An agent in the Aedes aegypti cell line (Peleg) which causes fusion of Aedes albopictus cells

Metatranscriptomic analysis of common mosquito vector species in the Canadian Prairies

The microbiome plays vital roles in the life history of mosquitoes, including their development, immunity, longevity, and vector competence. Recent advances in sequencing technologies have allowed for detailed exploration into the diverse microorganisms harbored by these medically important insects. Although these meta-studies have cataloged the microbiomes of mosquitoes in several continents, much of the information currently available for North America is limited to the state of California. In this study, we collected >35,000 mosquitoes throughout Manitoba, Canada, over a 3-year period and then harnessed RNA sequencing and targeted reverse transcriptase-PCR to characterize the microbiomes of the eight most pervasive and important vector and pest species. The consensus microbiome of each species was overwhelmingly composed of viruses but also included fungi, bacteria, protozoa, and parasitic invertebrates. The microbial assemblages were heterogeneous between species, even within the same genus. We detected notable pathogens, including the causal agents of Cache Valley Fever, avian malaria, and canine heartworm. The remaining microbiome consisted largely of putatively insect-specific viruses that are not well characterized, including 17 newly discovered viruses from 10 different families. Future research should focus on evaluating the potential application of these viruses in biocontrol, as biomarkers, and/or in disrupting mosquito vectorial capacity. Interestingly, we also detected viruses that naturally infect honeybees and thrips, which were presumably acquired indirectly through nectar foraging behaviors. Overall, we provide the first comprehensive catalog of the microorganisms harbored by the most common and important mosquito vectors and pests in the Canadian Prairies.

IMPORTANCE

Mosquitoes are the most dangerous animals on the planet, responsible for over 800,000 deaths per year globally. This is because they carry and transmit a plethora of human disease-causing microorganisms, such as West Nile virus and the malaria parasite. Recent innovations in nucleic acid sequencing technologies have enabled researchers unparalleled opportunities to characterize the suite of microorganisms harbored by different mosquito species, including the causal agents of disease. In our study, we carried out 3 years of intensive mosquito surveillance in Canada. We collected and characterized the microorganisms harbored by >35,000 mosquitoes, including the identification of the agents of Cache Valley fever, avian malaria, and canine heartworm. We also detected insect-specific viruses and discovered 17 new viruses that have never been reported. This study, which is the first of its kind in Canada and one of only a handful globally, will greatly aid in future infectious disease research.

Transovarial Transmission of Cell-Fusing Agent Virus in Naturally Infected Aedes aegypti Mosquitoes

Mosquito-borne arboviruses include several pathogens that are responsible for many diseases of significant public health burden. Mosquitoes also host many insect-specific viruses that cannot replicate in vertebrate cells. These insect-specific viruses persist in nature predominantly via vertical transmission (VT), and they exhibit high VT rates (VTRs). Cell-fusing agent virus (CFAV), an insect-specific orthoflavivirus, shows high VTRs in naturally infected mosquitoes but not in artificially infected mosquitoes. To determine whether the high VTRs are due to transovarial transmission, we investigated VT and ovary infection patterns in naturally CFAV-infected Aedes aegypti (Bangkok) mosquitoes. VT was monitored by detecting CFAV among the progeny by reverse-transcription polymerase chain reaction and ovary infection was determined by in situ hybridization using a virus-specific probe. We showed that in CFAV-positive mosquitoes, ovarian follicles were infected, suggesting that VT occurs by transovarial transmission in naturally infected mosquitoes. Additionally, mosquitoes harbored dormant, non-replicative CFAV that remained below the detection level. These results suggested that CFAV persists via VT in nature and has the potential to remain dormant in diapausing mosquitoes during unfavorable conditions. Understanding this VT mechanism is crucial for comprehending the persistence of insect-specific viruses (and potentially dual-host arboviruses) in their natural environment.

Diversity and molecular characterization of insect-specific flaviviruses in mosquitoes (Diptera: Culicidae) collected in central and northern Argentina

The genus Flavivirus comprises approximately 80 different viruses. Phylogenetic relationships among its members indicate a clear ecological separation between those viruses transmitted by mosquitoes, ticks, with no known vector, and insect-specific Flaviviruses. The diversity and phylogenetic relationships among insect-specific flaviviruses circulating in the central and northern regions of Argentina were studied by performing molecular detection and characterization of the NS5 protein gene in mosquitoes collected in Córdoba, Chaco and Tucumán provinces. Overall, 68 out of 1776 pools were positive. CxFV, KRV and CFAV circulate in the 3 studied provinces. Several mosquito species (Aedes aegypti, Culex bidens, Cx. dolosus, Cx. interfor, Cx. quinquefasciatus, Cx. saltanensis, Haemagogus spegazzini) were found infected. A wide circulation of CxFV was observed in the central-northern region of Argentina. CxFV strains detected in our study clustered with strains circulating in Santa Fe and Buenos Aires provinces (Argentina), and other countries such as Indonesia, Mexico, Uganda and Taiwan. The presence of these viruses in mosquitoes could play an important role from the public health perspective, because it has been shown that previous CxFV infection can increase or block the infection of the mosquito by other pathogenic flaviviruses.

Impact of symbiotic insect-specific viruses on mosquito vector competence for arboviruses

Mosquitoes are vectors for arboviruses, such as dengue, Zika, and Chikungunya. Symbiotic interactions can affect the intrinsic ability of mosquitoes to acquire and transmit arboviruses, referred to as vector competence. Insect-specific viruses (ISVs) are commonly found in symbiotic associations with mosquitoes in the wild and can affect many aspects of mosquito biology. Here, we review current knowledge on the effects of symbiotic ISV-mosquito interactions on vector competence. We discuss potential mechanisms underlying these interactions and their implications for shaping new biological control strategies. Finally, we highlight the need for field data analyzing the circulation of ISVs in mosquitoes associated with mechanistic studies in the laboratory.

Cell fusing agent virus rarely transmits vertically in artificially infected laboratory-colonized Aedes aegypti mosquitoes

BACKGROUND

Vertical transmission (VT) of arboviruses (arthropod-borne viruses) can serve as an essential link in the transmission cycle during adverse environmental conditions. The extent of VT among mosquito-borne arboviruses can vary significantly among different virus families and even among different viruses within the same genus. For example, orthobunyaviruses exhibit a higher VT rate than orthoflaviviruses and alphaviruses. Mosquitoes are also the natural hosts of a large number of insect-specific viruses (ISV) that belong to several virus families, including Bunyaviridae, Flaviviridae, and Togaviridae. Cell fusing agent virus (CFAV), an insect-specific orthoflavivirus, displays higher VT rates than other dual-host orthoflaviviruses, such as Zika and dengue viruses. High VT rates require establishment of stabilized infections in the germinal tissues of female vectors. To delve deeper into understanding the mechanisms governing these differences in VT rates and the establishment of stabilized infections, the ovary infection patterns and VT of Zika virus (ZIKV) and CFAV were compared.

METHODS

Laboratory colonized Aedes aegypti females were infected with either ZIKV or CFAV by intrathoracic injection. Ovary infection patterns were monitored by in situ hybridization using virus-specific probes, and VT was determined by detecting the presence of the virus among the progeny, using a reverse-transcription quantitative polymerase chain reaction (PCR) assay.

RESULTS

Both ZIKV and CFAV infect mosquito ovaries after intrathoracic injection. Infections then become widespread following a non-infectious blood meal. VT rates of ZIKV are similar to previously reported results (3.33%). CFAV, on the contrary transmits vertically very rarely. VT was not observed in the first gonotrophic cycle following intrathoracic injection, and only rarely in the second gonotrophic cycle. VT of CFAV is mosquito population independent, since similar results were obtained with Aedes aegypti collected from two different geographic locations.

CONCLUSIONS

Although CFAV infects mosquito ovaries, the occurrence of VT remains infrequent in artificially infected Ae. aegypti, despite the observation of high VT rates in field-collected mosquitoes. These results suggest that infections of insect-specific viruses are stabilized in mosquitoes by some as yet unidentified mechanisms.

