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

Evaluation of the vector competence for Batai virus of native Culex and exotic Aedes species in Central Europe

BACKGROUND

Batai virus (BATV) is a zoonotic arbovirus of veterinary importance. A high seroprevalence in cows, sheep and goats and infection in different mosquito species has been observed in Central Europe. Therefore, we studied indigenous as well as exotic species of the genera Culex and Aedes for BATV vector competence at different fluctuating temperature profiles.

METHODS

Field caught Culex pipiens biotype pipiens, Culex torrentium, Aedes albopictus and Aedes japonicus japonicus from Germany and Aedes aegypti laboratory colony were infected with BATV strain 53.3 using artificial blood meals. Engorged mosquitoes were kept under four (Culex species) or three (Aedes species) fluctuating temperature profiles (18 ± 5 °C, 21 ± 5 °C, 24 ± 5 °C, 27 ± 5 °C) at a humidity of 70% and a dark/light rhythm of 12:12 for 14 days. Transmission was measured by testing the saliva obtained by forced salivation assay for viable BATV particles. Infection rates were analysed by testing whole mosquitoes for BATV RNA by quantitative reverse transcription PCR.

RESULTS

No transmission was detected for Ae. aegypti, Ae. albopictus or Ae. japonicus japonicus. Infection was observed for Cx. p. pipiens, but only in the three conditions with the highest temperatures (21 ± 5 °C, 24 ± 5 °C, 27 ± 5 °C). In Cx. torrentium infection was measured at all tested temperatures with higher infection rates compared with Cx. p. pipiens. Transmission was only detected for Cx. torrentium exclusively at the highest temperature of 27 ± 5 °C.

CONCLUSIONS

Within the tested mosquito species, only Cx. torrentium seems to be able to transmit BATV if the climatic conditions are feasible.

Simultaneous Coinfections with West Nile Virus and Usutu Virus in Culex pipiens and Aedes vexans Mosquitoes

The mosquito-borne zoonotic flaviviruses West Nile virus (WNV) and Usutu virus (USUV) are endemic in many European countries and emerged in Germany in recent years. Due to the increasing overlap of their distribution areas and their similar epidemiology, coinfections of WNV and USUV are possible. Indeed, coinfections in vertebrate hosts as a rare event have already been reported from some countries including Germany. However, it is largely unknown whether and to what extent coinfections could affect the vector competence of mosquitoes for WNV and USUV. For this purpose, the mosquito species Culex pipiens biotype pipiens, Culex pipiens biotype molestus, and Aedes vexans were orally infected in mono- and simultaneous coinfections with German strains of WNV and USUV. Mosquitoes were incubated for 14 days at 26°C, 85% relative humidity, and a 16 : 8 light-dark photocycle, before they were dissected and forced to salivate. The results showed a decrease in USUV susceptibility in Culex pipiens biotype pipiens, an increase in USUV susceptibility in Aedes vexans, and no obvious interaction between both viruses in Culex pipiens biotype molestus. Vector competence for WNV appeared to be unaffected by a simultaneous occurrence of USUV in all tested mosquito species. Coinfections with both viruses were only found in Culex mosquitoes, and cotransmission of WNV and USUV was observed in Culex pipiens biotype molestus. Overall, our results show that viral interactions between WNV and USUV vary between mosquito species, and that the interaction mainly occurs during infection and replication in the mosquito midgut. The results of this study confirm that to fully understand the interaction between WNV and USUV, studies with various mosquito species are necessary. In addition, we found that even mosquito species with a low susceptibility to both viruses, such as Ae. vexans, can play a role in their transmission in areas with cocirculation.

Arboviruses in Mammals in the Neotropics: A Systematic Review to Strengthen Epidemiological Monitoring Strategies and Conservation Medicine

Arthropod-borne viruses (arboviruses) are a diverse group of ribonucleic acid (RNA) viruses, with the exception of African swine fever virus, that are transmitted by hematophagous arthropods to a vertebrate host. They are the important cause of many diseases due to their ability to spread in different environments and their diversity of vectors. Currently, there is no information on the geographical distribution of the diseases because the routes of transmission and the mammals (wild or domestic) that act as potential hosts are poorly documented or unknown. We conducted a systematic review from 1967 to 2021 to identify the diversity of arboviruses, the areas, and taxonomic groups that have been monitored, the prevalence of positive records, and the associated risk factors. We identified forty-three arboviruses in nine mammalian orders distributed in eleven countries. In Brazil, the order primates harbor the highest number of arbovirus records. The three most recorded arboviruses were Venezuelan equine encephalitis, Saint Louis encephalitis and West Nile virus. Serum is the most used sample to obtain arbovirus records. Deforestation is identified as the main risk factor for arbovirus transmission between different species and environments (an odds ratio of 1.46 with a 95% confidence interval: 1.34-1.59). The results show an increase in the sampling effort over the years in the neotropical region. Despite the importance of arboviruses for public health, little is known about the interaction of arboviruses, their hosts, and vectors, as some countries and mammalian orders have not yet been monitored. Long-term and constant monitoring allows focusing research on the analysis of the interrelationships and characteristics of each component animal, human, and their environment to understand the dynamics of the diseases and guide epidemiological surveillance and vector control programs. The biodiversity of the Neotropics should be considered to support epidemiological monitoring strategies.

