Aedes aegypti in the Black Sea: recent introduction or ancient remnant?

Mesocosm Studies Suggest Climate Change May Release Aedes aegypti (Diptera: Culicidae) Larvae from Cold Inhibition and Enable Year-Round Development in a Desert City

AbstractGlobal warming trends, human-assisted transport, and urbanization have allowed poleward expansion of many tropical vector species, but the specific mechanisms responsible for thermal mediation of range changes and ecological success of invaders remain poorly understood. Aedes aegypti (Diptera: Culicidae) is a tropical mosquito currently expanding into many higher-latitude regions, including the urban desert region of Maricopa County, Arizona. Here, adult populations virtually disappear in winter and spring and then increase exponentially through summer and fall, indicating that winter conditions remain a barrier to the development of some life stages of A. aegypti. To determine whether cold limits the winter development of A. aegypti larvae in Maricopa County, we surveyed for larval abundance and tested their capacity to develop in ambient and warmed conditions. Aedes aegypti larvae were not observed in artificial aquatic habitats in winter and spring but were abundant in summer and fall, suggesting winter suppression of adults, larvae, or both. Water temperatures in winter months fluctuated strongly; larvae were usually cold paralyzed at night but active during the day. Despite daytime temperatures that allowed activity and achieving similar degree-days as warmed mesocosms, larvae reared under ambient winter conditions were unable to develop to adulthood, perhaps due to repetitive cold damage. However, warming average temperature by 1.7°C allowed many larvae to successfully develop to adults. Because daytime highs in winter will often allow adult flight, it is likely that relatively minor additional winter warming may allow A. aegypti populations to develop and reproduce year-round in Maricopa County.

Robustness in population-structure and demographic-inference results derived from the Aedes aegypti genotyping chip and whole-genome sequencing data

The mosquito Aedes aegypti is the primary vector of many human arboviruses such as dengue, yellow fever, chikungunya, and Zika, which affect millions of people worldwide. Population genetic studies on this mosquito have been important in understanding its invasion pathways and success as a vector of human disease. The Axiom aegypti1 SNP chip was developed from a sample of geographically diverse A. aegypti populations to facilitate genomic studies on this species. We evaluate the utility of the Axiom aegypti1 SNP chip for population genetics and compare it with a low-depth shotgun sequencing approach using mosquitoes from the native (Africa) and invasive ranges (outside Africa). These analyses indicate that results from the SNP chip are highly reproducible and have a higher sensitivity to capture alternative alleles than a low-coverage whole-genome sequencing approach. Although the SNP chip suffers from ascertainment bias, results from population structure, ancestry, demographic, and phylogenetic analyses using the SNP chip were congruent with those derived from low-coverage whole-genome sequencing, and consistent with previous reports on Africa and outside Africa populations using microsatellites. More importantly, we identified a subset of SNPs that can be reliably used to generate merged databases, opening the door to combined analyses. We conclude that the Axiom aegypti1 SNP chip is a convenient, more accurate, low-cost alternative to low-depth whole-genome sequencing for population genetic studies of A. aegypti that do not rely on full allelic frequency spectra. Whole-genome sequencing and SNP chip data can be easily merged, extending the usefulness of both approaches.

Dengue Virus Serotype 1 Effects on Mosquito Survival Differ among Geographically Distinct Aedes aegypti Populations

The mosquito Aedes aegypti is distributed worldwide and is recognized as the primary vector for dengue in numerous countries. To investigate whether the fitness cost of a single DENV-1 isolate varies among populations, we selected four Ae. aegypti populations from distinct localities: Australia (AUS), Brazil (BRA), Pakistan (PAK), and Peru (PER). Utilizing simple methodologies, we concurrently assessed survival rates and fecundity. Overall, DENV-1 infection led to a significant decrease in mosquito survival rates, with the exception of the PER population. Furthermore, infected Ae. aegypti from PAK, the population with the lowest infection rate among those tested, exhibited a noteworthy reduction in egg laying. These findings collectively suggest that local mosquito-virus adaptations may influence dengue transmission in endemic settings.

