aedes aegypti is responsible for transmitting a range of arboviruses including dengue (DENV) and Zika (ZIKV). The global reach of these viruses is increasing due to an expansion of the mosquito's geographic range and increasing urbanization and human travel. Vector control remains the primary means for limiting these diseases. Wolbachia pipientis is an endosymbiotic bacterium of insects that has the ability to block the replication of pathogens, including flaviviruses such as DENV or ZIKV, inside the body of the vector. A strain of Wolbachia called wMel is currently being released into wild mosquito populations to test its potential to limit virus transmission to humans. The mechanism that underpins the virus blocking effect, however, remains elusive.

METHODS

We used a modified full-sib breeding design in conjunction with vector competence assays in wildtype and wMel-infected Aedes aegypti collected from the field. All individuals were injected with DENV-2 intrathoracically at 5-6 days of age. Tissues were dissected 7 days post-infection to allow quantification of DENV and Wolbachia loads.

RESULTS

We show the first evidence of family level variation in Wolbachia-mediated blocking in mosquitoes. This variation may stem from either genetic contributions from the mosquito and Wolbachia genomes or environmental influences on Wolbachia. In these families, we also tested for correlations between strength of blocking and expression level for several insect immunity genes with possible roles in blocking, identifying two genes of interest (AGO2 and SCP-2).

CONCLUSIONS

In this study we show variation in Wolbachia-mediated DENV blocking in Aedes aegypti that may arise from genetic contributions and environmental influences on the mosquito-Wolbachia association. This suggests that Wolbachia-mediated blocking may have the ability to evolve through time or be expressed differentially across environments. The long-term efficacy of Wolbachia in the field will be dependent on the stability of blocking. Understanding the mechanism of blocking will be necessary for successful development of strategies that counter the emergence of evolved resistance or variation in its expression under diverse field conditions.

The RNAi pathway plays a small part in Wolbachia-mediated blocking of dengue virus in mosquito cells

Wolbachia pipientis is an insect endosymbiont known to limit the replication of viruses including dengue and Zika in their primary mosquito vector, Aedes aegypti. Wolbachia is being released into mosquito populations globally in a bid to control the diseases caused by these viruses. It is theorized that Wolbachia’s priming of the insect immune system may confer protection against subsequent viral infection. Other hypotheses posit a role for competition between Wolbachia and viruses for host cellular resources. Using an A. aegypti cell line infected with Wolbachia, we tested the effects of targeting siRNAs against the major innate immune pathways on dengue virus loads. We show that while Wolbachia infection induces genes in the Toll, JAK/STAT and RNAi pathways, only reduced expression of RNAi leads to a rebound of dengue virus loads in Wolbachia-infected cells. The magnitude of the effect explained less than 10% of the total DENV load, demonstrating that blocking must be dependent on other factors in addition to the expression of RNAi. The findings bode well for the long-term stability of blocking given that immunity gene expression would likely be highly plastic and susceptible to rapid evolution.

Wolbachia-Based Dengue Virus Inhibition Is Not Tissue-Specific in Aedes aegypti

Background

Dengue fever, caused by the dengue virus (DENV), is now the most common arbovirus transmitted disease globally. One novel approach to control DENV is to use the endosymbiotic bacterium, Wolbachia pipientis, to limit DENV replication inside the primary mosquito vector, Aedes aegypti. Wolbachia that is naturally present in a range of insects reduces the capacity for viruses, bacteria, parasites and fungi to replicate inside insects. Wolbachia’s mode of action is not well understood but may involve components of immune activation or competition with pathogens for limited host resources. The strength of Wolbachia-based anti DENV effects appear to correlate with bacterial density in the whole insect and in cell culture. Here we aimed to determine whether particular tissues, especially those with high Wolbachia densities or immune activity, play a greater role in mediating the anti DENV effect.

Methodology/findings

Ae. aegypti mosquito lines with and without Wolbachia (Wildtype) were orally fed DENV 3 and their viral loads subsequently measured over two time points post infection in the midgut, head, salivary glands, Malpighian tubules, fat body and carcass. We did not find correlations between Wolbachia densities and DENV loads in any tissue, nor with DENV loads in salivary glands, the endpoint of infection. This is in contrast with strong positive correlations between DENV loads in a range of tissues and salivary gland loads for Wildtype mosquitoes. Lastly, there was no evidence of a heightened role for tissues with known immune function including the fat body and the Malpighian tubules in Wolbachia’s limitation of DENV.

