Arbovirus lifecycle in mosquito: acquisition, propagation and transmission.

Mosquitoes are haematophagous vectors for hundreds of pathogenic viruses that are aetiological agents of human diseases. In nature, mosquito-borne viruses maintain a lifecycle between mosquitoes and vertebrate animals. Viruses are acquired by a naive mosquito from an infected host by blood meals and then propagate extensively in the mosquito's tissues. This mosquito then becomes a virus reservoir and is competent to transmit the viruses to a naive vertebrate host through the next blood meal. To survive in and efficiently cycle between two distinct host environments, mosquito-borne viruses have evolved delicate and smart strategies to comprehensively exploit host and vector factors. Here, we provide an update on recent studies of the mechanisms of virus survival in, acquisition and transmission by mosquitoes.

Monitoring of alien mosquitoes of the genus Aedes (Diptera: Culicidae) in Austria.

Systematic, continuous mosquito surveillance is considered the most reliable tool to predict the spread and establishment of alien mosquito species such as the Asian tiger mosquito (Aedes albopictus), Japanese bush mosquito (Aedes japonicus), and the transmission risk of mosquito-borne arboviruses to humans. Only single individuals of Ae. albopictus have been found in Austria so far. However, it is likely that the species will be able to establish populations in the future due to global trade and traffic as well as increasing temperatures in the course of global climate change. In summer 2017, a project surveilling the oviposition of newly introduced Aedes mosquitoes, using ovitraps, was set up by means of citizen scientists and researchers and was performed in six federal provinces of Austria-Tyrol, Carinthia, Vienna, Lower Austria, Styria, and Burgenland. Eggs of Ae. albopictus were identified in Tyrol during the months August and September, while Ae. japonicus was found in Lower Austria, Styria, and Burgenland. In Vienna and Carinthia, all ovitraps were negative for Aedes eggs; however, Ae. japonicus was found for the first time in Vienna in July 2017 during routine sampling of adult mosquitoes. With this project, we demonstrated the benefits of citizen scientists for ovitrap-based mosquito surveillance. The finding of Ae. albopictus eggs in Northern Tyrol is not yet a proof of the establishment of a self-sustaining population, although it indicates the ongoing introduction of this species along main traffic routes from Italy, where this mosquito is well established. The risk of establishment of the tiger mosquito in the Lower Inn Valley is therefore a given and informing the public about preventive measures to hinder and delay this development is highly recommended.

Declining malaria, rising of dengue and Zika virus: insights for mosquito vector control.

The fight against mosquito-borne diseases is a challenge of huge public health importance. To our mind, 2015 was an extraordinary year for malaria control, due to three hot news: the Nobel Prize to Youyou Tu for the discovery of artemisinin, the development of the first vaccine against Plasmodium falciparum malaria [i.e. RTS,S/AS01 (RTS,S)], and the fall of malaria infection rates worldwide, with special reference to sub-Saharan Africa. However, there are major challenges that still deserve attention, in order to boost malaria prevention and control. Indeed, parasite strains resistant to artemisinin have been detected, and RTS,S vaccine does not offer protection against Plasmodium vivax malaria, which predominates in many countries outside of Africa. Furthermore, the recent outbreaks of Zika virus infections, occurring in South America, Central America and the Caribbean, represent the most recent of four arrivals of important arboviruses in the Western Hemisphere, over the last 20 years. Zika virus follows dengue (which slyly arrived in the hemisphere over decades and became more aggressive in the 1990s), West Nile virus (emerged in 1999) and chikungunya (emerged in 2013). Notably, there are no specific treatments for these arboviruses. The emerging scenario highlights that the effective and eco-friendly control of mosquito vectors, with special reference to highly invasive species such as Aedes aegypti and Aedes albopictus, is crucial. The concrete potential of screening plant species as sources of metabolites for parasitological purposes is worthy of attention, as elucidated by the Y. Tu's example. Notably, plant-borne molecules are often effective at few parts per million against Aedes, Ochlerotatus, Anopheles and Culex young instars, can be used for the rapid synthesis of mosquitocidal nanoformulations and even employed to prepare cheap repellents with low human toxicity. In addition, behaviour-based control tools relying to the employ of sound traps and the manipulation of swarming behaviour (i.e. "lure and kill" approach) are discussed. The importance of further research on the chemical cues routing mosquito swarming and mating dynamics is highlighted. Besides radiation, transgenic and symbiont-based mosquito control approaches, an effective option may be the employ of biological control agents of mosquito young instars, in the presence of ultra-low quantities of nanoformulated botanicals, which boost their predation rates.