Population genetic structure of Aedes aegypti subspecies in selected geographical locations in Sudan

Although knowledge of the composition and genetic diversity of disease vectors is important for their management, this is limiting in many instances. In this study, the population structure and phylogenetic relationship of the two Aedes aegypti subspecies namely Aedes aegypti aegypti (Aaa) and Aedes aegypti formosus (Aaf) in eight geographical areas in Sudan were analyzed using seven microsatellite markers. Hardy-Weinberg Equilibrium (HWE) for the two subspecies revealed that Aaa deviated from HWE among the seven microsatellite loci, while Aaf exhibited departure in five loci and no departure in two loci (A10 and M201). The Factorial Correspondence Analysis (FCA) plots revealed that the Aaa populations from Port Sudan, Tokar, and Kassala clustered together (which is consistent with the unrooted phylogenetic tree), Aaf from Fasher and Nyala populations clustered together, and Gezira, Kadugli, and Junaynah populations also clustered together. The Bayesian cluster analysis structured the populations into two groups suggesting two genetically distinct groups (subspecies). Isolation by distance test revealed a moderate to strong significant correlation between geographical distance and genetic variations (p = 0.003, r = 0.391). The migration network created using divMigrate demonstrated that migration and gene exchange between subspecies populations appear to occur based on their geographical proximity. The genetic structure of the Ae. aegypti subspecies population and the gene flow among them, which may be interpreted as the mosquito vector's capacity for dispersal, were revealed in this study. These findings will help in the improvement of dengue epidemiology research including information on the identity of the target vector/subspecies and the arboviruses vector surveillance program.

Invasive hematophagous arthropods and associated diseases in a changing world

Biological invasions have increased significantly with the tremendous growth of international trade and transport. Hematophagous arthropods can be vectors of infectious and potentially lethal pathogens and parasites, thus constituting a growing threat to humans-especially when associated with biological invasions. Today, several major vector-borne diseases, currently described as emerging or re-emerging, are expanding in a world dominated by climate change, land-use change and intensive transportation of humans and goods. In this review, we retrace the historical trajectory of these invasions to better understand their ecological, physiological and genetic drivers and their impacts on ecosystems and human health. We also discuss arthropod management strategies to mitigate future risks by harnessing ecology, public health, economics and social-ethnological considerations. Trade and transport of goods and materials, including vertebrate introductions and worn tires, have historically been important introduction pathways for the most prominent invasive hematophagous arthropods, but sources and pathways are likely to diversify with future globalization. Burgeoning urbanization, climate change and the urban heat island effect are likely to interact to favor invasive hematophagous arthropods and the diseases they can vector. To mitigate future invasions of hematophagous arthropods and novel disease outbreaks, stronger preventative monitoring and transboundary surveillance measures are urgently required. Proactive approaches, such as the use of monitoring and increased engagement in citizen science, would reduce epidemiological and ecological risks and could save millions of lives and billions of dollars spent on arthropod control and disease management. Last, our capacities to manage invasive hematophagous arthropods in a sustainable way for worldwide ecosystems can be improved by promoting interactions among experts of the health sector, stakeholders in environmental issues and policymakers (e.g. the One Health approach) while considering wider social perceptions.

Special Collection: Highlights of Medical, Urban and Veterinary Entomology. Highlights in Medical Entomology, 2021

Life remained far from normal as we completed the first year of the Covid-19 pandemic and entered a second year. Despite the challenges faced worldwide, together we continue to move the field of Medical Entomology forward. Here, I reflect on parallels between control of Covid-19 and vector-borne disease control, discuss the advantages and caveats of using new genotyping technologies for the study of invasive species, and proceed to highlight papers that were published between 2020 and 2021 with a focus on those related to mosquito surveillance and population genetics of mosquito vectors.