Deciphering the Tissue Tropism of the RNA Viromes Harbored by Field-Collected Anopheles sinensis and Culex quinquefasciatus

Arboviruses and insect-specific viruses (ISVs) are two major types of viruses harbored by mosquitoes that are distinguished by the involvement of vertebrate hosts in their transmission cycles. While intensive studies have focused on the transmission, tissue tropism, and evolution of arboviruses, these characteristics are poorly investigated in ISVs, which dominate the mosquito virome. Therefore, in this study, we collected two mosquito species, Anopheles sinensis and Culex quinquefasciatus, in the field and used a metatranscriptomics approach to characterize their RNA viromes in different tissues, such as the midgut, legs, salivary gland, eggs, and the remainder of the carcass. Blood-engorged individuals of these species were captured in 3 locations, and 60 mosquitoes were pooled from each species and location. A total of 40 viral species from diverse viral taxa associated with all viral RNA genome types were identified, among which 19 were newly identified in this study. According to the current viral taxonomy, some of these viruses, such as Yancheng Anopheles associated virus 2 (Narnaviridae) and Jiangsu Anopheles-related virus (Ghabrivirales), were novel. The two investigated mosquito species generally harbored distinct viromes. Nevertheless, the viruses were generally shared among different tissue types to various degrees. Specifically, the eggs possessed a viral community with significantly lower diversity and abundance than those in other tissues, whereas the legs and salivary glands exhibited higher viral abundance. The compositions and distributions of the viromes of different mosquito tissues were demonstrated for the first time in our study, providing important insight into the virome dynamics within individual mosquitoes. IMPORTANCE ISVs are considered to be ancestral to arboviruses. Because of their medical importance, arboviruses have been well studied from the aspects of their transmission mode, evolution of dual-host tropism, and genetic dynamics within mosquito vectors. However, the mode of ISV maintenance is poorly understood, even though many novel ISVs have been identified with the emergence of sequencing technology. In our study, in addition to the identification of a diverse virus community, the tissue tropism of RNA viromes harbored by two field-collected mosquito species was demonstrated for the first time. According to the results, the virus communities of different tissues, such as the salivary glands, midguts, legs, and eggs, can help us understand the evolution, transmission routes, and maintenance modes of mosquito-specific viruses in nature.

North American Arboviruses and White-Tailed Deer (Odocoileus virginianus): Associated Diseases and Role in Transmission

Background: Arboviral disease is of increasing concern to human and animal health professionals as emerging and re-emerging arboviruses are more frequently recognized. Wildlife species are known to play a role in the transmission and maintenance of arboviruses and infections can result in morbidity and mortality in wildlife hosts. Materials and Methods: In this review, we detail existing evidence of white-tailed deer (Odocoileus virginianus) as an important host to a diverse collection of arboviruses and evaluate the utility of this species as a resource to better understand the epidemiology of related viral diseases. Results: Relevant veterinary and zoonotic viral pathogens endemic to North America include epizootic hemorrhagic disease virus, bluetongue virus, orthobunyaviruses, vesicular stomatitis virus, Eastern equine encephalitis virus, West Nile virus, and Powassan virus. Exotic viral pathogens that may infect white-tailed deer are also identified with an emphasis on zoonotic disease risks. The utility of this species is attributed to the high degree of contact with humans and domestic livestock and evidence of preferential feeding by various insect vectors. Conclusions: There is mounting evidence that white-tailed deer are a useful, widely available source of information regarding arboviral circulation, and that surveillance and monitoring of deer populations would be of value to the understanding of certain viral transmission dynamics, with implications for improving human and domestic animal health.

Factors responsible for the emergence of novel viruses: An emphasis on SARS-CoV-2

Structural and genetic differences among various viruses play a significant factor in host infectivity and vulnerability to environmental stressors. Zoonoses of viruses require several recombinations and mutations in their genetic material and among several viruses allowing them to switch hosts and infect new species. Additionally, the host genetics play a significant role in successful viral transmission among various hosts. For example, human immunodeficiency virus (HIV), Ebola virus and influenza viruses. In efficient zoonotic events, selective stresses in the host milieu-interieur are critical during viral infection of the first human host. The genetic rearrangement of the virus and the selective environmental pressure of the host immune system dominate the emergence of new viral disease outbreaks.

The Genetic Basis for Salivary Gland Barriers to Arboviral Transmission

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

Evaluating the competence of the primary vector, Culex tritaeniorhynchus, and the invasive mosquito species, Aedes japonicus japonicus, in transmitting three Japanese encephalitis virus genotypes

Japanese encephalitis virus (JEV) is maintained in an enzootic cycle between swine, water birds, and mosquitoes. JEV has circulated indigenously in Asia, with Culex tritaeniorhynchus as the primary vector. In some areas where the primary vector is scarce or absent, sporadic cases of Japanese encephalitis have been reported, with Aedes japonicus japonicus presumed to have the potential as a secondary vector. As one of the world's most invasive culicid species, Ae. j. japonicus carries a considerable health risk for spreading diseases to wider areas, including Europe and North America. Thus, evaluation of its competency as a JEV vector, particularly in a native population, will be essential in preventing potential disease spread. In this study, the two mosquito species' vector competence in transmitting three JEV genotypes (I, III, and V) was assessed, with Cx. tritaeniorhynchus serving as a point of reference. The mosquitoes were virus-fed and the infection rate (IR), dissemination rate (DR), and transmission rate (TR) evaluated individually by either RT-qPCR or focus forming assay. Results showed striking differences between the two species, with IR of 95% (261/274) and 9% (16/177) in Cx. tritaeniorhynchus and Ae. j. japonicus, respectively. Both mosquitoes were susceptible to all three JEV genotypes with significant differences in IR and mean viral titer. Results confirm the primary vector's competence, but the fact that JEV was able to establish in Ae. j. japonicus is of public health significance, and with 2%-16% transmission rate it has the potential to successfully transmit JEV to the next host. This may explain the human cases and infrequent detection in primary vector-free areas. Importantly, Ae. j. japonicus could be a relevant vector spreading the disease into new areas, indicating the need for security measures in areas where the mosquito is distributed or where it may be introduced.

