Tissue Barriers to Arbovirus Infection in Mosquitoes

Life as a Vector of Dengue Virus: The Antioxidant Strategy of Mosquito Cells to Survive Viral Infection

Dengue fever is a mosquito-borne viral disease of increasing global importance. The disease has caused heavy burdens due to frequent outbreaks in tropical and subtropical areas of the world. The dengue virus (DENV) is generally transmitted between human hosts via the bite of a mosquito vector, primarily Aedes aegypti and Ae. albopictus as a minor species. It is known that the virus needs to alternately infect mosquito and human cells. DENV-induced cell death is relevant to the pathogenesis in humans as infected cells undergo apoptosis. In contrast, mosquito cells mostly survive the infection; this allows infected mosquitoes to remain healthy enough to serve as an efficient vector in nature. Overexpression of antioxidant genes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), glutaredoxin (Grx), thioredoxin (Trx), and protein disulfide isomerase (PDI) have been detected in DENV2-infected mosquito cells. Additional antioxidants, including GST, eukaryotic translation initiation factor 5A (eIF5a), and p53 isoform 2 (p53-2), and perhaps some others, are also involved in creating an intracellular environment suitable for cell replication and viral infection. Antiapoptotic effects involving inhibitor of apoptosis (IAP) upregulation and subsequent elevation of caspase-9 and caspase-3 activities also play crucial roles in the ability of mosquito cells to survive DENV infection. This article focused on the effects of intracellular responses in mosquito cells to infection primarily by DENVs. It may provide more information to better understand virus/cell interactions that can possibly elucidate the evolutionary pathway that led to the mosquito becoming a vector.

Vector Competence of Florida Culicoides insignis (Diptera: Ceratopogonidae) for Epizootic Hemorrhagic Disease Virus Serotype-2

Epizootic hemorrhagic disease virus (EHDV; family Reoviridae, genus Orbivirus) is an arthropod-borne virus of ungulates, primarily white-tailed deer in North America. Culicoides sonorensis, the only confirmed North American vector of EHDV, is rarely collected from Florida despite annual virus outbreaks. Culicoides insignis is an abundant species in Florida and is also a confirmed vector of the closely related Bluetongue virus. In this study, oral challenge of C. insignis was performed to determine vector competence for EHDV serotype-2. Field-collected female midges were provided bovine blood spiked with three different titers of EHDV-2 (5.05, 4.00, or 2.94 log₁₀PFUe/mL). After an incubation period of 10 days or after death, bodies and legs were collected. Saliva was collected daily from all females from 3 days post feeding until their death using honey card assays. All samples were tested for EHDV RNA using RT-qPCR. Our results suggest that C. insignis is a weakly competent vector of EHDV-2 that can support a transmissible infection when it ingests a high virus titer (29% of midges had virus positive saliva when infected at 5.05 log₁₀PFUe/mL), but not lower virus titers. Nevertheless, due to the high density of this species, particularly in peninsular Florida, it is likely that C. insignis plays a role in the transmission of EHDV-2.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Population bottlenecks and founder effects: implications for mosquito-borne arboviral emergence

Transmission of arthropod-borne viruses (arboviruses) involves infection and replication in both arthropod vectors and vertebrate hosts. Nearly all arboviruses are RNA viruses with high mutation frequencies, which leaves them vulnerable to genetic drift and fitness losses owing to population bottlenecks during vector infection, dissemination from the midgut to the salivary glands and transmission to the vertebrate host. However, despite these bottlenecks, they seem to avoid fitness declines that can result from Muller's ratchet. In addition, founder effects that occur during the geographic introductions of human-amplified arboviruses, including chikungunya virus and Zika virus, can affect epidemic and endemic circulation, as well as virulence. In this Review, we discuss the role of genetic drift following population bottlenecks and founder effects in arboviral evolution and spread, and the emergence of human disease.

Infection, Dissemination, and Transmission Potential of North American Culex quinquefasciatus, Culex tarsalis, and Culicoides sonorensis for Oropouche Virus

Oropouche virus (OROV), a vector-borne Orthobunyavirus circulating in South and Central America, causes a febrile illness with high rates of morbidity but with no documented fatalities. Oropouche virus is transmitted by numerous vectors, including multiple genera of mosquitoes and Culicoides biting midges in South America. This study investigated the vector competence of three North American vectors, Culex tarsalis, Culex quinquefasciatus, and Culicoides sonorensis, for OROV. Cohorts of each species were fed an infectious blood meal containing 6.5 log₁₀ PFU/mL OROV and incubated for 10 or 14 days. Culex tarsalis demonstrated infection (3.13%) but not dissemination or transmission potential at 10 days post infection (DPI). At 10 and 14 DPI, Cx. quinquefasciatus demonstrated 9.71% and 19.3% infection, 2.91% and 1.23% dissemination, and 0.97% and 0.82% transmission potential, respectively. Culicoides sonorensis demonstrated 86.63% infection, 83.14% dissemination, and 19.77% transmission potential at 14 DPI. Based on these data, Cx. tarsalis is unlikely to be a competent vector for OROV. Culex quinquefasciatus demonstrated infection, dissemination, and transmission potential, although at relatively low rates. Culicoides sonorensis demonstrated high infection and dissemination but may have a salivary gland barrier to the virus. These data have implications for the spread of OROV in the event of a North American introduction.

Dose dependence of susceptibility and transmission for an Argentinean West Nile virus strain in local Culex pipiens quinquefasciatus (Diptera: Culicidae)

BACKGROUND

West Nile virus (WNV) activity has been documented in the central region of Argentina since 2005, but its maintenance network has not been elucidated yet.

METHODS

We evaluated the susceptibility to WNV oral infection and transmission in a Culex pipiens quinquefasciatus mosquito population by means of a dose-dependent vector competence assay. Mosquitoes were orally infected with five different viral loads and evaluated for viral infection, dissemination and transmission.

RESULTS

The evaluated population was susceptible to WNV oral infection. Disseminated infections were detected in all the viral doses but transmission was only detected in the higher doses (6 and 7 log10 plaque-forming units/ml). A linear relationship between doses and transmission rates could be established.

CONCLUSIONS

A minimum transmission threshold suggests that the Cu. p. quinquefasciatus from Argentina could acquire WNV infection from local urban birds and transmit the virus.

Clustered rapid induction of apoptosis limits ZIKV and DENV-2 proliferation in the midguts of Aedes aegypti

Inter-host transmission of pathogenic arboviruses such as dengue virus (DENV) and Zika virus (ZIKV) requires systemic infection of the mosquito vector. Successful systemic infection requires initial viral entry and proliferation in the midgut cells of the mosquito followed by dissemination to secondary tissues and eventual entry into salivary glands¹. Lack of arbovirus proliferation in midgut cells has been observed in several Aedes aegypti strains², but the midgut antiviral responses underlying this phenomenon are not yet fully understood. We report here that there is a rapid induction of apoptosis (RIA) in the Aedes aegypti midgut epithelium within 2 hours of infection with DENV-2 or ZIKV in both in vivo blood-feeding and ex vivo midgut infection models. Inhibition of RIA led to increased virus proliferation in the midgut, implicating RIA as an innate immune mechanism mediating midgut infection in this mosquito vector.

Ayers et al. report rapid induction of apoptosis in the midgut of Aedes aegypti mosquitoes within 2 hours of infection by dengue and Zika viruses, and find that inhibiting apoptosis led to increased virus proliferation in the midgut. These results suggest rapid induction of apoptosis as an innate immune mechanism mediating midgut infection in this mosquito vector.