Past, present and future distribution of the yellow fever mosquito Aedes aegypti: The European paradox

The global distribution of the yellow fever mosquito Aedes aegypti is the subject of considerable attention because of its pivotal role as a biological vector of several high profile disease pathogens including dengue, chikungunya, yellow fever, and Zika viruses. There is also a lot of interest in the projected future species' distribution. However, less effort has been focused on its historical distribution, which has changed substantially over the past 100 years, especially in southern Europe where it was once widespread, but largely disappeared by the middle of the 20th century. The present work utilises all available historical records of the distribution of Ae. aegypti in southern Europe, the Near East within the Mediterranean Basin and North Africa from the late 19th century until the 1960's to construct a spatial distribution model using matching historical climatic and demographic data. The resulting model was then implemented using current climate and demographic data to assess the potential distribution of the vector in the present. The models were rerun with several different assumptions about the thresholds that determine habitat suitability for Ae. aegypti. The historical model matches the historical distributions well. When it is run with current climate values, the predicted present day distribution is somewhat broader than it used to be particularly in north-west France, North Africa and Turkey. Though it is beginning to reappear in the eastern Caucasus, this 'potential' distribution clearly does not match the actual distribution of the species, which suggests some other factors are responsible for its absence. Future distributions based on the historical model also do not match future distributions derived from models based only on present day vector distributions, which predict little or no presence in the Mediterranean Region. At the same time, the vector is widespread in the USA which is predicted to consolidate its range there in future. This contradiction and the implication for possible re-invasion of Europe are discussed.

An alien in Marseille: investigations on a single Aedes aegypti mosquito likely introduced by a merchant ship from tropical Africa to Europe

Control of invasive species relies partly on permanent surveillance at international points of entry. We report the exceptional trapping of one adult mosquito (Diptera: Culicidae) in the port of Marseille, France, in July 2018, during a routine survey conducted according to International Health Regulations. Morphological and molecular identification classified the specimen as a female Aedes (Stegomyia) aegypti (L.), vector of many arboviruses, absent from Europe and the Mediterranean rim since the 1950s. A world reference panel of approximately 23,000 genome-wide single nucleotide polymorphisms determined that the mosquito originated from Cameroon, west Africa. Cross-reference of this geographic location with boats traveling from Central Africa to Marseille during the trapping period suggests that the mosquito travelled within an identified merchant ship, a vehicles carrier connecting Douala, Cameroon to Marseille, France. This ship left Douala on June 25, 2018 and arrived 20 days later in Marseille on July 15. The mosquito was captured 350 m away from the dock. The interception of a propagule of an invasive species is a rare event that must be considered a priority to prevent its successful establishment.

Origins of high latitude introductions of Aedes aegypti to Nebraska and Utah during 2019

Aedes aegypti (L.), the yellow fever mosquito, is also an important vector of dengue and Zika viruses, and an invasive species in North America. Aedes aegypti inhabits tropical and sub-tropical areas of the world and in North America is primarily distributed throughout the southern US states and Mexico. The northern range of Ae. aegypti is limited by cold winter months and establishment in these areas has been mostly unsuccessful. However, frequent introductions of Ae. aegypti to temperate, non-endemic areas during the warmer months can lead to seasonal activity and disease outbreaks. Two Ae. aegypti incursions were reported in the late summer of 2019 into York, Nebraska and Moab, Utah. These states had no history of established populations of this mosquito and no evidence of previous seasonal activity. We genotyped a subset of individuals from each location at 12 microsatellite loci and ~ 14,000 single nucleotide polymorphic markers to determine their genetic affinities to other populations worldwide and investigate their potential source of introduction. Our results support a single origin for each of the introductions from different sources. Aedes aegypti from Utah likely derived from Tucson, Arizona, or a nearby location. Nebraska specimen results were not as conclusive, but point to an origin from southcentral or southeastern US. In addition to an effective, efficient, and sustainable control of invasive mosquitoes, such as Ae. aegypti, identifying the potential routes of introduction will be key to prevent future incursions and assess their potential health threat based on the ability of the source population to transmit a particular virus and its insecticide resistance profile, which may complicate vector control.

