The arbovirus vector Culex torrentium is more prevalent than Culex pipiens in northern and central Europe

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.

Catalase enzymatic activity in adult mosquitoes (Diptera: Culicidae): taxonomic distribution of the continuous trait suggests its relevance for phylogeny research

Molecular research based on gene sequence analysis and performed for decades, in general, supported morphology-based groupings of the species within the family Culicidae, but phylogenetic relationships between some genera and tribes remained uncertain for a long time. Interspecific differences in catalase, an antioxidant enzyme important for maintaining prolonged lifespan and reproduction, have not been studied extensively by estimating enzymatic activity levels. Here, catalase enzymatic activity was assayed in extracts of male mosquitoes belonging to 10 species of the subfamily Culicinae, including species from tribes of disputable phylogeny. Three species of Chaoboridae (nearest outgroup taxon) and mosquitoes from the subfamily Anophelinae (one species complex) were also added to the study. At least in Culicinae, immature adult males (less than one day after emergence) have distinctly elevated specific activity of catalase; therefore, only mature males of all species were used for the comparative study. As a result, significant differences in catalase activity were revealed between tribes, genera and particular species. Among culicids, the genera Coquillettidia and Culiseta were found to include the species with the highest and relatively high catalase activity, which is consistent with the affinity of the tribes Mansoniini and Culisetini to each other. Within Ochlerotatus, extremely low catalase activity in Oc. hexodontus suggests the more distant position of this species from Oc. cantans (Meigen) and Oc. communis (de Geer) than the positions of the latter two species from each other. Additional study of catalase activity in overwintering females of the genus Culex revealed significantly higher enzyme activity in Cx. torrentium in comparison with Cx. pipiens, which supports their quite distant positions from each other within the genus. Considering the distribution of catalase activity within the tree obtained, the preliminary outcome is that Culiseta retains the elevated level of catalase activity that was lost during the early separation of Anopheles and subsequent separation of Culex and Aedes/Ochlerotatus after Anopheles from their common branch with Culiseta/Coquillettidia. Overall, the use of taxonomic distribution of catalase activity levels appears to be effective for resolving disputed events of mosquito phylogeny.

Effects of Hibernation Site, Temperature, and Humidity on the Abundance and Survival of Overwintering Culex pipiens pipiens and Anopheles messeae (Diptera: Culicidae)

Knowledge of the hibernation site preferences and the factors which influence winter survival in these hibernation sites may enhance understanding of mosquito population dynamics after winter and how arboviruses persist in temperate regions. Our study quantified the number of adult overwintering mosquitoes in cellars and aboveground constructions and analyzed survival rates in relation to the environmental conditions in these sites. During the winters 2016/2017 and 2018/2019, 149 different constructions in Northwest Germany were sampled for mosquitoes. Mosquitoes were detected in 44% of the cellars and in 33% of the aboveground constructions. Culex p. pipiens Linnaeus was the most abundant species in cellars, whereas high numbers of Anopheles messeae Falleroni were collected from a single barn. Subsequently, an enclosure study was conducted during 2019/2020. Overwintering field-collected Cx. p. pipiens and An. messeae were divided into groups with or without fructose availability, and placed in cages with different man-made hibernations sites, where temperature and relative humidity were recorded hourly. For both species, increasing mean temperatures (5-16°C) but not mean relative humidity (58-94%) were correlated with winter mortality rates of the mosquitoes. The lipid measurements were greater and mortality rates were lower when both species were provided fructose. Larger specimens (determined by wing length) stored more lipids, and in Cx. p pipiens, but not in An. messeae, survival probability of large specimens was significantly greater than for small females. Mosquitoes showed a distinct pattern in the selection of overwintering sites, while temperature was an important driver for survival.

Diversity and distribution of larval habitats of mosquitoes (Diptera: Culicidae) in northern Spain: from urban to natural areas

Studies of the biodiversity of mosquito larval habitats are important for vector-borne disease control programs and help to improve vector distribution maps. This study was designed to investigate the geographical distribution of mosquito species and their larval habitats in urban, rural, and natural areas in northern Spain. Pre-imaginal stages were collected over two sampling periods (spring and summer) in 2019. In the laboratory, immature specimens were reared to the adult stage for species identification by morphological taxonomy and/or molecular methods. In total, 2,182 specimens belonging to 13 different native mosquito species of five genera were collected from 135 sampling points of which 59.2% harbored larvae. Culex pipiens s.l. was the most abundant species (45.1%), followed by Culex torrentium (12.3%), Anopheles maculipennis s.l. (10.2%), Culex hortensis (9.5%), and nine other species with lower frequencies that accounted for less than 25%. By molecular identification, An. maculipennis s.s. was identified as the only species within the An. maculipennis species complex and Cx. pipiens pipiens as the predominant subspecies of the Cx. pipiens species complex. Margins in large sunlit water bodies were the most suitable sites for An. maculipennis s.l., whereas Cx. pipiens s.l. was present in both natural and artificial habitats. The larval site index, species richness, and relative abundance of the mosquitoes were determined based on the characteristics of the sites where they were collected. The public health importance and ecology of some identified mosquitoes is also discussed.

Wolbachia prevalence in the vector species Culex pipiens and Culex torrentium in a Sindbis virus-endemic region of Sweden

Background

Wolbachia pipientis are endosymbiotic bacteria present in a large proportion of terrestrial arthropods. The species is known to sometimes affect the ability of its host to transmit vector-borne pathogens. Central Sweden is endemic for Sindbis virus (SINV), where it is mainly transmitted by the vector species Culex pipiens and Culex torrentium, with the latter established as the main vector. In this study we investigated the Wolbachia prevalence in these two vector species in a region highly endemic for SINV.

Methods

Culex mosquitoes were collected using CDC light traps baited with carbon dioxide over 9 years at 50 collection sites across the River Dalälven floodplains in central Sweden. Mosquito genus was determined morphologically, while a molecular method was used for reliable species determination. The presence of Wolbachia was determined through PCR using general primers targeting the wsp gene and sequencing of selected samples.

Results

In total, 676 Cx. pipiens and 293 Cx. torrentium were tested for Wolbachia. The prevalence of Wolbachia in Cx. pipiens was 97% (95% CI 94.8–97.6%), while only 0.7% (95% CI 0.19–2.45%) in Cx. torrentium. The two Cx. torrentium mosquitoes that were infected with Wolbachia carried different types of the bacteria.

Conclusions

The main vector of SINV in the investigated endemic region, Cx. torrentium, was seldom infected with Wolbachia, while it was highly prevalent in the secondary vector, Cx. pipiens. The presence of Wolbachia could potentially have an impact on the vector competence of these two species. Furthermore, the detection of Wolbachia in Cx. torrentium could indicate horizontal transmission of the endosymbiont between arthropods of different species.