Conclusion/significance

We conclude that the efficacy of DENV blocking in Wolbachia infected mosquitoes is not reliant on any particular tissue. This work therefore suggests that the mechanism of Wolbachia-based antiviral effects is either systemic or acts locally via processes that are fundamental to diverse cell types. We further conclude that the relationship between DENV blocking and Wolbachia density is not linear in mosquito tissues

Dengue fever caused by the dengue virus (DENV) is transmitted by the mosquito, Aedes aegypti. To control the disease, an intracellular bacterium called Wolbachia has been introduced into Ae. aegypti where it blocks/limits success of infection of DENV. The mechanistic basis of blocking is not well understood but may involve Wolbachia activating the host immune system or competing with DENV for host resources. The strength of blocking appears to correlate with Wolbachia density. Here, we aimed to determine if any particular tissues inside the mosquito play a greater role in blocking. Tissues were chosen based on their Wolbachia density and their roles in infection and immunity. Wolbachia infected and uninfected mosquitoes were orally infected with DENV and Wolbachia density and DENV load were assessed in midgut, salivary gland, head, Malpighian tubules, fat body and carcass. Wolbachia density did not correlate with DENV loads in the same tissues nor with DENV loads in the salivary glands. We also showed that no one tissue appeared to play a greater role in blocking. In summary, these finding suggest that in the mosquito a threshold Wolbachia density may be required for DENV blocking. Our findings also suggest that blocking may involve mechanisms that are fundamental to all cells.

Genome Investigations of Vector Competence in Aedes aegypti to Inform Novel Arbovirus Disease Control Approaches

Dengue (DENV), yellow fever, chikungunya, and Zika virus transmission to humans by a mosquito host is confounded by both intrinsic and extrinsic variables. Besides virulence factors of the individual arboviruses, likelihood of virus transmission is subject to variability in the genome of the primary mosquito vector, Aedes aegypti. The “vectorial capacity” of A. aegypti varies depending upon its density, biting rate, and survival rate, as well as its intrinsic ability to acquire, host and transmit a given arbovirus. This intrinsic ability is known as “vector competence”. Based on whole transcriptome analysis, several genes and pathways have been predicated to have an association with a susceptible or refractory response in A. aegypti to DENV infection. However, the functional genomics of vector competence of A. aegypti is not well understood, primarily due to lack of integrative approaches in genomic or transcriptomic studies. In this review, we focus on the present status of genomics studies of DENV vector competence in A. aegypti as limited information is available relative to the other arboviruses. We propose future areas of research needed to facilitate the integration of vector and virus genomics and environmental factors to work towards better understanding of vector competence and vectorial capacity in natural conditions.

Assessment of Local Mosquito Species Incriminates Aedes aegypti as the Potential Vector of Zika Virus in Australia

Background

Within the last 10 years Zika virus (ZIKV) has caused unprecedented epidemics of human disease in the nations and territories of the western Pacific and South America, and continues to escalate in both endemic and non-endemic regions. We evaluated the vector competence of Australian mosquitoes for ZIKV to assess their potential role in virus transmission.

Methodology/Principal Findings

Mosquitoes were exposed to infectious blood meals containing the prototype African ZIKV strain. After 14 days incubation at 28°C and high relative humidity, infection, dissemination and transmission rates were assessed. Infection in Culex annulirostris and Cx. sitiens could not be detected. 8% of Cx. quinquefasciatus were infected, but the virus did not disseminate in this species. Despite having infection rates > 50%, Aedes notoscriptus and Ae. vigilax did not transmit ZIKV. In contrast, Ae. aegypti had infection and transmission rates of 57% and 27%, respectively. In susceptibility trials, the virus dose required to infect 50% (ID₅₀) of Ae. aegypti was10^6.4 tissue culture infectious dose₅₀ (TCID₅₀)/mL. Additionally, a threshold viral load within the mosquito of at least 10^5.1 TCID₅₀ equivalents/mL had to be reached before virus transmission occurred.