The Genetic Basis for Salivary Gland Barriers to Arboviral Transmission

Arthropod-borne viruses (arboviruses) infect mosquito salivary glands and then escape to saliva prior to virus transmission. Arbovirus transmission from mosquitoes can be modulated by salivary gland infection barriers (SGIBs) and salivary gland escape barriers (SGEBs). We determined the influence of SGIBs and SGEBs by estimating the quantitative genetic contributions of Aedes aegypti half-sib families (Mapastepec, Mexico) infected with three dengue 2 (DENV2), two chikungunya (CHIKV), and two Zika (ZIKV) genotypes. We determined virus titer per salivary gland and saliva at seven days post-infection and virus prevalence in the half-sib population. CHIKV or ZIKV genotypes did not present SGIB, whereas DENV2 genotypes showed low rates of SGIB. However, virus titer and prevalence due to additive genetic factors in the half-sib family displayed a significant narrow-sense heritability (h²) for SGIB in two of the three DENV2 genotypes and one CHIKV and one ZIKV genotype. SGEBs were detected in all seven virus strains: 60-88% of DENV2 and 48-62% of CHIKV or ZIKV genotype infections. SGEB h² was significant for all CHIKV or ZIKV genotypes but not for any of the DENV2 genotypes. SGIBs and SGEBs exhibited classical gene-by-gene interaction dynamics and are influenced by genetic factors in the mosquito and the virus.

Chikungunya Virus Replication Rate Determines the Capacity of Crossing Tissue Barriers in Mosquitoes

Chikungunya virus (CHIKV) is a reemerging and rapidly spreading pathogen transmitted by mosquitoes. The emergence of new epidemic variants of the virus is associated with genetic evolutionary traits, including duplication of repeated RNA elements in the 3' untranslated region (UTR) that seemingly favor transmission by mosquitoes. The transmission potential of a given variant results from a complex interplay between virus populations and anatomical tissue barriers in the mosquito. Here, we used the wild-type CHIKV Caribbean strain and an engineered mutant harboring a deletion in the 3' UTR to dissect the interactions of virus variants with the anatomical barriers that impede transmission during the replication cycle of the virus in Aedes mosquitoes. Compared to the 3'-UTR mutant, we observed that the wild-type virus had a short extrinsic incubation period (EIP) after an infectious blood meal and was expectorated into mosquito saliva much more efficiently. We found that high viral titers in the midgut are not sufficient to escape the midgut escape barrier. Rather, viral replication kinetics play a crucial role in determining midgut escape and the transmission ability of CHIKV. Finally, competition tests in mosquitoes coinfected with wild-type and mutant viruses revealed that both viruses successfully colonized the midgut, but wild-type viruses effectively displaced mutant viruses during systemic infection due to their greater efficiency of escaping from the midgut into secondary tissues. Overall, our results uncover a link between CHIKV replication kinetics and the effect of bottlenecks on population diversity, as slowly replicating variants are less able to overcome the midgut escape barrier.IMPORTANCE It is well established that selective pressures in mosquito vectors impose population bottlenecks for arboviruses. Here, we used a CHIKV Caribbean lineage mutant carrying a deletion in the 3' UTR to study host-virus interactions in vivo in the epidemic mosquito vector Aedes aegypti We found that the mutant virus had a delayed replication rate in mosquitoes, which lengthened the extrinsic incubation period (EIP) and reduced fitness relative to the wild-type virus. As a result, the mutant virus displayed a reduced capacity to cross anatomical barriers during the infection cycle in mosquitoes, thus reducing the virus transmission rate. Our findings show how selective pressures act on CHIKV noncoding regions to select variants with shorter EIPs that are preferentially transmitted by the mosquito vector.

American Aedes japonicus japonicus, Culex pipiens pipiens, and Culex restuans mosquitoes have limited transmission capacity for a recent isolate of Usutu virus

Usutu virus (USUV; Flavivirus) has caused massive die-offs in birds across Europe since the 1950s. Although rare, severe neurologic disease in humans has been reported. USUV is genetically related to West Nile virus (WNV) and shares an ecological niche, suggesting it could spread from Europe to the Americas. USUV's risk of transmission within the United States is currently unknown. To this end, we exposed field-caught Aedes japonicus, Culex pipiens pipiens, and Culex restuans-competent vectors for WNV-to a recent European isolate of USUV. While infection rates for each species varied from 7%-21%, no dissemination or transmission was observed. These results differed from a 2018 report by Cook and colleagues, who found high dissemination rates and evidence of transmission potential using a different USUV strain, U.S. mosquito populations, temperature, and extrinsic incubation period. Future studies should evaluate the impact of these experimental conditions on USUV transmission by North American mosquitoes.

Heterogeneity of midgut cells and their differential responses to blood meal ingestion by the mosquito, Aedes aegypti

Mosquitoes are the most notorious hematophagous insects and due to their blood feeding behavior and genetic compatibility, numerous mosquito species are highly efficient vectors for certain human pathogenic parasites and viruses. The mosquito midgut is the principal organ of blood meal digestion and nutrient absorption. It is also the initial site of infection with blood meal acquired parasites and viruses. We conducted an analysis based on single-nucleus RNA sequencing (snRNA-Seq) to assess the cellular diversity of the midgut and how individual cells respond to blood meal ingestion to facilitate its digestion. Our study revealed the presence of 20 distinguishable cell-type clusters in the female midgut of Aedes aegypti. The identified cell types included intestinal stem cells (ISC), enteroblasts (EB), differentiating EB (dEB), enteroendocrine cells (EE), enterocytes (EC), EC-like cells, cardia cells, and visceral muscle (VM) cells. Blood meal ingestion dramatically changed the overall midgut cell type composition, profoundly increasing the proportions of ISC and three EC/EC-like clusters. In addition, transcriptional profiles of all cell types were strongly affected while genes involved in various metabolic processes were significantly upregulated. Our study provides a basis for further physiological and molecular studies on blood digestion, nutrient absorption, and cellular homeostasis in the mosquito midgut.

Risk of yellow fever virus transmission in the Asia-Pacific region

Historically endemic to Sub-Saharan Africa and South America, yellow fever is absent from the Asia-Pacific region. Yellow fever virus (YFV) is mainly transmitted by the anthropophilic Aedes mosquitoes whose distribution encompasses a large belt of tropical and sub tropical regions. Increasing exchanges between Africa and Asia have caused imported YFV incidents in non-endemic areas, which are threatening Asia with a new viral emergence. Here, using experimental infections of field-collected mosquitoes, we show that Asian-Pacific Aedes mosquitoes are competent vectors for YFV. We observe that Aedes aegypti populations from Singapore, Taiwan, Thailand, and New Caledonia are capable of transmitting YFV 14 days after oral infections, with a number of viral particles excreted from saliva reaching up to 23,000 viral particles. These findings represent the most comprehensive assessment of vector competence and show that Ae. aegypti mosquitoes from the Asia-Pacific region are highly competent to YFV, corroborating that vector populations are seemingly not a brake to the emergence of yellow fever in the region.

Yellow fever is absent from the Asia/Pacific region, despite presence of the mosquito vector. Here, the authors demonstrate that mosquitoes collected from field sites across the region are capable of transmitting yellow fever virus, indicating that vector competence is not a barrier to disease spread.

