Demonstration of efficient vertical and venereal transmission of dengue virus type-2 in a genetically diverse laboratory strain of Aedes aegypti

The potential role of the Asian bush mosquito Aedes japonicus as spillover vector for West Nile virus in the Netherlands

BACKGROUND

In recent years the Asian bush mosquito Aedes japonicus has invaded Europe, including the Netherlands. This species is a known vector for a range of arboviruses, possibly including West Nile virus (WNV). As WNV emerged in the Netherlands in 2020, it is important to investigate the vectorial capacity of mosquito species present in the Netherlands to estimate the risk of future outbreaks and further spread of the virus. Therefore, this study evaluates the potential role of Ae. japonicus in WNV transmission and spillover from birds to dead-end hosts in the Netherlands.

METHODS

We conducted human landing collections in allotment gardens (Lelystad, the Netherlands) in June, August and September 2021 to study the diurnal and seasonal host-seeking behaviour of Ae. japonicus. Furthermore, their host preference in relation to birds using live chicken-baited traps was investigated. Vector competence of field-collected Ae. japonicus mosquitoes for two isolates of WNV at two different temperatures was determined. Based on the data generated from these studies, we developed a Susceptible-Exposed-Infectious-Recovered (SEIR) model to calculate the risk of WNV spillover from birds to humans via Ae. japonicus, under the condition that the virus is introduced and circulates in an enzootic cycle in a given area.

RESULTS

Our results show that Ae. japonicus mosquitoes are actively host seeking throughout the day, with peaks in activity in the morning and evening. Their abundance in August was higher than in June and September. For the host-preference experiment, we documented a small number of mosquitoes feeding on birds: only six blood-fed females were caught over 4 full days of sampling. Finally, our vector competence experiments with Ae. japonicus compared to its natural vector Culex pipiens showed a higher infection and transmission rate when infected with a local, Dutch, WNV isolate compared to a Greek isolate of the virus. Interestingly, we also found a small number of infected Cx. pipiens males with virus-positive leg and saliva samples.

CONCLUSIONS

Combining the field and laboratory derived data, our model predicts that Ae. japonicus could act as a spillover vector for WNV and could be responsible for a high initial invasion risk of WNV when present in large numbers.

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.

Oxidative stress governs mosquito innate immune signalling to reduce chikungunya virus infection in Aedes-derived cells

Arboviruses such as chikungunya, dengue and zika viruses cause debilitating diseases in humans. The principal vector species that transmits these viruses is the Aedes mosquito. Lack of substantial knowledge of the vector species hinders the advancement of strategies for controlling the spread of arboviruses. To supplement our information on mosquitoes' responses to virus infection, we utilized Aedes aegypti-derived Aag2 cells to study changes at the transcriptional level during infection with chikungunya virus (CHIKV). We observed that genes belonging to the redox pathway were significantly differentially regulated. Upon quantifying reactive oxygen species (ROS) in the cells during viral infection, we further discovered that ROS levels are considerably higher during the early hours of infection; however, as the infection progresses, an increase in antioxidant gene expression suppresses the oxidative stress in cells. Our study also suggests that ROS is a critical regulator of viral replication in cells and inhibits intracellular and extracellular viral replication by promoting the Rel2-mediated Imd immune signalling pathway. In conclusion, our study provides evidence for a regulatory role of oxidative stress in infected Aedes-derived cells.

Mosquito transgenerational antiviral immunity is mediated by vertical transfer of virus DNA sequences and RNAi

Mosquitoes are important vectors for transmission of many viruses of public and veterinary health concern. These viruses most commonly have an RNA genome and infect mosquitoes for life. The principal mosquito antiviral response is the RNAi system which destroys virus RNA. Here, we confirm an earlier study that Aedes aegypti mosquitoes infected with positive-stranded RNA arboviruses can transmit specific immunity to their offspring. We show that this trans-generational immunity requires replication of virus RNA and reverse transcription of vRNA to vDNA in the infected parents and intergenerational transfer of vDNA. This vDNA is both genome-integrated and episomal. The episomal vDNA sequences are flanked by retrotransposon long-terminal repeats, predominantly Copia-like. Integrated vDNA sequences are propagated along several generations but specific immunity is effective only for a few generations and correlates with the presence of vRNA and episomal vDNA. This understanding raises new possibilities for the control of important mosquito-borne virus diseases.

Transovarial transmission of mosquito-borne viruses: a systematic review

Background

A number of mosquito-borne viruses (MBVs), such as dengue virus (DENV), zika virus (ZIKV), chikungunya (CHIKV), West Nile virus (WNV), and yellow fever virus (YFV) exert adverse health impacts on the global population. Aedes aegypti and Aedes albopictus are the prime vectors responsible for the transmission of these viruses. The viruses have acquired a number of routes for successful transmission, including horizontal and vertical transmission. Transovarial transmission is a subset/type of vertical transmission adopted by mosquitoes for the transmission of viruses from females to their offspring through eggs/ovaries. It provides a mechanism for these MBVs to persist and maintain their lineage during adverse climatic conditions of extremely hot and cold temperatures, during the dry season, or in the absence of susceptible vertebrate host when horizontal transmission is not possible.

Methods

The publications discussed in this systematic review were searched for using the PubMed, Scopus, and Web of Science databases, and websites such as those of the World Health Organization (WHO) and the European Centre for Disease Prevention and Control, using the search terms "transovarial transmission" and "mosquito-borne viruses" from 16 May 2023 to 20 September 2023.

Results

A total of 2,391 articles were searched, of which 123 were chosen for full text evaluation, and 60 were then included in the study after screening and removing duplicates.

