The evolution of virulence of West Nile virus in a mosquito vector: implications for arbovirus adaptation and evolution

Temperature influences West Nile virus evolution and adaptation

West Nile virus (WNV), the most common mosquito-borne disease in the continental U.S., is vectored by Culex spp. mosquitoes. Since its introduction to New York State (NYS) in 1999, WNV has become endemic. NYS temperatures have risen by 0.14°C per decade since 1900, with larger increases linked to higher WNV transmission. Using surveillance and sequencing data, we find a significant correlation between rising temperatures, increased WNV genetic diversity, and higher prevalence. Given the experimentally demonstrated role of temperature influencing WNV fitness, we hypothesized that contemporary strains should exhibit greater fitness in mosquitoes at higher temperatures compared to historic strains. To test this, we analyzed genetically distinct WNV strains from mosquitoes collected during recent warm summers (2017 and 2018) and cooler historic summers (2003 and 2004). Assessing Culex pipiens vector competence and calculating the relative R₀ at 20°C, 24°C, and 28°C, we found that contemporary strains exhibit higher transmission potential at increased temperatures. Our results show that contemporary WNV strains possess greater phenotypic and genotypic diversity, facilitating the emergence of strains with enhanced transmission potential in a warming climate.

A meta-analysis highlights the idiosyncratic nature of tradeoffs in laboratory models of virus evolution

Different theoretical frameworks have been invoked to guide the study of virus evolution. Three of the more prominent ones are (i) the evolution of virulence, (ii) life history theory, and (iii) the generalism-specialism dichotomy. All involve purported tradeoffs between traits that define the evolvability and constraint of virus-associated phenotypes. However, as popular as these frameworks are, there is a surprising paucity of direct laboratory tests of the frameworks that support their utility as broadly applicable theoretical pillars that can guide our understanding of disease evolution. In this study, we conduct a meta-analysis of direct experimental evidence for these three frameworks across several widely studied virus-host systems: plant viruses, fungal viruses, animal viruses, and bacteriophages. We extracted 60 datasets from 28 studies and found a range of relationships between traits in different analysis categories (e.g., frameworks, virus-host systems). Our work demonstrates that direct evidence for relationships between traits is highly idiosyncratic and specific to the host-virus system and theoretical framework. Consequently, scientists researching viral pathogens from different taxonomic groups might reconsider their allegiance to these canons as the basis for expectation, explanation, or prediction. Future efforts could benefit from consistent definitions, and from developing frameworks that are compatible with the evidence and apply to particular biological and ecological contexts.

Exposure to Zika and chikungunya viruses impacts aspects of the vectorial capacity of Aedes aegypti and Culex quinquefasciatus

Zika (ZIKV) and chikungunya (CHIKV) are arboviruses that cause infections in humans and can cause clinical complications, representing a worldwide public health problem. Aedes aegypti is the primary vector of these pathogens and Culex quinquefasciatus may be a potential ZIKV vector. This study aimed to evaluate fecundity, fertility, survival, longevity, and blood feeding activity in Ae. aegypti after exposure to ZIKV and CHIKV and, in Cx. quinquefasciatus exposed to ZIKV. Three colonies were evaluated: AeCamp (Ae. aegypti-field), RecL (Ae. aegypti-laboratory) and CqSLab (Cx. quinquefasciatus-laboratory). Seven to 10 days-old females from these colonies were exposed to artificial blood feeding with CHIKV or ZIKV. CHIKV caused reduction in fecundity and fertility in AeCamp and reduction in survival and fertility in RecL. ZIKV impacted survival in RecL, fertility in AeCamp and, fecundity and fertility in CqSLab. Both viruses had no effect on blood feeding activity. These results show that CHIKV produces a higher biological cost in Ae. aegypti, compared to ZIKV, and ZIKV differently alters the biological performance in colonies of Ae. aegypti and Cx. quinquefasciatus. These results provide a better understanding over the processes of virus-vector interaction and can shed light on the complexity of arbovirus transmission.

Arbovirus impact on mosquito behavior: the jury is still out

Parasites can manipulate host behavior to enhance transmission, but our understanding of arbovirus-induced changes in mosquito behavior is limited. Here, we explore current knowledge on such behavioral alterations in mosquito vectors, focusing on host-seeking and blood-feeding behaviors. Reviewing studies on dengue, Zika, La Crosse, Sindbis, and West Nile viruses in Aedes or Culex mosquitoes reveals subtle yet potentially significant effects. However, assay heterogeneity and limited sample sizes challenge definitive conclusions. To enhance robustness, we propose using deep-learning tools for automated behavior quantification and stress the need for standardized assays. Additionally, conducting longitudinal studies across the extrinsic incubation period and integrating diverse traits into modeling frameworks are crucial for understanding the nuanced implications of arbovirus-induced behavioral changes for virus transmission dynamics.

