Establishment of Culex modestus in Belgium and a Glance into the Virome of Belgian Mosquito Species

Lack of abundant core virome in Culex mosquitoes from a temperate climate region despite a mosquito species-specific virome

In arthropod-associated microbial communities, insect-specific viruses (ISVs) are prevalent yet understudied due to limited infectivity outside their natural hosts. However, ISVs might play a crucial role in regulating mosquito populations and influencing arthropod-borne virus transmission. Some studies have indicated a core virome in mosquitoes consisting of mostly ISVs. Employing single mosquito metagenomics, we comprehensively profiled the virome of native and invasive mosquito species in Belgium. This approach allowed for accurate host species determination, prevalence assessment of viruses and Wolbachia, and the identification of novel viruses. Contrary to our expectations, no abundant core virome was observed in Culex mosquitoes from Belgium. In that regard, we caution against rigidly defining mosquito core viromes and encourage nuanced interpretations of other studies. Nonetheless, our study identified 45 viruses of which 28 were novel, enriching our understanding of the mosquito virome and ISVs. We showed that the mosquito virome in this study is species-specific and less dependent on the location where mosquitoes from the same species reside. In addition, because Wolbachia has previously been observed to influence arbovirus transmission, we report the prevalence of Wolbachia in Belgian mosquitoes and the detection of several Wolbachia mobile genetic elements. The observed prevalence ranged from 83% to 92% in members from the Culex pipiens complex.IMPORTANCECulex pipiens mosquitoes are important vectors for arboviruses like West Nile virus and Usutu virus. Virome studies on individual Culex pipiens, and on individual mosquitoes in general, have been lacking. To mitigate this, we sequenced the virome of 190 individual Culex and 8 individual Aedes japonicus mosquitoes. We report the lack of a core virome in these mosquitoes from Belgium and caution the interpretation of other studies in this light. The discovery of new viruses in this study will aid our comprehension of insect-specific viruses and the mosquito virome in general in relation to mosquito physiology and mosquito population dynamics.

Metagenomic sequencing reveals viral diversity of mosquitoes from Shandong Province, China

Mosquitoes carry a large number of known and unknown viruses, some of which could cause serious diseases in humans or animals. Metagenomic sequencing for mosquito viromes is crucial for understanding the evolutionary history of viruses and preventing emerging mosquito-borne diseases. We collected 1,598 mosquitoes belonging to four species from five counties in Shandong Province, China in 2021. They were grouped by species and sampling locations and subjected to metagenomic next-generation sequencing for the analysis of the viromes. A total of 233,317,352 sequencing reads were classified into 30 viral families and an unclassified group. Comparative analysis showed that mosquitoes in Shandong Province generally possessed host-specific virome. We detected mosquito-borne viruses including Japanese encephalitis virus, Getah virus, and Kadipiro virus in Culex tritaeniorhynchus and Anopheles sinensis samples. Phylogenetic analysis showed that these pathogenic viruses may have existed in mosquitoes in Shandong Province for a long time. Meanwhile, we identified 22 novel viruses belonging to seven families and the genus Negevirus. Our study comprehensively described the viromes of several common mosquito species in Shandong Province, China, and demonstrated the major role of host species in shaping mosquito viromes. Furthermore, the metagenomic data provided valuable epidemiological information on multiple mosquito-borne viruses, highlighting the potential risk of infection transmission.

IMPORTANCE

Mosquitoes are known as the source of various pathogens for humans and animals. Culex tritaeniorhynchus, Armigeres subalbatus, and Anopheles sinensis have been found to transmit the Getah virus, which has recently caused increasing infections in China. Cx. tritaeniorhynchus and Culex pipiens are the main vectors of Japanese encephalitis virus and have caused epidemics of Japanese encephalitis in China in past decades. These mosquitoes are widely present in Shandong Province, China, leading to a great threat to public health and the breeding industry. This study provided a comprehensive insight into the viromes of several common mosquito species in Shandong Province, China. The metagenomic sequencing data revealed the risks of multiple pathogenic mosquito-borne viruses, including Japanese encephalitis virus, Getah virus, and Kadipiro virus, which are of great importance for preventing emerging viral epidemics.

Tips and tools to obtain and assess mosquito viromes

Due to their vectorial capacity, mosquitoes (Diptera: Culicidae) receive special attention from health authorities and entomologists. These cosmopolitan insects are responsible for the transmission of many viral diseases, such as dengue and yellow fever, causing huge impacts on human health and justifying the intensification of research focused on mosquito-borne diseases. In this context, the study of the virome of mosquitoes can contribute to anticipate the emergence and/or the reemergence of infectious diseases. The assessment of mosquito viromes also contributes to the surveillance of a wide variety of viruses found in these insects, allowing the early detection of pathogens with public health importance. However, the study of mosquito viromes can be challenging due to the number and complexities of steps involved in this type of research. Therefore, this article aims to describe, in a straightforward and simplified way, the steps necessary for obtention and assessment of mosquito viromes. In brief, this article explores: the capture and preservation of specimens; sampling strategies; treatment of samples before DNA/RNA extraction; extraction methodologies; enrichment and purification processes; sequencing choices; and bioinformatics analysis.