In vitro characterization of cell-fusing agent virus DNA forms in Aedes aegypti mosquitoes

How non-retroviral endogenous viral elements (EVEs) are established is a long-standing question. Viral DNA (vDNA) forms of RNA viruses are likely to be EVE precursors. Cell-fusing agent virus (CFAV) is a major insect-specific virus (ISV) in the Aedes aegypti mosquitoes and one of the few existing non-retroviral RNA viruses found as EVEs. We characterized CFAV-derived vDNA in the cell line to understand the mechanism of why current viruses are rarely endogenized. vDNA production was affected by cell culture media independent of CFAV replication. vDNAs that correspond to different regions covering the entire viral genome were detected, implying multiple initiation sites exist. A considerable proportion of vDNAs corresponded to ssDNA. Higher vDNA copies were detected in the cytoplasm than the nucleus. Our findings provide valuable insights into the intracellular characteristics of ISV-derived vDNAs, which will aid in understanding the underlying mechanisms of non-retroviral EVE formation.

Novel strains of Culex flavivirus and Hubei chryso-like virus 1 from the Anopheles mosquito in western Kenya

Surveillance of mosquito vectors is critical for early detection, prevention and control of vector borne diseases. In this study we used advanced molecular tools, such as DNA barcoding in combination with novel sequencing technologies to discover new and already known viruses in genetically identified mosquito species. Mosquitoes were captured using BG sentinel traps in Western Kenya during May and July 2019, and homogenized individually before pooled into groups of ten mosquitoes. The pools and individual samples were then used for molecular analysis and to infect cell cultures. Of a total of fifty-four (54) 10-pools, thirteen (13) showed cytopathic effect (CPE) on VeroB4 cells, eighteen (18) showed CPE on C6/36 cells. Eight (8) 10-pools out of the 31 CPE positive pools showed CPE on both VeroB4 and C6/36 cells. When using reverse transcriptase polymerase chain reaction (RT-PCR), Sanger sequencing and Twist Comprehensive Viral Research Panel (CVRP) (Twist Biosciences), all pools were found negative by RT-PCR when using genus specific primers targeting alphaviruses, orthobunyaviruses and virus specific primers towards o'nyong-nyong virus, chikungunya virus and Sindbis virus (previously reported to circulate in the region). Interestingly, five pools were RT-PCR positive for flavivirus. Two of the RT-PCR positive pools showed CPE on both VeroB4 and C6/36 cells, two pools showed CPE on C6/36 cells alone and one pool on VeroB4 cells only. Fifty individual mosquito homogenates from the five RT-PCR positive 10-pools were analyzed further for flavivirus RNA. Of these, 19 out of the 50 individual mosquito homogenates indicated the presence of flavivirus RNA. Barcoding of the flavivirus positive mosquitoes revealed the mosquito species as Aedes aegypti (1), Mansonia uniformis (6), Anopheles spp (3), Culex pipiens (5), Culex spp (1), Coquilletidia metallica (2) and Culex quinquefasciatus (1). Of the 19 flavivirus positive individual mosquitoes, five (5) virus positive homogenates were sequenced. Genome sequences of two viruses were completed. One was identified as the single-stranded RNA Culex flavivirus and the other as the double-stranded RNA Hubei chryso-like virus 1. Both viruses were found in the same Anopheles spp. homogenate extracted from a sample that showed CPE on both VeroB4 and C6/36 cells. The detection of both viruses in a single mosquito homogenate indicated coinfection. Phylogenetic analyses suggested that the Culex flavivirus sequence detected was closely related to a Culex flavivirus isolated from Uganda in 2008. All four Hubei chryso-like virus 1 segments clusters closely to Hubei chryso-like virus 1 strains isolated in Australia, China and USA. Two novel strains of insect-specific viruses in Anopheles mosquitoes were detected and characterized.

No detectable fitness cost of infection by cell-fusing agent virus in Aedes aegypti mosquitoes

Aedes mosquitoes are well-known vectors of arthropod-borne viruses (arboviruses). Mosquitoes are more frequently infected with insect-specific viruses (ISVs) that cannot infect vertebrates. Some ISVs interfere with arbovirus replication in mosquito vectors, which has gained attention for potential use against arbovirus transmission. Cell-fusing agent virus (CFAV), a widespread ISV, can reduce arbovirus dissemination in Ae. aegypti. However, vectorial capacity is largely governed by other parameters than pathogen load, including mosquito survival and biting behaviour. Understanding how ISVs impact these mosquito fitness-related traits is critical to assess the potential risk of using ISVs as biological agents. Here, we examined the effects of CFAV infection on Ae. aegypti mosquito fitness. We found no significant reduction in mosquito survival, blood-feeding behaviour and reproduction, suggesting that Ae. aegypti is tolerant to CFAV. The only detectable effect was a slight increase in human attraction of CFAV-infected females in one out of eight trials. Viral tolerance is beneficial for introducing CFAV into natural mosquito populations, whereas the potential increase in biting activity must be further investigated. Our results provide the first insight into the link between ISVs and Aedes mosquito fitness and highlight the importance of considering all aspects of vectorial capacity for arbovirus control using ISVs.

Translating mosquito viromes into vector management strategies

Mosquitoes are best known for transmitting human and animal viruses. However, they also harbour mosquito-specific viruses (MSVs) as part of their microbiota. These are a group of viruses whose diversity and prevalence overshadow their medically relevant counterparts. Although metagenomics sequencing has remarkably accelerated the discovery of these viruses, what we know about them is often limited to sequence information, leaving much of their fundamental biology to be explored. Understanding the biology and ecology of MSVs can enlighten our knowledge of virus-virus interactions and lead to new innovations in the management of mosquito-borne viral diseases. We retrace the history of their discovery and discuss research milestones that would line the path from mosquito virome knowledge to vector management strategies.

Intrinsic factors driving mosquito vector competence and viral evolution: a review

Mosquitoes are responsible for the transmission of numerous viruses of global health significance. The term "vector competence" describes the intrinsic ability of an arthropod vector to transmit an infectious agent. Prior to transmission, the mosquito itself presents a complex and hostile environment through which a virus must transit to ensure propagation and transmission to the next host. Viruses imbibed in an infectious blood meal must pass in and out of the mosquito midgut, traffic through the body cavity or hemocoel, invade the salivary glands, and be expelled with the saliva when the vector takes a subsequent blood meal. Viruses encounter physical, cellular, microbial, and immunological barriers, which are influenced by the genetic background of the mosquito vector as well as environmental conditions. Collectively, these factors place significant selective pressure on the virus that impact its evolution and transmission. Here, we provide an overview of the current state of the field in understanding the mosquito-specific factors that underpin vector competence and how each of these mechanisms may influence virus evolution.

Cell fusing agent virus isolated from Aag2 cells does not vertically transmit in Aedes aegypti via artificial infection

BACKGROUND

Cell fusing agent virus (CFAV) was the first insect-specific virus to be characterized, and has been reported to negatively influence the growth of arboviruses such as dengue, Zika, and La Cross, making it a promising biocontrol agent for mosquito-borne disease prevention. Aedes aegypti Aag2 cells were naturally infected with CFAV. However, the ability of this virus to stably colonize an Ae. aegypti population via artificial infection and how it influences the vector competence of this mosquito have yet to be demonstrated.

METHODS

CFAV used in this study was harvested from Aag2 cells and its complete genome sequence was obtained by polymerase chain reaction and rapid amplification of complementary DNA ends, followed by Sanger sequencing. Phylogenetic analysis of newly identified CFAV sequences and other sequences retrieved from GenBank was performed. CFAV stock was inoculated into Ae. aegypti by intrathoracic injection, the survival of parental mosquitoes was monitored and CFAV copies in the whole bodies, ovaries, and carcasses of the injected F0 generation and in the whole bodies of the F1 generation on different days were examined by reverse transcription-quantitative polymerase chain reaction.

RESULTS

The virus harvested from Aag2 cells comprised a mixture of three CFAV strains. All genome sequences of CFAV derived from Aag2 cells clustered into one clade but were far from those isolated or identified from Ae. aegypti. Aag2-derived CFAV efficiently replicated in the mosquito body and did not attenuate the survival of Ae. aegypti. However, the viral load in the ovarian tissues was much lower than that in other tissues and the virus could not passage to the offspring by vertical transmission.

CONCLUSIONS

The results of this study demonstrate that Aag2-derived CFAV was not vertically transmitted in Ae. aegypti and provide valuable information on the colonization of mosquitoes by this virus.

The identification and genetic characteristics of Quang Binh virus from field-captured Culex tritaeniorhynchus (Diptera: Culicidae) from Guizhou Province, China

BACKGROUND

Mosquitoes carry a variety of viruses that can cause disease in humans, animals and livestock. Surveys for viruses carried by wild mosquitoes can significantly contribute to surveillance efforts and early detection systems. In addition to mosquito-borne viruses, mosquitoes harbor many insect-specific viruses (ISVs). Quang Binh virus (QBV) is one such example, categorized as an ISV within the Flavivirus genus (family Flaviviridae). QBV has been specifically documented in Vietnam and China, with reports limited to several mosquito species.