Conclusions/Significance

We confirmed Ae. aegypti to be the most likely mosquito vector of ZIKV in Australia, although the restricted distribution of this species will limit the receptive zone to northern Queensland where this species occurs. Importantly, the role in ZIKV transmission of Culex and other Aedes spp. tested will be negligible. Despite being the implicated vector, the relatively high ID₅₀ and need for a high titer disseminated infection in Ae. aegypti suggest that high mosquito population densities will be required to facilitate epidemic ZIKV transmission among the currently immunologically naïve human population in Australia.

Zika virus was first isolated in Uganda in 1947 and exists in a transmission cycle between mosquitoes and non-human primates or humans. Whilst most clinical infections result in a self-limiting febrile illness, Zika virus has recently been linked to neurological syndromes, such as Guillain-Barré syndrome and congenital birth defects. Since 2007, Zika virus has undergone a dramatic range expansion, causing epidemics in nations and territories of the western Pacific and South America. To assess the emergence and transmission risk of Zika virus emerging in Australia, we evaluated the ability of local mosquitoes to become infected with and transmit the prototype African Zika virus strain. In agreement with its substantiated role in Zika virus transmission overseas, Australian Aedes aegypti were shown to be competent vectors. Coupled with its anthropophilic feeding behavior, this species should be considered the primary potential Zika virus vector in Australia. Although other common Australian species, such as Ae. notoscriptus and Ae. vigilax, were readily infected, they did not transmit the virus. The species of Culex tested were either refractory to infection or had a low infection rate. We also demonstrated that the Zika virus dose necessary to infect Ae. aegypti was higher than virus levels reported in infected humans. Finally, a high threshold level of virus circulating through the mosquito body was required before Ae. aegypti transmitted the virus. These results suggest that an outbreak of Zika virus in Australia would require high mosquito population densities and a susceptible human population.

Evolutionary potential of the extrinsic incubation period of dengue virus in Aedes aegypti

Dengue fever is the most common arboviral disease worldwide. It is caused by dengue viruses (DENV) and the mosquito Aedes aegypti is its primary vector. One of the most powerful determinants of a mosquito's ability to transmit DENV is the length of the extrinsic incubation period (EIP), the time it takes for a virus to be transmitted by a mosquito after consuming an infected blood meal. Here, we repeatedly measured DENV load in the saliva of individual mosquitoes over their lifetime and used this in combination with a breeding design to determine the extent to which EIP might respond to the evolutionary forces of drift and selection. We demonstrated that genetic variation among mosquitoes contributes significantly to transmission potential and length of EIP. We reveal that shorter EIP is genetically correlated with reduced mosquito lifespan, highlighting negative life-history consequences for virus-infected mosquitoes. This work highlights the capacity for local genetic variation in mosquito populations to evolve and to dramatically affect the nature of human outbreaks. It also provides the impetus for isolating mosquito genes that determine EIP. More broadly, our dual experimental approach offers new opportunities for studying the evolutionary potential of transmission traits in other vector/pathogen systems.

The Effect of Temperature on Wolbachia-Mediated Dengue Virus Blocking in Aedes aegypti

Dengue fever, caused by dengue virus (DENV), is endemic in more than 100 countries. The lack of effective treatment of patients and the suboptimal efficacies of the tetravalent vaccine in trials highlight the urgent need to develop alternative strategies to lessen the burden of dengue fever.Wolbachia pipientis, an obligate intracellular bacterium, is being developed as a biocontrol strategy against dengue because it limits the replication of the DENV in the mosquito vector,Aedes aegypti However, several recent studies have demonstrated the sensitivity of pathogens, vectors, and their symbionts to temperature. To understand how the tripartite interactions between the mosquito, DENV, and Wolbachia may change under different temperature regimes, we assessed the vector competence and transmission potential of DENV-infected mosquitoes reared at a common laboratory setting of a constant 25°C and at two diurnal temperature settings with mean of 25°C and 28°C and a fluctuating range of 8°C (±4°C). Temperature significantly affected DENV infection rate in the mosquitoes. Furthermore, temperature significantly influenced the proportion of mosquitoes that achieved transmission potential as measured by the presence of virus in the saliva. Regardless of the temperature regimes,Wolbachia significantly and efficiently reduced the proportion of mosquitoes achieving infection and transmission potential across all the temperature regimes studied. This work reinforces the robustness of the Wolbachia biocontrol strategy to field conditions in Cairns, Australia, and suggests that similar studies are required for local mosquito genotypes and field relevant temperatures for emerging field release sites globally.