Culex modestus: the overlooked mosquito vector

Culex (Barraudius) modestus (Ficalbi 1889) are found in temperate regions across Europe, Asia, and Northern Africa. These mosquitoes thrive during the summer and prefer to breed in permanent vegetative habitats such as rice paddies and marshes. Culex modestus feed on a wide range of bird species but are highly attracted to humans, which makes them a potential 'bridge' vector for enzootic pathogens. There is compelling evidence that Culex modestus is an efficient vector for West Nile virus, potentially capable of causing epidemics in humans and other mammals. This species is also a likely vector for Usutu virus, avian malaria (Plasmodium spp.), and parasitic heartworms (Dirofilaria spp.). Culex modestus can be morphologically identified at the larval and adult stages, and a distinctive phenotype of this species is their ability to overwinter. Despite the widespread establishment of this mosquito species and their role as vectors for human pathogens, we lack sufficient knowledge on this species to implement and evaluate targeted vector control measures. Since Culex modestus can be considered a potential public health threat, there is a need for a better understanding of this mosquito species.

Belgian Culex pipiens pipiens are competent vectors for West Nile virus while Culex modestus are competent vectors for Usutu virus

BACKGROUND

West Nile virus (WNV) and Usutu virus (USUV) are emerging arthropod-borne viruses (arboviruses) in Europe transmitted by Culex mosquitoes. In Belgium, it is currently unknown which Culex species are competent vectors for WNV or USUV and if these mosquitoes carry Wolbachia, an endosymbiotic bacterium that can block arbovirus transmission. The aims of our study were to measure the vector competence of Belgian Culex mosquitoes to WNV and USUV and determine if a naturally acquired Wolbachia infection can influence virus transmission.

METHODOLOGY/PRINCIPAL FINDINGS

Female Culex mosquitoes were captured from urban and peri-urban sites in Leuven, Belgium and offered an infectious bloodmeal containing WNV lineage 2, USUV European (EU) lineage 3, or USUV African (AF) lineage 3. Blood-fed females were incubated for 14 days at 25°C after which the body, head, and saliva were collected to measure infection, dissemination, and transmission rates as well as transmission efficiency. Mosquito species were identified by qRT-PCR or Sanger sequencing, the presence of infectious virus in mosquitoes was confirmed by plaque assays, and viral genome copies were quantified by qRT-PCR. Culex pipiens pipiens were able to transmit WNV (4.3% transmission efficiency, n = 2/47) but not USUV (EU lineage: n = 0/56; AF lineage: n = 0/37). In contrast, Culex modestus were able to transmit USUV (AF lineage: 20% transmission efficiency, n = 1/5) but not WNV (n = 0/6). We found that the presence or absence of Wolbachia was species-dependent and did not associate with virus transmission.

CONCLUSIONS/SIGNIFICANCE

This is the first report that Belgian Culex mosquitoes can transmit both WNV and USUV, forewarning the risk of human transmission. More research is needed to understand the potential influence of Wolbachia on arbovirus transmission in Culex modestus mosquitoes.

Mosquito community composition in two major stopover aquatic ecosystems used by migratory birds in northern Spain

Mosquitoes (Diptera: Culicidae) are common bloodsucking Diptera frequently found in aquatic environments, which are valuable ecosystems for many animal species, particularly migrating birds. Therefore, interactions between these animal species and mosquitoes may play a critical role in pathogen transmission. During 2018-2019, mosquitoes were collected from two aquatic ecosystems in northern Spain using different methodologies and identified using classical morphology and molecular tools. A total of 1529 males and females of 22 native mosquito species (including eight new records for the region) were trapped using CO2 -baited Centers for Disease Control and Prevention (CDC) traps and sweep netting. Among the blood-fed female mosquitoes, 11 vertebrate host species-six mammals and five birds-were identified using DNA barcoding. The developmental sites of eight mosquito species were determined across nine microhabitats, and 11 mosquito species were caught landing on humans. The flight period varied among mosquito species, with some peaking in the spring and others in the summer. Our study highlights the advantages of mosquito sampling using various techniques to comprehensively characterise species composition and abundance. Information on the trophic preferences, biting behaviour and influence of climatic variables on the ecology of mosquitoes is also provided.