DNA synthesis increases during the first hours post-emergence in Anopheles albimanus mosquito midgut

In hematophagous insects, the midgut is a fundamental barrier against infections and limits the development and transmission of pathogens. However, in mosquitoes, cell differentiation, proliferation, and cell cycle process in the midgut have not been characterized. Here we provide evidence of how cell cycle progression occurs in the newly emerged Anopheles albimanus mosquito midgut and describing cyclins expression as mediators of the cell cycle. The cell cycle at different post-emergence times was evaluated in disaggregated cells from midgut tissue using flow cytometry. Also, cyclins A, B, and E were identified by bioinformatics tools. These cyclins were used to analyze cell cycle progression. Flow cytometry data and the expression-pattern of the cyclins by qRT-PCR supported a polyploidy process, besides mitosis marker was marginally detected and only in newly emerged mosquitoes. Our results suggest that DNA increment in midguts occurs by polyploidy during the first hours post-emergence.

Vector Competence for Dengue-2 Viruses Isolated from Patients with Different Disease Severity

Dynamics of dengue serotype 2 virus isolated from patients with different disease severity, namely flu-like classic dengue fever (DF) and dengue shock syndrome (DSS) were studied in its mosquito vector Aedes aegypti. We compared isolate infectivity and vector competence (VC) among thirty two A. aegypti-viral isolate pairs. Mosquito populations from high dengue incidence area exhibited overall greater VC than those from low dengue incidence area at 58.1% and 52.5%, respectively. On the other hand, the overall infection rates for the isolates ThNR2/772 (DF, 62.3%) and ThNR2/391 (DSS, 60.9%), were significantly higher than those for isolates ThNR2/406 (DF, 55.2%) and ThNR2/479 (DSS, 54.8%). These results suggest that the efficacy of dengue virus circulation was likely to vary according to the combination between the virus strains and origin of the mosquito strains, and this may have epidemiologic implications toward the incidence of flu-like classic dengue fever (DF) and dengue shock syndrome (DSS).

Releasing Intracellular NS1 from Mosquito Cells for the Detection of Dengue Virus-Infected Mosquitoes

Dengue virus (DENV), the pathogen that causes dengue fever, is mainly transmitted by Aedes aegypti. Surveillance of infected mosquitoes is a major component of integrated mosquito control methods for reducing the risk of vector-born disease outbreaks. However, a specialized rapid test for DENV detection in mosquitoes is not currently available. Utilizing immunoblotting, we found that the secretion of NS1 from both a DENV-infected mosquito cell line and mosquito bodies was below the detection threshold. However, when Triton X-100 was used to lyse infected mosquitoes, intracellular NS1 was released, and could then be effectively detected by the NS1 rapid test. The distribution of DENV NS1 in intrathoracically infected mosquitoes was different from that of orally infected mosquitoes. Next, we performed sensitivity tests by bisecting mosquitoes longitudinally; one half of each mosquito was subjected to the NS1 rapid test while the other half was used for qPCR confirmation. This modified test had a sensitivity of nearly 90% from five days post-infection onwards, while DENV had escaped from the midgut barrier. This adapted test offers a valuable, easy-to-use tool for mosquito surveillance, which is a crucial component of DENV disease control.

The Role of Temperature in Shaping Mosquito-Borne Viruses Transmission

Mosquito-borne diseases having the greatest impact on human health are typically prevalent in the tropical belt of the world. However, these diseases are conquering temperate regions, raising the question of the role of temperature on their dynamics and expansion. Temperature is one of the most significant abiotic factors affecting, in many ways, insect vectors and the pathogens they transmit. Here, we debate the veracity of this claim by synthesizing current knowledge on the effects of temperature on arboviruses and their vectors, as well as the outcome of their interactions.

A Hyperactive Kunjin Virus NS3 Helicase Mutant Demonstrates Increased Dissemination and Mortality in Mosquitoes

The unwinding of double-stranded RNA intermediates is critical for the replication and packaging of flavivirus RNA genomes. This unwinding activity is achieved by the ATP-dependent nonstructural protein 3 (NS3) helicase. In previous studies, we investigated the mechanism of energy transduction between the ATP and RNA binding pockets using molecular dynamics simulations and enzymatic characterization. Our data corroborated the hypothesis that motif V is a communication hub for this energy transduction. More specifically, mutations T407A and S411A in motif V exhibit a hyperactive helicase phenotype, leading to the regulation of translocation and unwinding during replication. However, the effect of these mutations on viral infection in cell culture and in vivo is not well understood. Here, we investigated the role of motif V in viral replication using West Nile virus (Kunjin subtype) T407A and S411A mutants (T407A and S411A Kunjin, respectively) in cell culture and in vivo We were able to recover S411A Kunjin but unable to recover T407A Kunjin. Our results indicated that S411A Kunjin decreased viral infection and increased cytopathogenicity in cell culture compared to wild-type (WT) Kunjin. Similarly, decreased infection rates in surviving S411A Kunjin-infected Culex quinquefasciatus mosquitoes were observed, but S411A Kunjin infection resulted in increased mortality compared to WT Kunjin infection. Additionally, S411A Kunjin infection increased viral dissemination and saliva positivity rates in surviving mosquitoes compared to WT Kunjin infection. These data suggest that S411A Kunjin increases viral pathogenesis in mosquitoes. Overall, these data indicate that NS3 motif V may play a role in the pathogenesis, dissemination, and transmission efficiency of Kunjin virus.IMPORTANCE Kunjin and West Nile viruses belong to the arthropod-borne flaviviruses, which can result in severe symptoms, including encephalitis, meningitis, and death. Flaviviruses have expanded into new populations and emerged as novel pathogens repeatedly in recent years, demonstrating that they remain a global threat. Currently, there are no approved antiviral therapeutics against either Kunjin or West Nile viruses. Thus, there is a pressing need for understanding the pathogenesis of these viruses in humans. In this study, we investigated the role of the Kunjin virus helicase on infection in cell culture and in vivo This work provides new insight into how flaviviruses control pathogenesis and mosquito transmission through the nonstructural protein 3 helicase.

Transcriptome of the Aedes aegypti Mosquito in Response to Human Complement Proteins

Aedes aegypti is the primary mosquito vector of several human arboviruses, including the dengue virus (DENV). Vector control is the principal intervention to decrease the transmission of these viruses. The characterization of molecules involved in the mosquito physiological responses to blood-feeding may help identify novel targets useful in designing effective control strategies. In this study, we evaluated the in vivo effect of feeding adult female mosquitoes with human red blood cells reconstituted with either heat-inactivated (IB) or normal plasma (NB). The RNA-seq based transcript expression of IB and NB mosquitoes was compared against sugar-fed (SF) mosquitoes. In in vitro experiments, we treated Aag2 cells with a recombinant version of complement proteins (hC3 or hC5a) and compared transcript expression to untreated control cells after 24 h. The transcript expression analysis revealed that human complement proteins modulate approximately 2300 transcripts involved in multiple biological functions, including immunity. We also found 161 upregulated and 168 downregulated transcripts differentially expressed when human complement protein C3 (hC3) and human complement protein C5a (hC5a) treated cells were compared to the control untreated cells. We conclude that active human complement induces significant changes to the transcriptome of Ae. aegypti mosquitoes, which may influence the physiology of these arthropods.