Wolbachia: A biological control strategy against arboviral diseases.

Vector-borne diseases particularly those transmitted by mosquitoes like Dengue are among the leading causes of mortality and morbidity in human population. There are no effective vaccines or treatment against dengue fever till date and the control methods are limited. So, new approaches are urgently in need to reverse these trends. Vector control is currently the primary intervention tool. Strategies that reduce or block pathogen transmission by mosquitoes have been proposed as a means of augmenting current control measures to reduce the growing burden of vector-borne diseases. Wolbachia an endosymbiont of arthropod vectors is being explored as a novel ecofriendly control strategy. Studies in Drosophila have shown that Wolbachia can confer resistance to diverse RNA viruses and protect flies from virus-induced mortality. This review was focused on biology of the Wolbachia and its implication as a control measure for arboviral diseases mainly Dengue and Chikungunya.

Multiscale analysis for a vector-borne epidemic model.

Traditional studies about disease dynamics have focused on global stability issues, due to their epidemiological importance. We study a classical SIR-SI model for arboviruses in two different directions: we begin by describing an alternative proof of previously known global stability results by using only a Lyapunov approach. In the sequel, we take a different view and we argue that vectors and hosts can have very distinctive intrinsic time-scales, and that such distinctiveness extends to the disease dynamics. Under these hypothesis, we show that two asymptotic regimes naturally appear: the fast host dynamics and the fast vector dynamics. The former regime yields, at leading order, a SIR model for the hosts, but with a rational incidence rate. In this case, the vector disappears from the model, and the dynamics is similar to a directly contagious disease. The latter yields a SI model for the vectors, with the hosts disappearing from the model. Numerical results show the performance of the approximation, and a rigorous proof validates the reduced models.

Application of satellite precipitation data to analyse and model arbovirus activity in the tropics.

BACKGROUND: Murray Valley encephalitis virus (MVEV) is a mosquito-borne Flavivirus (Flaviviridae: Flavivirus) which is closely related to Japanese encephalitis virus, West Nile virus and St. Louis encephalitis virus. MVEV is enzootic in northern Australia and Papua New Guinea and epizootic in other parts of Australia. Activity of MVEV in Western Australia (WA) is monitored by detection of seroconversions in flocks of sentinel chickens at selected sample sites throughout WA. Rainfall is a major environmental factor influencing MVEV activity. Utilising data on rainfall and seroconversions, statistical relationships between MVEV occurrence and rainfall can be determined. These relationships can be used to predict MVEV activity which, in turn, provides the general public with important information about disease transmission risk. Since ground measurements of rainfall are sparse and irregularly distributed, especially in north WA where rainfall is spatially and temporally highly variable, alternative data sources such as remote sensing (RS) data represent an attractive alternative to ground measurements. However, a number of competing alternatives are available and careful evaluation is essential to determine the most appropriate product for a given problem. RESULTS: The Tropical Rainfall Measurement Mission (TRMM) Multi-satellite Precipitation Analysis (TMPA) 3B42 product was chosen from a range of RS rainfall products to develop rainfall-based predictor variables and build logistic regression models for the prediction of MVEV activity in the Kimberley and Pilbara regions of WA. Two models employing monthly time-lagged rainfall variables showed the strongest discriminatory ability of 0.74 and 0.80 as measured by the Receiver Operating Characteristics area under the curve (ROC AUC). CONCLUSIONS: TMPA data provide a state-of-the-art data source for the development of rainfall-based predictive models for Flavivirus activity in tropical WA. Compared to ground measurements these data have the advantage of being collected spatially regularly, irrespective of remoteness. We found that increases in monthly rainfall and monthly number of days above average rainfall increased the risk of MVEV activity in the Pilbara at a time-lag of two months. Increases in monthly rainfall and monthly number of days above average rainfall increased the risk of MVEV activity in the Kimberley at a lag of three months.