METHODS

The homogenate obtained from female mosquitoes was cultured on C6/36 (Aedes albopictus) and BHK-21 (baby hamster kidney) cell lines. Positive cultures were identified by reverse transcription-polymerase chain reaction (RT‒PCR) with taxon- or species-specific primers. Next-generation sequencing was employed to sequence the complete genomes of the identified positive samples. Subsequently, phylogenetic, gene homology, molecular evolutionary and genetic variation analyses were conducted.

RESULT

In 2021, a total of 32,177 adult female mosquitoes were collected from 15 counties in Guizhou Province, China. The predominant mosquito species identified were Culex tritaeniorhynchus, Armigeres subalbatus and Anopheles sinensis. Among the collected mosquitoes, three positive cultures were obtained from Cx. tritaeniorhynchus pools, revealing the presence of Quang Binh virus (QBV) RNA sequences. Phylogenetic analysis indicated that the three Guizhou isolates, along with the prototype isolate from Vietnam, formed distinct branches. These branches were primarily closely related to other QBV isolates reported in China. Comparative analysis revealed a high degree of nucleotide and amino acid homology between the Guizhou isolates and both Vietnamese and other indigenous Chinese isolates. Additionally, nonsynonymous single-nucleotide variants (SNVs) were observed in these strains compared to the QBV prototype strain.

CONCLUSION

This study represents the first report of QBV presences in Cx. tritaeniorhynchus mosquitoes in Guizhou Province, China. Phylogenetic tree analysis showed that the three Guizhou isolates were most closely related to the QBV genes found in China. In addition, the study of the genetic characteristics and variation of this virus provided a deeper understanding of QBV and enriched the baseline data of these insect-specific flaviviruses (ISFVs).

Longitudinal Study of Viral Diversity Associated with Mosquito Species Circulating in Cambodia

Arthropod-borne viruses (arboviruses) pose a significant global health threat and are primarily transmitted by mosquitoes. In Cambodia, there are currently 290 recorded mosquito species, with at least 17 of them considered potential vectors of arboviruses to humans. Effective surveillance of virome profiles in mosquitoes from Cambodia is vital, as it could help prevent and control arbovirus diseases in a country where epidemics occur frequently. The objective of this study was to identify and characterize the viral diversity in mosquitoes collected during a one-year longitudinal study conducted in various habitats across Cambodia. For this purpose, we used a metatranscriptomics approach and detected the presence of chikungunya virus in the collected mosquitoes. Additionally, we identified viruses categorized into 26 taxa, including those known to harbor arboviruses such as Flaviviridae and Orthomyxoviridae, along with a group of viruses not yet taxonomically identified and provisionally named "unclassified viruses". Interestingly, the taxa detected varied in abundance and composition depending on the mosquito genus, with no significant influence of the collection season. Furthermore, most of the identified viruses were either closely related to viruses found exclusively in insects or represented new viruses belonging to the Rhabdoviridae and Birnaviridae families. The transmission capabilities of these novel viruses to vertebrates remain unknown.

Holobiont perspectives on tripartite interactions among microbiota, mosquitoes, and pathogens

Mosquito-borne diseases like dengue and malaria cause a significant global health burden. Unfortunately, current insecticides and environmental control strategies aimed at the vectors of these diseases are only moderately effective in decreasing disease burden. Understanding and manipulating the interaction between the mosquito holobiont (i.e., mosquitoes and their resident microbiota) and the pathogens transmitted by these mosquitoes to humans and animals could help in developing new disease control strategies. Different microorganisms found in the mosquito's microbiota affect traits related to mosquito survival, development, and reproduction. Here, we review the physiological effects of essential microbes on their mosquito hosts; the interactions between the mosquito holobiont and mosquito-borne pathogen (MBP) infections, including microbiota-induced host immune activation and Wolbachia-mediated pathogen blocking (PB); and the effects of environmental factors and host regulation on the composition of the microbiota. Finally, we briefly overview future directions in holobiont studies, and how these may lead to new effective control strategies against mosquitoes and their transmitted diseases.

Culex Y Virus: A Native Virus of Culex Species Characterized In Vivo

Mosquitoes are vectors of various pathogens that cause diseases in humans and animals. To prevent the outbreak of mosquito-borne diseases, it is essential to control vector populations, as treatment or vaccination for mosquito-borne diseases are often unavailable. Insect-specific viruses (ISVs) have previously been described as being potentially helpful against arboviral disease outbreaks. In this study, we present the first in vivo characterization of the ISV Culex Y virus (CYV). CYV was first isolated from free-living Culex pipiens mosquitoes in 2010; then, it was found in several mosquito cell lines in a further study in 2018. For mammalian cells, we were able to confirm that CYV does not replicate as it was previously described. Additionally, we found that CYV does not replicate in honey bees or locusts. However, we detected replication in the Culex pipiens biotype molestus, Aedes albopictus, and Drosophila melanogaster, thus indicating dipteran specificity. We detected significantly higher mortality in Culex pipiens biotype molestus males and Drosophila melanogaster, but not in Aedes albopictus and female Culex pipiens biotype molestus. CYV could not be transmitted transovarially to offspring, but we detected venereal transmission as well as CYV in mosquitos' saliva, indicating that an oral route of infection would also be possible. CYV's dipteran specificity, transmission routes, and killing effect with respect to Culex males may be used as powerful tools with which to destabilize arbovirus vector populations in the future.

Contrasting the Practices of Virus Isolation and Characterization between the Early Period in History and Modern Times: The Case of Japanese Encephalitis Virus

Japanese encephalitis is a serious disease transmitted by mosquitoes. With its recent spread beyond the traditional territory of endemicity in Asia, the magnitude of global threat has increased sharply. While much of the current research are largely focused on changing epidemiology, molecular genetics of virus, and vaccination, little attention has been paid to the early history of virus isolation and phenotypic characterization of this virus. In this review, using this piece of history as an example, I review the transition of the concept and practice of virus isolation and characterization from the early period of history to modern times. The spectacular development of molecular techniques in modern times has brought many changes in practices as well as enormous amount of new knowledge. However, many aspects of virus characterization, in particular, transmission mechanism and host relationship, remain unsolved. As molecular techniques are not perfect in all respects, beneficial accommodation of molecular and biologic data is critically important in many branches of research. Accordingly, I emphasize exercising caution in applying only these modern techniques, point out unrecognized communication problems, and stress that JE research history is a rich source of interesting works still valuable even today and waiting to be discovered.

Chronic depletion of vertebrate lipids in Aedes aegypti cells dysregulates lipid metabolism and inhibits innate immunity without altering dengue infectivity

Aedes aegypti is the primary vector of dengue virus (DENV) and other arboviruses. Previous literature suggests that vertebrate and invertebrate lipids and the nutritional status of mosquitoes modify virus infection. Here, we developed a vertebrate lipid-depleted Ae. aegypti cell line to investigate if chronic depletion of vertebrate lipids normally present in a blood meal and insect cell culture medium would impact cell growth and virus infection. Chronic depletion of vertebrate lipids reduced cell size and proliferation, although cells retained equivalent total intracellular lipids per cell by reducing lipolysis and modifying gene expression related to sugar and lipid metabolism. Downregulation of innate immunity genes was also observed. We hypothesized that chronic depletion of vertebrate lipids would impact virus infection; however, the same amount of DENV was produced per cell. This study reveals how Ae. aegypti cells adapt in the absence of vertebrate lipids, and how DENV can replicate equally well in cells that contain predominately vertebrate or invertebrate lipids.

Aedes aegypti is a major threat to public health. Ae. aegypti is the primary vector of dengue virus types 1–4 (DENV 1–4), zika virus (ZIKV), chikungunya virus (CHIKV), and yellow fever virus (YFV). Ae. aegypti acquires arboviruses from a vertebrate host during blood feeding. Blood feeding introduces vertebrate-specific factors into the mosquito that may be important for both mosquito and virus. This study reveals that Ae. aegypti adapts to depletion of vertebrate lipids by inhibiting lipolysis and promoting de novo synthesis of invertebrate lipids, and that DENV can replicate equally well without high concentrations of cholesterol and other vertebrate lipid species. Understanding how disease vectors adapt to nutritional changes will identify novel strategies for vector control and disease mitigation.