Impact of Gut Bacteria on the Infection and Transmission of Pathogenic Arboviruses by Biting Midges and Mosquitoes

Tripartite interactions among insect vectors, midgut bacteria, and viruses may determine the ability of insects to transmit pathogenic arboviruses. Here, we investigated the impact of gut bacteria on the susceptibility of Culicoides nubeculosus and Culicoides sonorensis biting midges for Schmallenberg virus, and of Aedes aegypti mosquitoes for Zika and chikungunya viruses. Gut bacteria were manipulated by treating the adult insects with antibiotics. The gut bacterial communities were investigated using Illumina MiSeq sequencing of 16S rRNA, and susceptibility to arbovirus infection was tested by feeding insects with an infectious blood meal. Antibiotic treatment led to changes in gut bacteria for all insects. Interestingly, the gut bacterial composition of untreated Ae. aegypti and C. nubeculosus showed Asaia as the dominant genus, which was drastically reduced after antibiotic treatment. Furthermore, antibiotic treatment resulted in relatively more Delftia bacteria in both biting midge species, but not in mosquitoes. Antibiotic treatment and subsequent changes in gut bacterial communities were associated with a significant, 1.8-fold increased infection rate of C. nubeculosus with Schmallenberg virus, but not for C. sonorensis. We did not find any changes in infection rates for Ae. aegypti mosquitoes with Zika or chikungunya virus. We conclude that resident gut bacteria may dampen arbovirus transmission in biting midges, but not so in mosquitoes. Use of antimicrobial compounds at livestock farms might therefore have an unexpected contradictory effect on the health of animals, by increasing the transmission of viral pathogens by biting midges.

Novel Viruses in Mosquitoes from Brazilian Pantanal

Viruses are ubiquitous and diverse microorganisms arising as a result of interactions within their vertebrate and invertebrate hosts. Here we report the presence of different viruses in the salivary glands of 1657 mosquitoes classified over 28 culicinae species from the North region of the Brazilian Pantanal wetland through metagenomics, viral isolation, and RT-PCR. In total, 12 viruses were found, eight putative novel viruses with relatively low similarity with pre-existing species of viruses within their families, named Pirizal iflavirus, Furrundu phlebovirus, Pixé phlebovirus, Guampa vesiculovirus, Chacororé flavivirus, Rasqueado orbivirus, Uru chuvirus, and Bororo circovirus. We also found the already described Lobeira dielmorhabdovirus, Sabethes flavivirus, Araticum partitivirus, and Murici totivirus. Therefore, these findings underscore the vast diversity of culicinae and novel viruses yet to be explored in Pantanal, the largest wetland on the planet.

Mosquito-Specific Viruses-Transmission and Interaction

Mosquito-specific viruses (MSVs) are a subset of insect-specific viruses that are found to infect mosquitoes or mosquito derived cells. There has been an increase in discoveries of novel MSVs in recent years. This has expanded our understanding of viral diversity and evolution but has also sparked questions concerning the transmission of these viruses and interactions with their hosts and its microbiome. In fact, there is already evidence that MSVs interact with the immune system of their host. This is especially interesting, since mosquitoes can be infected with both MSVs and arthropod-borne (arbo) viruses of public health concern. In this review, we give an update on the different MSVs discovered so far and describe current data on their transmission and interaction with the mosquito immune system as well as the effect MSVs could have on an arboviruses-co-infection. Lastly, we discuss potential uses of these viruses, including vector and transmission control.

West Nile Virus and Usutu Virus Co-Circulation in Europe: Epidemiology and Implications

West Nile virus (WNV) and Usutu virus (USUV) are neurotropic mosquito-borne flaviviruses that may infect humans. Although WNV is much more widespread and plays a much larger role in human health, the two viruses are characterized by similar envelope antigens, clinical manifestations, and present overlapping in terms of geographic range of transmission, host, and vector species. This review highlights some of the most relevant aspects of WNV and USUV human infections in Europe, and the possible implications of their co-circulation.

Culex torrentium: A Potent Vector for the Transmission of West Nile Virus in Central Europe

The continuous circulation of West Nile virus (WNV) in Central, South and East Europe and its recent detection in several dead birds and two horses in Germany highlights the need for information on WNV vector competence of mosquitoes from Central Europe. Therefore, three common Culex species (Culex pipiens biotype pipiens, Culex pipiens biotype molestus and Culex torrentium) from Germany were orally infected with WNV and kept at 18 °C, 21 °C, 24 °C or 27 °C for 14 or 21 days post infection (dpi). Thereafter viable WNV was present in the saliva in all tested taxa, but only at incubation temperatures of 24 °C or 27 °C and predominantly at the extended incubation period of 21 dpi. Highest transmission efficiency rates of 17 % (24 °C) and 24% (27 °C) were found for Cx. torrentium. Culex p. pipiens and Cx. p. molestus showed low transmission efficiencies with a maximum of only 3%. Consequently, temperatures above 21 °C support transmission of WNV, which matches the predominant distribution of human WNV cases around the Mediterranean Sea and in South-East Europe. Culex torrentium has been identified as a potent vector for WNV in Central and Northern Europe, which highlights the need for surveillance of mosquito-borne viruses north of the Alps.