Wolbachia Reduces the Transmission Potential of Dengue-Infected Aedes aegypti

Background

Dengue viruses (DENV) are the causative agents of dengue, the world’s most prevalent arthropod-borne disease with around 40% of the world’s population at risk of infection annually. Wolbachia pipientis, an obligate intracellular bacterium, is being developed as a biocontrol strategy against dengue because it limits replication of the virus in the mosquito. The Wolbachia strain wMel, which has been introduced into the mosquito vector, Aedes aegypti, has been shown to invade and spread to near fixation in field releases. Standard measures of Wolbachia’s efficacy for blocking virus replication focus on the detection and quantification of virus in mosquito tissues. Examining the saliva provides a more accurate measure of transmission potential and can reveal the extrinsic incubation period (EIP), that is, the time it takes virus to arrive in the saliva following the consumption of DENV viremic blood. EIP is a key determinant of a mosquito’s ability to transmit DENVs, as the earlier the virus appears in the saliva the more opportunities the mosquito will have to infect humans on subsequent bites.

Methodology/Principal Findings

We used a non-destructive assay to repeatedly quantify DENV in saliva from wMel-infected and Wolbachia-free wild-type control mosquitoes following the consumption of a DENV-infected blood meal. We show that wMel lengthens the EIP, reduces the frequency at which the virus is expectorated and decreases the dengue copy number in mosquito saliva as compared to wild-type mosquitoes. These observations can at least be partially explained by an overall reduction in saliva produced by wMel mosquitoes. More generally, we found that the concentration of DENV in a blood meal is a determinant of the length of EIP, saliva virus titer and mosquito survival.

Conclusions/Significance

The saliva-based traits reported here offer more disease-relevant measures of Wolbachia’s effects on the vector and the virus. The lengthening of EIP highlights another means, in addition to the reduction of infection frequencies and DENV titers in mosquitoes, by which Wolbachia should operate to reduce DENV transmission in the field.

Dengue is endemic in more than 100 countries and is transmitted by the mosquito Aedes aegypti. The use of the symbiotic bacterium Wolbachia has become a potential biocontrol approach against dengue virus for two reasons. First, Wolbachia spreads rapidly through populations by manipulating host reproduction to its advantage. Second, Wolbachia limits viral replication in the mosquito by competing with the virus for essential host resources. Following field release in Cairns, Australia in 2011, the wMel strain of Wolbachia has successfully invaded wild mosquito populations, infecting nearly all individuals. To test whether limited dengue replication in wMel mosquitoes translates to a reduction in dengue transmission potential, we used a non-destructive assay to repeatedly quantify dengue virus in mosquito saliva. We found that wMel significantly delayed the time it took for mosquito saliva to become infectious, reduced the frequency of dengue virus that was expectorated by mosquitoes and lowered the virus titer in mosquito saliva. We also showed that wMel infection suppresses saliva production in mosquitoes that may, in part, explain our findings. The saliva-based nature of the work provides a more accurate assessment of Wolbachia’s ability to limit disease transmission and suggests that Wolbachia may have positive impacts on transmission not only by reducing the number of infectious mosquitoes in a population but also delaying the arrival of virus in the saliva.

Investigations of Koutango Virus Infectivity and Dissemination Dynamics in Aedes aegypti Mosquitoes

Aedes aegypti has already been implicated in the emergence of dengue and chikungunya viruses in the southern US. Vector competence is the ability of a mosquito species to support transmission of an arbovirus, which is bounded by its ability to support replication and dissemination of the virus through the mosquito body to the salivary glands to be expectorated in the saliva at the time of feeding on a vertebrate host. Here, we investigate the vector competence of A. aegypti for the arbovirus koutango by orally challenging mosquitoes with two titers of virus. We calculated the effective vector competence, a cumulative measure of transmission capability weighted by mosquito survival, and determined that A. aegypti was competent at the higher dose only. We conclude that further investigation is needed to determine the infectiousness of vertebrate hosts to fully assess the emergence potential of this virus in areas rich in A. aegypti.