Vertical and Horizontal Transmission of Cell Fusing Agent Virus in Aedes aegypti

Cell fusing agent virus (CFAV) is an insect-specific flavivirus (ISF) found in Aedes aegypti mosquitoes. ISFs have demonstrated the ability to modulate the infection or transmission of arboviruses such as dengue, West Nile, and Zika viruses. It is thought that vertical transmission is the main route for ISF maintenance in nature. This has been observed with CFAV, but there is evidence of horizontal and venereal transmission in other ISFs. Understanding the route of transmission can inform strategies to spread ISFs to vector populations as a method of controlling pathogenic arboviruses. We crossed individually reared male and female mosquitoes from both a naturally occurring CFAV-positive Ae. aegypti colony and its negative counterpart to provide information on maternal, paternal, and horizontal transmission. RT-PCR was used to detect CFAV in individual female pupal exuviae and was 89% sensitive, but only 42% in male pupal exuviae. This is a possible way to screen individuals for infection without destroying the adults. Female-to-male horizontal transmission was not observed during this study. However, there was a 31% transmission rate from mating pairs of CFAV-positive males to negative female mosquitoes. Maternal vertical transmission was observed with a filial infection rate of 93%. The rate of paternal transmission was 85% when the female remained negative, 61% when the female acquired CFAV horizontally, and 76% overall. Maternal and paternal transmission of CFAV could allow the introduction of this virus into wild Ae. aegypti populations through male or female mosquito releases, and thus provides a potential strategy for ISF-derived arbovirus control. IMPORTANCE Insect-specific flaviviruses (ISFs), are a group of nonpathogenic flaviviruses that only infect insects. ISFs can have a high prevalence in mosquito populations, but their transmission routes are not well understood. The results of this study confirm maternal transmission of cell fusing agent virus (CFAV) and demonstrate that paternal transmission is also highly efficient. Horizontal transmission of CFAV was also observed, aided by evaluation of the pupal infection status before mating with an infected individual. This technique of detecting infection in discarded pupae exuviae has not been evaluated previously and will be a useful tool for others in the field of studying viral transmission in mosquitoes. Identifying these routes of transmission provides information about how CFAV could be maintained in wild populations of mosquitoes and can aid future studies focusing on interactions of CFAV with their hosts and other viruses that infect mosquitoes.

From Mosquito Ovaries to Ecdysone; from Ecdysone to Wolbachia: One Woman's Career in Insect Biology

In anautogenous mosquitoes, synchronous development of terminal ovarian follicles after a blood meal provides an important model for studies on insect reproduction. Removal and implantation of ovaries, in vitro culture of dissected tissues and immunological assays for vitellogenin synthesis by the fat body showed that the Aedes aegypti (L.) (Diptera, Culicidae) mosquito ovary produces a factor essential for egg production. The discovery that the ovarian factor was the insect steroid hormone, ecdysone, provided a model for co-option of the larval hormones as reproductive hormones in adult insects. In later work on cultured mosquito cells, ecdysone was shown to arrest the cell cycle, resulting in an accumulation of diploid cells in G1, prior to initiation of DNA synthesis. Some mosquito species, such as Culex pipiens L. (Diptera, Culicidae), harbor the obligate intracellular bacterium, Wolbachia pipientis Hertig (Rickettsiales, Anaplasmataceae), in their reproductive tissues. When maintained in mosquito cell lines, Wolbachia abundance increases in ecdysone-arrested cells. This observation facilitated the recovery of high levels of Wolbachia from cultured cells for microinjection and genetic manipulation. In female Culex pipiens, it will be of interest to explore how hormonal cues that support initiation and progression of the vitellogenic cycle influence Wolbachia replication and transmission to subsequent generations via infected eggs.

Aedes aegypti and Ae. albopictus microbiome/virome: new strategies for controlling arboviral transmission?

Aedes aegypti and Aedes albopictus are the main vectors of highly pathogenic viruses for humans, such as dengue (DENV), chikungunya (CHIKV), and Zika (ZIKV), which cause febrile, hemorrhagic, and neurological diseases and remain a major threat to global public health. The high ecological plasticity, opportunistic feeding patterns, and versatility in the use of urban and natural breeding sites of these vectors have favored their dispersal and adaptation in tropical, subtropical, and even temperate zones. Due to the lack of available treatments and vaccines, mosquito population control is the most effective way to prevent arboviral diseases. Resident microorganisms play a crucial role in host fitness by preventing or enhancing its vectorial ability to transmit viral pathogens. High-throughput sequencing and metagenomic analyses have advanced our understanding of the composition and functionality of the microbiota of Aedes spp. Interestingly, shotgun metagenomics studies have established that mosquito vectors harbor a highly conserved virome composed of insect-specific viruses (ISV). Although ISVs are not infectious to vertebrates, they can alter different phases of the arboviral cycle, interfering with transmission to the human host. Therefore, this review focuses on the description of Ae. aegypti and Ae. albopictus as vectors susceptible to infection by viral pathogens, highlighting the role of the microbiota-virome in vectorial competence and its potential in control strategies for new emerging and re-emerging arboviruses.

Local-scale virome depiction in Medellín, Colombia, supports significant differences between Aedes aegypti and Aedes albopictus

Aedes spp. comprise the primary group of mosquitoes that transmit arboviruses such as dengue, Zika, and chikungunya viruses to humans, and thus these insects pose a significant burden on public health worldwide. Advancements in next-generation sequencing and metagenomics have expanded our knowledge on the richness of RNA viruses harbored by arthropods such as Ae. aegypti and Ae. albopictus. Increasing evidence suggests that vector competence can be modified by the microbiome (comprising both bacteriome and virome) of mosquitoes present in endemic zones. Using an RNA-seq-based metataxonomic approach, this study determined the virome structure, Wolbachia presence and mitochondrial diversity of field-caught Ae. aegypti and Ae. albopictus mosquitoes in Medellín, Colombia, a municipality with a high incidence of mosquito-transmitted arboviruses. The two species are sympatric, but their core viromes differed considerably in richness, diversity, and abundance; although the community of viral species identified was large and complex, the viromes were dominated by few virus species. BLAST searches of assembled contigs suggested that at least 17 virus species (16 of which are insect-specific viruses [ISVs]) infect the Ae. aegypti population. Dengue virus 3 was detected in one sample and it was the only pathogenic virus detected. In Ae. albopictus, up to 11 ISVs and one plant virus were detected. Therefore, the virome composition appears to be species-specific. The bacterial endosymbiont Wolbachia was identified in all Ae. albopictus samples and in some Ae. aegypti samples collected after 2017. The presence of Wolbachia sp. in Ae. aegypti was not related to significant changes in the richness, diversity, or abundance of this mosquito’s virome, although it was related to an increase in the abundance of Aedes aegypti To virus 2 (Metaviridae). The mitochondrial diversity of these mosquitoes suggested that the Ae. aegypti population underwent a change that started in the second half of 2017, which coincides with the release of Wolbachia-infected mosquitoes in Medellín, indicating that the population of wMel-infected mosquitoes released has introduced new alleles into the wild Ae. aegypti population of Medellín. However, additional studies are required on the dispersal speed and intergenerational stability of wMel in Medellín and nearby areas as well as on the introgression of genetic variants in the native mosquito population.

Metagenomic sequencing characterizes a wide diversity of viruses in field mosquito samples in Nigeria

Mosquito vectors are a tremendous public health threat. One in six diseases worldwide is vector-borne transmitted mainly by mosquitoes. In the last couple of years, there have been active Yellow fever virus (YFV) outbreaks in many settings in Nigeria, and nationwide, entomological surveillance has been a significant effort geared towards understanding these outbreaks. In this study, we used a metagenomic sequencing approach to characterize viruses present in vector samples collected during various outbreaks of Yellow fever (YF) in Nigeria between 2017 and 2020. Mosquito samples were grouped into pools of 1 to 50 mosquitoes, each based on species, sex and location. Twenty-five pools of Aedes spp and one pool of Anopheles spp collected from nine states were sequenced and metagenomic analysis was carried out. We identified a wide diversity of viruses belonging to various families in this sample set. Seven different viruses detected included: Fako virus, Phasi Charoen-like virus, Verdadero virus, Chaq like-virus, Aedes aegypti totivirus, cell fusing agent virus and Tesano Aedes virus. Although there are no reports of these viruses being pathogenic, they are an understudied group in the same families and closely related to known pathogenic arboviruses. Our study highlights the power of next generation sequencing in identifying Insect specific viruses (ISVs), and provide insight into mosquito vectors virome in Nigeria.