Molecular identification of blood meals in mosquitoes (Diptera, Culicidae) in urban and forested habitats in southern Brazil

The study of host associations of mosquitoes (Diptera, Culicidae) provides valuable information to assist in our understanding of a variety of related issues, from their life-history to the entomological surveillance of pathogens. In this study, we identified and characterized mosquito blood meals from both urban and forested areas in the city of Paranaguá, state of Paraná, Brazil, by analyzing the amplification of host DNA ingested by mosquitoes under different storage conditions and digestion levels. Host DNA preservation was evaluated in fresh blood meals according to storage duration (30 to 180 days) and temperature (-20°C / -80°C) and, in digested blood, according the degree of digestion classified on the Sella scale. Molecular analysis of blood meals was based on DNA extraction and amplification of a fragment of the mitochondrial COI gene. We determined that, up to180 days of storage, the evaluated temperatures did not influence the preservation of fresh blood meals DNA, whereas the amplification success was increasingly reduced over the course of the digestion process. The species Anopheles cruzii, Aedes fluviatilis, Aedes scapularis, Psorophora ferox, Culex quinquefasciatus, Culex mollis, and Culex intrincatus, together with specimens representing four subgenera and one genus of Culicidae [Ae. (Ochlerotatus), Cx. (Culex), Cx. (Melanoconion), Cx. (Microculex), and Limatus, respectively] had their blood meals identified. Their diverse host use was evidenced by the identification of 19 species of vertebrate host, namely two amphibians, three mammals and 14 birds. Birds were the most commonly identified host in blood meals. These results not only show the diversity of mosquito hosts, but also underscore the challenges involved in monitoring arboviruses of public health importance, given potential combinations of host use for each mosquito species.

Vector competence of biting midges and mosquitoes for Shuni virus

BACKGROUND

Shuni virus (SHUV) is an orthobunyavirus that belongs to the Simbu serogroup. SHUV was isolated from diverse species of domesticated animals and wildlife, and is associated with neurological disease, abortions, and congenital malformations. Recently, SHUV caused outbreaks among ruminants in Israel, representing the first incursions outside the African continent. The isolation of SHUV from a febrile child in Nigeria and seroprevalence among veterinarians in South Africa suggests that the virus may have zoonotic potential as well. The high pathogenicity, extremely broad tropism, potential transmission via both biting midges and mosquitoes, and zoonotic features of SHUV require further investigation. This is important to accurately determine the risk for animal and human health, and to facilitate preparations for potential epidemics. To gain first insight into the potential involvement of biting midges and mosquitoes in SHUV transmission we have investigated the ability of SHUV to infect two species of laboratory-colonised biting midges and two species of mosquitoes.

METHODOLOGY/PRINCIPAL FINDINGS

Culicoides nubeculosus, C. sonorensis, Culex pipiens pipiens, and Aedes aegypti were orally exposed to SHUV by providing an infectious blood meal. Biting midges showed high infection rates of approximately 40%-60%, whereas infection rates of mosquitoes were only 0-2%. Moreover, successful dissemination in both species of biting midges and no evidence for transmission by orally exposed mosquitoes was found.

CONCLUSIONS/SIGNIFICANCE

The results of this study suggest that different species of Culicoides midges are efficient in SHUV transmission, while the involvement of mosquitoes has not been supported.

Arbovirus coinfection and co-transmission: A neglected public health concern?

Epidemiological synergy between outbreaks of viruses transmitted by Aedes aegypti mosquitoes, such as chikungunya, dengue, and Zika viruses, has resulted in coinfection of humans with multiple viruses. Despite the potential impact on public health, we know only little about the occurrence and consequences of such coinfections. Here, we review the impact of coinfection on clinical disease in humans, discuss the possibility for co-transmission from mosquito to human, and describe a role for modeling transmission dynamics at various levels of co-transmission. Solving the mystery of virus coinfections will reveal whether they should be viewed as a serious concern for public health.

Vector competence of biting midges and mosquitoes for Shuni virus

Background

Shuni virus (SHUV) is an orthobunyavirus that belongs to the Simbu serogroup. SHUV was isolated from diverse species of domesticated animals and wildlife, and is associated with neurological disease, abortions, and congenital malformations. Recently, SHUV caused outbreaks among ruminants in Israel, representing the first incursions outside the African continent. The isolation of SHUV from a febrile child in Nigeria and seroprevalence among veterinarians in South Africa suggests that the virus may have zoonotic potential as well. The high pathogenicity, extremely broad tropism, potential transmission via both biting midges and mosquitoes, and zoonotic features warrants prioritization of SHUV for further research. Additional knowledge is essential to accurately determine the risk for animal and human health, and to assess the risk of future epizootics and epidemics. To gain first insights into the potential involvement of arthropod vectors in SHUV transmission, we have investigated the ability of SHUV to infect and disseminate in laboratory-reared biting midges and mosquitoes.

Methodology/Principal findings

Culicoides nubeculosus, C. sonorensis, Culex pipiens pipiens, and Aedes aegypti were orally exposed to SHUV by providing an infectious blood meal. Biting midges showed high infection rates of approximately 40–60%, whereas infection rates of mosquitoes were lower than 2%. SHUV successfully disseminated in both species of biting midges, but no evidence of transmission in orally exposed mosquitoes was found.