Influence of Pyrethroid Resistance on Vector Competency for Zika Virus by Aedes aegypti (Diptera: Culicidae).

The vector competence of mosquitoes for pathogens has been shown to be influenced by the status of insecticide resistance in the mosquito population. However, to date, only two studies has explored the impact of insecticide resistance on arbovirus transmission. The global and widespread use of pyrethroids has led to the development of insecticide resistance in many mosquito species, including Aedes aegypti (Linnaeus) (Diptera: Culicidae), the primary vector of Zika virus. Strains of Ae. aegypti that were genetically similar, but responded differently to pyrethroid exposure, were developed using backcrossing techniques. These populations were orally infected with Zika virus and susceptibility to infection, disseminated infection, and transmission potential were evaluated. Analyses revealed differences in susceptibility to infection and disseminated infection between the pyrethroid susceptible and resistant strains of Ae. aegypti during the infection period. Here, we identify an additional challenge to that of widespread pyrethroid resistance. Specifically, resistance is associated with altered phenotypic traits that influence susceptibility to arbovirus infection and progression of infection in the mosquito, factors which ultimately influence risk of arbovirus transmission. These findings support the need to 1) consider insecticide resistance status during times of arbovirus transmission and 2) to implement insecticide resistance management/ mitigation strategies in vector control programs.

Wolbachia strain wAu efficiently blocks arbovirus transmission in Aedes albopictus.

The global incidence of arboviral diseases transmitted by Aedes mosquitoes, including dengue, chikungunya, yellow fever, and Zika, has increased dramatically in recent decades. The release of Aedes aegypti carrying the maternally inherited symbiont Wolbachia as an intervention to control arboviruses is being trialled in several countries. However, these efforts are compromised in many endemic regions due to the co-localization of the secondary vector Aedes albopictus, the Asian tiger mosquito. Ae. albopictus has an expanding global distribution following incursions into a number of new territories. To date, only the wMel and wPip strains of Wolbachia have been reported to be transferred into and characterized in this vector. A Wolbachia strain naturally infecting Drosophila simulans, wAu, was selected for transfer into a Malaysian Ae. albopictus line to create a novel triple-strain infection. The newly generated line showed self-compatibility, moderate fitness cost and complete resistance to Zika and dengue infections.

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.

Progress towards Understanding the Mosquito-Borne Virus Life Cycle.

Mosquito-borne arboviruses are a group of heterogeneous viruses that are mainly transmitted to vertebrate hosts and are the aetiological agents of many human diseases. These viruses naturally maintain a life cycle between distinct hosts by transmission from an infected mosquito to a naive host, and acquisition from a viraemic host back to a fed mosquito. To survive in and maintain a cycle between different host environments, mosquito-borne arboviruses exploit sophisticated approaches, including subverting the immune system, hijacking host factors, and taking advantage of gut microbes. We summarize the recent progress towards understanding the mechanisms of arboviral transmission and acquisition by mosquitoes. This knowledge offers an insight into the emergence and re-emergence of arboviruses in nature and an avenue for disease prevention in the future.

Matching the genetics of released and local Aedes aegypti populations is critical to assure Wolbachia invasion.

