Colonization of the Mediterranean Basin by the vector biting midge species Culicoides imicola: an old story

Understanding the demographic history and genetic make-up of colonizing species is critical for inferring population sources and colonization routes. This is of main interest for designing accurate control measures in areas newly colonized by vector species of economically important pathogens. The biting midge Culicoides imicola is a major vector of Orbiviruses to livestock. Historically, the distribution of this species was limited to the Afrotropical region. Entomological surveys first revealed the presence of C. imicola in the south of the Mediterranean basin by the 1970's. Following recurrent reports of massive bluetongue outbreaks since the 1990s, the presence of the species was confirmed in northern areas. In this study, we addressed the chronology and processes of C. imicola colonization in the Mediterranean basin. We characterized the genetic structure of its populations across Mediterranean and African regions using both mitochondrial and nuclear markers, and combined phylogeographical analyses with population genetics and approximate Bayesian computation. We found a west/east genetic differentiation between populations, occurring both within Africa and within the Mediterranean basin. We demonstrated that three of these groups had experienced demographic expansions in the Pleistocene, probably because of climate changes during this period. Finally, we showed that C. imicola could have colonized the Mediterranean basin in the late Pleistocene or early Holocene through a single event of introduction; however we cannot exclude the hypothesis involving two routes of colonization. Thus, the recent bluetongue outbreaks are not linked to C. imicola colonization event, but rather to biological changes in the vector or the virus

Revealing potential bridge vectors for BTV and SBV: a study on <em>Culicoides</em> blood feeding preferences in natural ecosystems in Spain

International audienceSeveral species of Culicoides (Diptera: Ceratopogonidae) are vectors of pathogens, such as the bluetongue (BTV) and Schmallenberg (SBV) viruses, which cause important diseases in domestic and wild ruminants. As wild ruminants can contribute to overwintering and epizootics of both diseases, knowledge of the host-feeding behaviour of Culicoides in natural ecosystems is important to better understand their epidemiology. Blood-engorged Culicoides females trapped in natural areas inhabited by different wild ruminant species were genetically analysed to identify host species. The origin of bloodmeals was identified in 114 females of 14 species of Culicoides. A total of 104 (91.1%) Culicoides fed on mammals and 10 (8.9%) on birds. The most abundant host identified was red deer (66.7%), followed by humans (13%) and fallow deer (6.1%). Eleven of the 14 species of Culicoides fed exclusively on mammalian hosts. Among them, five are mammalophilic species considered to be important BTV and/or SBV vectors. The results of the present study confirm that Culicoides imicola, Culicoides obsoletus, Culicoides scoticus, Culicoides pulicaris and Culicoides punctatus fed on wild ruminants, and therefore support the hypothesis that these species can act as bridge vectors by facilitating the circulation of pathogens between wild and domestic ruminant communities

Molecular Determinants of West Nile Virus Virulence and Pathogenesis in Vertebrate and Invertebrate Hosts

International audienceWest Nile virus (WNV), like the dengue virus (DENV) and yellow fever virus (YFV), are major arboviruses belonging to the Flavivirus genus. WNV is emerging or endemic in many countries around the world, affecting humans and other vertebrates. Since 1999, it has been considered to be a major public and veterinary health problem, causing diverse pathologies, ranging from a mild febrile state to severe neurological damage and death. WNV is transmitted in a bird–mosquito–bird cycle, and can occasionally infect humans and horses, both highly susceptible to the virus but considered dead-end hosts. Many studies have investigated the molecular determinants of WNV virulence, mainly with the ultimate objective of guiding vaccine development. Several vaccines are used in horses in different parts of the world, but there are no licensed WNV vaccines for humans, suggesting the need for greater understanding of the molecular determinants of virulence and antigenicity in different hosts. Owing to technical and economic considerations, WNV virulence factors have essentially been studied in rodent models, and the results cannot always be transported to mosquito vectors or to avian hosts. In this review, the known molecular determinants of WNV virulence, according to invertebrate (mosquitoes) or vertebrate hosts (mammalian and avian), are presented and discussed. This overview will highlight the differences and similarities found between WNV hosts and models, to provide a foundation for the prediction and anticipation of WNV re-emergence and its risk of global spread

First detection of <em>Wolbachia</em>-infected <em>Culicoides</em> (Diptera: Ceratopogonidae) in Europe: <em>Wolbachia</em> and <em>Cardinium</em> infection across <em>Culicoides</em> communities revealed in Spain