Conclusions/Significance

The results of this study show that different species of Culicoides biting midges are susceptible to infection and dissemination of SHUV, whereas the two mosquito species tested were found not to be susceptible.

Arthropod-borne (arbo)viruses are notorious for causing unpredictable and large-scale epidemics and epizootics. Apart from viruses such as West Nile virus and Rift Valley fever virus that are well known to have a significant impact on human and animal health, many arboviruses remain neglected. Shuni virus (SHUV) is a neglected virus with zoonotic potential that was recently associated with severe disease in livestock and wildlife. Isolations of SHUV from field-collected biting midges and mosquitoes suggests that SHUV may be transmitted by these insects. Laboratory-reared biting midge species (Culicoides nubeculosus and C. sonorensis) and mosquito species (Culex pipiens pipiens and Aedes aegypti), that are known to transmit other arboviruses, were exposed to SHUV via an infectious blood meal. SHUV was able to successfully disseminate in both biting midge species, whereas no evidence of infection or transmission in both mosquito species was found. Our results show that SHUV infects and disseminates in two different Culicoides species, suggesting that these insects could play an important role in the disease transmission cycle.

Direct and indirect influences of virus-insect vector-plant interactions on non-circulative, semi-persistent virus transmission

Plant viruses that are transmitted in a non-circulative, semi-persistent (NCSP) manner have determinants on, and/or accessories to, their capsids that facilitate virion binding to specific retention sites in their insect vectors. Bilateral interactions and interactions occurring at the nexus of all three partners (virus, vector and plant) also contribute to transmission by influencing virus acquisition and inoculation. Vector feeding behavior lies at the core of this trio of virus transmission processes (retention-acquisition-inoculation), but transmission may also be mediated by virus infection-triggered and/or vector feeding-triggered plant cues that influence behavioral responses such as vector attraction, deterrence and dispersal. Insights into the multiphasic interactions and coordinated processes will lead to a better understanding of the mechanisms of NCSP transmission.

Vector competence of European mosquitoes for West Nile virus

West Nile virus (WNV) is an arthropod-borne flavivirus of high medical and veterinary importance. The main vectors for WNV are mosquito species of the Culex genus that transmit WNV among birds, and occasionally to humans and horses, which are ‘dead-end’ hosts. Recently, several studies have been published that aimed to identify the mosquito species that serve as vectors for WNV in Europe. These studies provide insight in factors that can influence vector competence of European mosquito species for WNV. Here, we review the current knowledge on vector competence of European mosquitoes for WNV, and the molecular knowledge on physical barriers, anti-viral pathways and microbes that influence vector competence based on studies with other flaviviruses. By comparing the 12 available WNV vector competence studies with European mosquitoes we evaluate the effect of factors such as temperature, mosquito origin and mosquito biotype on vector competence. In addition, we propose a standardised methodology to allow for comparative studies across Europe. Finally, we identify knowledge gaps regarding vector competence that, once addressed, will provide important insights into WNV transmission and ultimately contribute to effective strategies to control WNV.

Emerging arboviruses: Why today?

The recent global (re)emergence of arthropod-borne viruses (arboviruses), such as chikungunya and Zika virus, was widely reported in the media as though it was a new phenomenon. This is not the case. Arboviruses and other human microbial pathogens have been (re)emerging for centuries. The major difference today is that arbovirus emergence and dispersion are more rapid and geographically extensive, largely due to intensive growth of global transportation systems, arthropod adaptation to increasing urbanisation, our failure to contain mosquito population density increases and land perturbation. Here we select examples of (re)emerging pathogenic arboviruses and explain the reasons for their emergence and different patterns of dispersal, focusing particularly on the mosquito vectors which are important determinants of arbovirus emergence. We also attempt to identify arboviruses likely to (re)emerge in the future.

Identification of sympatric cryptic species of Aedes albopictus subgroup in Vietnam: new perspectives in phylosymbiosis of insect vector

Background

The Aedes (Stegomyia) albopictus subgroup includes 11 cryptic species of which Ae. albopictus is the most widely distributed. Its global expansion associated with a documented vector competence for several emerging arboviruses raise obvious concerns in the recently colonized regions. While several studies have provided important insights regarding medical importance of Ae. albopicus, the investigations of the other sibling species are scarce. In Asia, indigenous populations within the Ae. albopictus subgroup can be found in sympatry. In the present study, we aimed to describe and compare molecular, morphological and bacterial symbionts composition among sympatric individuals from the Ae. albopictus subgroup inhabiting a Vietnamese protected area.

Results

Based on morphological structure of the cibarial armarture, we identified a cryptic species in the forest park at Bù Gia Mập in the south-eastern region of Vietnam. Analysis of nuclear (ITS1-5.8S-ITS2) and mitochondrial (cox1, nad5) markers confirmed the divergence between the cryptic species and Ae. albopictus. Analysis of midgut bacterial microbiota revealed a strong similarity among the two species with a notable difference; contrary to Ae. albopictus, the cryptic species did not harbour any Wolbachia infection.

Conclusions

These results could reflect either a recent invasion of Wolbachia in Ae. albopictus or alternatively a loss of this symbiont in the cryptic species. We argue that neglected species of the Ae. albopictus subgroup are of main importance in order to estimate variation of host-symbionts interactions across evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2202-9) contains supplementary material, which is available to authorized users.

Emergent and Reemergent Arboviruses in South America and the Caribbean: Why So Many and Why Now?