Maintenance of a host-specific minority mutation in the West Nile virus NS3

West Nile virus (WNV), the most prevalent arthropod-borne virus (arbovirus) in the United States, is maintained in a cycle between Culex spp. mosquitoes and birds. Arboviruses exist within hosts and vectors as a diverse set of closely related genotypes. In theory, this genetic diversity can facilitate adaptation to distinct environments during host cycling, yet host-specific fitness of minority genotypes has not been assessed. Utilizing WNV deep-sequencing data, we previously identified a naturally occurring, mosquito-biased substitution, NS3 P319L. Using both cell culture and experimental infection in natural hosts, we demonstrated that this substitution confers attenuation in vertebrate hosts and increased transmissibility by mosquitoes. Biochemical assays demonstrated temperature-sensitive ATPase activity consistent with host-specific phenotypes. Together these data confirm the maintenance of host-specific minority variants in arbovirus mutant swarms, suggest a unique role for NS3 in viral fitness, and demonstrate that intrahost sequence data can inform mechanisms of host-specific adaptation.

Investigating and combatting the key drivers of viral zoonoses in Africa: an analysis of eight epidemics

Investigating the interplay of factors that result in a viral zoonotic outbreak is difficult, though it is increasingly important. As anthropogenic influences shift the delicate balance of ecosystems, new zoonoses emerge in humans. Sub-Saharan Africa is a notable hotspot for zoonotic disease due to abundant competent mammalian reservoir hosts. Furthermore, poverty, corruption, and an overreliance on natural resources play considerable roles in depleting biological resources, exacerbating the population's susceptibility. Unsurprisingly, viral zoonoses have emerged in Africa, including HIV/AIDS, Ebola, Avian influenza, Lassa fever, Zika, and Monkeypox. These diseases are among the principal causes of death in endemic areas. Though typically distinct in their manifestations, viral zoonoses are connected by underlying, definitive factors. This review summarises vital findings on viral zoonoses in Africa using nine notable case studies as a benchmark for future studies. We discuss the importance of ecological recuperation and protection as a central strategy to control zoonotic diseases. Emphasis was made on moderating key drivers of zoonotic diseases to forestall future pandemics. This is in conjunction with attempts to redirect efforts from reactive to pre-emptive through a multidisciplinary "one health" approach.

Trans-Generational Symbiont Transmission Reduced at High Temperatures in a West Nile Virus Vector Mosquito Culex quinquefasciatus

The influence of environmental factors on the efficacy of the endosymbiont Wolbachia used in mosquito and pathogen control are poorly characterized and may be critical for disease control. We studied the vector mosquito Culex pipiens quinquefasciatus (Say) to determine the effect of temperature on the composition of the relative abundance of Wolbachia spp. and the microbiome, as well as key immune genes of interest in the Toll and IMD pathways. 16S barcode sequencing was used to determine the microbiome composition and qPCR was used to determine the relative abundance of Wolbachia spp. based on the highly utilized marker Wolbachia surface protein (wsp) gene. We found no effect of temperature within a single generation on the relative abundance of Wolbachia or immune gene expression, nor on the alpha or beta diversity of the microbiome. However, there was a significant difference in the abundance of Wolbachia between generations at high temperatures (≥ 28°C), but not at lower temperatures (≤ 23°C). These results support the idea that Wolbachia are reduced at higher temperatures between generations, which has an influence on the establishment of pathogens including West Nile Virus (WNV). Modulation of the Toll or IMD mosquito immune pathways was not indicated. Wolbachia endosymbiosis and trans-generation transmission appears especially sensitive to high temperatures, which may have implications for Wolbachia-based vector control strategies under climate change scenarios.

Experimental Evolution of West Nile Virus at Higher Temperatures Facilitates Broad Adaptation and Increased Genetic Diversity

West Nile virus (WNV, Flaviviridae, Flavivirus) is a mosquito-borne flavivirus introduced to North America in 1999. Since 1999, the Earth's average temperature has increased by 0.6 °C. Mosquitoes are ectothermic organisms, reliant on environmental heat sources. Temperature impacts vector-virus interactions which directly influence arbovirus transmission. RNA viral replication is highly error-prone and increasing temperature could further increase replication rates, mutation frequencies, and evolutionary rates. The impact of temperature on arbovirus evolutionary trajectories and fitness landscapes has yet to be sufficiently studied. To investigate how temperature impacts the rate and extent of WNV evolution in mosquito cells, WNV was experimentally passaged 12 times in Culex tarsalis cells, at 25 °C and 30 °C. Full-genome deep sequencing was used to compare genetic signatures during passage, and replicative fitness was evaluated before and after passage at each temperature. Our results suggest adaptive potential at both temperatures, with unique temperature-dependent and lineage-specific genetic signatures. Further, higher temperature passage was associated with significantly increased replicative fitness at both temperatures and increases in nonsynonymous mutations. Together, these data indicate that if similar selective pressures exist in natural systems, increases in temperature could accelerate emergence of high-fitness strains with greater phenotypic plasticity.

Zika virus infects Aedes aegypti ovaries

Pathogens are transmitted from one host to another either by vertical transmission (VT) or horizontal transmission (HT). Mosquito-borne arboviruses (arthropod-borne viruses), including several clinically important viruses such as dengue, Zika, West Nile and chikungunya viruses persist in nature by both VT and HT. VT may also serve as an essential link in the transmission cycle during adverse environmental conditions. VT rates (VTRs) vary between virus families and even among viruses within the same genus. The mechanism behind these differences in VTRs among viruses is poorly understood. For efficient VT to occur, viruses must infect the mosquito germline. Here, we show that Zika virus infects mosquito ovaries and is transmitted vertically at a low rate. The infected progeny derive from mosquitoes with infected ovaries. The prevalence of ovary infection increases after a second non-infectious blood meal following an infectious blood meal.