Graphical abstract

Predicting Spatial Patterns of Sindbis Virus (SINV) Infection Risk in Finland Using Vector, Host and Environmental Data

Pogosta disease is a mosquito-borne infection, caused by Sindbis virus (SINV), which causes epidemics of febrile rash and arthritis in Northern Europe and South Africa. Resident grouse and migratory birds play a significant role as amplifying hosts and various mosquito species, including Aedes cinereus, Culex pipiens, Cx. torrentium and Culiseta morsitans are documented vectors. As specific treatments are not available for SINV infections, and joint symptoms may persist, the public health burden is considerable in endemic areas. To predict the environmental suitability for SINV infections in Finland, we applied a suite of geospatial and statistical modeling techniques to disease occurrence data. Using an ensemble approach, we first produced environmental suitability maps for potential SINV vectors in Finland. These suitability maps were then combined with grouse densities and environmental data to identify the influential determinants for SINV infections and to predict the risk of Pogosta disease in Finnish municipalities. Our predictions suggest that both the environmental suitability for vectors and the high risk of Pogosta disease are focused in geographically restricted areas. This provides evidence that the presence of both SINV vector species and grouse densities can predict the occurrence of the disease. The results support material for public-health officials when determining area-specific recommendations and deliver information to health care personnel to raise awareness of the disease among physicians.

Diversity of mosquito fauna (Diptera, Culicidae) in higher-altitude regions of Croatia

Global climate change and the accompanying rise in temperature could affect the biology and ecology of a number of vectors, including mosquitoes. High altitude areas that were previously unsuitable for the spread of mosquito vector populations could become suitable. The aim of this research was to study the distribution of mosquito species in higher altitude regions of Croatia. Samples were collected in three areas: Slavonian Mountains, Gorski Kotar, and Middle Velebit. Specimens were morphologically determined and confirmed by DNA barcoding and other genetic markers and showed the presence of 16 species belonging to six genera. The most abundant species were the Culex pipiens complex with 50% of the collected specimens. Both pipiens (Linnaeus, 1758) and molestus (Forskal, 1775) biotypes and their hybrids were identified within the complex, followed by Culex torrentium (Martini, 1925) (20.2%), Culiseta longiareolata (Macquart, 1838) (8.5%), and the invasive species Aedes japonicus (Theobald, 1901) (7.8% of the total number of collected specimens). The remaining 12 species made up 14.7% of the collected specimens. Intraspecific COI p-distances were within the standard barcoding threshold for OTUs, while interspecific genetic distances were much higher, confirming the existence of barcoding gaps. Mosquito fauna of Croatian mountains showed a moderate variety and made 30.8% of the total number of recorded mosquito species in Croatia thus far.

The mosquitoes of Finland: updated distributions and bionomics

Mosquitoes (Diptera: Culicidae) were collected in Finland between 2012 and 2018 to determine the species present and their distributions. In total, 52 466 specimens from 1031 collections formed the basis for the preparation of distribution maps for each of the 40 species that were collected. Anopheles maculipennis s.s., An. claviger, Aedes geminus and Ochlerotatus sticticus are confirmed on mainland Finland after previous records were uncertain or absent. Coquillettidia richiardii, Culiseta morsitans, Cs. ochroptera, Culex territans, Cx. torrentium, Oc. leucomelas, Oc. nigrinus, Oc. pullatus and Oc. punctodes occur more widely than previously reported. Three species, Ae. rossicus, Cs. subochrea and Oc. cyprius were not collected, although Ae. rossicus was subsequently found in Lapland by another researcher. No invasive species were collected. Ochlerotatus communis, an aggressive biter, was the most commonly encountered species. Larval collection data suggest that several species may have up to three generations per year in Finland, with Cx. torrentium and Cx. pipiens having at least two, and Oc. communis and Oc. punctor regularly found as larvae across the summer. These data, especially when coupled with historical records, are vital for monitoring species which have significant vector potential, particularly when faced with a warming climate.

German Culex pipiens biotype molestus and Culex torrentium are vector-competent for Usutu virus

BACKGROUND

Usutu virus (USUV) is a rapidly spreading zoonotic arbovirus (arthropod-borne virus) and a considerable threat to the global avifauna and in isolated cases to human health. It is maintained in an enzootic cycle involving ornithophilic mosquitoes as vectors and birds as reservoir hosts. Despite massive die-offs in wild bird populations and the detection of severe neurological symptoms in infected humans, little is known about which mosquito species are involved in the propagation of USUV.

METHODS

In the present study, the vector competence of a German (i.e. "Central European") and a Serbian (i.e. "Southern European") Culex pipiens biotype molestus laboratory colony was experimentally evaluated. For comparative purposes, Culex torrentium, a frequent species in Northern Europe, and Aedes aegypti, a primarily tropical species, were also tested. Adult female mosquitoes were exposed to bovine blood spiked with USUV Africa 2 and subsequently incubated at 25 °C. After 2 to 3 weeks saliva was collected from each individual mosquito to assess the ability of a mosquito species to transmit USUV.

RESULTS

Culex pipiens biotype molestus mosquitoes originating from Germany and the Republic of Serbia and Cx. torrentium mosquitoes from Germany proved competent for USUV, as indicated by harboring viable virus in their saliva 21 days post infection. By contrast, Ae. aegypti mosquitoes were relatively refractory to an USUV infection, exhibiting low infection rates and lacking virus in their saliva.

CONCLUSIONS

Consistent with the high prevalences and abundances of Cx. pipiens biotype molestus and Cx. torrentium in Central and Northern Europe, these two species have most likely played a historic role in the spread, maintenance, and introduction of USUV into Germany. Identification of the key USUV vectors enables the establishment and implementation of rigorous entomological surveillance programs and the development of effective, evidence-based vector control interventions.

An investigation of hibernating members from the Culex pipiens complex (Diptera, Culicidae) in subterranean habitats of central Germany

The Culex pipiens complex encompasses five species and subspecies of the genus Culex. Over time, a multitude of morphologically indistinguishable species has been assigned to this complex with several species being classified as important vectors for different diseases. Some species of this complex hibernate in subterranean habitats, and it has been proven that viruses can survive this phase of hibernation. However, studies focusing on the environmental requirements, ecology and spatial and temporal distribution patterns of mosquitos in underground habitats are sparse. Here, we investigate the main environmental factors and dependencies of Culex, considering the number of individuals and survival probabilities in underground habitats during the winter months. Methods. Since the State of Hesse, Germany harbors about 3500 to 4000 subterranean shelters ample availability of subterranean habitats there provides a good opportunity to conduct detailed investigations of the Culex pipiens complex. In this study, we identified a sample of 727 specimens of overwintering females within the Culex pipiens complex from 52 different underground sites collected over a period of 23 years using qPCR. A complete data set of samplings of hibernating mosquitos from 698 subterranean habitats in Central Germany over the same period was available to study the spatial and temporal patterns and the effect of temperature and precipitation conditions on these hibernating populations using a generalized linear model (GLM). Results. Our qPCR-results show, similar to aboveground studies of mosquitos, that Culex pipiens pipiens and Culex torrentium occur sympatrically. On the other hand, Culex pipiens molestus occurred very rarely. The GLM revealed no shifts in species composition over time, but different preferences for subterranean hibernacula, chemical effects on overwintering populations as well as effects of annual and seasonal mean temperature and precipitation during the active phase from March to November. Cx. p. pipiens and Cx. torrentium are the most common species within Hessian caves and other underground habitats during winter. They co-occur with different frequency without any patterns in species composition. Weather conditions influence the number of overwintering mosquitos during the activity phase. Depending on cave parameters, the number of mosquitos decreases during the winter months.