Varios arbovirus han emergido y/o reemergido en el Nuevo Mundo en las últimas décadas. Los virus Zika y chikungunya, anteriormente restringidos a África y quizás Asia, invadieron el continente, causando gran preocupación; además siguen ocurriendo brotes causados por el virus dengue en casi todos los países, con millones de casos por año. El virus West Nile invadió rápidamente América del Norte, y ya se han encontrado casos en América Central y del Sur. Otros arbovirus, como Mayaro y el virus de la encefalitis equina del este han aumentado su actividad y se han encontrado en nuevas regiones. Se han documentado cambios en la patogenicidad de algunos virus que conducen a enfermedades inesperadas. Una fauna diversa de mosquitos, cambios climáticos y en la vegetación, aumento de los viajes, y urbanizaciones no planificadas que generan condiciones adecuadas para la proliferación de Aedes aegypti (L.), Culex quinquefasciatus Say y otros mosquitos vectores, se han combinado para influir fuertemente en los cambios en la distribución y la incidencia de varios arbovirus. Se enfatiza la necesidad de realizar estudios exhaustivos de la fauna de mosquitos y modificaciones de las condiciones ambientales, sobre todo en las zonas urbanas fuertemente influenciadas por factores sociales, políticos y económicos.

Overview on the Current Status of Zika Virus Pathogenesis and Animal Related Research

There is growing evidence that Zika virus (ZIKV) infection is linked with activation of Guillan-Barré syndrome (GBS) in adults infected with the virus and microcephaly in infants following maternal infection. With the recent outpour in publications by numerous research labs, the association between microcephaly in newborns and ZIKV has become very apparent in which large numbers of viral particles were found in the central nervous tissue of an electively aborted microcephalic ZIKV-infected fetus. However, the underlying related mechanisms remain poorly understood. Thus, development of ZIKV-infected animal models are urgently required. The need to develop drugs and vaccines of high efficacy along with efficient diagnostic tools for ZIKV treatment and management raised the demand for a very selective animal model for exploring ZIKV pathogenesis and related mechanisms. In this review, we describe recent advances in animal models developed for studying ZIKV pathogenesis and evaluating potential interventions against human infection, including during pregnancy. The current research directions and the scientific challenges ahead in developing effective vaccines and therapeutics are also discussed.

A Semipersistent Plant Virus Differentially Manipulates Feeding Behaviors of Different Sexes and Biotypes of Its Whitefly Vector

It is known that plant viruses can change the performance of their vectors. However, there have been no reports on whether or how a semipersistent plant virus manipulates the feeding behaviors of its whitefly vectors. Cucurbit chlorotic yellows virus (CCYV) (genus Crinivirus, family Closteroviridae) is an emergent plant virus in many Asian countries and is transmitted specifically by B and Q biotypes of tobacco whitefly, Bemisia tabaci (Gennadius), in a semipersistent manner. In the present study, we used electrical penetration graph (EPG) technique to investigate the effect of CCYV on the feeding behaviors of B. tabaci. The results showed that CCYV altered feeding behaviors of both biotypes and sexes of B. tabaci with different degrees. CCYV had stronger effects on feeding behaviors of Q biotype than those of B biotype, by increasing duration of phloem salivation and sap ingestion, and could differentially manipulate feeding behaviors of males and females in both biotype whiteflies, with more phloem ingestion in Q biotype males and more non-phloem probing in B biotype males than their respective females. With regard to feeding behaviors related to virus transmission, these results indicated that, when carrying CCYV, B. tabaci Q biotype plays more roles than B biotype, and males make greater contribution than females.

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

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

Complexity of virus-vector interactions

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

Noncoding Subgenomic Flavivirus RNA Is Processed by the Mosquito RNA Interference Machinery and Determines West Nile Virus Transmission by Culex pipiens Mosquitoes

Flaviviruses, such as Zika virus, yellow fever virus, dengue virus, and West Nile virus (WNV), are a serious concern for human health. Flaviviruses produce an abundant noncoding subgenomic flavivirus RNA (sfRNA) in infected cells. sfRNA results from stalling of the host 5'-3' exoribonuclease XRN1/Pacman on conserved RNA structures in the 3' untranslated region (UTR) of the viral genomic RNA. sfRNA production is conserved in insect-specific, mosquito-borne, and tick-borne flaviviruses and flaviviruses with no known vector, suggesting a pivotal role for sfRNA in the flavivirus life cycle. Here, we investigated the function of sfRNA during WNV infection of Culex pipiens mosquitoes and evaluated its role in determining vector competence. An sfRNA1-deficient WNV was generated that displayed growth kinetics similar to those of wild-type WNV in both RNA interference (RNAi)-competent and -compromised mosquito cell lines. Small-RNA deep sequencing of WNV-infected mosquitoes indicated an active small interfering RNA (siRNA)-based antiviral response for both the wild-type and sfRNA1-deficient viruses. Additionally, we provide the first evidence that sfRNA is an RNAi substrate in vivo Two reproducible small-RNA hot spots within the 3' UTR/sfRNA of the wild-type virus mapped to RNA stem-loops SL-III and 3' SL, which stick out of the three-dimensional (3D) sfRNA structure model. Importantly, we demonstrate that sfRNA-deficient WNV displays significantly decreased infection and transmission rates in vivo when administered via the blood meal. Finally, we show that transmission and infection rates are not affected by sfRNA after intrathoracic injection, thereby identifying sfRNA as a key driver to overcome the mosquito midgut infection barrier. This is the first report to describe a key biological function of sfRNA for flavivirus infection of the arthropod vector, providing an explanation for the strict conservation of sfRNA production.