Evolution of resistance to fluoroquinolones by dengue virus serotype 4 provides insight into mechanism of action and consequences for viral fitness

Drugs against flaviviruses such as dengue (DENV) and Zika (ZIKV) virus are urgently needed. We previously demonstrated that three fluoroquinolones, ciprofloxacin, enoxacin, and difloxacin, suppress replication of six flaviviruses. To investigate the barrier to resistance and mechanism(s) of action of these drugs, DENV-4 was passaged in triplicate in HEK-293 cells in the presence or absence of each drug. Resistance to ciprofloxacin was detected by the seventh passage and to difloxacin by the tenth, whereas resistance to enoxacin did not occur within ten passages. Two putative resistance-conferring mutations were detected in the envelope gene of ciprofloxacin and difloxacin-resistant DENV-4. In the absence of ciprofloxacin, ciprofloxacin-resistant viruses sustained a significantly higher viral titer than control viruses in HEK-293 and HuH-7 cells and resistant viruses were more stable than control viruses at 37 °C. These results suggest that the mechanism of action of ciprofloxacin and difloxacin involves interference with virus binding or entry.

Do seasonal microbiome changes affect infection susceptibility, contributing to seasonal disease outbreaks?

The aim of the present paper is to explore whether seasonal outbreaks of infectious diseases may be linked to changes in host microbiomes. This is a very important issue, because one way to have more control over seasonal outbreaks is to understand the factors that underlie them. In this paper, I will evaluate the relevance of the microbiome as one of such factors. The paper is based on two pillars of reasoning. Firstly, on the idea that microbiomes play an important role in their hosts' defence against infectious diseases. Secondly, on the idea that microbiomes are not stable, but change seasonally. These two ideas are combined in order to argue that seasonal changes in a given microbiome may influence the functionality of the host's immune system and consequently make it easier for infectious agents to infect the host at certain times of year. I will argue that, while this is only a theoretical possibility, certain studies may back up such claims. Furthermore, I will show that this does not necessarily contradict other hypotheses aimed at explaining seasonal outbreaks; in fact, it may even enhance them.

Sequencing of ZIKV genomes directly from Ae. aegypti and Cx. quinquefasciatus mosquitoes collected during the 2015-16 epidemics in Recife

Zika virus (ZIKV) is a negative sense RNA virus from the Flaviviridae family, which was relatively unknown until the first human epidemic in Micronesia, in 2007. Since then, it spread to French Polynesia and the Americas. Recife, the capital of Pernambuco state and epicenter of the Zika epidemic in Brazil, experienced a large number of microcephaly cases and other congenital abnormalities associated to the ZIKV infection from, 2015 to 16. Evidences suggest that both Aedes aegypti and Culex quinquefasciatus mosquitoes from Recife are capable of replicating and transmitting the virus. Here, we conducted high throughput sequencing of ZIKV genomes directly from Ae. aegypti and Cx. quinquefasciatus mosquitoes collected during the ZIKV epidemics in Recife, in order to investigate the variability and evolution of the virus. We obtained 11 draft ZIKV genomes derived from 5 pools from each Ae. aegypti and Cx. quinquefasciatus species. Genome coverage breadth ranged from 16 to 100% and average depth from 45 to 46,584×. Two of these genomes were obtained from pools of Cx. quinquefasciatus females with no sign of blood in the abdomen. Amino acid substitutions found here were not species-specific. In addition, molecular clock dating estimated that ZIKV draft genomes obtained here were co-circulating in other regions of the country during the epidemics. Overall results highlight that viral mutations and even minor variants can be detected in genomes directly sequenced from mosquito samples and insights about natural viral genomic variability and viral evolution can be useful when designing tools for mosquito control programs.

Increased temperatures reduce the vectorial capacity of Aedes mosquitoes for Zika virus

Rapid and significant range expansion of both Zika virus (ZIKV) and its Aedes vector species has resulted in ZIKV being declared a global health threat. Mean temperatures are projected to increase globally, likely resulting in alterations of the transmission potential of mosquito-borne pathogens. To understand the effect of diurnal temperature range on the vectorial capacity of Ae. aegypti and Ae. albopictus for ZIKV, longevity, blood-feeding and vector competence were assessed at two temperature regimes following feeding on infectious blood meals. Higher temperatures resulted in decreased longevity of Ae. aegypti [Log-rank test, χ2, df 35.66, 5, P < 0.001] and a decrease in blood-feeding rates of Ae. albopictus [Fisher's exact test, P < 0.001]. Temperature had a population and species-specific impact on ZIKV infection rates. Overall, Ae. albopictus reared at the lowest temperature regime demonstrated the highest vectorial capacity (0.53) and the highest transmission efficiency (57%). Increased temperature decreased vectorial capacity across groups yet more significant effects were measured with Ae. aegypti relative to Ae. albopictus. The results of this study suggest that future increases in temperature in the Americas could significantly impact vector competence, blood-feeding and longevity, and potentially decrease the overall vectorial capacity of Aedes mosquitoes in the Americas.