Conclusion

The present systematic review focuses on understanding the above diseases, their pathogenesis, epidemiology and host-parasite interactions. The factors affecting transovarial transmission, potential implications, mosquito antiviral defense mechanism, and the control strategies for these mosquito-borne viral diseases (MBVDs) are also be included in this review.

Natural vertical transmission of dengue virus in Latin America and the Caribbean: highlighting its detection limitations and potential significance

Dengue continues to be a major public health concern in Latin America and the Caribbean with many countries in the region having experienced drastic increases in the incidence of dengue over the past few years. Dengue virus is predominantly transmitted by the bite of an infected female Aedes aegypti mosquito via a process called horizontal transmission. However, the virus may also be transmitted from an infected female mosquito to her offspring by vertical transmission, which occurs via viral invasion of the ovary either at the time of fertilization or during oviposition. In this way, mosquitoes may become dengue virus infected before ever encountering a human host. While some researchers have reported this phenomenon and suggested it may serve as a reservoir for the dengue virus in nature, others have questioned its epidemiological significance because of the low frequency at which it has been observed. Several researchers have either altogether failed to detect it or observed its occurrence at low frequencies. However, some studies have attributed these failures to small sample sizes as well as poor sensitivities of screening methods employed. Therefore, an overview of the occurrence, significance and limitations of detection of vertical transmission of dengue virus in Aedes mosquitoes in nature within Latin America and the Caribbean will be the focus of this review.

Detection of La Crosse Virus In Situ and in Individual Progeny to Assess the Vertical Transmission Potential in Aedes albopictus and Aedes aegypti

La Crosse virus (LACV) is circulating in the midwestern and southeastern states of the United States and can cause human encephalitis. The main vector of the virus is the eastern tree-hole mosquito, Aedes triseriatus. Ae. albopictus has been also described as a natural LACV vector, while Ae. aegypti has been infected with the virus under laboratory conditions. Here, we compare the vertical transmission potential of LACV in Ae. albopictus and Ae. aegypti, with emphasis given to the ovarian infection patterns that the virus generates in both species. Both mosquito species received artificial bloodmeals containing LACV. At defined time points post-infection/bloodmeal, midguts, head tissue, and ovaries were analyzed for the presence of virus. Viral infection patterns in the ovaries were visualized via immunofluorescence confocal microscopy and immunohistopathology assays using an LACV-specific monoclonal antibody. In Ae. aegypti, LACV was confronted with midgut infection and escape barriers, which were much less pronounced in Ae. albopictus, resulting in a significantly higher prevalence of infection in the latter. Following the ingestion of a single virus-containing bloodmeal, no progeny larvae were found to be virus-infected. Regardless, females of both species showed the presence of LACV antigen in their ovariole sheaths. Furthermore, in a single Ae. albopictus female, viral antigen was associated with the nurse cells inside the primary follicles. Following the ingestion of a second non-infectious bloodmeal at 7- or 10-days post-ingestion of an LACV-containing bloodmeal, more progeny larvae of Ae. albopictus than of Ae. aegypti were virus-infected. LACV antigen was detected in the egg chambers and ovariole sheaths of both mosquito species. Traces of viral antigen were also detected in a few oocytes from Ae. albopictus. The low level of vertical transmission and the majority of the ovarian infection patterns suggested the transovum rather than transovarial transmission (TOT) of the virus in both vector species. However, based on the detection of LACV antigen in follicular tissue and oocytes, there was the potential for TOT among several Ae. albopictus females. Thus, TOT is not a general feature of LACV infection in mosquitoes. Instead, the TOT of LACV seems to be dependent on its particular interaction with the reproductive tissues of a female.

Surveillance of Aedes aegypti and Aedes albopictus (Diptera: Culicidae) as a Method for Prevention of Arbovirus Transmission in Urban and Seaport Areas of the Southern Coast of Brazil

Entomological surveillance is a traditional method to measure presence, distribution, and seasonal variation of vectors in urban areas, and is essential to targeted control activities to prevent arbovirus transmission. Ovitraps as one of the main components of surveillance programs, enable determination of female oviposition behavior, as well as identification of seasonal variations of the vector. The goals of this study were 1) to detect the mosquitos (Aedes aegypti Linnaeus) and (Aedes albopictus Skuse) (Diptera:Culicidae), in Paranaguá city, 2) to assess ovitrap positive index (OPI), egg density index (EDI), and their relationship with meteorological variables, and 3) to evaluate the vertical transmission of dengue, Zika, and chikungunya in Ae. aegypti and Ae. albopictus. The study was carried out in urban areas of Paranaguá city, an important port region of Brazil, from June 2017 to November 2018. The city was divided into 16 area-clusters. Three-hundred and thirty-one ovitraps were installed monthly, remaining for four days in selected places. Kernel density maps were done to compare the spatiotemporal distribution of collected eggs. Areas which maintained constant oviposition associated with vector activity were identified and were found to overlap the area-clusters with the highest EDI. As viral RNA was not detected, vertical transmission was likely not a maintenance mechanism of arbovirus circulation in Ae. aegypti and Ae. albopictus vectors. This study reiterates the importance, efficiency, and feasibility of ovitraps to monitor the presence and dynamics of Aedes spp. populations.