Evidence for serial founder events during the colonization of North America by the yellow fever mosquito, Aedes aegypti

The Aedes aegypti mosquito first invaded the Americas about 500 years ago and today is a widely distributed invasive species and the primary vector for viruses causing dengue, chikungunya, Zika, and yellow fever. Here, we test the hypothesis that the North American colonization by Ae. aegypti occurred via a series of founder events. We present findings on genetic diversity, structure, and demographic history using data from 70 Ae. aegypti populations in North America that were genotyped at 12 microsatellite loci and/or ~20,000 single nucleotide polymorphisms, the largest genetic study of the region to date. We find evidence consistent with colonization driven by serial founder effect (SFE), with Florida as the putative source for a series of westward invasions. This scenario was supported by (1) a decrease in the genetic diversity of Ae. aegypti populations moving west, (2) a correlation between pairwise genetic and geographic distances, and (3) demographic analysis based on allele frequencies. A few Ae. aegypti populations on the west coast do not follow the general trend, likely due to a recent and distinct invasion history. We argue that SFE provides a helpful albeit simplified model for the movement of Ae. aegypti across North America, with outlier populations warranting further investigation.

Impact of temperature on dengue and chikungunya transmission by the mosquito Aedes albopictus

The mosquito Aedes albopictus is an invasive species first detected in Europe in Albania in 1979, and now established in 28 European countries. Temperature is a limiting factor in mosquito activities and in the transmission of associated arboviruses namely chikungunya (CHIKV) and dengue (DENV). Since 2007, local transmissions of CHIKV and DENV have been reported in mainland Europe, mainly in South Europe. Thus, the critical question is how far north transmission could occur. In this context, the Albanian infestation by Ae. albopictus is of interest because the species is present up to 1200 m of altitude; this allows using altitude as a proxy for latitude. Here we show that Ae. albopictus can transmit CHIKV at 28 °C as well as 20 °C, however, the transmission of DENV is only observed at 28 °C. We conclude that if temperature is the key environmental factor limiting transmission, then transmission of CHIKV, but not DENV is feasible in much of Europe.

Aedes aegypti in the Mediterranean container ports at the time of climate change: A time bomb on the mosquito vector map of Europe

In the past, Aedes aegypti was present in Southern Europe. Although the mosquito was eradicated from the Mediterranean region, its regional ecotype survived the second half of the 20th century in the eastern Black Sea area. The aim of the study was to model the changes in the altering climatic suitability, ontogenetic development time and the survival rate of Aedes aegypti from first-stage larvae to adulthood in Southern Europe. The modelled present climatic suitability patterns of the mosquito show that large areas of the lower altitude Mediterranean regions, including the coastal areas of the Balkan Peninsula, South France, and large regions of the Apennines and the Iberian Peninsulas could be suitable for Ae. aegypti. The future (2041-2060 and 2061-2080) projections predict the potential northward shift of the northern occurrence of the species in the circum-Mediterranean and Black Sea areas. Both, the potential development time, and survival rate of Ae. aegypti in the late 19th and the early 20th century could be like in the present times along the Mediterranean coast. The current climatic conditions cannot explain the absence of the mosquito in wide areas of the Mediterranean and sub-Mediterranean ecoregions. The future models predict the notable increase in the development time and survival rate of the mosquito in the southern and central regions of Europe. In general, the container ports of the Alboran, Balearic, and Aegean seas seem to be the most suitable sites for the re-colonization of the mosquito, and such northern parts of the Mediterranean Sea like the Gulf of Lion, the Ligurian, and Adriatic Seas are in less extent.

The effects of genetic drift and genomic selection on differentiation and local adaptation of the introduced populations of Aedes albopictus in southern Russia

Background

Asian tiger mosquito Aedes albopictus is an arbovirus vector that has spread from its native habitation areal in Southeast Asia throughout North and South Americas, Europe, and Africa. Ae. albopictus was first detected in the Southern Federal District of the Russian Federation in the subtropical town of Sochi in 2011. In subsequent years, this species has been described in the continental areas with more severe climate and lower winter temperatures.

Methods

Genomic analysis of pooled Ae. albopictus samples collected in the mosquito populations in the coastal and continental regions of the Krasnodar Krai was conducted to look for the genetic changes associated with the spread and potential cold adaptation in Ae. albopictus.