The Role of Temperature in Transmission of Zoonotic Arboviruses

We reviewed the literature on the role of temperature in transmission of zoonotic arboviruses. Vector competence is affected by both direct and indirect effects of temperature, and generally increases with increasing temperature, but results may vary by vector species, population, and viral strain. Temperature additionally has a significant influence on life history traits of vectors at both immature and adult life stages, and for important behaviors such as blood-feeding and mating. Similar to vector competence, temperature effects on life history traits can vary by species and population. Vector, host, and viral distributions are all affected by temperature, and are generally expected to change with increased temperatures predicted under climate change. Arboviruses are generally expected to shift poleward and to higher elevations under climate change, yet significant variability on fine geographic scales is likely. Temperature effects are generally unimodal, with increases in abundance up to an optimum, and then decreases at high temperatures. Improved vector distribution information could facilitate future distribution modeling. A wide variety of approaches have been used to model viral distributions, although most research has focused on the West Nile virus. Direct temperature effects are frequently observed, as are indirect effects, such as through droughts, where temperature interacts with rainfall. Thermal biology approaches hold much promise for syntheses across viruses, vectors, and hosts, yet future studies must consider the specificity of interactions and the dynamic nature of evolving biological systems.

On the Fly: Interactions Between Birds, Mosquitoes, and Environment That Have Molded West Nile Virus Genomic Structure Over Two Decades

West Nile virus (WNV) was first identified in North America almost 20 yr ago. In that time, WNV has crossed the continent and established enzootic transmission cycles, resulting in intermittent outbreaks of human disease that have largely been linked with climatic variables and waning avian seroprevalence. During the transcontinental dissemination of WNV, the original genotype has been displaced by two principal extant genotypes which contain an envelope mutation that has been associated with enhanced vector competence by Culex pipiens L. (Diptera: Culicidae) and Culex tarsalis Coquillett vectors. Analyses of retrospective avian host competence data generated using the founding NY99 genotype strain have demonstrated a steady reduction in viremias of house sparrows over time. Reciprocally, the current genotype strains WN02 and SW03 have demonstrated an inverse correlation between house sparrow viremia magnitude and the time since isolation. These data collectively indicate that WNV has evolved for increased avian viremia while house sparrows have evolved resistance to the virus such that the relative host competence has remained constant. Intrahost analyses of WNV evolution demonstrate that selection pressures are avian species-specific and purifying selection is greater in individual birds compared with individual mosquitoes, suggesting that the avian adaptive and/or innate immune response may impose a selection pressure on WNV. Phylogenomic, experimental evolutionary systems, and models that link viral evolution with climate, host, and vector competence studies will be needed to identify the relative effect of different selective and stochastic mechanisms on viral phenotypes and the capacity of newly evolved WNV genotypes for transmission in continuously changing landscapes.

Predicting West Nile virus transmission in North American bird communities using phylogenetic mixed effects models and eBird citizen science data

BACKGROUND

West Nile virus (WNV) is a mosquito-transmitted disease of birds that has caused bird population declines and can spill over into human populations. Previous research has identified bird species that infect a large fraction of the total pool of infected mosquitoes and correlate with human infection risk; however, these analyses cover small spatial regions and cannot be used to predict transmission in bird communities in which these species are rare or absent. Here we present a mechanistic model for WNV transmission that predicts WNV spread (R0) in any bird community in North America by scaling up from the physiological responses of individual birds to transmission at the level of the community. We predict unmeasured bird species' responses to infection using phylogenetic imputation, based on these species' phylogenetic relationships with bird species with measured responses.

RESULTS

We focused our analysis on Texas, USA, because it is among the states with the highest total incidence of WNV in humans and is well sampled by birders in the eBird database. Spatio-temporal patterns: WNV transmission is primarily driven by temperature variation across time and space, and secondarily by bird community composition. In Texas, we predicted WNV R0 to be highest in the spring and fall when temperatures maximize the product of mosquito transmission and survival probabilities. In the most favorable months for WNV transmission (April, May, September and October), we predicted R0 to be highest in the "Piney Woods" and "Oak Woods & Prairies" ecoregions of Texas, and lowest in the "High Plains" and "South Texas Brush County" ecoregions. Dilution effect: More abundant bird species are more competent hosts for WNV, and predicted WNV R0 decreases with increasing species richness. Keystone species: We predicted that northern cardinals (Cardinalis cardinalis) are the most important hosts for amplifying WNV and that mourning doves (Zenaida macroura) are the most important sinks of infection across Texas.

CONCLUSIONS

Despite some data limitations, we demonstrate the power of phylogenetic imputation in predicting disease transmission in heterogeneous host communities. Our mechanistic modeling framework shows promise both for assisting future analyses on transmission and spillover in heterogeneous multispecies pathogen systems and for improving model transparency by clarifying assumptions, choices and shortcomings in complex ecological analyses.