International audienceBiting midges of the genus Culicoides (Diptera: Ceratopogonidae) transmit pathogens that cause important diseases. No effective technique has been found to properly control either Culicoides spp. abundance or their likelihood to transmit pathogens. Endosymbionts, particularly Wolbachia, represent powerful alternatives to control arthropods of health interest. In arthropods, Wolbachia can reduce vector fitness and vector's pathogen transmission capacity, thus being a potential target for population reduction and replacement strategies. The presence of Wolbachia and Cardinium endosymbionts was screened in Spanish Culicoides spp. populations at livestock premises and natural habitats. The first detection of Wolbachia-infected Culicoides spp. in Europe is reported. The putative Palaearctic vectors for bluetongue and Schmallenberg diseases, C. imicola, C. obsoletus (s.s.) and C. pulicaris (s.l.), were infected with Wolbachia. Four genetic clusters of closely-related Wolbachia strains from A and B supergroups were detected infecting Culicoides. Cardinium strain of the C-group was detected in C. obsoletus (s.l.). Both endosymbionts, Wolbachia and Cardinium, were detected in Culicoides species of minor epidemiological relevance as well. Higher prevalence of Wolbachia infection was detected in natural habitats, while livestock premises lead to higher prevalence of Cardinium. Significant differences in the prevalence of Wolbachia, but not Cardinium, were also detected between some Culicoides species and between locations. The presence of Wolbachia and Cardinium endosymbionts in Culicoides is expected to trigger new research towards the control of Culicoides-transmitted diseases. The results of the present study could have an impact beyond the Culicoides arena because successful Wolbachia transfection is possible even across genus and species barriers

pWCP is a widely distributed and highly conserved Wolbachia plasmid in Culex pipiens and Culex quinquefasciatus mosquitoes worldwide

Abstract Mosquitoes represent the most important pathogen vectors and are responsible for the spread of a wide variety of poorly treatable diseases. Wolbachia are obligate intracellular bacteria that are widely distributed among arthropods and collectively represents one of the most promising solutions for vector control. In particular, Wolbachia has been shown to limit the transmission of pathogens, and to dramatically affect the reproductive behavior of their host through its phage WO. While much research has focused on deciphering and exploring the biocontrol applications of these WO-related phenotypes, the extent and potential impact of the Wolbachia mobilome remain poorly appreciated. Notably, several Wolbachia plasmids, carrying WO-like genes and Insertion Sequences (IS), thus possibly interrelated to other genetic units of the endosymbiont, have been recently discovered. Here we investigated the diversity and biogeography of the first described plasmid of Wolbachia in Culex pipiens (pWCP) in several islands and continental countries around the world—including Cambodia, Guadeloupe, Martinique, Thailand, and Mexico—together with mosquito strains from colonies that evolved for 2 to 30 years in the laboratory. We used PCR and qPCR to determine the presence and copy number of pWCP in individual mosquitoes, and highly accurate Sanger sequencing to evaluate potential variations. Together with earlier observation, our results show that pWCP is omnipresent and strikingly conserved among Wolbachia populations within mosquitoes from distant geographies and environmental conditions. These data suggest a critical role for the plasmid in Wolbachia ecology and evolution, and the potential of a great tool for further genetic dissection and possible manipulation of this endosymbiont

The mosquito microbiome includes habitat-specific but rare symbionts

Microbial communities are known to influence mosquito lifestyles by modifying essential metabolic and behavioral processes that affect reproduction, development, immunity, digestion, egg survival, and the ability to transmit pathogens. Many studies have used 16S rRNA gene amplicons to characterize mosquito microbiota and investigate factors that influence host-microbiota dynamics. However, a relatively low taxonomic resolution due to clustering methods based on arbitrary threshold and the overall dominance of Wolbachia or Asaia symbionts obscured the investigation of rare members of mosquito microbiota in previous studies. Here, we used high resolution Shannon entropy-based oligotyping approach to analyze the microbiota of Culex pipiens, Culex quinquefasciatus and Aedes individuals from continental Southern France and overseas Guadeloupe as well as from laboratories with or without antibiotics treatment. Our experimental design that resulted in a series of mosquito samples with a gradient of Wolbachia density and relative abundance along with high-resolution analyses of amplicon sequences enabled the recovery of a robust signal from typically less accessible bacterial taxa. Our data confirm species-specific mosquito-bacteria associations with geography as a primary factor that influences bacterial community structure. But interestingly, they also reveal co-occurring symbiotic bacterial variants within single individuals for both Elizabethkingia and Erwinia genera, distinct and specific Asaia and Chryseobacterium in continental and overseas territories, and a putative rare Wolbachia variant. Overall, our study reveals the presence of previously overlooked microdiversity and multiple closely related symbiotic strains within mosquito individuals with a remarkable habitat-specificity