BACKGROUND: Traditional vector control approaches such as source reduction and insecticide spraying have limited effect on reducing Aedes aegypti population. The endosymbiont Wolbachia is pointed as a promising tool to mitigate arbovirus transmission and has been deployed worldwide. Models predict a rapid increase on the frequency of Wolbachia-positive Ae. aegypti mosquitoes in local settings, supported by cytoplasmic incompatibility (CI) and high maternal transmission rate associated with the wMelBr strain. METHODOLOGY/PRINCIPLE FINDINGS: Wolbachia wMelBr strain was released for 20 consecutive weeks after receiving >87% approval of householders of the isolated community of Tubiacanga, Rio de Janeiro. wMelBr frequency plateued~40% during weeks 7-19, peaked 65% but dropped as releases stopped. A high (97.56%) maternal transmission was observed. Doubling releases and deploying mosquitoes with large wing length and low laboratory mortality produced no detectable effects on invasion trend. By investigating the lab colony maintenance procedures backwardly, pyrethroid resistant genotypes in wMelBr decreased from 68% to 3.5% after 17 generations. Therefore, we initially released susceptible mosquitoes in a local population highly resistant to pyrethroids which, associated with the over use of insecticides by householders, ended jeopardizing Wolbachia invasion. A new strain (wMelRio) was produced after backcrossing wMelBr females with males from field to introduce mostly pyrethroid resistance alleles. The new strain increased mosquito survival but produced relevant negative effects on Ae. aegypti fecundity traits, reducing egg clutche size and egg hatch. Despite the cost on fitness, wMelRio successful established where wMelBr failed, revealing that matching the local population genetics, especially insecticide resistance background, is critical to achieve invasion. CONCLUSIONS/SIGNIFICANCE: Local householders support was constantly high, reaching 90% backing on the second release (wMelRio strain). Notwithstanding the drought summer, the harsh temperature recorded (daily average above 30°C) did not seem to affect the expression of maternal transmission of wMel on a Brazilian background. Wolbachia deployment should match the insecticide resistance profile of the wild population to achieve invasion. Considering pyrethroid-resistance is a widely distributed phenotype in natural Ae. aegypti populations, future Wolbachia deployments must pay special attention in maintaining insecticide resistance in lab colonies for releases.

Aedes aegypti (Aag2)-derived clonal mosquito cell lines reveal the effects of pre-existing persistent infection with the insect-specific bunyavirus Phasi Charoen-like virus on arbovirus replication.

BACKGROUND: Aedes aegypti is a vector mosquito of major public health importance, transmitting arthropod-borne viruses (arboviruses) such as chikungunya, dengue, yellow fever and Zika viruses. Wild mosquito populations are persistently infected at high prevalence with insect-specific viruses that do not replicate in vertebrate hosts. In experimental settings, acute infections with insect-specific viruses have been shown to modulate arbovirus infection and transmission in Ae. aegypti and other vector mosquitoes. However, the impact of persistent insect-specific virus infections, which arboviruses encounter more commonly in nature, has not been investigated extensively. Cell lines are useful models for studying virus-host interactions, however the available Ae. aegypti cell lines are poorly defined and heterogenous cultures. METHODOLOGY/PRINCIPLE FINDINGS: We generated single cell-derived clonal cell lines from the commonly used Ae. aegypti cell line Aag2. Two of the fourteen Aag2-derived clonal cell lines generated harboured markedly and consistently reduced levels of the insect-specific bunyavirus Phasi Charoen-like virus (PCLV) known to persistently infect Aag2 cells. In contrast to studies with acute insect-specific virus infections in cell culture and in vivo, we found that pre-existing persistent PCLV infection had no major impact on the replication of the flaviviruses dengue virus and Zika virus, the alphavirus Sindbis virus, or the rhabdovirus vesicular stomatitis virus. We also performed a detailed characterisation of the morphology, transfection efficiency and immune status of our Aag2-derived clonal cell lines, and have made a clone that we term Aag2-AF5 available to the research community as a well-defined cell culture model for arbovirus-vector interaction studies. CONCLUSIONS/SIGNIFICANCE: Our findings highlight the need for further in vivo studies that more closely recapitulate natural arbovirus transmission settings in which arboviruses encounter mosquitoes harbouring persistent rather than acute insect-specific virus infections. Furthermore, we provide the well-characterised Aag2-derived clonal cell line as a valuable resource to the arbovirus research community.

VECTOS: An Integrated System for Monitoring Risk Factors Associated With Urban Arbovirus Transmission.