Nora Virus VP4b and ORF1 Circulate in Hemolymph of Infected D. melanogaster with Coordinate Expression of Vago and Vir-1

Study of the novel RNA virus, Nora virus, which is a persistent, picorna-like virus that replicates in the gut of Drosophila melanogaster offers insight into human innate immunity and other picorna-like viruses. Nora virus infection leads to a locomotor abnormality and upregulation of two candidate target proteins, Vago and Virus-induced RNA 1 (Vir-1). These proteins are uncharacterized in response to Nora virus. We hypothesize that Nora virus is circulating in the hemolymph of Nora virus-infected D. melanogaster, allowing for migration beyond the primary site of replication in the gut. Analysis by qRT-PCR demonstrated biphasic viral load and corresponding vago and vir-1 transcription levels, suggesting transcription of vago and vir-1 occurs in response to viral infection. However, Vir-1 is also present in virus-free D. melanogaster suggesting basal expression or alternative functions. Presence of Nora virus RNA and the Viral Protein 4b (VP4b), in hemolymph of infected D. melanogaster supports the hypothesized circulation of Nora virus in the hemolymph. The study suggests that impaired locomotor function may be due to transport of Nora virus from the gut to the brain via the hemolymph.

Understanding the Mechanisms Underlying Host Restriction of Insect-Specific Viruses

Arthropod-borne viruses contribute significantly to global mortality and morbidity in humans and animals. These viruses are mainly transmitted between susceptible vertebrate hosts by hematophagous arthropod vectors, especially mosquitoes. Recently, there has been substantial attention for a novel group of viruses, referred to as insect-specific viruses (ISVs) which are exclusively maintained in mosquito populations. Recent discoveries of novel insect-specific viruses over the past years generated a great interest not only in their potential use as vaccine and diagnostic platforms but also as novel biological control agents due to their ability to modulate arbovirus transmission. While arboviruses infect both vertebrate and invertebrate hosts, the replication of insect-specific viruses is restricted in vertebrates at multiple stages of virus replication. The vertebrate restriction factors include the genetic elements of ISVs (structural and non-structural genes and the untranslated terminal regions), vertebrate host factors (agonists and antagonists), and the temperature-dependent microenvironment. A better understanding of these bottlenecks is thus warranted. In this review, we explore these factors and the complex interplay between ISVs and their hosts contributing to this host restriction phenomenon.

Detailed Analyses of Zika Virus Tropism in Culex quinquefasciatus Reveal Systemic Refractoriness

The role of Culex quinquefasciatus in Zika virus transmission has been debated since the epidemic of Zika occurred in the Americas in 2015 to 2016. The majority of studies have found no evidence that C. quinquefasciatus or other Culex species are competent vectors of Zika virus, and the few studies that have proposed Zika vector status for C. quinquefasciatus have relied predominantly on quantitative real-time PCR (qRT-PCR) for viral detection. We assessed the infectious range of pre- and post-epidemic Zika virus isolates in order to classify mosquito samples based on titer infectiousness and demonstrated that two strains of C. quinquefasciatus, including one previously found to be competent, are highly resistant to infection with these Zika isolates compared to Aedes aegypti and are not competent for virus transmission. Further dissection of the dynamics of Zika exposure in both A. aegypti and C. quinquefasciatus revealed that while virus transmission by C. quinquefasciatus is blocked at the levels of the midgut and salivary glands, viral RNA persists in these tissues for prolonged periods post-exposure. We assessed Zika entry dynamics in both Aedes and Culex cells, and our results suggest that Zika virus infection in Culex cells may be blocked downstream of cell entry. These findings strongly suggest that C. quinquefasciatus is not a vector of Zika virus and additionally inform the use of qRT-PCR in vector competence assays as well as our understanding of barriers to arbovirus infection in non-susceptible mosquito species.IMPORTANCE Understanding which mosquito species transmit an emerging arbovirus is critical to effective vector control. During the Zika virus epidemic in 2015 to 2016, Aedes mosquitoes were confirmed as vectors. However, studies addressing the vector status of Culex quinquefasciatus mosquitoes presented conflicting evidence and remain an outstanding source of confusion in the field. Here, we established a robust cell-based assay to identify infectious titers of Zika virus and assessed the virus titers in C. quinquefasciatus by quantitative real-time PCR (qRT-PCR). We found that while low levels of virus were detected in C. quinquefasciatus, these titers did not correspond to infectious virus, and these mosquitoes did not transmit virus in the saliva. We also present evidence that the virus may enter Culex cells before infection is disrupted. Our findings are important for future studies incriminating vector species using qRT-PCR for virus detection and offer new information on how virus transmission is blocked by mosquitoes.

Contrasted transmission efficiency of Zika virus strains by mosquito species Aedes aegypti, Aedes albopictus and Culex quinquefasciatus from Reunion Island

BACKGROUND

Zika virus (ZIKV) is a mosquito-borne flavivirus that recently emerged in the South Pacific islands and Americas where unprecedented outbreaks were reported. Although Aedes aegypti is considered to be the main vector for ZIKV, other mosquito species have been shown to be potential vectors and differences in vector competence with respect to mosquito strain and ZIKV strain have been demonstrated. In this study we compared the vector competence of three mosquito species Aedes albopictus, Ae. aegypti and Culex quinquefasciatus from Reunion Island for three ZIKV strains.

METHODS

Five mosquito strains (2 strains of Ae. albopictus, 1 of Ae. aegypti and 2 of Cx. quinquefasciatus) were exposed to three ZIKV strains: one African strain (Dak84) and two Asian strains (PaRi_2015 and MAS66). The vector competence parameters (infection rate, dissemination efficiency and transmission efficiency) and viral loads were examined at 14 and 21 days post-infection.

RESULTS

The two Cx. quinquefasciatus strains did not become infected and were therefore unable to either disseminate or transmit any of the three ZIKV strains. Aedes albopictus and Ae. aegypti strains were poorly competent for the two Asian ZIKV strains, while both mosquito species displayed higher infection rates, dissemination and transmission efficiencies for the African ZIKV Dak84 strain. However, this African ZIKV strain was better transmitted by Ae. aegypti as compared to Ae. albopictus.

CONCLUSIONS

Our results show that both Ae. albopictus and Ae. aegypti, from Reunion Island, are more likely to be competent for ZIKV in contrast to Cx. quinquefasciatus which appeared refractory to all tested ZIKV strains. This improves our understanding of the role of mosquito species in the risk of the ZIKV emergence on Reunion Island.

Vertical Transmission of Zika Virus in Aedes aegypti Produces Potentially Infectious Progeny

Vertical transmission, or pathogen transfer from female to offspring, can facilitate the persistence of emerging arboviruses, such as Zika virus (ZIKV), through periods of low horizontal transmission or adverse environmental conditions. We aimed at determining the rate of vertical transmission for ZIKV in its principal vector, Aedes aegypti, and the vector competence of vertically infected progeny. Aedes aegypti females that consumed a blood meal provisioned with ZIKV were maintained under three temperature conditions (27°C, 30°C, and 33°C) following the infectious blood meal and allowed to complete three reproductive cycles. The overall vertical transmission rate was 6.5% (95% CI = 3.9-9.9). Vertical transmission of ZIKV was observed across all temperature conditions and virus detected in adult progeny up to 2 weeks postemergence. In total, 3.4% (95% CI = 1.6-6.2) of adult progeny produced saliva with ZIKV, indicating their vector competence. These results suggest the virus may be maintained in Ae. aegypti populations without a vertebrate host for short periods.