IMPORTANCE

Understanding the flavivirus transmission cycle is important to identify novel targets to interfere with disease and to aid development of virus control strategies. Flaviviruses produce an abundant noncoding viral RNA called sfRNA in both arthropod and mammalian cells. To evaluate the role of sfRNA in flavivirus transmission, we infected mosquitoes with the flavivirus West Nile virus and an sfRNA-deficient mutant West Nile virus. We demonstrate that sfRNA determines the infection and transmission rates of West Nile virus in Culex pipiens mosquitoes. Comparison of infection via the blood meal versus intrathoracic injection, which bypasses the midgut, revealed that sfRNA is important to overcome the mosquito midgut barrier. We also show that sfRNA is processed by the antiviral RNA interference machinery in mosquitoes. This is the first report to describe a pivotal biological function of sfRNA in arthropods. The results explain why sfRNA production is evolutionarily conserved.

Emergence and Spreading Potential of Zika Virus

Zika virus (ZIKV) is an arthropod-borne Flavivirus (family Flaviviridae) closely related to dengue, yellow fever and West Nile viruses. ZIKV remained neglected, confined to enzootic transmission cycles in Africa and Asia, until the first significant outbreak was reported in Micronesia in 2007. Subsequent epidemics of growing incidence occurred in French Polynesia and other South Pacific Islands, and recently, in the Americas. The latter and currently ongoing outbreak of unprecedented incidence revealed the association of ZIKV infection with the occurrence of severe congenital malformations and neurological diseases, leading to a widespread concern about its potential to pose a global public health threat. Serological and molecular data suggest that the genetic and geographic diversification of ZIKV may be greatly underestimated. Here we discuss several ecological and epidemiological aspects, together with the evolutionary processes that may have driven the emergence and abrupt spread of ZIKV in the Americas.

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.

Overlap in the Seasonal Infection Patterns of Avian Malaria Parasites and West Nile Virus in Vectors and Hosts

Multiple vector-borne pathogens often circulate in the same vector and host communities, and seasonal infection dynamics influence the potential for pathogen interactions. Here, we explore the seasonal infection patterns of avian malaria (Haemosporida) parasites (Plasmodium and Haemoproteus) and West Nile virus (WNV) in birds and mosquitoes in suburban Chicago. We show that both pathogens vary seasonally in Culex mosquitoes and avian hosts, but that patterns of covariation are complex. Different putative Plasmodium species varied asynchronously across the season in mosquitoes and birds, suggesting that different forces may govern their transmission. Infections of Culex mosquitoes with Plasmodium parasites were positively associated with WNV infections in pools of individuals aggregated from the same time and site, suggesting that these pathogens respond to common environmental drivers and co-circulate among the same host and vector populations. Future research should focus on these common drivers, and whether these pathogens interact in vectors and hosts.

Zika Virus

Zika virus (ZIKV) is an arthropod-borne virus (arbovirus) in the genus Flavivirus and the family Flaviviridae. ZIKV was first isolated from a nonhuman primate in 1947 and from mosquitoes in 1948 in Africa, and ZIKV infections in humans were sporadic for half a century before emerging in the Pacific and the Americas. ZIKV is usually transmitted by the bite of infected mosquitoes. The clinical presentation of Zika fever is nonspecific and can be misdiagnosed as other infectious diseases, especially those due to arboviruses such as dengue and chikungunya. ZIKV infection was associated with only mild illness prior to the large French Polynesian outbreak in 2013 and 2014, when severe neurological complications were reported, and the emergence in Brazil of a dramatic increase in severe congenital malformations (microcephaly) suspected to be associated with ZIKV. Laboratory diagnosis of Zika fever relies on virus isolation or detection of ZIKV-specific RNA. Serological diagnosis is complicated by cross-reactivity among members of the Flavivirus genus. The adaptation of ZIKV to an urban cycle involving humans and domestic mosquito vectors in tropical areas where dengue is endemic suggests that the incidence of ZIKV infections may be underestimated. There is a high potential for ZIKV emergence in urban centers in the tropics that are infested with competent mosquito vectors such as Aedes aegypti and Aedes albopictus.

Genome Sequences of Five Arboviruses in Field-Captured Mosquitoes in a Unique Rural Environment of South Korea

Here, we present the genome sequences of one mesonivirus and four novel arboviruses observed in Culex bitaeniorhynchus and Culex pipiens, captured in and near the demilitarized zone, Republic of Korea. Results suggest the ubiquitous presence of mesoniviruses and the discovery of a potentially new species of arboviruses in field-captured mosquitoes.

French invasive Asian tiger mosquito populations harbor reduced bacterial microbiota and genetic diversity compared to Vietnamese autochthonous relatives

The Asian tiger mosquito Aedes albopictus is one of the most significant pathogen vectors of the twenty-first century. Originating from Asia, it has invaded a wide range of eco-climatic regions worldwide. The insect-associated microbiota is now recognized to play a significant role in host biology. While genetic diversity bottlenecks are known to result from biological invasions, the resulting shifts in host-associated microbiota diversity has not been thoroughly investigated. To address this subject, we compared four autochthonous Ae. albopictus populations in Vietnam, the native area of Ae. albopictus, and three populations recently introduced to Metropolitan France, with the aim of documenting whether these populations display differences in host genotype and bacterial microbiota. Population-level genetic diversity (microsatellite markers and COI haplotype) and bacterial diversity (16S rDNA metabarcoding) were compared between field-caught mosquitoes. Bacterial microbiota from the whole insect bodies were largely dominated by Wolbachia pipientis. Targeted analysis of the gut microbiota revealed a greater bacterial diversity in which a fraction was common between French and Vietnamese populations. The genus Dysgonomonas was the most prevalent and abundant across all studied populations. Overall genetic diversities of both hosts and bacterial microbiota were significantly reduced in recently established populations of France compared to the autochthonous populations of Vietnam. These results open up many important avenues of investigation in order to link the process of geographical invasion to shifts in commensal and symbiotic microbiome communities, as such shifts may have dramatic impacts on the biology and/or vector competence of invading hematophagous insects.