Updated checklist of the mosquitoes (Diptera: Culicidae) of Croatia

Improvement of morphological and molecular identification methods allows the detection of new species of mosquitoes. The mosquito fauna of Croatia currently includes 52 species, belonging to eight genera, including Anopheles (12 species), Aedes (24 species), Coquillettidia (one species), Culex (seven species), Culiseta (six species), Orthopodomyia (one species), and Uranotaenia (one species). This is an updated checklist, which includes five new species found in Croatian mosquito fauna. Two of these are invasive mosquito species, Aedes albopictus (Skuse, 1895) and Aedes japonicus (Theobald 1901), which are spreading across Europe and Croatia. The other three species, Culex laticinctus (Edwards 1913), Culex torrentium (Martini 1925), and Anopheles daciae (Linton, Nicolescu & Harbach 2004) are autochthonous species which haven't been recorded so far. Since there are several more invasive species spreading across Europe, we assume that this is not the final list.

A Comparison of Adult Mosquito Trapping Methods to Assess Potential West Nile Virus Mosquito Vectors in Greece during the Onset of the 2018 Transmission Season

West Nile virus (WNV) threatens the health of humans and equines worldwide. Culex (Cx.) pipiens complex mosquitoes are major vectors but numerous other species have been implicated. Due to variations in blood-feeding behaviour, Cx. pipiens biotypes and hybrids influence transmission, from enzootic cycles (between mosquitoes and birds), to spill-over transmission to humans and equines. In this study, mosquitoes were collected in May–June 2018 during the early period of the transmission season from two regional units of Greece, where WNV cases had been reported in the previous four years (Palaio Faliro and Argolida). A total of 1062 mosquitoes were collected with Biogents Sentinel 2 traps collecting both a greater number of all mosquito species and the Cx. pipiens complex than CDC miniature light traps or Heavy Duty EVS traps. Molecular identification confirmed additional species including Aedes albopictus. The proportion of Cx. pipiens biotypes in Palaio Faliro was 54.5% pipiens, 20.0% molestus and 25.5% hybrids. In Argolida, the collection comprised 68.1% pipiens biotype, 8.3% molestus biotype and 23.6% hybrids. Screening resulted in WNV detection in three females of the pipiens biotype and in one hybrid. As hybrids play a role in spill-over transmission, these findings highlight the importance of entomological surveillance programs incorporating molecular xenomonitoring as an early warning before human cases at the onset of the transmission season.

The knowns and unknowns of West Nile virus in Europe: what did we learn from the 2018 outbreak?

Introduction: West Nile virus (WNV) is a mosquito-borne human and animal pathogen with nearly worldwide distribution. In Europe, the virus is endemic with seasonal regional outbreaks that have increased in frequency over the last 10 years. A massive outbreak occurred across southern and central Europe in 2018 with the number of confirmed human cases increasing up to 7.2-fold from the previous year, and expanding to include previously virus-free regions.Areas covered: This review focuses on potential causes that may explain the 2018 European WNV outbreak. We discuss the role genetic, ecological, and environmental aspects may have played in the increased activity during the 2018 transmission season, summarizing the latest epidemiological and virological publications.Expert opinion: Optimal environmental conditions, specifically increased temperature, were most likely responsible for the observed outbreak. Other factors cannot be ruled out due to limited available information, including factors that may influence host/vector abundance and contact. Europe will likely experience even larger-scale outbreaks in the coming years. Increased surveillance efforts should be implemented with a focus on early-warning detection methods, and large-scale host and vector surveys should continue to fill gaps in knowledge.

Surveillance of mosquito vectors in Southern Sweden for Flaviviruses and Sindbis virus

There are three human pathogenic bird-viruses transmitted by Culex mosquitoes in Europe: the alphavirus Sindbis and the flaviviruses West Nile virus and Usutu virus. Cases of Sindbis fever occur in the north while the flaviviruses are reported from southern Europe. In this study, 7933 Culex pipiens/torrentium mosquitoes from southern Sweden were screened by RTqPCR for these viruses. None of the mosquitoes were positive for viral RNA. The importance of mosquito species composition is discussed as a potential explanation to the lack of detection of mosquito-borne viruses in southern Sweden. However, continued surveillance of mosquitoes for Flaviviruses would be valuable as an early warning for public health awareness.

Meta-Transcriptomic Comparison of the RNA Viromes of the Mosquito Vectors Culex pipiens and Culex torrentium in Northern Europe

Mosquitoes harbor an extensive diversity of ‘insect-specific’ RNA viruses in addition to those important to human and animal health. However, because most studies of the mosquito virome have been conducted at lower latitudes, little is known about the diversity and evolutionary history of RNA viruses sampled from mosquitoes in northerly regions. Here, we compared the RNA virome of two common northern mosquito species, Culex pipiens and Culex torrentium, collected in south-central Sweden. Following bulk RNA-sequencing (meta-transcriptomics) of 12 libraries, comprising 120 specimens of Cx. pipiens and 150 specimens of Cx. torrentium, we identified 40 viruses (representing 14 virus families) of which 28 were novel based on phylogenetic analysis of the RNA-dependent RNA polymerase (RdRp) protein. Hence, we documented similar levels of virome diversity as in mosquitoes sampled from the more biodiverse lower latitudes. Many viruses were also related to those sampled on other continents, indicative of a widespread global movement and/or long host–virus co-evolution. Although the two mosquito species investigated have overlapping geographical distributions and share many viruses, several viruses were only found at a specific location at this scale of sampling, such that local habitat and geography may play an important role in shaping viral diversity in Culex mosquitoes.

Introduction and Dispersal of Sindbis Virus from Central Africa to Europe

This study shows that only a single introduction of SINV into a new geographical area is required for spread and establishment, provided that the requisite vector(s) and reservoir(s) of epizootological and epidemiological importance are present. Furthermore, we present the first report of recombination between two strains of SINV in nature. Our study increases the knowledge on new introductions and dispersal of arboviruses in general and of SINV in particular.