Leveraging insect-specific viruses to elucidate mosquito population structure and dynamics

Several aspects of mosquito ecology that are important for vectored disease transmission and control have been difficult to measure at epidemiologically important scales in the field. In particular, the ability to describe mosquito population structure and movement rates has been hindered by difficulty in quantifying fine-scale genetic variation among populations. The mosquito virome represents a possible avenue for quantifying population structure and movement rates across multiple spatial scales. Mosquito viromes contain a diversity of viruses, including several insect-specific viruses (ISVs) and "core" viruses that have high prevalence across populations. To date, virome studies have focused on viral discovery and have only recently begun examining viral ecology. While nonpathogenic ISVs may be of little public health relevance themselves, they provide a possible route for quantifying mosquito population structure and dynamics. For example, vertically transmitted viruses could behave as a rapidly evolving extension of the host's genome. It should be possible to apply established analytical methods to appropriate viral phylogenies and incidence data to generate novel approaches for estimating mosquito population structure and dispersal over epidemiologically relevant timescales. By studying the virome through the lens of spatial and genomic epidemiology, it may be possible to investigate otherwise cryptic aspects of mosquito ecology. A better understanding of mosquito population structure and dynamics are key for understanding mosquito-borne disease ecology and methods based on ISVs could provide a powerful tool for informing mosquito control programs.

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

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

Blood meal source identification and RNA virome determination in Japanese encephalitis virus vectors collected in Ishikawa Prefecture, Japan, show distinct avian/mammalian host preference

In Asia, Culex mosquitoes are of particular interest because of their role in maintaining endemic mosquito-borne viral diseases, including the Japanese encephalitis virus (JEV). Nonetheless, host-feeding preferences, along with naturally infecting RNA viruses in certain Culex species, remain understudied. In this study, selected blood-fed mosquitoes were processed for avian and mammalian blood meal source identification. Concurrently, cell culture propagation and high-throughput sequencing (HTS) approaches were used to determine the RNA virome of Culex mosquitoes collected in Ishikawa Prefecture, Japan. The identification of blood meal sources from wild-caught Culex spp. revealed that Culex (Culex) tritaeniorhynchus Giles, 1901, has a robust preference toward wild boar (62%, 26/42), followed by heron (21%, 9/42). The other two species, Culex (Oculeomyia) bitaeniorhynchus Giles, 1901, and Culex (Culex) orientalis Edwards, 1921, showed a distinct preference for avian species, including migratory birds. From the HTS results, 34 virus sequences were detected, four of which were newly identified virus sequences of unclassified Aspiviridae, Qinviridae, Iflaviridae, and Picornaviridae. The absence of observable cytopathic effects in mammalian cells and phylogenetic analysis suggested that all identified virus sequences were insect-specific. Further investigations involving other mosquito populations collected in different areas are warranted to explore previously unknown vertebrate hosts that may be linked to JEV dispersal in nature.

Isolation of a novel rhabdovirus and detection of multiple novel viral sequences in Culex species mosquitoes in the United States

To increase our understanding of the diversity of the mosquito virome, 6956 mosquitoes of five species (Culex erraticus, Culex pipiens, Culex restuans, Culex tarsalis, and Culex territans) collected in Iowa in the United States in 2017 and 2020 were assayed for novel viruses by performing polyethylene glycol precipitation, virus isolation in cell culture, and unbiased high-throughput sequencing. A novel virus, provisionally named "Walnut Creek virus", was isolated from Cx. tarsalis, and its genomic sequence and organization are characteristic of viruses in the genus Hapavirus (family Rhabdoviridae). Replication of Walnut Creek virus occurred in avian, mammalian, and mosquito, but not tick, cell lines. A novel virus was also isolated from Cx. restuans, and partial genome sequencing revealed that it is distantly related to an unclassified virus of the genus Phytoreovirus (family Sedoreoviridae). Two recognized viruses were also isolated: Culex Y virus (family Birnaviridae) and Houston virus (family Mesoniviridae). We also identified sequences of eight novel viruses from six families (Amalgaviridae, Birnaviridae, Partitiviridae, Sedoreoviridae, Tombusviridae, and Totiviridae), two viruses that do not belong to any established families, and many previously recognized viruses. In summary, we provide evidence of multiple novel and recognized viruses in Culex spp. mosquitoes in the United States.