Study of Dengue Virus Transovarial Transmission in Aedes spp. in Ternate City Using Streptavidin-Biotin-Peroxidase Complex Immunohistochemistry

Aedes aegypti is the most dominant vector in the transmission of dengue hemorrhagic fever (DHF). In addition to Ae. aegypti, Ae. albopictus is a secondary vector of the dengue virus, and both species are widespread in Indonesia. The dengue virus is transmitted from person to person through the bite of an Aedes spp. The vertical (transovarial) transmission of the dengue virus from infective female mosquitoes to their offspring is one of the means by which the dengue virus maintains its existence in nature. Transovarial dengue virus transmission in Aedes spp. mosquitoes contributes to the spread and maintenance of the dengue epidemic. This study employed a qualitative survey to detect dengue virus transovarial transmission in Ternate using the streptavidin-biotin-peroxidase complex (ISBPC) immunohistochemical test. The ISBPC examination of samples collected from the four subdistricts in Ternate revealed a positive result for transovarial transmission of dengue virus. Four Aedes spp., including two Ae. aegypti females, one Ae. albopictus female, and one Ae. albopictus male, tested positive for transovarial transmission of dengue virus in the district of North Ternate. Four Aedes spp., including three Ae. aegypti females and one Ae. aegypti male, were found to be positive for the transovarial transmission of dengue virus in the Central Ternate district. Seven Aedes spp., including five Ae. aegypti females, one Ae. aegypti male, and one Ae. albopictus female, tested positive for transovarial transmission of the dengue virus in the district of South Ternate city. One Ae. aegypti male showed positive results for transovarial transmission of dengue virus in the Ternate Island District. In this study, the transovarial transmission of the dengue virus occurred in both Aedes spp. female and male mosquitoes. It was demonstrated that Aedes spp. carry the dengue virus in their ovaries and can pass it on to their offspring. As a result, the cycle of passing the dengue virus on to local mosquito populations in the city of Ternate is not going to end just yet.

Coupled small molecules target RNA interference and JAK/STAT signaling to reduce Zika virus infection in Aedes aegypti

The recent global Zika epidemics have revealed the significant threat that mosquito-borne viruses pose. There are currently no effective vaccines or prophylactics to prevent Zika virus (ZIKV) infection. Limiting exposure to infected mosquitoes is the best way to reduce disease incidence. Recent studies have focused on targeting mosquito reproduction and immune responses to reduce transmission. Previous work has evaluated the effect of insulin signaling on antiviral JAK/STAT and RNAi in vector mosquitoes. Specifically, insulin-fed mosquitoes resulted in reduced virus replication in an RNAi-independent, ERK-mediated JAK/STAT-dependent mechanism. In this work, we demonstrate that targeting insulin signaling through the repurposing of small molecule drugs results in the activation of both RNAi and JAK/STAT antiviral pathways. ZIKV-infected Aedes aegypti were fed blood containing demethylasterriquinone B1 (DMAQ-B1), a potent insulin mimetic, in combination with AKT inhibitor VIII. Activation of this coordinated response additively reduced ZIKV levels in Aedes aegypti. This effect included a quantitatively greater reduction in salivary gland ZIKV levels up to 11 d post-bloodmeal ingestion, relative to single pathway activation. Together, our study indicates the potential for field delivery of these small molecules to substantially reduce virus transmission from mosquito to human. As infections like Zika virus are becoming more burdensome and prevalent, understanding how to control this family of viruses in the insect vector is an important issue in public health.

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.

First Evidence of Horizontal Transmission by Fecal Shedding of Dengue Virus 4 Among Aedes aegypti Larvae (Diptera: Culicidae) Under Laboratory Conditions

The transmission pathways of dengue virus (DENV) among mosquitoes are a topic that has gained relevance in recent years because they could explain the maintenance of the virus in the wild independently of the human-mosquito horizontal transmission cycle. In this regard, Aedes aegypti larvae exposed to supernatants of C6/36 cells infected with DENV-4 were evaluated for virus excretion in feces and viability of infection in immature stages (larvae). The results demonstrate that larvae excrete DENV-4 in their feces with the potential to at least infect immature individuals of the same species. A horizontal transmission pathway of larvae-larvae DENV-4 under laboratory conditions is suggested.

Dengue Virus-2 Infection Affects Fecundity and Elicits Specific Transcriptional Changes in the Ovaries of Aedes aegypti Mosquitoes

Dengue fever (DF), caused by the dengue virus (DENV), is the most burdensome arboviral disease in the world, with an estimated 400 million infections each year. The Aedes aegypti mosquito is the main vector of DENV and transmits several other human pathogens, including Zika, yellow fever, and chikungunya viruses. Previous studies have shown that the pathogen infection of mosquitoes can alter reproductive fitness, revealing specific vector-pathogen interactions that are key determinants of vector competence. However, only a handful of studies have examined the effect of DENV infection in A. aegypti, showing a reduction in lifespan and fecundity over multiple blood meals. To provide a more comprehensive analysis of the impact of DENV infection on egg laying and fecundity, we assessed egg laying timing in DENV-2 blood-fed mosquitoes (infected group) compared to mock blood-fed mosquitoes (control group). We confirmed a significant decrease in fecundity during the first gonadotrophic cycle. To further investigate this phenotype and the underlying DENV-2 infection-dependent changes in gene expression, we conducted a transcriptomic analysis for differentially expressed genes in the ovaries of A. aegypti infected with DENV-2 vs. mock-infected mosquitoes. This analysis reveals several DENV-2-regulated genes; among them, we identified a group of 12 metabolic genes that we validated using reverse transcription-quantitative PCR (RT-qPCR). Interestingly, two genes found to be upregulated in DENV-infected mosquito ovaries exhibited an antiviral role for DENV-2 in an Aedes cell line. Altogether, this study offers useful insights into the virus-vector interface, highlighting the importance of gene expression changes in the mosquito's ovary during DENV-2 infection in the first gonadotrophic  cycle,  triggering  antiviral  responses  that  may  possibly  interfere  with mosquito reproduction. This information is extremely relevant for further investigation of A. aegypti's ability to tolerate viruses since virally infected mosquitoes in nature constitute a powerful source of supporting viruses during intra-epidemic periods, causing a huge burden on the public health system.