Bird-hosted viruses have the potential to be transported over large areas of the world and to be transmitted in distant geographical regions. Sindbis virus (SINV) is a mosquito-borne alphavirus that is locally amplified in a bird-mosquito enzootic cycle and distributed all over the Old World and Australia/Oceania. Sindbis virus genotype I (SINV-I) is the cause of disease outbreaks in humans in South Africa as well as in northern Europe. To trace the evolutionary history and potential strain-disease association of SINV-I, we sequenced 36 complete genomes isolated from field material in Europe, as well as in Africa and the Middle East, collected over 58 years. These were analyzed together with 30 additional published whole SINV-I genomes using Bayesian analysis. Our results suggested that SINV-I was introduced only once to northern Europe from central Africa, in the 1920s. After its first introduction to Sweden, it spread east and southward on two separate occasions in the 1960s and 1970s. Another introduction from central Africa to southern/central Europe seems to have occurred, and where these two introductions meet, one recombination event was detected in central Europe. In addition, another recombinant strain was found in central Africa, where the most divergent SINV-I strains also originated.

IMPORTANCE This study shows that only a single introduction of SINV into a new geographical area is required for spread and establishment, provided that the requisite vector(s) and reservoir(s) of epizootological and epidemiological importance are present. Furthermore, we present the first report of recombination between two strains of SINV in nature. Our study increases the knowledge on new introductions and dispersal of arboviruses in general and of SINV in particular.

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.

Experimental Infection and Transmission Competence of Sindbis Virus in Culex torrentium and Culex pipiens Mosquitoes from Northern Sweden

INTRODUCTION

Sindbis virus (SINV) is a mosquito-borne Alphavirus known to infect birds and cause intermittent outbreaks among humans in Fenno-Scandia. In Sweden, the endemic area has mainly been in central Sweden. Recently, SINV infections have emerged to northern Sweden, but the vectorial efficiency for SINV of mosquito species in this northern region has not yet been ascertained.

OBJECTIVE

Mosquito larvae were sampled from the Umeå region in northern Sweden and propagated in a laboratory to adult stage to investigate the infection, dissemination, and transmission efficiency of SINV in mosquitoes.

MATERIALS AND METHODS

The mosquito species were identified by DNA barcoding of the cytochrome oxidase I gene. Culex torrentium was the most abundant (82.2%) followed by Culex pipiens (14.4%), Aedes annulipes (1.1%), Anopheles claviger (1.1%), Culiseta bergrothi (1.1%), or other unidentified species (1.1%). Mosquitoes were fed with SINV-infected blood and monitored for 29 days to determine the viral extrinsic incubation period. Infection and dissemination were determined by RT-qPCR screening of dissected body parts of individual mosquitoes. Viral transmission was determined from saliva collected from individual mosquitoes at 7, 14, and 29 days. SINV was detected by cell culture using BHK-21 cells, RT-qPCR, and sequencing.

RESULTS

Cx. torrentium was the only mosquito species in our study that was able to transmit SINV. The overall transmission efficiency of SINV in Cx. torrentium was 6.8%. The rates of SINV infection, dissemination, and transmission in Cx. torrentium were 11%, 75%, and 83%, respectively.

CONCLUSIONS

Cx. torrentium may be the key vector involved in SINV transmission in northern Sweden.

Diversity and seasonal abundances of mosquitoes at potential arboviral transmission sites in two different climate zones in Switzerland

Pathogens of medical or veterinary significance that are transmitted by mosquitoes (Diptera: Culicidae) are (re-)emerging in Europe [e.g. West Nile virus (WNV), Dirofilaria nematodes]. Little is known about the spatiotemporal abundances of mosquito species in Switzerland. Therefore, mosquito population dynamics were investigated, focusing on areas of risk for sylvatic or synanthropic transmission, such as natural sites and suburban sites on either side of the Alpine crest. Repeated collections were made using Centers for Disease Control (CDC) traps, juvenile sampling and ovitrapping. A total of 122 831 mosquito specimens of 21 taxa were identified. Levels of mosquito species richness were similar at suburban sites and in natural zones in Switzerland. Mosquito abundances and seasonality were analysed with generalized linear mixed models based on 382 CDC trap samples (29 454 females) and revealed Aedes annulipes/cantans, Aedes geniculatus, Aedes japonicus, Aedes sticticus, Aedes vexans, Coquillettidia richiardii and Culex pipiens/torrentium as the dominant species overall. Abundances of these species were season-dependent in most cases. There was an effect of site with regard to abundance (higher in natural zones), but not with respect to seasonality. Together with data on vector competence and the host preferences of different species, the present data contribute to assessments of risk for pathogen transmission. For example, both natural and suburban environments seem feasible as sites for amplification cycles of WNV and transmission to mammals.

Culex torrentium mosquitoes from Germany are negative for Wolbachia

Wolbachia (Rickettsiales: Anaplasmataceae) infects a wide range of arthropods, including several mosquito species. The bacterium is known to induce a plethora of phenotypes in its host, examples being the reproductive phenotype cytoplasmic incompatibility or resistance against infection with arboviruses. The latter is especially relevant when assessing the vector competence of mosquito species for emerging arboviruses. Thus, knowledge of Wolbachia infection status is important for the assessment of vector competence. To facilitate Wolbachia screening in mosquito populations, a quantitative polymerase chain reaction (qPCR) assay was developed to enable high-throughput analysis of mosquito samples. Using this assay, the Wolbachia infection status of the two most common Culex mosquito species in Germany, Culex pipiens biotype pipiens Linnaeus (Diptera: Culicidae) and Culex torrentium Martini (Diptera: Culicidae), was assessed. About 93% of all tested C. pipiens biotype pipiens individuals were positive for Wolbachia, whereas none of the C. torrentium samples was found to be infected. Furthermore, other applications of the qPCR assay were explored by assessing a potential link between the levels of Wolbachia and West Nile virus (WNV) infections in German C. pipiens biotype pipiens mosquitoes. No relationship was found between the two variables, indicating that a Wolbachia-induced antiviral phenotype in this mosquito population is not exclusively attributable to the general level of bacterial infection.

The Role of Culex pipiens L. (Diptera: Culicidae) in Virus Transmission in Europe

Over the past three decades, a range of mosquito-borne viruses that threaten public and veterinary health have emerged or re-emerged in Europe. Mosquito surveillance activities have highlighted the Culex pipiens species complex as being critical for the maintenance of a number of these viruses. This species complex contains morphologically similar forms that exhibit variation in phenotypes that can influence the probability of virus transmission. Critical amongst these is the choice of host on which to feed, with different forms showing different feeding preferences. This influences the ability of the mosquito to vector viruses and facilitate transmission of viruses to humans and domestic animals. Biases towards blood-feeding on avian or mammalian hosts have been demonstrated for different Cx. pipiens ecoforms and emerging evidence of hybrid populations across Europe adds another level of complexity to virus transmission. A range of molecular methods based on DNA have been developed to enable discrimination between morphologically indistinguishable forms, although this remains an active area of research. This review provides a comprehensive overview of developments in the understanding of the ecology, behaviour and genetics of Cx. pipiens in Europe, and how this influences arbovirus transmission.