First Report of Mutations Associated With Pyrethroid (L1014F) and Organophosphate (G119S) Resistance in Belgian Culex (Diptera: Culicidae) Mosquitoes

The emergence of West Nile virus and Usutu virus in Europe poses a significant risk to public health. In the absence of efficient antiviral therapy or vaccine candidates, the only strategy to control these arboviruses is to target the Culex (Diptera: Culicidae) mosquito vector. However, the selection pressure caused by exposure to insecticides for vector control or agricultural pest control can lead to insecticide resistance, thereby reducing the efficacy of insecticide-based vector control interventions. In Culex mosquitoes, two of the most common amino acid substitutions associated with insecticide resistance are the kdr L1014F in voltage gated sodium channels and G119S in acetylcholinesterase. In this study, Culex pipiens biotype pipiens, Culex torrentium, and Culex modestus were sampled from 2019 to 2021 in three distinct environmental habitats (urban, peri-urban, and agricultural) in and around the city of Leuven, Belgium. Individual mosquitoes were screened for two mutations resulting in L1014F and G119S amino acid substitutions. Both mutations were observed in Cx. pipiens and Cx. modestus but not in Cx. torrentium mosquitoes across the four collection sites. Furthermore, multi-resistance or cross-resistance in Cx. pipiens could be a threat in these areas, as both mutations were observed at low frequencies. These results provide the first report of kdr L1014F and ace-1 G119S resistance mutations in Cx. pipiens and Cx. modestus mosquitoes from Belgium, highlighting the importance of mosquito surveillance to design effective arbovirus outbreak control strategies.

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

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

Characterisation of the RNA Virome of Nine Ochlerotatus Species in Finland

RNA viromes of nine commonly encountered Ochlerotatus mosquito species collected around Finland in 2015 and 2017 were studied using next-generation sequencing. Mosquito homogenates were sequenced from 91 pools comprising 16-60 morphologically identified adult females of Oc. cantans, Oc. caspius, Oc. communis, Oc. diantaeus, Oc. excrucians, Oc. hexodontus, Oc. intrudens, Oc. pullatus and Oc. punctor/punctodes. In total 514 viral Reverse dependent RNA polymerase (RdRp) sequences of 159 virus species were recovered, belonging to 25 families or equivalent rank, as follows: Aliusviridae, Aspiviridae, Botybirnavirus, Chrysoviridae, Chuviridae, Endornaviridae, Flaviviridae, Iflaviridae, Negevirus, Partitiviridae, Permutotetraviridae, Phasmaviridae, Phenuiviridae, Picornaviridae, Qinviridae, Quenyavirus, Rhabdoviridae, Sedoreoviridae, Solemoviridae, Spinareoviridae, Togaviridae, Totiviridae, Virgaviridae, Xinmoviridae and Yueviridae. Of these, 147 are tentatively novel viruses. One sequence of Sindbis virus, which causes Pogosta disease in humans, was detected from Oc. communis from Pohjois-Karjala. This study greatly increases the number of mosquito-associated viruses known from Finland and presents the northern-most mosquito-associated viruses in Europe to date.

Updated occurrence and bionomics of potential malaria vectors in Europe: a systematic review (2000-2021)

Despite the eradication of malaria across most European countries in the 1960s and 1970s, the anopheline vectors are still present. Most of the malaria cases that have been reported in Europe up to the present time have been infections acquired in endemic areas by travelers. However, the possibility of acquiring malaria by locally infected mosquitoes has been poorly investigated in Europe, despite autochthonous malaria cases having been occasionally reported in several European countries. Here we present an update on the occurrence of potential malaria vector species in Europe. Adopting a systematic review approach, we selected 288 papers published between 2000 and 2021 for inclusion in the review based on retrieval of accurate information on the following Anopheles species: An. atroparvus, An. hyrcanus sensu lato (s.l.), An. labranchiae, An. maculipennis sensu stricto (s.s.), An. messeae/daciae, An. sacharovi, An. superpictus and An. plumbeus. The distribution of these potential vector species across Europe is critically reviewed in relation to areas of major presence and principal bionomic features, including vector competence to Plasmodium. Additional information, such as geographical details, sampling approaches and species identification methods, are also reported. We compare the information on each species extracted from the most recent studies to comparable information reported from studies published in the early 2000s, with particular reference to the role of each species in malaria transmission before eradication. The picture that emerges from this review is that potential vector species are still widespread in Europe, with the largest diversity in the Mediterranean area, Italy in particular. Despite information on their vectorial capacity being fragmentary, the information retrieved suggests a re-definition of the relative importance of potential vector species, indicating An. hyrcanus s.l., An. labranchiae, An. plumbeus and An. sacharovi as potential vectors of higher importance, while An. messeae/daciae and An. maculipennis s.s. can be considered to be moderately important species. In contrast, An. atroparvus and An. superpictus should be considered as vectors of lower importance, particularly in relation to their low anthropophily. The presence of gaps in current knowledge of vectorial systems in Europe becomes evident in this review, not only in terms of vector competence but also in the definition of sampling approaches, highlighting the need for further research to adopt the appropriate surveillance system for each species.