Lipids fluctuations in mosquitoes upon arboviral infections

Arboviruses are responsible for several emerging and re-emerging infectious diseases, with dengue, Zika virus disease and Chikungunya fever being the most important arboviral diseases nowadays. Infection of these viruses depends primarily on its ability to replicate and disseminate in mosquitoes. Since these viruses are enveloped, viral replication, assembly and release occurs in the cellular membranes, which depends on the manipulation of host lipid metabolism. Specifically in mammalian cells replication, they use host lipids to establish a compartment known as replication complex that contains the replicase complex. This complex includes viral RNA, proteins and host factors necessary for a successful replication in mammalian cells. Although little is known about extrinsic factor(s) needed for arbovirus replication in vectors,recent reports show that high lipid concentrations are related with increased viral replication in mosquito cells infected with dengue, Zika and Chikungunya viruses. Here, we present a review that focuses on the cellular mechanisms and the lipid environment alteration in mosquito vector after arbovirus infection and their relationship with arbovirus replication.

Geographic Partitioning of Dengue Virus Transmission Risk in Florida

Dengue viruses (DENVs) cause the greatest public health burden globally among the arthropod-borne viruses. DENV transmission risk has also expanded from tropical to subtropical regions due to the increasing range of its principal mosquito vector, Aedes aegypti. Focal outbreaks of dengue fever (dengue) in the state of Florida (FL) in the USA have increased since 2009. However, little is known about the competence of Ae. aegypti populations across different regions of FL to transmit DENVs. To understand the effects of DENV genotype and serotype variations on vector susceptibility and transmission potential in FL, we orally infected a colony of Ae. aegypti (Orlando/ORL) with low passage or laboratory DENV-1 through -4. Low passage DENVs were more infectious to and had higher transmission potential by ORL mosquitoes. We used these same DENVs to examine natural Ae. aegypti populations to determine whether spatial distributions correlated with differential vector competence. Vector competence across all DENV serotypes was greater for mosquitoes from areas with the highest dengue incidence in south FL compared to north FL. Vector competence for low passage DENVs was significantly higher, revealing that transmission risk is influenced by virus/vector combinations. These data support a targeted mosquito-plus-pathogen screening approach to more accurately estimate DENV transmission risk.

Zika virus infects Aedes aegypti ovaries

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

Evidence of Zika virus horizontal and vertical transmission in Aedes albopictus from Spain but not infectious virus in saliva of the progeny

Aedes albopictus mosquitoes have been experimentally demonstrated to be a competent vector for Zika virus (ZIKV) in different countries, but there are still some gaps related to the importance of Ae. albopictus in ZIKV transmission. Recent studies on Spanish Ae. albopictus populations showed controversial results for ZIKV transmission and no studies have been performed yet to detect infectious ZIKV in saliva of progeny of infected female mosquitoes. Herein, the horizontal transmission (HT) and vertical transmission (VT) of ZIKV in field-collected Ae. albopictus mosquitoes from Spain were evaluated for ZIKV strains (African I and Asian lineages) to better estimate the risk of ZIKV transmission by Ae. albopictus. The two field-collected Ae. albopictus populations assayed were infected by all tested ZIKV strains, however differences in terms of vector competence were detected depending on strain-population combination. Moreover, a higher susceptibility to the African I lineage strain than to the Asian lineage strain was observed in both mosquito populations. On the other hand, VT was demonstrated for both ZIKV lineages, detecting the virus in both males and females of the progeny of infected females, although importantly ZIKV dissemination and transmission were not detected in the infected females from the offspring. The results of the present study demonstrate that Spanish Ae. albopictus populations could sustain virus transmission in case of ZIKV introduction, but VT would play a poor role in the ZIKV epidemiology. Overall, our results provide helpful information to health authorities to establish efficient surveillance and vector control programmes for ZIKV.

RNA-Seq analysis of blood meal induced gene-expression changes in Aedes aegypti ovaries

Background

Transmission of pathogens by vector mosquitoes is intrinsically linked with mosquito’s reproductive strategy because anautogenous mosquitoes require vertebrate blood to develop a batch of eggs. Each cycle of egg maturation is tightly linked with the intake of a fresh blood meal for most species. Mosquitoes that acquire pathogens during the first blood feeding can transmit the pathogens to susceptible hosts during subsequent blood feeding and also vertically to the next generation via infected eggs. Large-scale gene-expression changes occur following each blood meal in various tissues, including ovaries. Here we analyzed mosquito ovary transcriptome following a blood meal at three different time points to investigate blood-meal induced changes in gene expression in mosquito ovaries.

Results

We collected ovaries from Aedes aegypti that received a sugar meal or a blood meal on days 3, 10 and 20 post blood meal for transcriptome analysis. Over 4000 genes responded differentially following ingestion of a blood meal on day 3, and 660 and 780 genes on days 10 and 20, respectively. Proteins encoded by differentially expressed genes (DEGs) on day 3 include odorant binding proteins (OBPs), defense-specific proteins, and cytochrome P450 detoxification enzymes. In addition, we identified 580 long non-coding RNAs that are differentially expressed at three time points. Gene ontology analysis indicated that genes involved in peptidase activity, oxidoreductase activity, extracellular space, and hydrolase activity, among others were enriched on day 3. Although most of the DEGs returned to the nonsignificant level compared to the sugar-fed mosquito ovaries following oviposition on days 10 and 20, there remained differences in the gene expression pattern in sugar-fed and blood-fed mosquitoes.