Mosquitoes as Arbovirus Vectors: From Species Identification to Vector Competence

Mosquitoes and other arthropods transmit a large number of medically important pathogens, in particular viruses. These arthropod-borne viruses (arboviruses) include a wide variety of RNA viruses belonging to the Flaviviridae family (West Nile virus (WNV), Usutu virus (USUV), Dengue virus (DENV), Japanese encephalitis virus (JEV), Zika virus (ZIKV)), the Togaviridae family (Chikungunya virus (CHIKV)), and Bunyavirales order (Rift Valley fever virus (RVFV)) (please refer also to Table 9.1). Arboviral transmission to humans and livestock constitutes a major threat to public health and economy as illustrated by the emergence of ZIKV in the Americas, RVFV outbreaks in Africa, and the worldwide outbreaks of DENV. To answer the question if those viral pathogens also pose a risk to Europe, we need to first answer the key questions (summarized in Fig. 9.1):

Who could contribute to such an outbreak? Information about mosquito species resident or imported, potential hosts and viruses able to infect vectors and hosts in Germany is needed.

Where would competent mosquito species meet favorable conditions for transmission? Information on the minimum requirements for efficient replication of the virus in a given vector species and subsequent transmission is needed.

How do viruses and vectors interact to facilitate transmission? Information on the vector immunity, vector physiology, vector genetics, and vector microbiomes is needed.

City-Dwellers and Country Folks: Lack of Population Differentiation Along an Urban-Rural Gradient in the Mosquito Culex pipiens (Diptera: Culicidae)

Mosquitoes (Diptera, Culicidae) occur in natural, urban, and peri-urban areas throughout the globe. Although the characteristics of urban and peri-urban habitats differ from those of natural habitats in many ways (e.g., fragmentation, pollution, noise, and light), few studies have examined the population connectivity of mosquitoes in urban areas. To obtain an overview of the species composition, we sampled mosquitoes from 23 sites in and around the city of Berlin, Germany. Of 23 species, five occurred in urban, 10 in peri-urban, and 20 in rural areas. Culex pipiens Linnaeus (Diptera: Culicidae) was the most common species collected (75% of all individuals) and occurred in all habitats. Hence this species was selected to be analysed at 10 microsatellite markers. There was no significant differentiation (FST = 0.016, P = 0.9) or isolation by distance (P = 0.06) among Cx. pipiens populations along an urban-rural gradient. The only significant differences detected were between Cx. pipiens and a laboratory population of Cx. pipiens f. molestus (pairwise FST = 0.114-0.148, P ≤ 0.001 in all comparisons), suggesting that the markers chosen were suitable for the identification of population differentiation. Our results indicate that Cx. pipiens gene flow is widespread within and among urban, peri-urban, and rural areas and that urban habitat does not necessarily impede or enhance gene flow among these populations.

Bacterial diversity of cosmopolitan Culex pipiens and invasive Aedes japonicus from Germany

Symbiotic bacteria have gained significant attention in recent years. For example, microbiota of some mosquito species seems to influence the development and transmission of pathogens. Furthermore, several attempts using bacteria as a paratransgenetic tool have been made in order to assist the control of mosquito-borne diseases. In this study, we examined the bacterial diversity of wild-caught adult Culex (Cx.) pipiens and laboratory-reared adult Aedes japonicus (Ae. japonicus) in Germany using a culture-independent method. Genomic DNA was extracted from each specimen and submitted to PCR amplification of eubacterial 16S rDNA. After the cloning reaction, 28 bacterial transformants per sample containing the 16S rDNA inserts were selected per each sample for sequencing. The analysed specimens of Cx. pipiens as well as of Ae. japonicus showed a diverse bacterial community including some common bacterial genera. Blast analysis allowed to identify 21 bacterial genera belonging to 2 phyla among the 23 specimens of Cx. pipiens. The 14 analysed Ae. japonicus revealed 11 bacterial genera belonging to 3 phyla. In both mosquito species, identified isolates were mainly Proteobacteria. Only 4 of the bacterial genera were found in both mosquito species, with the most prevalent genera Sphingomonas and Rahnella in Cx. pipiens and in Ae. japonicus respectively. Most of the bacterial genera found in our study have been identified in other mosquito species before. Due to the currently scarce data situation, ongoing examinations on the very abundant bacterial genera or species are strongly required to determine their relevance for the biology and adaptiveness of mosquitoes including pathogen-host relationship.

Wide recognition of Culex pipiens and lack of detection of Culex torrentium through biomolecular differentiation of mosquitoes in the Emilia-Romagna region, Northern Italy

The Culex pipiens complex includes species with reported differences in vector competence for arthropod-borne viruses, many of which are of significant importance to human health such as the West Nile virus and the Sindbis virus. This group of mosquitoes is difficult to distinguish morphologically; particularly as adult females. In Europe, the two species of the complex, Culex pipiens Linnaeus 1758 and Culex torrentium Martini 1925, are often found sympatrically. With the aim to characterize the presence and spread of both species in the Emilia-Romagna region, Northern Italy, mosquitoes of the complex - collected during the West Nile virus surveillance plans - were tested by multiplex real-time PCR for the detection of the two species Cx. pipiens and Cx. torrentium. A total of 24 165 mosquitoes, collected between 2012 and 2014 from 105 sites, and sorted in 204 pools, were tested. All tested pools were found to be composed of Cx. pipiens, whereas Cx. torrentium was not detected. These results indicate a likely absence of Cx. torrentium mosquitoes within the surveyed territory, whereas Cx. pipiens is widely distributed in the area mentioned. This is in line with previous reports, which describe a pre-alpine distribution of Cx. torrentium in Italy.

Latitudinal Diversity of Culex pipiens Biotypes and Hybrids in Farm, Peri-Urban, and Wetland Habitats in Europe

Despite the presence of Culex (Cx.) pipiens mosquitoes and circulation of West Nile virus (WNV), WNV outbreaks have so far not occurred in northern Europe. The species Cx. pipiens consists of two morphologically identical biotypes, pipiens and molestus, which can form hybrids. Until now, population dynamic studies of Cx. pipiens have not differentiated between biotypes and hybrids at the European scale, nor have they used comparative surveillance approaches. We therefore aimed to elucidate the relative abundance of Cx. pipiens biotypes and hybrids in three habitat types at different latitudes across Europe, using two different surveillance traps. BG-Sentinel and Mosquito-Magnet Liberty Plus traps were placed in three habitat types (farms, peri-urban, wetlands), in three European countries (Sweden, The Netherlands, Italy). Collected Cx. pipiens mosquitoes were identified to biotype with real-time PCR. Both trap types collected equal ratios of the biotypes and their hybrids. From northern to southern latitudes there was a significant decrease of pipiens and an increase of molestus. Habitat types influenced the relative ratios of biotypes and hybrids, but results were not consistent across latitudes. Our results emphasize the need to differentiate Cx. pipiens to the biotype level, especially for proper future WNV risk assessments for Europe.