Results

The results of the phylogenetic analysis based on mitochondrial genomes corresponded well with the hypothesis that Ae. albopictus haplotype A1a2a1 was introduced into the region from a single source. Population analysis revealed the role of dispersal and genetic drift in the local adaptation of the Asian tiger mosquito. The absence of shared haplotypes between the samples and high fixation indices suggest that gene flow between samples was heavily restricted. Mitochondrial and genomic differentiation together with different distances between dispersal routes, natural and anthropogenic barriers and local effective population size reduction could lead to difficulties in local climatic adaptations due to reduced selection effectiveness. We have found genomic regions with selective sweep patterns which can be considered as having been affected by recent selection events. The genes located in these regions participate in neural protection, lipid conservation, and cuticle formation during diapause. These processes were shown to be important for cold adaptation in the previous transcriptomic and proteomic studies. However, the population history and relatively low coverage obtained in the present article could have negatively affect sweep detection.

Population structure and ancestry prediction of Aedes aegypti (Diptera: Culicidae) supports a single African origin of Colombian populations

BACKGROUND

A previous phylogeographic study revealed two Aedes aegypti African-related mitochondrial lineages distributed in Colombian's cities with different eco-epidemiologic characteristics with regard to dengue virus (DENV). It has been proposed these lineages might indicate independent invasion sources.

OBJECTIVES

Assessing to Colombian population structure and to support evidence of its probable source origin.

METHODS

We analysed a total of 267 individuals from cities of Bello, Riohacha and Villavicencio, which 241 were related to the West and East African mitochondrial lineages (termed here as WAL and EAL, respectively). Eight polymorphic microsatellite loci were analysed aiming population structure.

FINDINGS

Results indicate substantial gene flow among distant and low-connected cities composing a panmictic population with incipient local differentiation of Ae. aegypti is placed in Colombia. Likewise, genetic evidence indicates no significant differences among individuals related to WAL and EAL is placed.

MAIN CONCLUSIONS

Minimal genetic differentiation in low-connected Ae. aegypti populations of Colombia, and lack concordance between mitochondrial and nuclear genealogies suggest that Colombian Ae. aegypti shared a common demographic history. Under this scenario, we suggest current Ae. aegypti population structure reflects a single origin instead of contemporary migration, which founding populations have a single source from a mitochondrial polymorphic African ancient.

The genomic signal of local environmental adaptation in Aedes aegypti mosquitoes

Local adaptation is important when predicting arthropod-borne disease risk because of its impacts on vector population fitness and persistence. However, the extent that vector populations are adapted to the environment generally remains unknown. Despite low population structure and high gene flow in Aedes aegypti mosquitoes across Panama, excepting the province of Bocas del Toro, we identified 128 candidate SNPs, clustered within 17 genes, which show a strong genomic signal of local environmental adaptation. This putatively adaptive variation occurred across fine geographical scales with the composition and frequency of candidate adaptive loci differing between populations in wet tropical environments along the Caribbean coast and dry tropical conditions typical of the Pacific coast. Temperature and vegetation were important predictors of adaptive genomic variation in Ae. aegypti with several potential areas of local adaptation identified. Our study lays the foundations of future work to understand whether environmental adaptation in Ae. aegypti impacts the arboviral disease landscape and whether this could either aid or hinder efforts of population control.

Increased size and energy reserves in diapausing eggs of temperate Aedes aegypti populations

Many insects overwinter in diapause, a pre-programmed anticipated response to unfavorable environmental conditions, often induced by a short-day photoperiod. Diapause involves morphological changes and increased energy stores required for metabolic demands during winter. In diapausing mosquito eggs, the accumulation of lipids plays an important role, because these molecules are the primary fuel consumed during embryogenesis and pharate larvae metabolism, and have a key role in egg desiccation resistance. The supposed inability of the mosquito Aedes aegypti to lay diapausing eggs has been recently challenged by a study on a temperate population, which showed that the inhibition of egg hatching in response to short days is possible in this species. Thus, the aim of the present study was to assess the effects of parental photoperiod on embryonic diapause-related traits, such as the triglyceride content and size of eggs laid, of two populations whose localities of origin differ in their winter length. Two colonies were maintained for each population: one under a Short-Day Photoperiod (SD: 10 h:14 h - Light:Dark) and the other under a Long-Day Photoperiod (LD: 14 h:10 h - Light:Dark). The eggs obtained from each combination of population and light treatment were used for size measurement (length, width and volume) and for the quantification of triglyceride content. Egg size showed differences between photoperiod treatments, with larger width and volume in eggs from the SD treatment. Remarkably, eggs from the SD treatment accumulated twice as many triglycerides as those from the LD treatment. Also, the eggs derived from the population having the longer winter accumulated larger amounts of triglycerides. The higher lipid content is probably contributing to a better survival during the cold season in both populations. The photoperiod-induced response in egg size and amount of triglycerides observed in this study support the hypothesis that the Ae. aegypti populations studied are able to lay diapausing eggs, a fact that provides physiological bases for the further expansion of this species to colder regions.