Conclusions

Enrichment of OBPs following blood meal ingestion suggests that these genes may have other functions besides being part of the olfactory system. The enrichment of immune-specific genes and cytochrome P450 genes indicates that ovaries become well prepared to protect their germ line from any pathogens that may accompany the blood meal or from environmental contamination during oviposition, and to deal with the detrimental effects of toxic metabolites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07551-z.

Natural Vertical Transmission of Dengue Virus Serotype 4 in Aedes aegypti Larvae from Urban Areas in Sinaloa, Mexico

Dengue virus (DENV) is transmitted to humans by the bite of the vector Aedes aegypti. Several researchers have suggested that the mechanism of vertical transmission of DENV in the vector is a key aspect for the prevalence of the virus in the environment and the potentiation of epidemic outbreaks of the disease. In this context and as part of an integrated study of DENV serotypes in mosquitoes of urban areas in Sinaloa, Mexico, the presence of DENV-4 in larval stages of Ae. aegypti was evaluated to demonstrate the vertical transmission of this serotype. In total, 672 larvae of Ae. aegypti were collected in 16 sectors and were grouped into 36 pools, of which 41.66% (15/36 pools) tested positive for DENV-4, with a minimum infection rate = 22.32. The analysis of the obtained sequences showed a 98% similarity to the DENV-4 with sequences previously reported in GenBank. These results show that Ae. aegypti acts as a natural reservoir for DENV-4 in this region.

Vertical transmission of zika virus in Aedes albopictus

Background

Zika virus (ZIKV) is an arthropod-borne flavivirus transmitted by Aedes mosquitoes. Aedes albopictus is an important vector of ZIKV worldwide. To date, most experiments have focused on the vertical transmission of ZIKV in Ae. aegypti, while studies on Ae. albopictus are very limited. To explore vertical transmission in Ae. albopictus, a series of laboratory studies were carried out.

Methodology/Principal findings

In this study, Ae. albopictus were blood-fed with ZIKV-infectious blood, and the ovaries and offspring viral infection rates were analyzed by reverse transcription PCR (RT-PCR), real-time reverse transcription PCR (RT-qPCR) and immunohistochemistry (IHC). ZIKV was detected in the ovaries and oviposited eggs in two gonotrophic cycles. The minimum filial egg infection rates in two gonotrophic cycles were 2.06% and 0.69%, and the effective population transmission rate was 1.87%. The hatching, pupation, and emergence rates of infected offspring were not significantly different from those of uninfected offspring, indicating that ZIKV did not prevent the offspring from completing the growth and development process. ZIKV was detected in three of thirteen C57BL/6 suckling mice bitten by ZIKV-positive F₁ females, and the viremia persisted for at least seven days.

Conclusions/Significance

ZIKV can be vertically transmitted in Ae. albopictus via transovarial transmission. The vertical transmission rates in F₁ eggs and adults were 2.06% and 1.87%, respectively. Even though the vertical transmission rates were low, the female mosquitoes infected via the congenital route horizontally transmitted ZIKV to suckling mice through bloodsucking. This is the first experimental evidence of offspring with vertically transmitted ZIKV initiating new horizontal transmission. The present study deepens the understanding of the vertical transmission of flaviviruses in Aedes mosquitoes and sheds light on the prevention and control of mosquito-borne diseases.

Zika virus (ZIKV) is a mosquito-borne flavivirus that poses a serious threat worldwide because of its associated serious neurological complications, such as Guillain-Barré syndrome in adults and microcephaly in newborns. Vertical transmission of ZIKV in humans has been confirmed. Furthermore, there have been reports of ZIKV infection in field-collected eggs, larvae and adult mosquitoes, which implies that ZIKV can also be vertically transmitted in mosquito vectors. However, the characteristics of vertical transmission of ZIKV in Aedes albopictus remain unclear. Here, we infected mosquitoes by allowing them to feed on an infectious blood meal. F₁ progeny (eggs and adults) from mosquitoes with ZIKV-positive ovaries were studied. Our results demonstrate that ZIKV can be vertically transmitted in Ae. albopictus via transovarial transmission. The female mosquitoes infected via the congenital route can horizontally transmit ZIKV to suckling mice through bloodsucking. These updated findings can be used for ZIKV disease prevention and vector control strategies.

Assessing the vertical transmission potential of dengue virus in field-reared Aedes aegypti using patient-derived blood meals in Ho Chi Minh City, Vietnam

BACKGROUND

Dengue viruses (DENV) can be transmitted from an adult female Aedes aegypti mosquito through the germ line to the progeny; however, there is uncertainty if this occurs at a frequency that is epidemiologically significant. We measured vertical transmission of DENV from field-reared Ae. aegypti to their F1 progeny after feeding upon blood from dengue patients. We also examined the transmission potential of F1 females.

METHODS

We examined the frequency of vertical transmission in field-reared mosquitoes, who fed upon blood from acutely viremic dengue patients, and the capacity for vertically infected females to subsequently transmit virus horizontally, in two sets of experiments: (i) compared vertical transmission frequency of field-reared Ae. aegypti and Ae. albopictus, in individual progeny; and (ii) in pooled progeny derived from field- and laboratory-reared Ae. aegypti.