Symbionts and gene drive: two strategies to combat vector-borne disease

Mosquitoes bring global health problems by transmitting parasites and viruses such as malaria and dengue. Unfortunately, current insecticide-based control strategies are only moderately effective because of high cost and resistance. Thus, scalable, sustainable, and cost-effective strategies are needed for mosquito-borne disease control. Symbiont-based and genome engineering-based approaches provide new tools that show promise for meeting these criteria, enabling modification or suppression approaches. Symbiotic bacteria like Wolbachia are maternally inherited and manipulate mosquito host reproduction to enhance their vertical transmission. Genome engineering-based gene drive methods, in which mosquitoes are genetically altered to spread drive alleles throughout wild populations, are also proving to be a potentially powerful approach in the laboratory. Here, we review the latest developments in both symbionts and gene drive-based methods. We describe some notable similarities, as well as distinctions and obstacles, relating to these promising technologies.

Biology and Transmission Dynamics of Aedes flavivirus

Aedes albopictus (Skuse) and Aedes aegypti (Linnaeus) (Diptera: Culicidae) mosquitoes transmit pathogenic arthropod-borne viruses, including dengue, chikungunya, and Zika viruses, with significant global health consequences. Both Ae. albopictus and Ae. aegypti also are susceptible to Aedes flavivirus (AEFV), an insect-specific flavivirus (ISF) first isolated in Japan from Ae. albopictus and Ae. flavopictus. ISFs infect only insect hosts and evidence suggests that they are maintained by vertical transmission. In some cases, ISFs interfere with pathogenic flavivirus infection, and may have potential use in disease control. We explored the host range of AEFV in 4 genera of mosquitoes after intrathoracic injection and observed greater than 95% prevalence in the species of Aedes and Toxorhynchites tested. Anopheles and Culex species were less permissive to infection. Vertical transmission studies revealed 100% transovarial transmission and a filial infection rate of 100% for AEFV in a persistently-infected colony of Ae. albopictus. Horizontal transmission potential was assessed for adult and larval mosquitoes following per os exposures and in venereal transmission experiments. No mosquitoes tested positive for AEFV infection after blood feeding, and infection with AEFV after sucrose feeding was rare. Similarly, 2% of adult mosquitoes tested positive for AEFV after feeding on infected cells in culture as larvae. Venereal transmission of AEFV was most frequently observed from infected males to uninfected females as compared with transmission from infected females to uninfected males. These results reveal new information on the infection potential of AEFV in mosquitoes and expand our understanding of both vertical and horizontal transmission of ISFs.

Vector Specificity of Arbovirus Transmission

More than 25% of human infectious diseases are vector-borne diseases (VBDs). These diseases, caused by pathogens shared between animals and humans, are a growing threat to global health with more than 2.5 million annual deaths. Mosquitoes and ticks are the main vectors of arboviruses including flaviviruses, which greatly affect humans. However, all tick or mosquito species are not able to transmit all viruses, suggesting important molecular mechanisms regulating viral infection, dissemination, and transmission by vectors. Despite the large distribution of arthropods (mosquitoes and ticks) and arboviruses, only a few pairings of arthropods (family, genus, and population) and viruses (family, genus, and genotype) successfully transmit. Here, we review the factors that might limit pathogen transmission: internal (vector genetics, immune responses, microbiome including insect-specific viruses, and coinfections) and external, either biotic (adult and larvae nutrition) or abiotic (temperature, chemicals, and altitude). This review will demonstrate the dynamic nature and complexity of virus–vector interactions to help in designing appropriate practices in surveillance and prevention to reduce VBD threats.

Detection of Insect-Specific Flaviviruses in Mosquitoes (Diptera: Culicidae) in Northeastern Regions of South Africa

Mosquitoes in the Aedes and Culex genera are considered the main vectors of pathogenic flaviviruses worldwide. Entomological surveillance using universal flavivirus sets of primers in mosquitoes can detect not only pathogenic viruses but also insect-specific ones. It is hypothesized that insect-specific flaviviruses, which naturally infect these mosquitoes, may influence their vector competence for zoonotic arboviruses. Here, entomological surveillance was performed between January 2014 and May 2018 in five different provinces in the northeastern parts of South Africa, with the aim of identifying circulating flaviviruses. Mosquitoes were sampled using different carbon dioxide trap types. Overall, 64,603 adult mosquitoes were collected, which were screened by RT-PCR and sequencing. In total, 17 pools were found positive for insect-specific Flaviviruses in the mosquito genera Aedes (12/17, 70.59%) and Anopheles (5/17, 29.41%). No insect-specific viruses were detected in Culex species. Cell-fusing agent viruses were detected in Aedes aegypti and Aedes caballus. A range of anopheline mosquitoes, including Anopheles coustani, An. squamosus and An. maculipalpis, were positive for Culex flavivirus-like and Anopheles flaviviruses. These results confirm the presence of insect-specific flaviviruses in mosquito populations in South Africa, expands their geographical range and indicates potential mosquito species as vector species.

Chimeric Vaccines Based on Novel Insect-Specific Flaviviruses

Vector-borne flaviviruses are responsible for nearly half a billion human infections worldwide each year, resulting in millions of cases of debilitating and severe diseases and approximately 115,000 deaths. While approved vaccines are available for some of these viruses, the ongoing efficacy, safety and supply of these vaccines are still a significant problem. New technologies that address these issues and ideally allow for the safe and economical manufacture of vaccines in resource-poor countries where flavivirus vaccines are in most demand are urgently required. Preferably a new vaccine platform would be broadly applicable to all flavivirus diseases and provide new candidate vaccines for those diseases not yet covered, as well as the flexibility to rapidly pivot to respond to newly emerged flavivirus diseases. Here, we review studies conducted on novel chimeric vaccines derived from insect-specific flaviviruses that provide a potentially safe and simple system to produce highly effective vaccines against a broad spectrum of flavivirus diseases.

Development of genus-specific universal primers for the detection of flaviviruses

Background

Flaviviruses are representative arboviruses carried by arthropods and/or vertebrates; these viruses can pose a public health concern in many countries. By contrast, it is known that a novel virus group called insect-specific flaviviruses (ISFs) also infects arthropods, although no such virus has yet been isolated from vertebrates. The characteristics of ISFs, which affect replication of human-pathogenic flaviviruses within co-infected mosquito cells or mosquitoes without affecting the mosquitoes themselves, mean that we should pay attention to both ISFs and human-pathogenic flaviviruses, despite the fact that ISFs appear not to be directly hazardous to human health. To assess the risk of diseases caused by flaviviruses, and to better understand their ecology, it is necessary to know the extent to which flaviviruses are harbored by arthropods.

Methods

We developed a novel universal primer for use in a PCR-based system to detect a broad range of flaviviruses. We then evaluated its performance. The utility of the novel primer pair was evaluated in a PCR assay using artificially synthesized oligonucleotides derived from a template viral genome sequence. The utility of the primer pair was also examined by reverse transcription PCR (RT-PCR) using cDNA templates prepared from virus-infected cells or crude supernatants prepared from virus-containing mosquito homogenates.

Results

The novel primer pair amplified the flavivirus NS5 sequence (artificially synthesized) in all samples tested (six species of flavivirus that can cause infectious diseases in humans, and flaviviruses harbored by insects). In addition, the novel primer pair detected viral genomes in cDNA templates prepared from mosquito cells infected with live flavivirus under different infectious conditions. Finally, the viral genome was detected with high sensitivity in crude supernatants prepared from pooled mosquito homogenates.

Conclusion

This PCR system based on a novel primer pair makes it possible to detect arthropod-borne flaviviruses worldwide (the primer pair even detected viruses belonging to different genetic subgroups). As such, an assay based on this primer pair may help to improve public health and safety, as well as increase our understanding of flavivirus ecology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-021-01646-5.

Growth and Maintenance of Wolbachia in Insect Cell Lines

Simple Summary

Wolbachia is an intracellular bacterium that occurs in arthropods and in filarial worms. First described nearly a century ago in the reproductive tissues of Culex pipiens mosquitoes, Wolbachia is now known to occur in roughly 50% of insect species, and has been considered the most abundant intracellular bacterium on earth. In insect hosts, Wolbachia modifies reproduction in ways that facilitate spread of the microbe within the host population, but otherwise is relatively benign. In this “gene drive” capacity, Wolbachia provides a tool for manipulating mosquito populations. In mosquitoes, Wolbachia causes cytoplasmic incompatibility, in which the fusion of egg and sperm nuclei is disrupted, and eggs fail to hatch, depending on the presence/absence of Wolbachia in the parent insects. Recent findings demonstrate that Wolbachia from infected insects can be transferred into mosquito species that do not host a natural infection. When transinfected into Aedes aegypti, an important vector of dengue and Zika viruses, Wolbachia causes cytoplasmic incompatibility and, in addition, decreases the mosquito’s ability to transmit viruses to humans. This review addresses the maintenance of Wolbachia in insect cell lines, which provide a tool for high-level production of infectious bacteria. In vitro technologies will improve use of Wolbachia for pest control, and provide the microbiological framework for genetic engineering of this promising biocontrol agent.