Arthropod exosomes as bubbles with message(s) to transmit vector-borne diseases

Ticks and mosquitoes are medically important vectors that transmit several pathogens, including arboviruses, to humans. Understanding how these blood-feeding arthropods transmit pathogens to humans requires knowledge on the molecular and cellular interplay at vector-host interface. Recent studies have highlighted the role of tick and mosquito small extracellular vesicles (EVs), including exosomes, facilitating arbovirus transmission within arthropod cells and from arthropod to mammalian cells. In this review, we summarize this emerging line of investigation in understanding the role of tick and mosquito exosomes in vector-pathogen-host tripartite interactions. Understanding the role of arthropod exosomes in pathogen interactions could lead to the discovery of novel therapeutic targets to interfere with the life cycle of several pathogens transmitted by vectors.

Experimental study of dengue virus infection in Aedes aegypti and Aedes albopictus: A comparative analysis on susceptibility, virus transmission and reproductive success

Epidemiology of dengue fever has substantially changed over the years with respect to prevalent strains, affected geographical locations and severity of disease. Mosquito vectors show variable response in terms of susceptibility to four different serotypes of dengue virus. Although studies have postulated that, the vectors Ae. aegypti and Ae. albopictus are crucial for transmission of dengue virus, comparative efficacy of these species for viral transmission and tolerance is still enigmatic. In this study, these two vectors were infected orally with four serotypes of the dengue virus viz. DENV-1 to DENV-4 and their co-infection. It was observed that Ae. aegypti harbors multiple serotype infections more efficiently than Ae. albopictus. We suggest that transovarial transmission is of low importance in the epidemiology of the virus due to low infection rates in the filial generation, and also that reduced fecundity and fertility in both vectors after dengue virus infection affect the ecology of the pathogen.

Impact of alphavirus 3'UTR plasticity on mosquito transmission

Alphaviruses such as chikungunya and western equine encephalitis viruses are important human pathogens transmitted by mosquitoes that have recently caused large epidemic and epizootic outbreaks. The epidemic potential of alphaviruses is often related to enhanced mosquito transmission. Tissue barriers and antiviral responses impose bottlenecks to viral populations in mosquitoes. Substitutions in the envelope proteins and the presence of repeated sequence elements (RSEs) in the 3'UTR of epidemic viruses were proposed to be specifically associated to efficient replication in mosquito vectors. Here, we discuss the molecular mechanisms that originated RSEs, the evolutionary forces that shape the 3'UTR of alphaviruses, and the significance of RSEs for mosquito transmission. Finally, the presence of RSEs in the 3'UTR of viral genomes appears as evolutionary trait associated to mosquito adaptation and emerges as a common feature among viruses from the alphavirus and flavivirus genera.

AN IMMUNO-EPIDEMIOLOGICAL VECTOR–HOST MODEL WITH WITHIN-VECTOR VIRAL KINETICS

A current challenge for disease modeling and public health is understanding pathogen dynamics across scales since their ecology and evolution ultimately operate on several coupled scales. This is particularly true for vector-borne diseases, where within-vector, within-host, and between vector–host populations all play crucial roles in diversity and distribution of the pathogen. Despite recent modeling efforts to determine the effect of within-host virus-immune response dynamics on between-host transmission, the role of within-vector viral dynamics on disease spread is overlooked. Here, we formulate an age-since-infection-structured epidemic model coupled to nonlinear ordinary differential equations describing within-host immune-virus dynamics and within-vector viral kinetics, with feedbacks across these scales. We first define the within-host viral-immune response and within-vector viral kinetics-dependent basic reproduction number [Formula: see text] Then we prove that whenever [Formula: see text] the disease-free equilibrium is locally asymptotically stable, and under certain biologically interpretable conditions, globally asymptotically stable. Otherwise, if [Formula: see text] it is unstable and the system has a unique positive endemic equilibrium. In the special case of constant vector to host inoculum size, we show the positive equilibrium is locally asymptotically stable and the disease is weakly uniformly persistent. Furthermore, numerical results suggest that within-vector-viral kinetics and dynamic inoculum size may play a substantial role in epidemics. Finally, we address how the model can be utilized to better predict the success of control strategies such as vaccination and drug treatment.

Temperature Dramatically Shapes Mosquito Gene Expression With Consequences for Mosquito-Zika Virus Interactions

Vector-borne flaviviruses are emerging threats to human health. For successful transmission, the virus needs to efficiently enter mosquito cells and replicate within and escape several tissue barriers while mosquitoes elicit major transcriptional responses to flavivirus infection. This process will be affected not only by the specific mosquito-pathogen pairing but also by variation in key environmental variables such as temperature. Thus far, few studies have examined the molecular responses triggered by temperature and how these responses modify infection outcomes, despite substantial evidence showing strong relationships between temperature and transmission in a diversity of systems. To define the host transcriptional changes associated with temperature variation during the early infection process, we compared the transcriptome of mosquito midgut samples from mosquitoes exposed to Zika virus (ZIKV) and non-exposed mosquitoes housed at three different temperatures (20, 28, and 36°C). While the high-temperature samples did not show significant changes from those with standard rearing conditions (28°C) 48 h post-exposure, the transcriptome profile of mosquitoes housed at 20°C was dramatically different. The expression of genes most altered by the cooler temperature involved aspects of blood-meal digestion, ROS metabolism, and mosquito innate immunity. Further, we did not find significant differences in the viral RNA copy number between 24 and 48 h post-exposure at 20°C, suggesting that ZIKV replication is limited by cold-induced changes to the mosquito midgut environment. In ZIKV-exposed mosquitoes, vitellogenin, a lipid carrier protein, was most up-regulated at 20°C. Our results provide a deeper understanding of the temperature-triggered transcriptional changes in Aedes aegypti and can be used to further define the molecular mechanisms driven by environmental temperature variation.

Dynamic proteomic analysis of Aedes aegypti Aag-2 cells infected with Mayaro virus

Background

Mayaro virus (MAYV) is responsible for a mosquito-borne tropical disease with clinical symptoms similar to dengue or chikungunya virus fevers. In addition to the recent territorial expansion of MAYV, this virus may be responsible for an increasing number of outbreaks. Currently, no vaccine is available. Aedes aegypti is promiscuous in its viral transmission and thus an interesting model to understand MAYV-vector interactions. While the life-cycle of MAYV is known, the mechanisms by which this arbovirus affects mosquito host cells are not clearly understood.

Methods

After defining the best conditions for cell culture harvesting using the highest virus titer, Ae. aegypti Aag-2 cells were infected with a Brazilian MAYV isolate at a MOI of 1 in order to perform a comparative proteomic analysis of MAYV-infected Aag-2 cells by using a label-free semi-quantitative bottom-up proteomic analysis. Time-course analyses were performed at 12 and 48 h post-infection (hpi). After spectrum alignment between the triplicates of each time point and changes of the relative abundance level calculation, the identified proteins were annotated and using Gene Ontology database and protein pathways were annotated using the Kyoto Encyclopedia of Genes and Genomes.

Results

After three reproducible biological replicates, the total proteome analysis allowed for the identification of 5330 peptides and the mapping of 459, 376 and 251 protein groups, at time 0, 12 hpi and 48 hpi, respectively. A total of 161 mosquito proteins were found to be differentially abundant during the time-course, mostly related to host cell processes, including redox metabolism, translation, energy metabolism, and host cell defense. MAYV infection also increased host protein expression implicated in viral replication.

Conclusions

To our knowledge, this first proteomic time-course analysis of MAYV-infected mosquito cells sheds light on the molecular basis of the viral infection process and host cell response during the first 48 hpi. Our data highlight several mosquito proteins modulated by the virus, revealing that MAYV manipulates mosquito cell metabolism for its propagation.