RESULTS

Of 41 DENV-infected and isofemaled females who laid eggs, only a single female (2.43%) transmitted virus to one of the F1 progeny, but this F1 female did not have detectable virus in the saliva when 14 days-old. We complemented this initial study by testing for vertical transmission in another 460 field-reared females and > 900 laboratory-reared counterparts but failed to provide any further evidence of vertical virus transmission.

CONCLUSIONS

In summary, these results using field-reared mosquitoes and viremic blood from dengue cases suggest that vertical transmission is uncommon. Field-based studies that build on these observations are needed to better define the contribution of vertical DENV transmission to dengue epidemiology.

C-Type Lectins Link Immunological and Reproductive Processes in Aedes aegypti

Physiological trade-offs between mosquito immune response and reproductive capability can arise due to insufficient resource availability. C-type lectin family members may be involved in these processes. We established a GCTL-3-/- mutant Aedes aegypti using CRISPR/Cas9 to investigate the role of GCTL-3 in balancing the costs associated with immune responses to arboviral infection and reproduction. GCTL-3-/- mutants showed significantly reduced DENV-2 infection rate and gut commensal microbiota populations, as well as upregulated JAK/STAT, IMD, Toll, and AMPs immunological pathways. Mutants also had significantly shorter lifespans than controls and laid fewer eggs due to defective germ line development. dsRNA knock-down of Attacin and Gambicin, two targets of the AMPs pathway, partially rescued this reduction in reproductive capabilities. Upregulation of immune response following GCTL-3 knock-out therefore comes at a cost to reproductive fitness. Knock-out of other lectins may further improve our knowledge of the molecular and genetic mechanisms underlying reproduction-immunity trade-offs in mosquitoes.

Shifts in the microbiota associated with male mosquitoes (Aedes aegypti) exposed to an obligate gut fungal symbiont (Zancudomyces culisetae)

Research characterizing arthropod-associated microbiota has revealed that microbial dynamics can have an important impact on host phenotypic traits. The influence of fungi on these interactions are emerging as targets for research, especially in organisms associated with global human health. A recent study demonstrated colonization of a widespread gut fungus (Zancudomyces culisetae) in a larval mosquito (Aedes aegypti) digestive tract affected microbiomes in larvae and newly emerged adult females (Frankel-Bricker et al. Appl Environ Microbiol, 2020. 10.1128/AEM.02334-19) but did not investigate these processes in males. The objective of the study presented here was to assess fungal influences on adult male mosquito microbiomes to enable a more complete assessment of fungal–bacterial–host interactions in the A. aegypti–Z. culisetae system. Sequencing of 16S rRNA gene amplicons from microbiomes harbored in adult males directly after emerging from pupae revealed larval fungal exposure significantly decreased overall microbial community diversity, altered microbiome composition and structure, and decreased within-group microbiome variation across individuals. Further, bacteria in the family Burkholderiaceae were present in high abundance in fungal-exposed males, likely contributing to the disparate microbiota between treatment groups. Comparisons between male and the female microbiomes analyzed in Frankel-Bricker et al. (2020), showed distinct shifts in bacterial communities incurred by larval exposure to fungi, potentially revealing sex-specific fungal–bacterial–host dynamics in A. aegypti. These findings highlight the complex role a gut fungus can play in influencing the microbial communities harbored in an important insect and emphasize the significance of accounting for an organism’s sex when studying fungal–bacterial–host dynamics.

Highly Efficient Vertical Transmission for Zika Virus in Aedes aegypti after Long Extrinsic Incubation Time

While the Zika virus (ZIKV) 2014-2017 pandemic has subsided, there remains active transmission. Apart from horizontal transmission to humans, the main vector Aedes aegypti can transmit the virus vertically from mother to offspring. Large variation in vertical transmission (VT) efficiency between studies indicates the influence of parameters, which remain to be characterized. To determine the roles of extrinsic incubation time and gonotrophic cycle, we deployed an experimental design that quantifies ZIKV in individual progeny and larvae. We observed an early infection of ovaries that exponentially progressed. We quantified VT rate, filial infection rate, and viral load per infected larvae at 10 days post oral infection (d.p.i.) on the second gonotrophic cycle and at 17 d.p.i. on the second and third gonotrophic cycle. As compared to previous reports that studied pooled samples, we detected a relatively high VT efficiency from 1.79% at 10 d.p.i. and second gonotrophic cycle to 66% at 17 d.p.i. and second gonotrophic cycle. At 17 d.p.i., viral load largely varied and averaged around 800 genomic RNA (gRNA) copies. Longer incubation time and fewer gonotrophic cycles promoted VT. These results shed light on the mechanism of VT, how environmental conditions favor VT, and whether VT can maintain ZIKV circulation.

Impact of venereal transmission on the dynamics of vertically transmitted viral diseases among mosquitoes

Despite centuries of enormous control efforts, mosquito-borne diseases continue to show upward trend of morbidity. According to WHO reports, malaria caused 438000 deaths in the year 2015 and dengue cases have been increased 30-fold over the last five decades. To control these diseases, it is necessary to understand the transmission dynamics of them among mosquitoes. There are some vertically transmitted mosquito-borne diseases which can also be spread among mosquitoes through sexual contact (e.g., dengue, zika, chikungunya). Recent experimental observations indicate that for virus persistence in mosquito population, the role of venereal transmission cannot be ignored. It is therefore important to investigate which transmission route is more responsible for the persistence of the virus when there is no host. To this aim, we propose and analyze a novel compartmental model considering mosquito population only. To the best of authors knowledge, this is the first attempt to take into account both vertical and sexual transmission of the virus in a mathematical model. Expression representing the basic reproduction number is derived using Jacobian approach. Local stability conditions for disease-free equilibrium and complete infection equilibrium are obtained. Global sensitivity analysis of the system is performed with respect to an epidemiologically important response. While investigating the impact of sexual transmission in presence of vertical transmission, we observed that sexual transmission route has the potential to drive the equilibrium from disease free to endemic states. Further numerical experiments reveal that the virus will have higher half life in fertilized infected female mosquitoes for vertical transmission only than for venereal transmission alone. Furthermore, when both transmission pathways are active, a variety of parameters indicate threshold like behavior of the infection.