Detection of the Invasive Mosquito Species Aedes (Stegomyia) aegypti and Aedes (Hulecoeteomyia) koreicus on the Southern Coast of the Crimean Peninsula

Background:

The incidence and area of arbovirus infections is increasing around the world. It is largely linked to the spread of the main arbovirus vectors, invasive mosquito of the genus Aedes. Previously, it has been reported that Aedes aegypti reemerged in Russia after a 50-year absence. Moreover, in 2011, Ae. albopictus was registered in the city of Sochi (South Russia, Black Sea coast) for the first time. In 2013, Asian Ae. koreicus was found in Sochi for the first time.

Methods:

Mosquitoes were collected using the following methods: larvae with a dip net, imago on volunteers and using bait traps. The mosquitoes were identified using both morphology and sequencing of the second internal transcribed spacer of the nuclear ribosomal RNA gene cluster.

Results:

In August 2016, Ae. koreicus larvae and imago and a single male of Ae. aegypti were found on the southern coast of the Crimean Peninsula, where they were not registered before. Newly obtained DNA sequences were registered in GenBank with the accession numbers MF072936 and MF072937.

Conclusion:

Detection of invasive mosquito species (Ae. aegypti and Ae. koreicus) implies the possibility of their area expansion. Intensive surveillance is required at the Crimean Peninsula to evaluate the potential for the introduction of vector-borne diseases.

Two Haplotypes of Aedes aegypti Detected by ND4 Mitochondrial Marker in Three Regions of Ecuador

Simple Summary

The yellow fever mosquito, Aedes aegypti, is a widespread species associated with the transmission of vector-borne diseases across tropical and subtropical areas of the world. The genetic variability of its populations has been assessed with the use of several molecular markers to understand aspects of the population dynamics and their implication in disease transmission. However, the genetic diversity of Ecuadorian populations of the vector have not been investigated. In this study, we evaluated the genetic diversity of Ecuadorian populations of Ae. aegypti from 17 sites (Galapagos Islands, Amazon basin, and Coastal regions). These analyses revealed the presence of only two haplotypes among the Ecuadorian population of the vector. Haplotype 1, appears to be related to previously reported haplotypes from America, Asia, and West Africa. While haplotype 2 is only related to samples from America. The genetic diversity of Ecuadorian populations seems to be low, according to different statistical analyses, which show only one main population across sampled localities and no effect of the main geographical barriers. Understanding the genetic diversity of local populations is a key element in vector control strategies.

Abstract

Aedes aegypti, also known as the yellow fever mosquito, is the main vector of several arboviruses. In Ecuador, dengue and chikungunya are the most prevalent mosquito-borne diseases. Hence, there is a need to understand the population dynamics and genetic structure of the vector in tropical areas for a better approach towards effective vector control programs. This study aimed to assess the genetic diversity of Ae. aegypti, through the analyses of the mitochondrial gene ND4, using a combination of phylogenetic and population genetic structure from 17 sites in Ecuador. Results showed two haplotypes in the Ecuadorian populations of Ae. aegypti. Haplotype 1 was closely related to Ae. aegypti reported from America, Asia, and West Africa. Haplotype 2 was only related to samples from America. The sampled vectors from the diverse localities showed low nucleotide diversity (π = 0–0.01685) and genetic differentiation (FST = 0.152). AMOVA analyses indicated that most of the variation (85–91%) occurred within populations, suggesting that geographical barriers have little effect on the genetic structure of Ecuadorian populations of Ae. aegypti. These results agree with the one main population (K = 1) detected by Structure. Vector genetic identity may be a key factor in the planning of vector control strategies.