Abstract

The obligate intracellular microbe, Wolbachia pipientis (Rickettsiales; Anaplasmataceae), is a Gram-negative member of the alpha proteobacteria that infects arthropods and filarial worms. Although closely related to the genera Anaplasma and Ehrlichia, which include pathogens of humans, Wolbachia is uniquely associated with invertebrate hosts in the clade Ecdysozoa. Originally described in Culex pipiens mosquitoes, Wolbachia is currently represented by 17 supergroups and is believed to occur in half of all insect species. In mosquitoes, Wolbachia acts as a gene drive agent, with the potential to modify vector populations; in filarial worms, Wolbachia functions as a symbiont, and is a target for drug therapy. A small number of Wolbachia strains from supergroups A, B, and F have been maintained in insect cell lines, which are thought to provide a more permissive environment than the natural host. When transferred back to an insect host, Wolbachia produced in cultured cells are infectious and retain reproductive phenotypes. Here, I review applications of insect cell lines in Wolbachia research and describe conditions that facilitate Wolbachia infection and replication in naive host cells. Progress in manipulation of Wolbachia in vitro will enable genetic and biochemical advances that will facilitate eventual genetic engineering of this important biological control agent.

Uncovering the Worldwide Diversity and Evolution of the Virome of the Mosquitoes Aedes aegypti and Aedes albopictus

Aedes aegypti, the yellow fever mosquito, and Aedes albopictus, the Asian tiger mosquito, are the most significant vectors of dengue, Zika, and Chikungunya viruses globally. Studies examining host factors that control arbovirus transmission demonstrate that insect-specific viruses (ISVs) can modulate mosquitoes’ susceptibility to arbovirus infection in both in vivo and in vitro co-infection models. While research is ongoing to implicate individual ISVs as proviral or antiviral factors, we have a limited understanding of the composition and diversity of the Aedes virome. To address this gap, we used a meta-analysis approach to uncover virome diversity by analysing ~3000 available RNA sequencing libraries representing a worldwide geographic range for both mosquitoes. We identified ten novel viruses and previously characterised viruses, including mononegaviruses, orthomyxoviruses, negeviruses, and a novel bi-segmented negev-like group. Phylogenetic analysis suggests close relatedness to mosquito viruses implying likely insect host range except for one arbovirus, the multi-segmented Jingmen tick virus (Flaviviridae) in an Italian colony of Ae. albopictus. Individual mosquito transcriptomes revealed remarkable inter-host variation of ISVs within individuals from the same colony and heterogeneity between different laboratory strains. Additionally, we identified striking virus diversity in Wolbachia infected Aedes cell lines. This study expands our understanding of the virome of these important vectors. It provides a resource for further assessing the ecology, evolution, and interaction of ISVs with their mosquito hosts and the arboviruses they transmit.

Investigating Virus-Host Interactions in Cultured Primary Honey Bee Cells

Honey bee (Apis mellifera) health is impacted by viral infections at the colony, individual bee, and cellular levels. To investigate honey bee antiviral defense mechanisms at the cellular level we further developed the use of cultured primary cells, derived from either larvae or pupae, and demonstrated that these cells could be infected with a panel of viruses, including common honey bee infecting viruses (i.e., sacbrood virus (SBV) and deformed wing virus (DWV)) and an insect model virus, Flock House virus (FHV). Virus abundances were quantified over the course of infection. The production of infectious virions in cultured honey bee pupal cells was demonstrated by determining that naïve cells became infected after the transfer of deformed wing virus or Flock House virus from infected cell cultures. Initial characterization of the honey bee antiviral immune responses at the cellular level indicated that there were virus-specific responses, which included increased expression of bee antiviral protein-1 (GenBank: MF116383) in SBV-infected pupal cells and increased expression of argonaute-2 and dicer-like in FHV-infected hemocytes and pupal cells. Additional studies are required to further elucidate virus-specific honey bee antiviral defense mechanisms. The continued use of cultured primary honey bee cells for studies that involve multiple viruses will address this knowledge gap.

The Possible Role of Microorganisms in Mosquito Mass Rearing

Simple Summary

One of the most promising control methods against Aedes albopictus is the sterile insect technique, which consists of mass rearing the target species, separation of males from females, and male exposure to sterilizing ionizing radiation. Once released in the environment, the sterile males are expected to search for wild females to mate with. The quality of sterile males is a crucial aspect in SIT programs in order to optimize effectiveness and limit production costs. The integration of probiotic microorganisms in larval and adult mosquito diets could enhance the quality parameters of the released sterile males.

Abstract

In Europe, one of the most significant mosquitoes of public health importance is Aedes albopictus (Skuse), an allochthonous species of Asian origin. One of the most promising control methods against Aedes albopictus is the sterile insect technique (SIT), which consists of mass rearing the target species, separation of males from females, and male exposure to sterilizing ionizing radiation. Once released in the environment, the sterile males are expected to search for wild females to mate with. If mating occurs, no offspring is produced. The quality of sterile males is a crucial aspect in SIT programs in order to optimize effectiveness and limit production costs. The integration of probiotic microorganisms in larval and adult mosquito diets could enhance the quality parameters of the released sterile males. In this review, we attempt to give the most representative picture of the present knowledge on the relationships between gut microbiota of mosquitoes and the natural or artificial larval diet. Furthermore, the possible use of probiotic microorganisms for mosquito larvae rearing is explored. Based on the limited amount of data found in the literature, we hypothesize that a better understanding of the interaction between mosquitoes and their microbiota may bring significant improvements in mosquito mass rearing for SIT purposes.

Genetic lineage characterization and spatiotemporal dynamics of classical insect-specific flaviviruses: outcomes and limitations

The genus Flavivirus incorporates bona fide arboviruses, as well as others viruses with restricted replication in insect cells. Among the latter, a large monophyletic cluster of viruses, known as cISF (classical insect-specific flaviviruses), has been sampled in many species of mosquitoes collected over a large geographic range. In this study, we investigated nucleotide and protein sequences with a suite of molecular characterization approaches including genetic distance, Shannon entropy, selective pressure analysis, polymorphism identification, principal coordinate analysis, likelihood mapping, phylodynamic reconstruction, and spatiotemporal dispersal, to further characterize this diverse group of insect-viruses. The different lineages and sub-lineages of viral sequences presented low sequence diversity and entropy (though some displayed lineage-specific polymorphisms), did not show evidence of frequent recombination and evolved under strong purifying selection. Moreover, the reconstruction of the evolutionary history and spatiotemporal dispersal was highly impacted by overall low signals of sequence divergence throughout time but suggested that cISF distribution in space and time is dynamic and may be dependent on human activities, including commercial trading and traveling.

Transmission Potential of Floridian Aedes aegypti Mosquitoes for Dengue Virus Serotype 4: Implications for Estimating Local Dengue Risk

Dengue virus serotype 4 (DENV-4) circulated in Aedes aegypti in 2016 and 2017 in Florida in the absence of human index cases, compelling a full assessment of local mosquito vector competence and DENV-4 risk. To better understand DENV-4 transmission risk in Florida, we used an expanded suite of tests to measure and compare the vector competencies of both an established colony of A. aegypti (Orlando strain [ORL]) and a field-derived colony from Collier County, FL, in 2018 (COL) for a Haitian DENV-4 human field isolate and a DENV-4 laboratory strain (Philippines H241). We immediately noted that ORL saliva positivity was higher for the field than for laboratory DENV-4 strains. In a subsequent comparison with the recent COL mosquito colony, we also observed significantly higher midgut infection of COL and ORL by the Haitian DENV-4 field strain and a significantly higher saliva positivity rate for COL, although overall saliva virus titers were similar between the two. These data point to a potential midgut infection barrier for the DENV-4 laboratory strain for both mosquito colonies and indicate that the marked differences in transmission potential estimates hinge on virus-vector combinations. Our study highlights the importance of leveraging an expanded suite of testing methods with emphasis on utilizing local mosquito populations and field-relevant dengue virus serotypes and strains to accurately estimate transmission risk in a given setting.