Venereal Transmission of Vesicular Stomatitis Virus by Culicoides sonorensis Midges

Culicoides sonorensis biting midges are well-known agricultural pests and transmission vectors of arboviruses such as vesicular stomatitis virus (VSV). The epidemiology of VSV is complex and encompasses a broad range of vertebrate hosts, multiple routes of transmission, and diverse vector species. In temperate regions, viruses can overwinter in the absence of infected animals through unknown mechanisms, to reoccur the next year. Non-conventional routes for VSV vector transmission may help explain viral maintenance in midge populations during inter-epidemic periods and times of adverse conditions for bite transmission. In this study, we examined whether VSV could be transmitted venereally between male and female midges. Our results showed that VSV-infected females could venereally transmit virus to uninfected naïve males at a rate as high as 76.3% (RT-qPCR), 31.6% (virus isolation) during the third gonotrophic cycle. Additionally, VSV-infected males could venereally transmit virus to uninfected naïve females at a rate as high as 76.6% (RT-qPCR), 49.2% (virus isolation). Immunofluorescent staining of micro-dissected reproductive organs, immunochemical staining of midge histological sections, examination of internal reproductive organ morphology, and observations of mating behaviors were used to determine relevant anatomical sites for virus location and to hypothesize the potential mechanism for VSV transmission in C. sonorensis midges through copulation.

Deducing the Role of Virus Genome-Derived PIWI-Associated RNAs in the Mosquito-Arbovirus Arms Race

The P-element-induced wimpy testis (PIWI)-associated RNA (piRNA) pathway is known for its role in the protection of genome integrity in the germline of Drosophila melanogaster by silencing transposable elements. The piRNAs that target transposons originate from piRNA clusters in transposon-rich regions of the Drosophila genome and are processed by three PIWI family proteins. In Aedes aegypti and Aedes albopictus mosquitoes, which are two of the most important vectors of arthropod-borne viruses (arboviruses), the number of PIWI family genes has expanded and some are expressed in somatic, as well as germline, tissues. These discoveries have led to active research to explore the possible expanded functional roles of the piRNA pathway in vector mosquitoes. Virus genome-derived piRNAs (which will be referred to as (virus name) vpiRNAs) have been demonstrated in Aedes spp. cultured cells and mosquitoes after infection by arthropod-borne alpha-, bunya-, and flaviviruses. However, although Culex quinquefasciatus also is an important arbovirus vector and has an expansion of PIWI family genes, vpiRNAs have seldom been documented in this genus after arbovirus infection. Generation of complementary DNA (cDNA) fragments from RNA genomes of alpha-, bunya-, and flaviviruses (viral-derived cDNAs, vDNAs) has been demonstrated in cultured Aedes spp. cells and mosquitoes, and endogenous viral elements (EVEs), cDNA fragments of non-retroviral RNA virus genomes, are found more abundantly in genomes of Ae. aegypti and Ae. albopictus than other vector mosquitoes. These observations have led to speculation that vDNAs are integrated into vector genomes to form EVEs, which serve as templates for the transcription of antiviral vpiRNA precursors. However, no EVEs derived from alphavirus genomes have been demonstrated in genomes of any vector mosquito. In addition, although EVEs have been shown to be a source of piRNAs, the preponderance of EVEs described in Aedes spp. vectors are more closely related to the genomes of persistently infecting insect-specific viruses than to acutely infecting arboviruses. Furthermore, the signature patterns of the “ping-pong” amplification cycle that maintains transposon-targeting piRNAs in Drosophila are also evident in alphavirus and bunyavirus vpiRNAs, but not in vpiRNAs of flaviviruses. These divergent observations have rendered deciphering the mechanism(s) of biogenesis and potential role of vpiRNAs in the mosquito–arbovirus arms race difficult, and the focus of this review will be to assemble major findings regarding vpiRNAs and antiviral immunity in the important arbovirus vectors from Aedes and Culex genera.

Zika Virus Dissemination from the Midgut of Aedes aegypti is Facilitated by Bloodmeal-Mediated Structural Modification of the Midgut Basal Lamina

The arboviral disease cycle requires that key tissues in the arthropod vector become persistently infected with the virus. The midgut is the first organ in the mosquito that needs to be productively infected with an orally acquired virus. Following midgut infection, the virus then disseminates to secondary tissues including the salivary glands. Once these are productively infected, the mosquito is able to transmit the virus to a vertebrate host. Recently, we described the midgut dissemination pattern for chikungunya virus in Aedes aegypti. Here we assess the dissemination pattern in the same mosquito species for Zika virus (ZIKV), a human pathogenic virus belonging to the Flaviviridae. ZIKV infection of secondary tissues, indicative of dissemination from the midgut, was not observed before 72 h post infectious bloodmeal (pibm). Virion accumulation at the midgut basal lamina (BL) was only sporadic, although at 96–120 h pibm, virions were frequently observed between strands of the BL indicative of their dissemination. Our data suggest that ZIKV dissemination from the mosquito midgut occurs after digestion of the bloodmeal. Using gold-nanoparticles of 5 nm and 50 nm size, we show that meal ingestion leads to severe midgut tissue distention, causing the mesh width of the BL to remain enlarged after complete digestion of the meal. This could explain how ZIKV can exit the midgut via the BL after bloodmeal digestion. Ingestion of a subsequent, non-infectious bloodmeal five days after acquisition of an initial, dengue 4 virus containing bloodmeal resulted in an increased number of virions present in the midgut epithelium adjacent to the BL. Thus, subsequent bloodmeal ingestion by an infected mosquito may primarily stimulate de novo synthesis of virions leading to increased viral titers in the vector.