Administration of Defective Virus Inhibits Dengue Transmission into Mosquitoes

The host-vector shuttle and the bottleneck in dengue transmission is a significant aspect with regard to the study of dengue outbreaks. As mosquitoes require 100–1000 times more virus to become infected than human, the transmission of dengue virus from human to mosquito is a vulnerability that can be targeted to improve disease control. In order to capture the heterogeneity in the infectiousness of an infected patient population towards the mosquito population, we calibrate a population of host-to-vector virus transmission models based on an experimentally quantified infected fraction of a mosquito population. Once the population of models is well-calibrated, we deploy a population of controls that helps to inhibit the human-to-mosquito transmission of the dengue virus indirectly by reducing the viral load in the patient body fluid. We use an optimal bang-bang control on the administration of the defective virus (transmissible interfering particles (TIPs)) to symptomatic patients in the course of their febrile period and observe the dynamics in successful reduction of dengue spread into mosquitoes.

Flavivirus Envelope Protein Glycosylation: Impacts on Viral Infection and Pathogenesis

Flaviviruses encode one, two, or no N-linked glycosylation sites on their envelope proteins. Glycosylation can impact virus interactions with cell surface attachment factors and also may impact virion stability and virus replication. Envelope protein glycosylation has been identified as a virulence determinant for multiple flaviviruses, but the mechanisms by which glycosylation mediates pathogenesis remain unclear. In this Gem, we summarize current knowledge on flavivirus envelope protein glycosylation and its impact on viral infection and pathogenesis.

Vector competence of Aedes aegypti from New Caledonia for the four recent circulating dengue virus serotypes

In New Caledonia (NC), Aedes aegypti is the only proven vector of dengue virus (DENV), which is the most prevalent arbovirosis in NC. Since World War II, the four DENV serotypes have circulated regularly in NC. The epidemiological profile, however, has evolved over the last ten years, with the persistence of DENV-1 circulation and the co-circulation of several DENV serotypes. The current study evaluated the ability of Ae. aegypti from NC to transmit four DENV serotypes (and two DENV-1 genotypes) isolated during recent outbreaks in NC. An Ae. aegypti F1 generation was twice independently orally challenged with each DENV strain (10⁷ FFU/ml). Infection, dissemination and transmission rates and transmission efficiency were measured at day 7 and 14 post-exposure, as well as the quantity of infectious virus particles. Mosquito infection was observed as early as 7 days post-infection. Infection rates between 18 and 58% were measured for all DENV serotypes/genotypes tested. Although dissemination rates ranged from 78 to 100%, transmission efficiencies were low, with values not exceeding 21% at 14 days post-infection for all DENV strains. This study shows that NC Ae. aegypti are moderately competent for DENV in laboratory conditions. In link with epidemiological data, these results suggest implication of other factors in the sustained circulation of DENV-1 in New Caledonia.

Aedes aegypti is the only known vector for dengue virus (DENV) in New Caledonia (NC). DENV are divided into four serotypes (DENV-1 to -4), based on their antigenic properties, these being subdivided into different genotypes. All four DENV serotypes have regularly circulated in New Caledonia in the past. An unusual persistence of DENV-1 has been observed during the last ten years, suggesting a possible preferential transmission of this DENV serotype by Ae. aegypti. In this study, we examine for the first time the ability of Ae. aegypti from New Caledonia to transmit the four circulating DENV serotypes, and more precisely two genotypes of DENV-1. Our results demonstrate that this vector is moderately competent for DENV, with slight differences observed between DENV serotype/genotype in terms of transmission. These findings suggest that other factors are at play in the sustained circulation of DENV-1 and that further vector-virus interactions studies should be undertaken to better understand the DENV epidemiological profile in NC.

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.

Risk of arbovirus emergence via bridge vectors: case study of the sylvatic mosquito Aedes malayensis in the Nakai district, Laos

Many emerging arboviruses of global public health importance, such as dengue virus (DENV) and yellow fever virus (YFV), originated in sylvatic transmission cycles involving wild animals and forest-dwelling mosquitoes. Arbovirus emergence in the human population typically results from spillover transmission via bridge vectors, which are competent mosquitoes feeding on both humans and wild animals. Another related, but less studied concern, is the risk of 'spillback' transmission from humans into novel sylvatic cycles. We colonized a sylvatic population of Aedes malayensis from a forested area of the Nakai district in Laos to evaluate its potential as an arbovirus bridge vector. We found that this Ae. malayensis population was overall less competent for DENV and YFV than an urban population of Aedes aegypti. Olfactometer experiments showed that our Ae. malayensis colony did not display any detectable attraction to human scent in laboratory conditions. The relatively modest vector competence for DENV and YFV, combined with a lack of detectable attraction to human odor, indicate a low potential for this sylvatic Ae. malayensis population to act as an arbovirus bridge vector. However, we caution that opportunistic blood feeding on humans by sylvatic Ae. malayensis may occasionally contribute to bridge sylvatic and human transmission cycles.

Zika virus noncoding RNA suppresses apoptosis and is required for virus transmission by mosquitoes

Flaviviruses, including Zika virus (ZIKV), utilise host mRNA degradation machinery to produce subgenomic flaviviral RNA (sfRNA). In mammalian hosts, this noncoding RNA facilitates replication and pathogenesis of flaviviruses by inhibiting IFN-signalling, whereas the function of sfRNA in mosquitoes remains largely elusive. Herein, we conduct a series of in vitro and in vivo experiments to define the role of ZIKV sfRNA in infected Aedes aegypti employing viruses deficient in production of sfRNA. We show that sfRNA-deficient viruses have reduced ability to disseminate and reach saliva, thus implicating the role for sfRNA in productive infection and transmission. We also demonstrate that production of sfRNA alters the expression of mosquito genes related to cell death pathways, and prevents apoptosis in mosquito tissues. Inhibition of apoptosis restored replication and transmission of sfRNA-deficient mutants. Hence, we propose anti-apoptotic activity of sfRNA as the mechanism defining its role in ZIKV transmission.

The function on subgenomic flaviviral RNA (sfRNA) in the mosquito vector is not well understood. Here, Slonchak et al. show that sfRNA affects virus-induced apoptosis and dissemination of ZIKV in Aedes aegypti mosquitoes, suggesting a role of sfRNA in Zika virus replication and transmission.

Vector competence of Virginia mosquitoes for Zika and Cache Valley viruses

Background

Vector-borne diseases are a major public health concern and cause significant morbidity and mortality. Zika virus (ZIKV) is the etiologic agent of a massive outbreak in the Americas that originated in Brazil in 2015 and shows a strong association with congenital ZIKV syndrome in newborns. Cache Valley virus (CVV) is a bunyavirus that causes mild to severe illness in humans and ruminants. In this study, we investigated the vector competence of Virginia mosquitoes for ZIKV and CVV to explore their abilities to contribute to potential outbreaks.

Methods

To determine vector competence, mosquitoes were fed a blood meal comprised of defibrinated sheep blood and virus. The presence of midgut or salivary gland barriers to ZIKV infection were determined by intrathoracic inoculation vs oral infection. After 14-days post-exposure, individual mosquitoes were separated into bodies, legs and wings, and saliva expectorant. Virus presence was detected by plaque assay to determine midgut infection, dissemination, and transmission rates.