Non-structural protein NS3/NS3a is required for propagation of bluetongue virus in Culicoides sonorensis

BACKGROUND

Bluetongue virus (BTV) causes non-contagious haemorrhagic disease in ruminants and is transmitted by Culicoides spp. biting midges. BTV encodes four non-structural proteins of which NS3/NS3a is functional in virus release. NS3/NS3a is not essential for in vitro virus replication. However, deletion of NS3/NS3a leads to delayed virus release from mammalian cells and largely reduces virus release from insect cells. NS3/NS3a knockout BTV in sheep causes no viremia, but induces sterile immunity and is therefore proposed to be a Disabled Infectious Single Animal (DISA) vaccine candidate. In the absence of viremia, uptake of this vaccine strain by blood-feeding midges would be highly unlikely. Nevertheless, unintended replication of vaccine strains within vectors, and subsequent recombination or re-assortment resulting in virulent phenotypes and transmission is a safety concern of modified-live vaccines.

METHODS

The role of NS3/NS3a in replication and dissemination of BTV1, expressing VP2 of serotype 2 within colonized Culicoides sonorensis midges was investigated. Virus strains were generated using reverse genetics and their growth was examined in vitro. A laboratory colony of C. sonorensis, a known competent BTV vector, was fed or injected with BTV with or without expressing NS3/NS3a and replication in the midge was examined using RT PCR. Crossing of the midgut infection barrier was examined by separate testing of midge heads and bodies.

RESULTS

Although the parental NS3/NS3a expressing strain was not able to replicate and disseminate within C. sonorensis after oral feeding, this virus was able to replicate efficiently when the midgut infection barrier was bypassed by intrathoracic injection, whereas the NS3/NS3a knockout mutant was unable to replicate. This demonstrates that NS3/NS3a is required for viral replication within Culicoides.

CONCLUSION

The lack of viremia and the inability to replicate within the vector, clearly demonstrate the inability of NS3/NS3a knockout DISA vaccine strains to be transmitted by midges.

Differential profiles of direct and indirect modification of vector feeding behaviour by a plant virus

Plant viruses interact with their insect vectors directly and indirectly via host plants, and this tripartite interaction may produce fitness benefits to both the vectors and the viruses. Our previous studies show that the Middle East-Asia Minor 1 (MEAM1) species of the whitefly Bemisia tabaci complex improved its performance on tobacco plants infected by the Tomato yellow leaf curl China virus (TYLCCNV), which it transmits, although virus infection of the whitefly per se reduced its performance. Here, we use electrical penetration graph recording to investigate the direct and indirect effects of TYLCCNV on the feeding behaviour of MEAM1. When feeding on either cotton, a non-host of TYLCCNV, or uninfected tobacco, a host of TYLCCNV, virus-infection of the whiteflies impeded their feeding. Interestingly, when viruliferous whiteflies fed on virus-infected tobacco, their feeding activities were no longer negatively affected; instead, the virus promoted whitefly behaviour related to rapid and effective sap ingestion. Our findings show differential profiles of direct and indirect modification of vector feeding behaviour by a plant virus, and help to unravel the behavioural mechanisms underlying a mutualistic relationship between an insect vector and a plant virus that also has features reminiscent of an insect pathogen.

French invasive Asian tiger mosquito populations harbor reduced bacterial microbiota and genetic diversity compared to Vietnamese autochthonous relatives

The Asian tiger mosquito Aedes albopictus is one of the most significant pathogen vectors of the twenty-first century. Originating from Asia, it has invaded a wide range of eco-climatic regions worldwide. The insect-associated microbiota is now recognized to play a significant role in host biology. While genetic diversity bottlenecks are known to result from biological invasions, the resulting shifts in host-associated microbiota diversity has not been thoroughly investigated. To address this subject, we compared four autochthonous Ae. albopictus populations in Vietnam, the native area of Ae. albopictus, and three populations recently introduced to Metropolitan France, with the aim of documenting whether these populations display differences in host genotype and bacterial microbiota. Population-level genetic diversity (microsatellite markers and COI haplotype) and bacterial diversity (16S rDNA metabarcoding) were compared between field-caught mosquitoes. Bacterial microbiota from the whole insect bodies were largely dominated by Wolbachia pipientis. Targeted analysis of the gut microbiota revealed a greater bacterial diversity in which a fraction was common between French and Vietnamese populations. The genus Dysgonomonas was the most prevalent and abundant across all studied populations. Overall genetic diversities of both hosts and bacterial microbiota were significantly reduced in recently established populations of France compared to the autochthonous populations of Vietnam. These results open up many important avenues of investigation in order to link the process of geographical invasion to shifts in commensal and symbiotic microbiome communities, as such shifts may have dramatic impacts on the biology and/or vector competence of invading hematophagous insects.