IMPORTANCE DENV-4 was found circulating in Florida (FL) A. aegypti mosquitoes in the absence of human index cases in the state (2016 to 2017). How DENV-4 was maintained locally is unclear, presenting a major gap in our understanding of DENV-4 public health risk. We determined the baseline arbovirus transmission potential of laboratory and field colonies of A. aegypti for both laboratory and field isolates of DENV-4. We observed a high transmission potential of field populations of A. aegypti and evidence of higher vertical transmission of the DENV-4 field isolate, providing clues to the possible mechanism of undetected DENV-4 maintenance in the state. Our findings also move the field forward in the development of best practices for evaluating arbovirus vector competence, with evidence that transmission potential estimates vary depending on the mosquito-virus combinations. These data emphasize the poor suitability of laboratory-established virus strains and the high relevance of field-derived mosquito populations in estimating transmission risk.

Mosquito-infecting virus Espirito Santo virus inhibits replication and spread of dengue virus

The primary vector of dengue virus (DENV) is Aedes aegypti. The mosquito-infecting virus, Espirito Santo virus (ESV), does not infect Vero (mammalian) cells and grows in mosquito (C6/36) cells without cytopathic effects. Effects of ESV infection on replication of DENV were explored in vitro and in vivo, analyzing protein, RNA genome expression, and plaque formation. ESV and DENV simultaneous coinfection did not block protein synthesis from either virus but did result in inhibition of DENV replication in mosquito cells. Furthermore, ESV superinfected with DENV resulted in inhibition of DENV replication and spread in A. aegypti, thus reducing vector competence. Tissue culture experiments on viral kinetics of ESV and DENV coinfection showed that neither virus significantly affects the replication of the other in Vero, HeLa, or HEK cells. Hence, ESV blocks DENV replication in insect cells, but not the mammalian cells evaluated here. Our study provides new insights into ESV-induced suppression of DENV, a globally important pathogen impacting public health.

The virome of vector mosquitoes

Mosquitoes are the major vectors for arthropod-borne viruses (arboviruses) of medical importance. Aedes aegypti and A. albopictus are the most prolific and widespread mosquito vectors being responsible for global transmission of dengue, Zika and Chikungunya viruses. Characterizing the collection of viruses circulating in mosquitoes, the virome, has long been of special interest. In addition to arboviruses, mosquitoes carry insect-specific viruses (ISVs) that do not directly infect vertebrates. Mounting evidence indicates that ISVs interact with arboviruses and may affect mosquito vector competence. Here, we review our current knowledge about the virome of vector mosquitoes and discuss the challenges for the field that may lead to novel strategies to prevent outbreaks of arboviruses.

Argonaute-CLIP delineates versatile, functional RNAi networks in Aedes aegypti, a major vector of human viruses

Argonaute (AGO) proteins bind small RNAs to silence complementary RNA transcripts, and they are central to RNA interference (RNAi). RNAi is critical for regulation of gene expression and antiviral defense in Aedes aegypti mosquitoes, which transmit Zika, chikungunya, dengue, and yellow fever viruses. In mosquitoes, AGO1 mediates miRNA interactions, while AGO2 mediates siRNA interactions. We applied AGO-crosslinking immunoprecipitation (AGO-CLIP) for both AGO1 and AGO2, and we developed a universal software package for CLIP analysis (CLIPflexR), identifying 230 small RNAs and 5,447 small RNA targets that comprise a comprehensive RNAi network map in mosquitoes. RNAi network maps predicted expression levels of small RNA targets in specific tissues. Additionally, this resource identified unexpected, context-dependent AGO2 target preferences, including endogenous viral elements and 3'UTRs. Finally, contrary to current thinking, mosquito AGO2 repressed imperfect targets. These findings expand our understanding of small RNA networks and have broad implications for the study of antiviral RNAi.

Chimeric Zika viruses containing structural protein genes of insect-specific flaviviruses cannot replicate in vertebrate cells due to entry and post-translational restrictions

Long Pine Key virus (LPKV) and Lammi virus are insect-specific flaviviruses that phylogenetically affiliate with dual-host flaviviruses. The goal of this study was to provide insight into the genetic determinants that condition this host range restriction. Chimeras were initially created by replacing select regions of the Zika virus genome, including the premembrane and envelope protein (prM-E) genes, with the corresponding regions of the LPKV genome. Of the four chimeras produced, one (the prM-E swap) yielded virus that replicated in mosquito cells. Another chimeric virus with a mosquito replication-competent phenotype was created by inserting the prM-E genes of Lammi virus into a Zika virus genetic background. Vertebrate cells did not support the replication of either chimeric virus although trace to modest amounts of viral antigen were produced, consistent with suboptimal viral entry. These data suggest that dual-host affiliated insect-specific flaviviruses cannot replicate in vertebrate cells due to entry and post-translational restrictions.

Discovery of a Novel Flavivirus (Flaviviridae) From the Horse Fly, Tabanus rufidens (Diptera: Tabanidae): The Possible Coevolutionary Relationships Between the Classical Insect-Specific Flaviviruses and Host Dipteran Insects

Tabanid flies (Tabanidae: Diptera) are common hematophagous insects known to transmit some pathogens mechanically or biologically to animals; they are widely distributed throughout the world. However, no tabanid-borne viruses, except mechanically transmitted viruses, have been reported to date. In this study, we conducted RNA virome analysis of several human-biting tabanid species in Japan, to discover and characterize viruses associated with tabanids. A novel flavivirus was encountered during the study in the Japanese horse fly, Tabanus rufidens (Bigot, 1887). The virus was detected only in T. rufidens, but not in other tabanid species, and as such was designated Tabanus rufidens flavivirus (TrFV). TrFV could not be isolated using a mammalian cell line and showed a closer phylogenetic relationship to the classical insect-specific flaviviruses (cISFs) rather than the vertebrate-infecting flaviviruses (VIFs), suggesting that it is a novel member of the cISFs. The first discovery of a cISF from Brachycera provides new insight into the evolutionary history and dynamics of flaviviruses.

Detection of DENV-2 and Insect-Specific Flaviviruses in Mosquitoes Collected From Jeddah, Saudi Arabia

Background

Mosquito-borne diseases are rapidly spreading due to increasing international travel and trade. Routine mosquito surveillance and screening for mosquito-borne pathogens can be early indicators for local disease transmission and outbreaks. However, arbovirus detection in mosquito vectors has rarely been reported in Saudi Arabia.

Methods

A total of 769,541 Aedes and Culex mosquitoes were collected by Black Hole traps during routine mosquito surveillance in the first half of 2016. Culex. quinquefasciatus and Ae. aegypti were the most prevalent species observed. Twenty-five and 24 randomly selected pools of Ae. aegypti and Cx. quinquefasciatus, respectively, were screened for arboviruses by RT-PCR.

Results

Dengue 2 (DENV-2) and four strains of insect-specific flaviviruses, including one of cell-fusing agent virus (CFAV) and three of Phlebotomus-associated flavivirus (PAFV) were detected in pools of Ae. aegypti. We also detected 10 strains of Culex flavivirus (CxFV) in pools of Cx. quinquefasciatus. Phylogenetic analysis using whole genome sequences placed the DENV strain into the cosmopolitan 1 sub-DENV-2 genotype, and the CxFVs into the African/Caribbean/Latin American genotype. These analyses also showed that the DENV-2 strain detected in the present study was closely related to strains detected in China in 2014 and in Japan in 2018, which suggests frequent movement of DENV-2 strains among these countries. Furthermore, the phylogenetic analysis suggested at least five introductions of DENV-2 into Saudi Arabia from 2014 through 2018, most probably from India.

Conclusions

To our knowledge, this study reports the first detection of four arboviruses DENV, CFAV, PAFV, and CxFV in mosquitoes in Saudi Arabia, which shows that they are co-circulating in Jeddah. Our findings show a need for widespread mosquito-based arbovirus surveillance programs in Saudi Arabia, which will improve our understanding of the transmission dynamics of the mosquito-borne arboviruses within the country and help early predict and mitigate the risk of human infections and outbreaks.