Results

Transmission rates for Ae. albopictus orally infected (24%) and intrathoracically inoculated (63%) with ZIKV was similar to Ae. aegypti (48% and 71%, respectively). Transmission rates of ZIKV in Ae. japonicus were low, and showed evidence of a midgut infection barrier demonstrated by low midgut infection and dissemination rates from oral infection (3%), but increased transmission rates after intrathoracic inoculation (19%). Aedes triseriatus was unable to transmit ZIKV following oral infection or intrathoracic inoculation. CVV transmission was dose-dependent where mosquitoes fed high titer (ht) virus blood meals developed higher rates of midgut infection, dissemination, and transmission compared to low titer (lt) virus blood meals. CVV was detected in the saliva of Ae. albopictus (ht: 68%, lt: 24%), Ae. triseriatus (ht: 52%, lt: 7%), Ae. japonicus (ht: 22%, lt: 0%) and Ae. aegypti (ht: 10%; lt: 7%). Culex pipiens and Cx. restuans were not competent for ZIKV or CVV.

Conclusions

This laboratory transmission study provided further understanding of potential ZIKV and CVV transmission cycles with Aedes mosquitoes from Virginia. The ability for these mosquitoes to transmit ZIKV and CVV make them a public health concern and suggest targeted control programs by mosquito and vector abatement districts.

Confocal Analysis of the Distribution and Persistence of Sindbis Virus (TaV-GFP) Infection in Midguts of Aedes aegypti Mosquitoes

Biological transmission of arthropod-borne viruses (arboviruses) to vertebrate hosts by hematophagous insects poses a global threat because such arboviruses can result in a range of serious public health infectious diseases. Sindbis virus (SINV), the prototype Alphavirus, was used to track infections in the posterior midgut (PMG) of Aedes aegypti adult mosquitoes. Females were fed viremic blood containing a virus reporter, SINV [Thosea asigna virus-green fluorescent protein (TaV-GFP)], that leaves a fluorescent signal in infected cells. We assessed whole-mount PMGs to identify primary foci, secondary target tissues, distribution, and virus persistence. Following a viremic blood meal, PMGs were dissected and analyzed at various days of post blood-feeding. We report that virus foci indicated by GFP in midgut epithelial cells resulted in a 9.8% PMG infection and a 10.8% dissemination from these infected guts. The number of virus foci ranged from 1 to 3 per individual PMG and was more prevalent in the PMG-middle > PMG-frontal > PMG-caudal regions. SINV TaV-GFP was first observed in the PMG (primary target tissue) at 3 days post blood-feeding, was sequestered in circumscribed foci, replicated in PMG peristaltic muscles (secondary target tissue) following dissemination, and GFP was observed to persist in PMGs for 30 days postinfection.

Comparison of vector competence of Aedes vexans Green River and Culex pipiens biotype pipiens for West Nile virus lineages 1 and 2

West Nile virus (WNV), a zoonotic arbovirus, has recently established an autochthonous transmission cycle in Germany. In dead-end hosts like humans and horses the WNV infection may cause severe symptoms in the central nervous system. In nature, WNV is maintained in an enzootic transmission cycle between birds and ornithophilic mosquitoes. Bridge vector species, such as members of the Culex pipiens complex and Aedes spp., also widely distributed in Germany, might transmit WNV to other vertebrate host species. This study determined and compared the vector competence of field-collected northern-German Cx. pipiens biotype pipiens and laboratory-reared Ae. vexans Green River (GR) for WNV lineage 1 (strain: Magpie/Italy/203204) and WNV lineage 2 (strain: "Austria") under temperatures typical for northern Germany in spring/summer and autumn. For assessment of vector competence, 7- to 14-day-old female mosquitoes were offered a WNV containing blood meal via Hemotek membrane feeding system or cotton-stick feeding. After incubation at 18°C respectively 24°C for 14 days engorged female mosquitoes were salivated and dissected for determination of infection, dissemination and transmission rates by reverse transcriptase quantitative real-time PCR (RT-qPCR). Both Ae. vexans GR and Cx. pipiens biotype pipiens were infected with both tested WNV strains and tested 14 days post-inoculation. Disseminated infections were detected only in Ae. vexans GR incubated at 18°C and in Cx. pipiens pipiens incubated at 24°C after infection with WNV lineage 1. Transmission of WNV lineage 1 was detected in Cx. pipiens pipiens incubated at 24°C. These results indicate that Cx. pipiens pipiens from Northern Germany may be involved in the transmission of WNV, also to dead-end hosts like humans and horses.

Impact of temperature on the extrinsic incubation period of Zika virus in Aedes aegypti

Since Zika virus (ZIKV) emerged as a global human health threat, numerous studies have pointed to Aedes aegypti as the primary vector due to its high competence and propensity to feed on humans. The majority of vector competence studies have been conducted between 26-28°C, but arboviral extrinsic incubation periods (EIPs), and therefore transmission efficiency, are known to be affected strongly by temperature. To better understand the relationship between ZIKV EIPs and temperature, we evaluated the effect of adult mosquito exposure temperature on ZIKV infection, dissemination, and transmission in Ae. aegypti at four temperatures: 18°C, 21°C, 26°C, and 30°C. Mosquitoes were exposed to viremic mice infected with a 2015 Puerto Rican ZIKV strain, and engorged mosquitoes were sorted into the four temperatures with 80% RH and constant access to 10% sucrose. ZIKV infection, dissemination, and transmission rates were assessed via RT-qPCR from individual mosquito bodies, legs and wings, and saliva, respectively, at three to five time points per temperature from three to 31 days, based on expectations from other flavivirus EIPs. The median time from ZIKV ingestion to transmission (median EIP, EIP50) at each temperature was estimated by fitting a generalized linear mixed model for each temperature. EIP50 ranged from 5.1 days at 30°C to 24.2 days at 21°C. At 26°C, EIP50 was 9.6 days. At 18°C, only 15% transmitted by day 31 so EIP50 could not be estimated. This is among the first studies to characterize the effects of temperature on ZIKV EIP in Ae. aegypti, and the first to do so based on feeding of mosquitoes on a live, viremic host. This information is critical for modeling ZIKV transmission dynamics to understand geographic and seasonal limits of ZIKV risk; it is especially relevant for determining risk in subtropical regions with established Ae. aegypti populations and relatively high rates of return travel from the tropics (e.g. California or Florida), as these regions typically experience cooler temperature ranges than tropical regions.

Extracellular vesicles restrict dengue virus fusion in Aedes aegypti cells

Aedes aegypti is the primary vector of dengue virus (DENV), and acquires this virus from a vertebrate host during blood feeding. Previous literature has shown that vertebrate blood factors such as complement protein C5a and low-density lipoprotein (LDL) influence DENV acquisition in the mosquito. Here, we show that extracellular vesicles in cell culture medium inhibit DENV infection in mosquito cells. Specifically, extracellular vesicles enter into mosquito cells and inhibit an early stage of infection. Extracellular vesicles had no effect on virus cell attachment or entry. Instead, extracellular vesicles restricted virus membrane fusion. Extracellular vesicles only inhibited DENV infection in mosquito cells and not vertebrate cells. These data highlight a novel virus-vector-host interaction that limits virus infection in mosquito cells by restricting virus membrane fusion.