Culex pipiens and Culex torrentium populations from Central Europe are susceptible to West Nile virus infection

West Nile virus (WNV), a Flavivirus with an avian primary host, is already widespread in Europe and might also pose an infection risk to Germany, should competent mosquito vectors be present. Therefore, we analysed the ability of WNV to infect German Culex mosquitoes with special emphasis on field collected specimens of Culex torrentium and Culex pipiens biotype pipiens. We collected egg rafts of Culex mosquitoes over two subsequent seasons at two geographically distinct sampling areas in Germany and differentiated the samples by molecular methods. Adult females, reared from the various egg rafts, were challenged with WNV by feeding of artificial blood meals. WNV infection was confirmed by real-time RT-PCR and virus titration. The results showed that field collected C. pipiens biotype pipiens and C. torrentium mosquitoes native to Germany are susceptible to WNV infection at 25 °C as well as 18 °C incubation temperature. C. torrentium mosquitoes, which have not been established as WNV vector so far, were the most permissive species tested with maximum infection rates of 96% at 25 °C. Furthermore, a disseminating infection was found in up to 94% of tested C. pipiens biotype pipiens and 100% of C. torrentium. Considering geographical variation of susceptibility, C. pipiens biotype pipiens mosquitoes from Southern Germany were more susceptible to WNV infection than corresponding populations from Northern Germany. All in all, we observed high infection and dissemination rates even at a low average ambient temperature of 18 °C. The high susceptibility of German Culex populations for WNV indicates that an enzootic transmission cycle in Germany could be possible.

Ecological characterization and molecular differentiation of Culex pipiens complex taxa and Culex torrentium in eastern Austria

BACKGROUND

Culex pipiens complex taxa differ in behaviour, ecophysiology and epidemiologic importance. Despite their epidemiologic significance, information on genetic diversity, occurrence and seasonal and spatial distribution patterns of the Cx. pipiens complex is still insufficient. Assessment of seasonal and spatial distribution patterns of Culex pipiens forms and their congener Cx. torrentium is crucial for the understanding of their vector-pathogen dynamics.

METHODS

Female mosquitoes were trapped from April-October 2014 twice a month for a 24-h time period with BG-sentinel traps at 24 sampling sites in eastern Austria, using carbon dioxide as attractant. Ecological forms of Cx. pipiens s.l. and their hybrids were differentiated using the CQ11 locus, and Cx. pipiens forms and their congener Cx. torrentium using the ACE-2 gene. Differential exploitation of ecological niches by Cx. pipiens forms and Cx. torrentium was analysed using likelihood ratio tests. Possible effects of environmental parameters on these taxa were tested using PERMANOVA based on distance matrices and, if significant, were modelled in nMDS ordination space to estimate non-linear relationships.

RESULTS

For this study, 1476 Culex spp. were sampled. Culex pipiens f. pipiens representing 87.33 % of the total catch was most abundant, followed by hybrids of both forms (5.62 %), Cx. torrentium (3.79 %) and Cx. pipiens f. molestus (3.25 %). Differences in proportional abundances were found between land cover classes. Ecological parameters affecting seasonal and spatial distribution of these taxa in eastern Austria are precipitation duration, air temperature, sunlight and the interaction term of precipitation amount and the Danube water level, which can be interpreted as a proxy for breeding habitat availability.

CONCLUSIONS

The Cx. pipiens complex of eastern Austria comprises both ecologically different forms, the mainly ornithophilic form pipiens and the mainly mammalophilic and anthropophilic form molestus. Heterogeneous agricultural areas as areas of coexistence may serve as hybridization zones, resulting in potential bridge vectors between birds and humans. Occurrence, seasonal and spatial distribution patterns of the Cx. pipiens complex and Cx. torrentium and the presence of hybrids between both forms were quantified for the first time in Austria. These findings will improve the knowledge of their vector-pathogen dynamics in this country.

Assessment of productivity of Culex spp. larvae (Diptera: Culicidae) in urban storm water catch basin system in Wrocław (SW Poland)

In urban environments, catch basins serve as major developmental and resting sites for anthropophilic and zoophilic mosquitoes. However, the use of this habitat is inconsistent, with abundance of larvae varying significantly across catch basins at a fine spatial scale. During seasonal summer investigations on mosquito species composition, their spatial and temporal distribution and the environmental characteristic of the breeding sites in the underground storm drain systems of the Wrocław urban area (SW Poland) were assessed from May to September in 2012-2013. The study was conducted in order to develop a rational strategy to control mosquito populations and prevent the potential human exposure to mosquito-transmitted pathogens. Mosquito larvae and pupae were collected and identified weekly from 100 regularly inspected street catch basins located in the town center. All existing and potential breeding habitats in the study area were recorded using a GPS receiver (Magellan MobileMapper CX) and transferred to the computer database. Collected data on the geographical location of inspected breeding places, water quality parameters in inspected catch basins, daily temperature, and precipitation were imposed on orthophotomap in ArcGIS (ESRI, USA). Water quality parameters including pH, electrical conductivity, and water temperature were measured by standard methods. Chemical water analysis of cations (Na(+), NH4 (+), K(+), Mg(2+), Ca(2+)) and anions (Cl(-), NO2 (-), NO3 (-), SO4 (2-)) were carried out using Waters Alliance high-performance liquid chromatograph (HPLC) 2695 with 432 Conductivity Detector and 2998 Photodiode Array Detector, an IC-Pak Anion HR column (glauconate/borate eluent) and IC-Pak Cation M/D column (EDTA/HNO3 eluent). Over two seasonal studies and 3739 samplings in total, 3669 mosquito larvae and 274 pupae/1 dip (from 0 to 110 individuals/dip) were collected by dipper. Culex pipiens s.l. (L.) and Cx. torrentium (Martini) prevailed at all catch basins of the study area as the predominant species. In all examined catch basins, autogenous individuals dominated by far. Breeding activity was first detected in early May. Peak abundance of Culex spp. population in many catch basins was observed in June 2012 and August 2013 when average daily temperatures were increasing and rainfall had declined. Dry periods between rainfalls varied during 2 years of the study period and were noted on June 2012 as well as on July and August 2013. Organically enriched catch basins with significant higher concentrations of Na(+) and NO3 (-) were found to be more productive breeding habitats. Differences in the Culex immature stage density based on the variables of habitat type, temperature, and precipitation support the need for ongoing surveillance in communities to guide public health officials in planning for and prioritizing mosquito control efforts.