In Colombia, as in many Latin American countries, decision making and development of effective strategies for vector control of urban diseases such as dengue, Zika, and chikungunya is challenging for local health authorities. The heterogeneity of transmission in urban areas requires an efficient risk-based allocation of resources to control measures. With the objective of strengthening the capacity of local surveillance systems to identify variables that favor urban arboviral transmission, a multidisciplinary research team collaborated with the local Secretary of Health officials of 3 municipalities in Colombia (Giron, Yopal, and Buga), in the design of an integrated information system called VECTOS from 2015 to 2018. Information and communication technologies were used to develop 2 mobile applications to capture entomological and social information, as well as a web-based system for the collection, geo-referencing, and integrated information analysis using free geospatial software. This system facilitates the capture and analysis of epidemiological information from the Colombian national surveillance system (SIVIGILA), periodic entomological surveys-mosquito larvae and pupae in premises and peridomestic breeding sites-and surveys of knowledge, attitudes, and practices (KAP) in a spatial and temporal context at the neighborhood level. The data collected in VECTOS are mapped and visualized in graphical reports. The system enables real-time monitoring of weekly epidemiological indicators, entomological indices, and social surveys. Additionally, the system enables risk stratification of neighborhoods, using selected epidemiological, entomological, demographic, and environmental variables. This article describes the VECTOS system and the lessons learned during its development and use. The joint analysis of epidemiological and entomological data within a geographic information system in VECTOS gives better insight to the routinely collected data and identifies the heterogeneity of risk factors between neighborhoods. We expect the system to continue to strengthen vector control programs in evidence-based decision making and in the design and enhanced follow-up of vector control strategies.

Factors contributing to and strategies to combat emerging arboviruses.

Less than half a century ago infectious diseases appeared to be destined to be extinguished via as a culmination of medical triumphs. As focus turned towards combating non-communicable diseases, emerging and re-emerging diseases (EIDs) have bloomed from those ashes. Five epidemic mosquito-borne arboviruses (Yellow Fever virus (YFV), Dengue virus, West Nile virus, Chikungunya virus, and Zika virus) have emerged in the recent past. Arboviruses are of the utmost importance with respect to EIDs due to intensive growth of globalisation, arthropod urban fitness/adaption, and environmental changes. We focus on recent outbreaks of the arthropod borne viruses (arboviruses) Zika virus and YFV. Factors contributing to the blossoming of EIDs (environmental, globalisation, and urbanisation) and combating strategies (surveillance, containment, and prevention) will be discussed. Specifically, Zika virus and YFV will be used in the context of these factors and strategies. YFV is discussed in detail as it pertains to these factors and strategies in the United States (US), 2017 Brazil Outbreak, 2016 Africa Outbreak, and global risk. Vigilance is needed to focus on, prevent, and control the current and next arbovirus EIDs.

Mapping Arbovirus-Vector Interactions Using Systems Biology Techniques.

Studying how arthropod-borne viruses interact with their arthropod vectors is critical to understanding how these viruses replicate and are transmitted. Until recently, these types of studies were limited in scale because of the lack of classical tools available to study virus-host interaction for non-model viruses and non-model organisms. Advances in systems biology "-omics"-based techniques such as next-generation sequencing (NGS) and mass spectrometry can rapidly provide an unbiased view of arbovirus-vector interaction landscapes. In this mini-review, we discuss how arbovirus-vector interaction studies have been advanced by systems biology. We review studies of arbovirus-vector interactions that occur at multiple time and length scales, including intracellular interactions, interactions at the level of the organism, viral and vector populations, and how new techniques can integrate systems-level data across these different scales.

[Mosquitoes as vectors of arboviruses: an endless story]. / Les moustiques vecteurs d'arbovirus : une histoire sans fin.

The recent emergence or re-emergence of vector-borne diseases (VBD) and, more specifically, VBD associated with arboviruses such as dengue, chikungunya, Zika or yellow fever are not new events. The globalization of trade and travels as well as the unplanned urbanization of many tropical and subtropical cities have created the conditions suitable for the establishment of vector mosquitoes offering opportunities for arbovirus introduction. This review describes the major arboviruses important for human health and their epidemic vectors, and the conditions leading to their emergence.