Aedes aegypti (Diptera: Culicidae) Immune Responses with Different Feeding Regimes Following Infection by the Entomopathogenic Fungus Metarhizium anisopliae

The mosquito Aedes aegypti is the most notorious vector of illness-causing viruses. The use of entomopathogenic fungi as bioinsecticides is a promising alternative for the development of novel mosquito control strategies. We investigate whether differences in immune responses could be responsible for modifications in survival rates of insects following different feeding regimes. Sucrose and blood-fed adult A. aegypti females were sprayed with M. anisopliae 1 × 106 conidia mL-1, and after 48 h, the midgut and fat body were dissected. We used RT-qPCR to monitor the expression of Cactus and REL1 (Toll pathway), IMD, REL2, and Caspar (IMD pathway), STAT and PIAS (JAK-STAT pathway), as well as the expression of antimicrobial peptides (Defensin A, Attacin and Cecropin G). REL1 and REL2 expression in both the midgut and fat body were higher in blood-fed fungus-challenged A. aegypti than in sucrose-fed counterparts. Interestingly, infection of sucrose-fed insects induced Cactus expression in the fat body, a negative regulator of the Toll pathway. The IMD gene was upregulated in the fat body in response to fungal infection after a blood meal. Additionally, we observed the induction of antimicrobial peptides in the blood-fed fungus-challenged insects. This study suggests that blood-fed A. aegypti are less susceptible to fungal infection due to the rapid induction of Toll and IMD immune pathways.

Insecticide resistance genes affect Culex quinquefasciatus vector competence for West Nile virus

Insecticide resistance has been reported to impact the interactions between mosquitoes and the pathogens they transmit. However, the effect on vector competence for arboviruses still remained to be investigated. We examined the influence of two insecticide resistance mechanisms on vector competence of the mosquito Culex quinquefasciatus for two arboviruses, Rift Valley Fever virus (RVFV) and West Nile virus (WNV). Three Cx. quinquefasciatus lines sharing a common genetic background were used: two insecticide-resistant lines, one homozygous for amplification of the Ester² locus (SA2), the other homozygous for the acetylcholinesterase ace-1 G119S mutation (SR) and the insecticide-susceptible reference line Slab. Statistical analyses revealed no significant effect of insecticide-resistant mechanisms on vector competence for RVFV. However, both insecticide resistance mechanisms significantly influenced the outcome of WNV infections by increasing the dissemination of WNV in the mosquito body, therefore leading to an increase in transmission efficiency by resistant mosquitoes. These results showed that insecticide resistance mechanisms enhanced vector competence for WNV and may have a significant impact on transmission dynamics of arboviruses. Our findings highlight the importance of understanding the impacts of insecticide resistance on the vectorial capacity parameters to assess the overall consequence on transmission.

Low vector competence in sylvatic mosquitoes limits Zika virus to initiate an enzootic cycle in South America

Zika virus (ZIKV) has spread in the Americas since 2015 and the potential establishment of a sylvatic transmission cycle in the continent has been hypothesized. We evaluated vector competence of five sylvatic Neotropical mosquito species to two ZIKV isolates. Distinct batches of Haemagogus leucoceleanus, Sabethes albiprivus, Sabethes identicus, Aedes terrens and Aedes scapularis females were respectively orally challenged and inoculated intrathoracically with ZIKV. Orally challenged mosquitoes were refractory or exhibited low infection rates. Viral dissemination was detected only in Hg. leucocelaenus, but with very low rates. Virus was not detected in saliva of any mosquito orally challenged with ZIKV, regardless of viral isolate and incubation time. When intrathoracically injected, ZIKV disseminated in high rates in Hg. leucocelaenus, Sa. identicus and Sa. albpiprivus, but low transmission was detected in these species; very low dissemination and no transmission was detected in Ae. terrens and Ae. scapularis. Together these results suggest that genetically determined tissue barriers, especially in the midgut, play a vital role in inhibiting ZIKV for transmission in the tested sylvatic mosquito species. Thus, an independent enzootic transmission cycle for ZIKV in South America is very unlikely.

Successive blood meals enhance virus dissemination within mosquitoes and increase transmission potential

The recent Zika virus (ZIKV) and chikungunya virus (CHIKV) epidemics highlight the explosive nature of arthropod-borne (arbo)viruses transmitted by Aedes spp. mosquitoes^1,2. Vector competence and the extrinsic incubation period (EIP) are two key entomological parameters used to assess the public health risk posed by arboviruses³. These are typically measured empirically by offering mosquitoes an infectious bloodmeal and temporally sampling mosquitoes to determine infection and transmission status. This approach has been used for the better part of a century; however, it does not accurately capture the biology and behavior of many mosquito vectors which refeed frequently (every 2–3 days)⁴. Here we demonstrate that acquisition of a second non-infectious bloodmeal significantly shortens the EIP of ZIKV-infected Ae. aegypti by enhancing virus dissemination from the mosquito midgut. Similarly, a second bloodmeal increases the competence of this species for dengue virus and CHIKV as well as Ae. albopictus for ZIKV, suggesting that this phenomenon may be common among other virus-vector pairings and that Ae. albopictus might be a more important vector than once thought. Bloodmeal-induced microperforations in the virus-impenetrable basal lamina which surrounds the midgut provide a mechanism for enhanced virus escape. Modeling of these findings reveals that a shortened EIP would result in a significant increase in the basic reproductive number, R0, estimated from experimental data. This helps explain how Ae. aegypti can sustain explosive epidemics like ZIKV despite relatively poor vector competence in single-feed laboratory trials. Together, these data demonstrate a direct and unrecognized link between mosquito feeding behavior, EIP, and vector competence.

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.

Use of MALDI-TOF MS to identify the culturable midgut microbiota of laboratory and wild mosquitoes

Mosquitoes are responsible for transmitting many pathogens to humans and Aedes aegypti, Aedes albopictus and Culex quinquefasciatus are important vectors in the world. The microbiota plays an important role in developmental studies that involve impacts on the biological cycle of mosquitoes and vector control strategies. In this study, the aim was to understand the environment plays in the microbiota culturable diversity of Aedes aegytpi, Aedes albopictus and Culex quinquefasciatus. Midgut of studied mosquitoes (laboratory-reared and wild) were dissected and analyzed by MALDI-TOF MS to identify the microbiota. Most of the bacteria identified in the microbiota of mosquitoes from the laboratory and field belong to the phylum Proteobacteria. We reported on the microbial diversity among the mosquito species studied where Cx. quinquefasciatus and Ae. albopictus show greater bacterial similarity. The genus Rahnella was present in all mosquito species studied, both in those from the laboratory and those from the wild. Bacillus, Ewingella, Microccocus, Klebsiella and Pantoea are genera was predominant among the mosquitoes studied. The difference of microbiota diversity between mosquitoes laboratory-reared and wild shows that the environment plays an important role in the acquisition of bacteria, mainly in Ae. aegypti and Cx. quinquefasciatus.

Estimating the risk of arbovirus transmission in Southern Europe using vector competence data

Arboviral diseases such as chikungunya, dengue, and Zika viruses have been threatening the European countries since the introduction in 1979 of the major vector Aedes albopictus. In 2017, more than three hundred of CHIKV autochthonous cases were reported in Italy, highlighting the urgent need for a risk assessment of arboviral diseases in European countries. In this study, the vector competence for three major arboviruses were analyzed in eight Ae. albopictus populations from Europe. Here we show that Southern European Ae. albopictus were susceptible to CHIKV, DENV-1 and ZIKV with the highest vector competence for CHIKV. Based on vector competence data and vector distribution, a prediction risk map for CHIKV was generated stressing the fear of CHIKV and to a lesser extent, of other arboviruses for Europe, calling us for new public health strategies.