Host preferences in host-seeking and blood-fed mosquitoes in Switzerland

The avian zoonotic agent for West Nile virus (WNV) can cause neuroinvasive disease in horses and humans and is expanding its range in Europe. Analyses of the risk for transmission to these hosts in non-endemic areas are necessary. Host preferences of mosquitoes (Diptera: Culicidae), the main vectors of WNV, were determined in Switzerland using animal-baited trap (horse, chickens) experiments at a natural and a periurban site. This was undertaken on four occasions during May-September 2014. In addition, the hosts of 505 blood-fed mosquitoes collected in a zoo and in the field were determined. Mosquito data obtained in the animal bait experiments were corrected for host weight and body surface area and by Kleiber's scaling factor. Collections of 11-14 different mosquito species were achieved with these approaches. Statistically significant host preferences were identified in three species in both approaches. The other species showed opportunistic feeding behaviours to varying extents. Specifically, the invasive species Hulecoeteomyia japonica (= Aedes japonicus) was identified for the first time as feeding on avians in nature. Abundance data, spatiotemporal activity and laboratory vector competence for WNV suggested that, in addition to the main WNV vector Culex pipiens, H. japonica and Aedimorphus vexans (= Aedes vexans) are the most likely candidate bridge vectors for WNV transmission in Switzerland.

Culex torrentium Mosquito Role as Major Enzootic Vector Defined by Rate of Sindbis Virus Infection, Sweden, 2009

We isolated Sindbis virus (SINV) from the enzootic mosquito vectors Culex torrentium, Cx. pipiens, and Culiseta morsitans collected in an area of Sweden where SINV disease is endemic. The infection rate in Cx. torrentium mosquitoes was exceptionally high (36 infections/1,000 mosquitoes), defining Cx. torrentium as the main enzootic vector of SINV in Scandinavia.

Ecological Distribution and CQ11 Genetic Structure of Culex pipiens Complex (Diptera: Culicidae) in Italy

Mosquitoes in the Culex pipiens complex are considered to be involved in the transmission of a range of pathogens, including West Nile virus (WNV). Although its taxonomic status is still debated, the complex includes species, both globally distributed or with a more limited distribution, morphologically similar and characterised by different physiological and behavioural traits, which affect their ability as vectors. In many European countries, Cx. pipiens and its sibling species Culex torrentium occur in sympatry, exhibiting similar bionomic and morphological characters, but only Cx. pipiens appears to play a vector role in WNV transmission. This species consists of two biotypes, pipiens and molestus, which can interbreed when in sympatry, and their hybrids can act as WNV-bridge vectors, due to intermediate ecological features. Considering the yearly WNV outbreaks since 2008 and given the morphological difficulties in recognising species and biotypes, our aim was to molecularly identify and characterised Cx. pipiens and Cx. torrentium in Italy, using recently developed molecular assays. Culex torrentium was not detected; as in other European countries, the pipiens and molestus biotypes were widely found in sympatry with hybrids in most environments. The UPGMA cluster analysis applied to CQ11 genotypic frequencies mainly revealed two groups of Cx. pipiens populations that differed in ecological features. The high propensity of the molestus biotype to exist in hypogean environments, where the habitat’s physical characteristics hinder and preclude the gene flow, was shown. These results confirmed the CQ11 assay as a reliable diagnostic method, consistent with the ecological and physiological aspects of the populations analysed. Since the assessment of the actual role of three biotypes in the WNV circulation remains a crucial point to be elucidated, this extensive molecular screening of Cx. pipiens populations can provide new insights into the ecology of the species and may give useful indications to plan and implement WNV surveillance activities in Italy.

Diversity and ecology survey of mosquitoes potential vectors in Belgian equestrian farms: A threat prevention of mosquito-borne equine arboviruses

Emergence of West Nile Virus was recently recorded in several European countries, which can lead to severe health problems in horse populations. Europe is also at risk of introduction of mosquito-borne equine alphavirus from Americas. Prevention of these arboviruses requires a clear understanding of transmission cycles, especially their vectors. To characterize mosquito fauna, their ecology and identify potential vectors of equine arboviruses in Belgium, entomological surveys of six equestrian farms located in the Wolloon Region were conducted during 2011-2012. The harvest of mosquitoes was based on larval sampling (272 samples from 111 breeding sites) and monthly adults trapping (CO2-baited traps, Mosquito Magnet Liberty Plus). Among 51,493 larvae and 319 adult mosquitoes collected, morphological identification showed the presence of 11 species: Anopheles claviger (Meigen), An. maculipennis s.l. (Meigen), An. plumbeus (Stephens), Culex hortensis (Ficalbi), Cx. territans (Walker), Cx. pipiens s.l. L., Cx. torrentium (Martini), Coquillettidia richiardii (Ficalbi), Culiseta annulata (Schrank), Aedes cantans (Meigen), Ae. geniculatus (Olivier). Molecular identification of Cx. pipiens species complex allowed the detection of three molecular forms, Pipiens (92.3%), Molestus (4.6%) and Hybrid (3.1%). Larvae of Cx. pipiens sl and Cx. torrentium were omnipresent and the most abundant species. Water troughs, ponds and slurry (liquid manure) were the most favorable breeding sites of mosquito larvae. Based upon behavior and ecology of the identified mosquito species, Studied Belgian equestrian farms seem to provide a suitable environment and breeding sites for the proliferation of potential vectors of arboviruses and those being a real nuisance problem for horses and neighboring inhabitants.

Physico-Chemical Characteristics of Culex pipiens sensu lato and Culex torrentium (Diptera: Culicidae) Breeding Sites in Germany

Culex pipiens sensu lato (L.) and Culex torrentium Martini, 1925 are two widely distributed mosquito species in Europe. Due to difficulties in morphological discrimination, the current knowledge on differences in the breeding ecology is fragmentary. Therefore, this study evaluated the relation between the presence-absence of both species at various types of breeding habitats in response to physico-chemical parameters, using a recently developed molecular assay that allows reliable species-specific typing of larvae and pupae. The results revealed that the two species often occur in sympatry at the studied breeding sites, and there were no substantial differences concerning presence-absence of the two species with regard to the various environmental parameters investigated.

Seasonal dynamics and habitat specificity of mosquitoes in an English wetland: implications for UK wetland management and restoration

We engaged in field studies of native mosquitoes in a Cambridgeshire Fen, investigating a) the habitat specificity and seasonal dynamics of our native fauna in an intensively managed wetland, b) the impact of water-level and ditch management, and c) their colonization of an arable reversion to flooded grassland wetland expansion project. Studies from April to October, 2010 collected 14,000 adult mosquitoes (15 species) over 292 trap-nights and ∼4,000 pre-imaginal mosquitoes (11 species). Open floodwater species (Aedes caspius and Aedes cinereus, 43.3%) and wet woodland species (Aedes cantans/annulipes and Aedes rusticus, 32.4%) dominated, highlighting the major impact of seasonal water-level management on mosquito populations in an intensively managed wetland. In permanent habitats, managing marginal ditch vegetation and ditch drying significantly affect densities of pre-imaginal anophelines and culicines, respectively. This study presents the first UK field evidence of the implications of wetland expansion through arable reversion on mosquito colonization. Understanding the heterogeneity of mosquito diversity, phenology, and abundance in intensively managed UK wetlands will be crucial to mitigating nuisance and vector species through habitat management and biocidal control.