Evolutionary dynamics and molecular epidemiology of West Nile virus in New York State: 1999-2015

Following its introduction into New York State (NYS) in 1999, West Nile virus (WNV; Flavivirus, Flaviviridae) underwent a rapid expansion throughout the USA and into Canada and Latin America. WNV has been characterized as being evolutionarily stable, with weak geographic structure, a dominance of purifying selection and limited adaptive change. We analyzed all available full-genome WNV sequences, focusing on the 543 available sequences from NYS, which included 495 newly sequenced 2000-15 isolates. In addition, we analyzed deep-sequencing data from 317 of these isolates. While our data are generally in agreement with the limited pace of evolutionary change and broad geographic and temporal mixing identified in other studies, we have identified some important exceptions. Most notably, there are 14 codons which demonstrated evidence of positive selection as determined by multiple models, including some positions with evidence of selection in NYS exclusively. Coincident with increased WNV activity, genotypes possessing one or more of these mutations, designated NY01, NY07, and NY10, have increased in prevalence in recent years and displaced historic strains. In addition, we have found a geographical bias with many of these mutations, which suggests selective pressures and adaptations could be regional. Lastly, our deep-sequencing data suggest both increased overall diversity in avian tissue isolates relative to mosquito isolates and multiple non-synonymous minority variants that are both host-specific and retained over time and space. Together, these data provide novel insight into the evolutionary pressures on WNV and the need for continued genetic surveillance and characterization of emergent strains.

The role of co-infection and swarm dynamics in arbovirus transmission

Arthropod-borne viruses (arboviruses) are transmitted by hematophagous insects, primarily mosquitoes. The geographic range and prevalence of mosquito-borne viruses and their vectors has dramatically increased over the last 50 years. As a result, the most medically important arboviurses now co-exist in many regions, resulting in an increased frequency of co-infections in hosts and vectors. In addition to concurrent infections with human pathogens, mosquito-only viruses and/or enzootic viruses not associated with human disease are ubiquitous in mosquito populations. Moreover, mosquito-borne viruses are largely RNA viruses that exist within individual hosts as a diverse and dynamic swarm of closely related genotypes. Interactions among co-infecting viruses and genotypes can have profound effects on virulence, fitness and evolution. Here, we review our understanding of how these complex interactions influence transmission of mosquito-borne viruses, focusing on the often-neglected virus interactions in the mosquito vector, and identify gaps in our knowledge that should guide future studies.

Differential Effects of Temperature and Mosquito Genetics Determine Transmissibility of Arboviruses by Aedes aegypti in Argentina

Aedes aegypti (L.) (Diptera: Culicidae) have a global distribution and are the primary vector of a number of mosquito-borne viruses responsible for epidemics throughout the Americas. As in much of South America, the threat from pathogens including dengue virus (DENV; Flaviviridae, Flavivirus) and chikungunya virus (CHIKV; Togaviridae, Alphavirus) has increased in Argentina in recent years. The complexity of transmission cycles makes predicting the occurrence and intensity of arbovirus outbreaks difficult. To gain a better understanding of the risk of DENV and CHIKV in Argentina and the factors influencing this risk, we evaluated the role of population and temperature in the vector competence and vectorial capacity (VC) of Ae. aegypti from geographically and ecologically distinct locations. Our results demonstrate that intrinsic and extrinsic factors including mosquito population, viral species, and temperature significantly influence both vector competence and overall VC of Ae. aegypti in Argentina, yet also that the magnitude of these influences is highly variable. Specifically, results suggest that CHIKV competence is more dependent on mosquito genetics than is DENV competence, whereas temperature has a greater effect on DENV transmission. In addition, although there is an overall positive correlation between temperature and competence for both viruses, there are exceptions to this for individual virus-population combinations. Together, these data establish large variability in VC for these pathogens among distinct Ae. aegypti populations in Argentina and demonstrate that accurate assessment of arbovirus risk will require nuanced models that fully consider the complexity of interactions between virus, temperature, mosquito genetics, and hosts.

West Nile virus and its vectors

West Nile virus (WNV Flaviviridae; Flavivrus) is the most geographically widespread arbovirus in the world and the leading cause of arboviral encephalitis globally. Worldwide, WNV is maintained in an enzootic cycle between primarily Culex spp. mosquitoes and birds, with human infection and disease resulting from enzootic spillover. Dynamic and complex intrinsic and extrinsic factors contribute to the temporal and spatial variability in WNV transmission. The most current information on the relative contribution of each of these factors is reviewed and a case to incorporate detailed and localized environmental and genetic data into predictive models is presented.

Vertical Transmission of Zika Virus by Aedes aegypti and Ae. albopictus Mosquitoes

To determine the potential role of vertical transmission in Zika virus expansion, we evaluated larval pools of perorally infected Aedes aegypti and Ae. albopictus adult female mosquitoes; ≈1/84 larvae tested were Zika virus–positive; and rates varied among mosquito populations. Thus, vertical transmission may play a role in Zika virus spread and maintenance.