Insect-Specific Viruses: An overview and their relationship to arboviruses of concern to humans and animals

The group of Insect-specific viruses (ISVs) includes viruses apparently restricted to insects based on their inability to replicate in the vertebrates. Increasing numbers of ISVs have been discovered and characterized representing a diverse number of viral families. However, most studies have focused on those ISVs belonging to the family Flaviviridae, which highlights the importance of ISV study from other viral families, which allow a better understanding for the mechanisms of transmission and evolution used for this diverse group of viruses. Some ISVs have shown the potential to modulate arboviruses replication and vector competence of mosquitoes. Based on this, ISVs may be used as an alternative tool for biological control, development of vaccines, and diagnostic platforms for arboviruses. In this review, we provide an update of the general characteristics of ISVs and their interaction with arboviruses that infect vertebrates.

Novel Flavivirus Attenuation Markers Identified in the Envelope Protein of Alfuy Virus

Alfuy (ALFV) is an attenuated flavivirus related to the Murray Valley encephalitis virus (MVEV). We previously identified markers of attenuation in the envelope (E) protein of the prototype strain (ALFV₃₉₂₉), including the hinge region (E273-277) and lack of glycosylation at E154-156. To further determine the mechanisms of attenuation we assessed ALFV₃₉₂₉ binding to glycosaminoglycans (GAG), a known mechanism of flaviviruses attenuation. Indeed, ALFV₃₉₂₉ exhibited reduced binding to GAG-rich cells in the presence of heparin; however, low-passage ALFV isolates were relatively unaffected. Sequence comparisons between ALFV strains and structural modelling incriminated a positively-charged residue (K327) in ALFV₃₉₂₉ as a GAG-binding motif. Substitution of this residue to the corresponding uncharged residue in MVEV (L), using a previously described chimeric virus containing the prM & E genes of ALFV₃₉₂₉ in the backbone of MVEV (MVEV/ALFV-prME), confirmed a role for K327 in enhanced GAG binding. When the wild type residues at E327, E273-277 and E154-156 of ALFV₃₉₂₉ were replaced with the corresponding residues from virulent MVEV, it revealed each motif contributed to attenuation of ALFV₃₉₂₉, with the E327/E273-277 combination most dominant. These data demonstrate that attenuation of ALFV₃₉₂₉ is multifactorial and provide new insights for the rational design of attenuated flavivirus vaccines.

Diverse mosquito-specific flaviviruses in the Bolivian Amazon basin

The genus Flavivirus includes a range of mosquito-specific viruses in addition to well-known medically important arboviruses. Isolation and comprehensive genomic analyses of viruses in mosquitoes collected in Bolivia resulted in the identification of three novel flavivirus species. Psorophora flavivirus (PSFV) was isolated from Psorophora albigenu. The coding sequence of the PSFV polyprotein shares 60 % identity with that of the Aedes-associated lineage II insect-specific flavivirus (ISF), Marisma virus. Isolated PSFV replicates in both Aedes albopictus- and Aedes aegypti-derived cells, but not in mammalian Vero or BHK-21 cell lines. Two other flaviviruses, Ochlerotatus scapularis flavivirus (OSFV) and Mansonia flavivirus (MAFV), which were identified from Ochlerotatus scapularis and Mansonia titillans, respectively, group with the classical lineage I ISFs. The protein coding sequences of these viruses share only 60 and 40 % identity with the most closely related of known lineage I ISFs, including Xishuangbanna aedes flavivirus and Sabethes flavivirus, respectively. Phylogenetic analysis suggests that MAFV is clearly distinct from the groups of the current known Culicinae-associated lineage I ISFs. Interestingly, the predicted amino acid sequence of the MAFV capsid protein is approximately two times longer than that of any of the other known flaviviruses. Our results indicate that flaviviruses with distinct features can be found at the edge of the Bolivian Amazon basin at sites that are also home to dense populations of human-biting mosquitoes.

Transmission competence of a new mesonivirus, Yichang virus, in mosquitoes and its interference with representative flaviviruses

Advances in technology have greatly stimulated the understanding of insect-specific viruses (ISVs). Unfortunately, most of these findings are based on sequencing technology, and laboratory data are scarce on the transmission dynamics of ISVs in nature and the potential effects of these viruses on arboviruses. Mesonivirus is a class of ISVs with a wide geographical distribution. Recently, our laboratory reported the isolation of a novel strain of mesonivirus, Yichang virus (YCV), from Culex mosquitoes, China. In this study, the experimental infection of YCV by the oral route for adult and larvae mosquitoes, and the vertical transmission has been conducted, which suggests that YCV could adopt a mixed-mode transmission. Controlled experiments showed that the infectivity of YCV depends on the mosquito species, virus dose, and infection route. The proliferation curve and tissue distribution of YCV in Cx. quinquefasciatus and Ae. albopictus showed that YCV is more susceptible to Ae. albopictus and is located in the midgut. Furthermore, we also assessed the interference of YCV with flaviviruses both in vitro and in vivo. YCV significantly inhibited the proliferation of DENV-2 and ZIKV, in cell culture, and reduced transmission rate of DENV-2 in Ae. albopictus. Our work provides insights into the transmission of ISVs in different mosquito species during ontogeny and their potential ability to interact with mosquito-borne viruses.

Superinfection Exclusion in Mosquitoes and Its Potential as an Arbovirus Control Strategy

The continuing emergence of arbovirus disease outbreaks around the world, despite the use of vector control strategies, warrants the development of new strategies to reduce arbovirus transmission. Superinfection exclusion, a phenomenon whereby a primary virus infection prevents the replication of a second closely related virus, has potential to control arbovirus disease emergence and outbreaks. This phenomenon has been observed for many years in plants, insects and mammalian cells. In this review, we discuss the significance of identifying novel vector control strategies, summarize studies exploring arbovirus superinfection exclusion and consider the potential for this phenomenon to be the basis for novel arbovirus control strategies.

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.

Chimeric viruses of the insect-specific flavivirus Palm Creek with structural proteins of vertebrate-infecting flaviviruses identify barriers to replication of insect-specific flaviviruses in vertebrate cells

Here we report the generation of novel chimeric flaviviruses, which express the prM and E proteins of either dengue or Zika viruses on the genomic backbone of Palm Creek virus (PCV), an insect-specific flavivirus. The chimeric virus particles were antigenically indistinguishable from their parental prM-E donors, but were unable to infect vertebrate cells. An additional chimera (PCV structural genes in the backbone of West Nile virus - WNV/PCV-prME) was also unable to infect vertebrate cells, but transfection with RNA from this virus resulted in detectable RNA replication and translation but no infectious virion production. These data suggest multiple blocks at the entry, RNA replication and assembly/release stages of insect-specific flavivirus (ISF) infection in vertebrate cells. Serial passaging of these chimeric viruses in mosquito cells identified amino acid substitutions that may lead to increased replication efficiency. These chimeric viruses provide unique tools to further dissect the mechanisms of the host restriction of ISFs.

Exploiting insect-specific viruses as a novel strategy to control vector-borne disease

Novel insect-specific viruses (ISVs) are being discovered in many important vectors due to advances in sequencing technology and a growing awareness of the virome. Several in vitro and in vivo studies indicate that ISVs are capable of modulating pathogenic arboviruses. In addition, there is growing evidence that both vertical and horizonal transmission strategies maintain ISVs in vector populations. As such there is potential to exploit ISVs for stand-alone vector control strategies and deploying them in synergy with other symbiont control approaches such as Wolbachia-mediated control. However, before the applied potential can be realized, a greater understanding of their basic biology is required, including their species range, ability to be maintained and transmitted in native and non-native vector hosts, and the effect of infection on a range of pathogens.

Cell fusing agent virus (Flavivirus) infection in Aedes aegypti in Texas: seasonality, comparison by trap type, and individual viral loads

South Texas has experienced local transmission of Zika virus and of other mosquito-borne viruses such as chikungunya virus and dengue virus in the last decades. Using a mosquito surveillance program in the Lower Rio Grande Valley (LRGV) and San Antonio, TX, from 2016 to 2018, we detected the presence of an insect-specific virus, cell fusing agent virus (CFAV), in the Aedes aegypti mosquito population. We tested 6,326 females and 1,249 males from the LRGV and 659 females from San Antonio for CFAV by RT-PCR using specific primers. Infection rates varied from 0 to 261 per 1,000 mosquitoes in the LRGV and 115 to 208 per 1,000 in San Antonio depending on the month of collection. Infection rates per 1,000 individuals appeared higher in females collected from BG Sentinel 2 traps compared to Autocidal Gravid Ovitraps, but the ratio of the percentage of infected pools did not differ by trap type. The natural viral load in individual males ranged from 1.25 x 10² to 5.50 x 10⁶ RNA copies and in unfed females from 5.42 x 10³ to 8.70 x 10⁶ RNA copies. Gravid females were found to harbor fewer viral particles than males and unfed females.