Trapping biases of Culex torrentium and Culex pipiens revealed by comparison of captures in CDC traps, ovitraps, and gravid traps

We evaluate three trapping methods for their effectiveness at capturing Culex pipiens and Culex torrentium, both enzootic vectors of bird-associated viruses in Europe. The comparisons, performed in two regions in Sweden, were among CDC traps baited with carbon dioxide, gravid traps, and ovitraps baited with hay infusion. The proportions of the two Culex species in a catch differed between trap types, with CDC traps catching a lower proportion of Cx. torrentium than both gravid traps and ovitraps. Between gravid traps and ovitraps, there was no difference in the proportions of the two species. The results indicate that Cx. torrentium may go undetected or underestimated compared to Cx. pipiens when using carbon dioxide baited CDC traps. The new insight of trap bias presented here adds an important dimension to consider when investigating these vectors of bird-associated viruses in the field.

Colonization of a newly constructed urban wetland by mosquitoes in England: implications for nuisance and vector species

Urban wetlands are being created in the UK as part of sustainable urban drainage strategies, to create wetland habitats lost during development, to provide a habitat for protected species, and to increase the public's access to 'blue-space' for the improvement of health and well-being. Sewage treatment reedbeds are also being incorporated into newly constructed wetlands to offer an alternative approach to dealing with sewage. This field study aims to provide the first UK evidence of how such newly constructed aquatic habitats are colonized by mosquitoes. A number of new aquatic habitats were surveyed for immature mosquitoes every fortnight over the first two years following wetland construction. The majority of mosquitoes collected were Culex sp. and were significantly associated with the sewage treatment reedbed system, particularly following storm events and sewage inflow. Other more natural aquatic habitats that were subject to cycles of drying and re-wetting contributed the majority of the remaining mosquitoes colonizing. Colonization of permanent habitats was slow, particularly where fluctuations in water levels inhibited emergent vegetation growth. It is recommended that during the planning process for newly constructed wetlands consideration is given on a case-by-case basis to the impact of mosquitoes, either as a cause of nuisance or as potential vectors. Although ornithophagic Culex dominated in this wetland, their potential role as enzootic West Nile virus vectors should not be overlooked.

Culex pipiens s.l. and Culex torrentium (Culicidae) in Wrocław area (Poland): occurrence and breeding site preferences of mosquito vectors

Both ornithophilic mosquito species, Culex pipiens s.l. (L.) and Culex torrentium (Martini, 1925), occur sympatric in temperate Europe. They are presumed to be primary vectors of West Nile and Sindbis viruses. Differentiation of these morphologically similar Culex species is essential for evaluation of different vector roles, for mosquito surveillance and integrated control strategies. Cx. torrentium has been neglected or erroneously determined as Cx. pipiens s.l. in some previous studies, because only males of both species can be diagnosed reliably by morphology. Thus, knowledge about species abundance, geographical distribution, breeding site preferences and the zoonotic risk assessment is incomplete also in Poland. In Wrocław area (Silesian Lowland), besides typical urban breeding sites, huge sewage irrigation fields provide suitable breeding conditions for Culex species. They are also inhabited by 180 resident and migratory bird species serving as potential virus reservoirs. In this study, morphology of larvae and males as well as species diagnostic enzyme markers, namely adenylate kinase (AK) and 2-hydroxybutyrate dehydrogenase (HBDH), were used to discriminate Cx. pipiens s.l. and Cx. torrentium. In a total of 650 Culex larvae from 24 natural and artificial breeding sites, Cx. pipiens s.l. had a proportion of 94.0% and Cx. torrentium only 6.0%. It could be shown that both species are well adapted to various breeding site types like ditches, catch basins, flower pots and buckets with diverse water quality. Cx. torrentium preferred more artificial water containers in urban surrounding (12% species proportion), whereas in semi-natural breeding sites, Cx. torrentium was rare (3%). In 12 of 24 breeding sites, larvae of both species have been found associated.

Population structure and distribution patterns of the sibling mosquito species Culex pipiens and Culex torrentium (Diptera: Culicidae) reveal different evolutionary paths

Nowadays a number of endemic mosquito species are known to possess vector abilities for various diseases, as e.g. the sibling species Culex pipiens and Culex torrentium. Due to their morphological similarity, ecology, distribution and vector abilities, knowledge about these species' population structure is essential. Culicidae from 25 different sampling sites were collected from March till October 2012. All analyses were performed with aligned cox1 sequences with a total length of 658 bp. Population structure as well as distribution patterns of both species were analysed using molecular methods and different statistical tests like distance based redundancy analysis (dbDRA), analysis of molecular variances (AMOVA) or McDonald & Kreitman test and Tajima's D. Within both species, we could show a genetic variability among the cox1 fragment. The construction of haplotype networks revealed one dominating haplotype for Cx. pipiens, widely distributed within Germany and a more homogeneous pattern for Cx. torrentium. The low genetic differences within Cx. pipiens could be a result of an infection with Wolbachia which can induce a sweep through populations by passively taking the also maternally inherited mtDNA through the population, thereby reducing the mitochondrial diversity as an outcome of reproductive incompatibility. Pairwise population genetic differentiation (F_ST) ranged significantly from moderate to very great between populations of Cx. pipiens and Cx. torrentium. Analyses of molecular variances revealed for both species that the main genetic variability exists within the populations (Cx. pipiens [88.38%]; Cx. torrentium [66.54%]). Based on a distance based redundancy analysis geographical origin explained a small but significant part of the species' genetic variation. Overall, the results confirm that Cx. pipiens and Cx. torrentium underlie different factors regarding their mitochondrial differentiation, which could be a result of endosymbiosis, dispersal between nearly located populations or human introduction.

The use of morphometric wing characters to discriminate female Culex pipiens and Culex torrentium

The reliability of the length of wing radial vein r(2/3) as a character for the morphological discrimination of the two potential arbovirus vectors Culex pipiens s.s. and Cx. torrentium from Germany was reassessed, after this character had been neglected for more than 40 years. Additionally, multivariate morphometric analyses were applied to evaluate wing shape variation between both species. Although high-throughput molecular tools are now available to differentiate the two species, a simple, low-cost routine alternative may be useful in the absence of a molecular laboratory, such as under semi-field conditions. A thin-plate splines transformation confirmed that primarily the shrinkage of vein r(2/3) is responsible for the wing differences between the two species. In the bivariate analysis, the r(2/3)/r3 indices of Cx. pipiens s.s. and Cx. torrentium were 0.185 and 0.289, respectively, resulting in a correct classification of more than 91% of all tested specimens. Using the absolute length of vein r(2/3) alone still allowed for more than 90% accurate discrimination. Furthermore, classification accuracy of linear discriminant analysis exceeded 97%.