Amplification due to spatial clustering in an individual-based model of mosquito-avian arbovirus transmission.

Theory and observations indicate that spatial clustering of birds and mosquitoes may be necessary for epizootic amplification of arboviruses with avian zoonoses. In this paper, I present an individual-based model of zoonotic arbovirus transmission among birds and mosquitoes. The results of initial ensemble model simulations indicate that the co-location of a vector mosquito oviposition site with an infected bird roost increases the local vector-to-host density and increases the likelihood of arbovirus amplification within the infected roost. Such amplification also increases the likelihood of secondary amplification at other roost sites, produces higher vector and host infection rates, increases the time to virus extinction within the model population, and increases the total number of birds infected. Additional oviposition locations within the model domain also increase the likelihood of secondary amplification. These findings support the idea that spatial clustering of mosquitoes and birds may facilitate arbovirus amplification. This model provides a basis for future exploration of specific zoonotic transmission cycles, including West Nile virus, and could be used to test the efficacy of various control strategies.

Experimental infections of mosquitoes with severe fever with thrombocytopenia syndrome virus.

BACKGROUND: Severe fever with thrombocytopenia syndrome (SFTS) is a newly identified emerging infectious disease, which is caused by a novel bunyavirus (termed SFTSV) in Asia. Although mosquitoes have not been identified as the primary vectors, as revealed by epidemiological surveys, their role in transmitting this SFTSV as a suspicious vector has not been validated. FINDINGS: In this study, we conducted experimental infections of mosquitoes with SFTSV to examine the role of mosquitoes in the transmission of the virus. We did not detect viral replication in Culex pipiens pallens, Aedes aegyptis and Anopheles sinensis as revealed by qRT-PCR assay. In addition, we failed to isolate SFTSV from the Vero cells cultured with suspensions of SFTSV-infected mosquitoes. CONCLUSION: The results of the present study demonstrate little possibility that mosquitoes act as vectors for the emerging pathogen SFTSV.

Ecological effects on arbovirus-mosquito cycles of transmission.

Mosquitoes transmit many viruses to a variety of hosts. Cycles of mosquito borne arbovirus transmission are the result of complex interactions between the mosquito, the arbovirus and the host that are influenced by genetic variations in a variety of traits in each that are all influenced by many environmental factors. R0, the basic reproduction number or mean number of individuals infected from a single infected individual, is a measure of mosquito borne arbovirus transmission. Understanding the causes for the distribution of R0 in any transmission cycle is a daunting challenge due to the lack of information on the genetic and environmental variances that influence R0. Information about the major factors influencing R0 for specific transmission cycles is essential to develop efficient and effective strategies to reduce transmission in different cycles and locations.

Arbovirus infection of vertebrate and insect cell cultures, with special emphasis on Mokola, Obodhiang, and kotonkan viruses of the rabies serogroup.

Multiplication of rabies serogroup viruses, Obodhiang and kotonkan (two presumptive arboviruses), was induced in vertebrate cell cultures with Singh's A. albopictus cell cultures used as "helper cells" in cocultivation experiments. Plaque formation without prior in vitro adaptation was induced in Vero cell cultures with eight rabies serogroup viruses: in all five instances by cocultivation of either infected BHK-21 or A. albopictus cells with Vero cells under agar overlay and in three of eight instances by direct plaque assay of infected mouse brain suspensions. In cross-plaque reduction neutralization tests with cloned viruses that represented human pathogens, rabies, Duvenhage, and Mokola, on the one hand, and the presumptive arboviruses Obodhiang and kotonkan, on the other hand, Mokola virus shared common antigenic components with both the nonarboviruses and the arboviruses. Biologically, Mokola virus was different from the other two human pathogens, rabies and Duvenhage, in that it multiplied in both vertebrate and invertebrate cell cultures. Mokola virus thus appears to be the biologic and serologic bridging agent.