Mosquito Microbiome Dynamics, a Background for Prevalence and Seasonality of West Nile Virus

Symbiotic microbial communities augment host phenotype, including defense against pathogen carriage and infection. We sampled the microbial communities in 11 adult mosquito host species from six regions in southern Ontario, Canada over 3 years. Of the factors examined, we found that mosquito species was the largest driver of the microbiota, with remarkable phylosymbiosis between host and microbiota. Seasonal shifts of the microbiome were consistently repeated over the 3-year period, while region had little impact. Both host species and seasonal shifts in microbiota were associated with patterns of West Nile virus (WNV) in these mosquitoes. The highest prevalence of WNV, with a seasonal spike each year in August, was in the Culex pipiens/restuans complex, and high WNV prevalence followed a decrease in relative abundance of Wolbachia in this species. Indeed, mean temperature, but not precipitation, was significantly correlated with Wolbachia abundance. This suggests that at higher temperatures Wolbachia abundance is reduced leading to greater susceptibility to WNV in the subsequent generation of C. pipiens/restuans hosts. Different mosquito genera harbored significantly different bacterial communities, and presence or abundance of Wolbachia was primarily associated with these differences. We identified several operational taxonomic units (OTUs) of Wolbachia that drive overall microbial community differentiation among mosquito taxa, locations and timepoints. Distinct Wolbachia OTUs were consistently found to dominate microbiomes of Cx. pipiens/restuans, and of Coquilletidia perturbans. Seasonal fluctuations of several other microbial taxa included Bacillus cereus, Enterococcus, Methylobacterium, Asaia, Pantoea, Acinetobacter johnsonii, Pseudomonas, and Mycoplasma. This suggests that microbiota may explain some of the variation in vector competence previously attributed to local environmental processes, especially because Wolbachia is known to affect carriage of viral pathogens.

Vector-Borne Pathogen and Host Evolution in a Structured Immuno-Epidemiological System

Vector-borne disease transmission is a common dissemination mode used by many pathogens to spread in a host population. Similar to directly transmitted diseases, the within-host interaction of a vector-borne pathogen and a host's immune system influences the pathogen's transmission potential between hosts via vectors. Yet there are few theoretical studies on virulence-transmission trade-offs and evolution in vector-borne pathogen-host systems. Here, we consider an immuno-epidemiological model that links the within-host dynamics to between-host circulation of a vector-borne disease. On the immunological scale, the model mimics antibody-pathogen dynamics for arbovirus diseases, such as Rift Valley fever and West Nile virus. The within-host dynamics govern transmission and host mortality and recovery in an age-since-infection structured host-vector-borne pathogen epidemic model. By considering multiple pathogen strains and multiple competing host populations differing in their within-host replication rate and immune response parameters, respectively, we derive evolutionary optimization principles for both pathogen and host. Invasion analysis shows that the [Formula: see text] maximization principle holds for the vector-borne pathogen. For the host, we prove that evolution favors minimizing case fatality ratio (CFR). These results are utilized to compute host and pathogen evolutionary trajectories and to determine how model parameters affect evolution outcomes. We find that increasing the vector inoculum size increases the pathogen [Formula: see text], but can either increase or decrease the pathogen virulence (the host CFR), suggesting that vector inoculum size can contribute to virulence of vector-borne diseases in distinct ways.

Complexity of virus-vector interactions

The inter-relationships among viruses, vectors and vertebrate hosts are complex and dynamic and shaped by biotic (e.g., viral strain, vector genetics, host susceptibility) and abiotic (e.g., temperature, rainfall, human land use) factors. It is anticipated that changes in climate, as predicted by the most recent Report of the Intergovernmental Panel on Climate Change, will result in landscape changes and consequent changes in spatiotemporal patterns of arbovirus transmission. To anticipate evolving patterns of virus activity in a dynamically changing environment, it is important to understand how interconnectedness of mosquito and virus biology together with climate influence arbovirus transmission intensity. Vector competence, survivorship, and feeding behavior, among other aspects of vectorial capacity are intrinsically important to estimate risk and design control approaches.

The wMel Strain of Wolbachia Reduces Transmission of Chikungunya Virus in Aedes aegypti

Background

New approaches to preventing chikungunya virus (CHIKV) are needed because current methods are limited to controlling mosquito populations, and they have not prevented the invasion of this virus into new locales, nor have they been sufficient to control the virus upon arrival. A promising candidate for arbovirus control and prevention relies on the introduction of the intracellular bacterium Wolbachia into Aedes aegypti mosquitoes. This primarily has been proposed as a tool to control dengue virus (DENV) transmission; however, evidence suggests Wolbachia infections confer protection for Ae. aegypti against CHIKV. Although this approach holds much promise for limiting virus transmission, at present our understanding of the ability of CHIKV to infect, disseminate, and be transmitted by wMel-infected Ae. aegypti currently being used at Wolbachia release sites is limited.

Methodology/Principal Findings

Using Ae. aegypti infected with the wMel strain of Wolbachia that are being released in Medellin, Colombia, we report that these mosquitoes have reduced vector competence for CHIKV, even with extremely high viral titers in the bloodmeal. In addition, we examined the dynamics of CHIKV infection over the course of four to seven days post feeding. Wolbachia-infected mosquitoes remained non-infective over the duration of seven days, i.e., no infectious virus was detected in the saliva when exposed to bloodmeals of moderate viremia, but CHIKV-exposed, wild type mosquitoes did have viral loads in the saliva consistent with what has been reported elsewhere. Finally, the presence of wMel infection had no impact on the lifespan of mosquitoes as compared to wild type mosquitoes following CHIKV infection.

Conclusions/Significance

These results could have an impact on vector control strategies in areas where Ae. aegypti are transmitting both DENV and CHIKV; i.e., they argue for further exploration, both in the laboratory and the field, on the feasibility of expanding this technology beyond DENV.