Entomo-virological surveillance strategy for dengue, Zika and chikungunya arboviruses in field-caught Aedes mosquitoes in an endemic urban area of the Northeast of Brazil

Aedes spp. are considered the main vectors of dengue (DENV), Zika (ZIKV) and chikungunya (CHIKV) viruses in the world. Arbovirus detection in Aedes mosquitoes can alert authorities to possible outbreaks, reducing the impact of these diseases. The purpose of this study was to perform an operational strategy for virological surveillance of DENV, ZIKV and CHIKV in adult Aedes aegypti and Aedes albopictus mosquitoes captured at different key-sites in an endemic urban area of the Northeast Region of Brazil, with the prospect of discussing its role as part of an alert system for outbreaks in critical areas. Residential and non-residential premises located in areas of recent of transmission of these arboviruses were selected for adult mosquito collection in the rainy season (July) of 2018. A total of 1068 adult mosquitoes were collected: 946 Culex quinquefasciatus (88.6%), 118 Ae. aegypti (11.0%), two Ae. albopictus (0.2%) and two Aedes taeniorhynchus (0.2%). Among the premises surveyed, recycling points (N = 48, 40.7%), municipal schools (N = 36, 30.5%) and junkyards (N = 31, 26.2%) were the places with the highest frequency of adult Ae. aegypti. Health units (including primary health care facilities and one hospital) (N = 23; 19.5%) together with residential premises (N = 11; 9.3%) presented the lowest frequencies. Total RNAs of the samples were extracted from Aedes mosquitoes and a nested reverse transcription (RT) polymerase chain reaction (PCR) assay for detecting and typing DENV, ZIKV and CHIKV was performed. From the 37 Aedes spp. pools analyzed (35 Ae. aegypti, one Ae. albopictus and one Ae. taeniorhynchus), seven were positive for DENV-3, including three pools containing Ae. aegypti females, one containing an Ae. aegypti engorged female and three comprised of Ae. aegypti males. The positive pools were composed of mosquitoes collected in public schools, health units, junkyards, recycling points and residential premises. Our findings reinforce the importance of continuous virological surveillance in Aedes mosquitoes, as a useful tool for detecting arboviruses circulation in vulnerable areas, even in low infestation seasons.

The electronic song "Scary Monsters and Nice Sprites" reduces host attack and mating success in the dengue vector Aedes aegypti

Sound and its reception are crucial for reproduction, survival, and population maintenance of many animals. In insects, low-frequency vibrations facilitate sexual interactions, whereas noise disrupts the perception of signals from conspecifics and hosts. Despite evidence that mosquitoes respond to sound frequencies beyond fundamental ranges, including songs, and that males and females need to struggle to harmonize their flight tones, the behavioral impacts of music as control targets remain unexplored. In this study, we examined the effects of electronic music (Scary Monsters and Nice Sprites by Skrillex) on foraging, host attack, and sexual activities of the dengue vector Aedes aegypti. Adults were presented with two sound environments (music-off or music-on). Discrepancies in visitation, blood feeding, and copulation patterns were compared between environments with and without music. Ae. aegypti females maintained in the music-off environment initiated host visits earlier than those in the music-on environment. They visited the host significantly less often in the music-on than the music-off condition. Females exposed to music attacked hosts much later than their non-exposed peers. The occurrence of blood feeding activity was lower when music was being played. Adults exposed to music copulated far less often than their counterparts kept in an environment where there was no music. In addition to providing insight into the auditory sensitivity of Ae. aegypti to sound, our results indicated the vulnerability of its key vectorial capacity traits to electronic music. The observation that such music can delay host attack, reduce blood feeding, and disrupt mating provides new avenues for the development of music-based personal protective and control measures against Aedes-borne diseases.

Molecular Basis for Arbovirus Transmission by Aedes aegypti Mosquitoes

Mosquitoes are considered the most important vectors for the transmission of pathogens to humans. Aedes aegypti is a unique species, not only by its highly anthropophilic and peridomestic habits but also because it can transmit an important variety of pathogenic viruses. Examples are dengue, yellow fever, chikungunya, Zika, and Mayaro viruses. After ingesting viremic blood, a wide range of mechanisms are activated in the mosquito to counteract viral infection. Nevertheless, these arboviruses possess strategies to overcome barriers in the mosquito and eventually reach the salivary glands to continue the transmission cycle. However, the infection and eventual transmission of arbovirus depends on multiple factors. The current review focuses in detail on the anatomic, physiological, and molecular characteristics of the mosquito A. aegypti that participate in response to a viral infection. In the past decades, the awareness of the importance of this mosquito as a disease vector and its impact on human health was largely recognized. We need to improve our comprehension of molecular mechanisms that determine the outcome of successful virus replication or control of infection for each arbovirus in the vector; this could lead to the design of effective control strategies in the future.