New approaches to preventing chikungunya virus (CHIKV) infection are needed because the endemic range of this virus is expanding and because current methods are limited to controlling mosquito populations, and this approach has not effectively controlled this virus. A promising candidate for arbovirus control and prevention relies on the introduction of the intracellular bacterium Wolbachia into Aedes aegypti mosquitoes. Wolbachia biocontrol has advanced from laboratory experiments demonstrating that Wolbachia reduces virus replication to small-scale field trials demonstrating that Wolbachia are capable of spreading through wild Ae. aegypti populations. This primarily has been proposed as a tool to control dengue virus (DENV) transmission; however, Wolbachia infections confer protection for their insect hosts against a range of pathogens including CHIKV in Ae. aegypti. Medium-scale Wolbachia deployments are imminent or in certain instances have commenced. Therefore, assessing whether or not Wolbachia-infected Ae. aegypti are effective against CHIKV will help inform the viability of Wolbachia biocontrol for CHIKV control. Our study provides valuable evidence that could justify expanding this type of control program to other Ae. aegypti-transmitted arboviruses, primarily CHIKV.

Dissecting vectorial capacity for mosquito-borne viruses

The inter-relationship between mosquitoes and the viruses they transmit is complex. While previously understood barriers to infection and transmission remain valid, additional factors have been uncovered that suggest an 'arms race' between mosquito and virus. These include the mosquito microbiota and interplay between mosquito and viral genetics. Following an infectious blood meal, the mosquito mounts an immune and transcriptional response, leading to altered expression of multiple genes. These complex interactions, specific to vector and virus genotypes, combine with external influences, particularly temperature, to determine vector competence. The mosquito's response to the infecting agent may have consequences in terms of longevity, feeding behavior and/or fecundity. These factors, together with population density and the frequency of host contact determine vectorial capacity.

Exploring Genomic, Geographic and Virulence Interactions among Epidemic and Non-Epidemic St. Louis Encephalitis Virus (Flavivirus) Strains

St. Louis encephalitis virus (SLEV) is a re-emerging arbovirus in South America. In 2005, an encephalitis outbreak caused by SLEV was reported in Argentina. The reason for the outbreak remains unknown, but may have been related to virological factors, changes in vectors populations, avian amplifying hosts, and/or environmental conditions. The main goal of this study was to characterize the complete genome of epidemic and non-epidemic SLEV strains from Argentina. Seventeen amino acid changes were detected; ten were non-conservative and located in proteins E, NS1, NS3 and NS5. Phylogenetic analysis showed two major clades based on geography: the North America and northern Central America (NAnCA) clade and the South America and southern Central America (SAsCA) clade. Interestingly, the presence of SAsCA genotype V SLEV strains in the NAnCA clade was reported in California, Florida and Texas, overlapping with known bird migration flyways. This work represents the first step in understanding the molecular mechanisms underlying virulence and biological variation among SLEV strains.

West Nile virus adaptation to ixodid tick cells is associated with phenotypic trade-offs in primary hosts

West Nile virus (WNV; Flaviviridae, Flavivirus) is the most geographically widespread arthropod-borne virus (arbovirus) in the world and is found in multiple ecologically distinct settings. Despite the likelihood of frequent exposure to novel hosts, studies evaluating the capacity and correlates of host range expansions or shifts of WNV and other arboviruses are generally lacking. We utilized experimental evolution of WNV in an Amblyomma americanum tick cell line to model an invertebrate host shift and evaluate the adaptive potential of WNV outside of its primary transmission cycle. Our results demonstrate that highly significant gains in replicative ability in ixodid tick cells are attainable for WNV but are also associated with widespread genetic change and significant phenotypic costs in vitro. Decreased fitness in primary hosts could represent a barrier to frequent exploitation of hard ticks by WNV in nature.

New insights into innate immune restriction of West Nile virus infection

West Nile virus (WNV) is an encephalitic flavivirus that has provided a valuable experimental system for studying viral pathogenesis and immunity. Although in vitro approaches and mouse models of infection have identified pattern recognition receptor and interferon pathways that control WNV infection, our appreciation of specific antiviral effectors has been more limited. In this review, we highlight recent advances in our understanding of the host factors that restrict WNV infection in mammals and insects, especially those resulting from large-scale screening approaches.

The role of environmental, virological and vector interactions in dictating biological transmission of arthropod-borne viruses by mosquitoes

Arthropod-borne viruses (arboviruses) are transmitted between vertebrate hosts and arthropod vectors. An inherently complex interaction among virus, vector, and the environment determines successful transmission of the virus. Once believed to be "flying syringes," recent advances in the field have demonstrated that mosquito genetics, microbiota, salivary components, and mosquito innate immune responses all play important roles in modulating arbovirus transmissibility. The literature on the interaction among virus, mosquito, and environment has expanded dramatically in the preceding decade and the utilization of next-generation sequencing and transgenic vector methodologies assuredly will increase the pace of knowledge acquisition in this field. This chapter outlines the interplay among the three factors in both direct physical and biochemical manners as well as indirectly through superinfection barriers and altered induction of innate immune responses in mosquito vectors. The culmination of the aforementioned interactions and the arms race between the mosquito innate immune response and the capacity of arboviruses to antagonize such a response ultimately results in the subjugation of mosquito cells for viral replication and subsequent transmission.

Arboviral bottlenecks and challenges to maintaining diversity and fitness during mosquito transmission

The term arbovirus denotes viruses that are transmitted by arthropods, such as ticks, mosquitoes, and other biting arthropods. The infection of these vectors produces a certain set of evolutionary pressures on the virus; involving migration from the midgut, where the blood meal containing the virus is processed, to the salivary glands, in order to transmit the virus to the next host. During this process the virus is subject to numerous bottlenecks, stochastic events that significantly reduce the number of viral particles that are able to infect the next stage. This article reviews the latest research on the bottlenecks that occur in arboviruses and the way in which these affect the evolution and fitness of these viruses. In particular we focus on the latest research on three important arboviruses, West Nile virus, Venezuelan equine encephalitis virus and Chikungunya viruses and compare the differing effects of the mosquito bottlenecks on these viruses as well as other evolutionary pressures that affect their evolution and transmission.

Mosquito cell lines: history, isolation, availability and application to assess the threat of arboviral transmission in the United Kingdom

Mosquitoes are highly effective vectors for transmission of human and animal pathogens. Understanding the relationship between pathogen and vector is vital in developing strategies to predict and prevent transmission. Cell lines derived from appropriate mosquito hosts provide a relatively simple tool for investigating the interaction between the host and viruses transmitted by mosquitoes. This review provides a brief overview of the development of mosquito cell lines, methods of isolation, their availability and application for investigating insect-virus interactions.

Vertical transmission selects for reduced virulence in a plant virus and for increased resistance in the host

For the last three decades, evolutionary biologists have sought to understand which factors modulate the evolution of parasite virulence. Although theory has identified several of these modulators, their effect has seldom been analysed experimentally. We investigated the role of two such major factors—the mode of transmission, and host adaptation in response to parasite evolution—in the evolution of virulence of the plant virus Cucumber mosaic virus (CMV) in its natural host Arabidopsis thaliana. To do so, we serially passaged three CMV strains under strict vertical and strict horizontal transmission, alternating both modes of transmission. We quantified seed (vertical) transmission rate, virus accumulation, effect on plant growth and virulence of evolved and non-evolved viruses in the original plants and in plants derived after five passages of vertical transmission. Our results indicated that vertical passaging led to adaptation of the virus to greater vertical transmission, which was associated with reductions of virus accumulation and virulence. On the other hand, horizontal serial passages did not significantly modify virus accumulation and virulence. The observed increases in CMV seed transmission, and reductions in virus accumulation and virulence in vertically passaged viruses were due also to reciprocal host adaptation during vertical passages, which additionally reduced virulence and multiplication of vertically passaged viruses. This result is consistent with plant-virus co-evolution. Host adaptation to vertically passaged viruses was traded-off against reduced resistance to the non-evolved viruses. Thus, we provide evidence of the key role that the interplay between mode of transmission and host-parasite co-evolution has in determining the evolution of virulence.

Virulence is a key property of parasites, and is linked to the emergence of new diseases and to the reduction of ecosystem biodiversity. Consequently, scientists have devoted a great effort to build theoretical models that predict which factors may modulate virulence evolution. However, whether (and how) these factors affect virulence evolution has been seldom analysed experimentally. Using the plant virus Cucumber mosaic virus (CMV) and its natural host Arabidopsis thaliana, we studied the role of two such factors: the mode of transmission, and host adaptation in response to parasite evolution. We serially passaged CMV under strict vertical and strict horizontal transmission, and a combination of both. Subsequently, we analysed differences in CMV seed (vertical) transmission rate, accumulation and virulence between evolved and non-evolved viruses. We also compared whether these differences varied in original plants and in plants evolved during vertical passaging. Vertical passaging increased CMV seed transmission, and reduced accumulation and virulence, while horizontal passaging had no effect. Changes during vertical passaging were determined also by reciprocal host adaptation, which additionally reduced virulence and accumulation of vertically transmitted viruses. Hence, we provide evidence that the interplay between the transmission mode and host-parasite co-evolution is central in determining virulence evolution.

Vector-virus interactions and transmission dynamics of West Nile virus

West Nile virus (WNV; Flavivirus; Flaviviridae) is the cause of the most widespread arthropod-borne viral disease in the world and the largest outbreak of neuroinvasive disease ever observed. Mosquito-borne outbreaks are influenced by intrinsic (e.g., vector and viral genetics, vector and host competence, vector life-history traits) and extrinsic (e.g., temperature, rainfall, human land use) factors that affect virus activity and mosquito biology in complex ways. The concept of vectorial capacity integrates these factors to address interactions of the virus with the arthropod host, leading to a clearer understanding of their complex interrelationships, how they affect transmission of vector-borne disease, and how they impact human health. Vertebrate factors including host competence, population dynamics, and immune status also affect transmission dynamics. The complexity of these interactions are further exacerbated by the fact that not only can divergent hosts differentially alter the virus, but the virus also can affect both vertebrate and invertebrate hosts in ways that significantly alter patterns of virus transmission. This chapter concentrates on selected components of the virus-vector-vertebrate interrelationship, focusing specifically on how interactions between vector, virus, and environment shape the patterns and intensity of WNV transmission.