The Absence of Yellow Fever in Asia: History, Hypotheses, Vector Dispersal, Possibility of YF in Asia, and Other Enigmas

Since the recent epidemics of yellow fever in Angola and Brazil as well as the importation of cases to China in 2016, there has been an increased interest in the century-old enigma, absence of yellow fever in Asia. Although this topic has been repeatedly reviewed before, the history of human intervention has never been considered a critical factor. A two-stage literature search online for this review, however, yielded a rich history indispensable for the debate over this medical enigma. As we combat the pandemic of COVID-19 coronavirus worldwide today, we can learn invaluable lessons from the historical events in Asia. In this review, I explore the history first and then critically examine in depth major hypotheses proposed in light of accumulated data, global dispersal of the principal vector, patterns of YF transmission, persistence of urban transmission, and the possibility of YF in Asia. Through this process of re-examination of the current knowledge, the subjects for research that should be conducted are identified. This review also reveals the importance of holistic approach incorporating ecological and human factors for many unresolved subjects, such as the enigma of YF absence in Asia, vector competence, vector dispersal, spillback, viral persistence and transmission mechanisms.

Genetic evidence for the origin of Aedes aegypti, the yellow fever mosquito, in the southwestern Indian Ocean

Aedes aegypti is among the best-studied mosquitoes due to its critical role as a vector of human pathogens and ease of laboratory rearing. Until now, this species was thought to have originated in continental Africa, and subsequently colonized much of the world following the establishment of global trade routes. However, populations of this mosquito on the islands in the southwestern Indian Ocean (SWIO), where the species occurs with its nearest relatives referred to as the Aegypti Group, have received little study. We re-evaluated the evolutionary history of Ae. aegypti and these relatives, using three data sets: nucleotide sequence data, 18,489 SNPs and 12 microsatellites. We found that: (a) the Aegypti Group diverged 16 MYA (95% HPD: 7-28 MYA) from its nearest African/Asian ancestor; (b) SWIO populations of Ae. aegypti are basal to continental African populations; (c) after diverging 7 MYA (95% HPD: 4-15 MYA) from its nearest formally described relative (Ae. mascarensis), Ae. aegypti moved to continental Africa less than 85,000 years ago, where it recently (<1,000 years ago) split into two recognized subspecies Ae. aegypti formosus and a human commensal, Ae. aegypti aegypti; (d) the Madagascar samples form a clade more distant from all other Ae. aegypti than the named species Ae. mascarensis, implying that Madagascar may harbour a new cryptic species; and (e) there is evidence of introgression between Ae. mascarensis and Ae. aegypti on Réunion, and between the two subspecies elsewhere in the SWIO, a likely consequence of recent introductions of domestic Ae. aegypti aegypti from Asia.

Genetic structure of the mosquito Aedes aegypti in local forest and domestic habitats in Gabon and Kenya

Background

The mosquito Aedes aegypti is a devastating disease vector transmitting several important human arboviral diseases. In its native range in Africa, the mosquito can be found in both the ancestral forest habitat and anthropogenic habitats such as villages. How do the different habitats impact the population genetic structure of the local mosquito populations?

Methods

To address this question, we simultaneously sampled Ae. aegypti from the forest and local villages in La Lopé, Gabon and Rabai, Kenya. The mosquitoes were genotyped at 12 microsatellite loci and a panel of ~25,000 single nucleotide polymorphisms (SNPs), which allowed us to estimate their genetic ancestries and the population genetic structure related to habitats and sampling sites.

Results

In the context of the global population genetic structure of Ae. aegypti, clustering analysis showed that mosquitoes from the same locality (La Lopé or Rabai) have similar genetic ancestry, regardless of their habitats. Further analysis at the local scale also found no strong genetic differentiation between the forest and village mosquitoes in both La Lopé and Rabai. Interestingly, these results from our 2017 samples from Rabai, Kenya contrast to the documentation of genetic differentiation between village and forest mosquito collections from 1975–1976 and 2009. Between-habitat measures of genetic difference (F_st) vary across the genome, with a peak of high divergence observed at the third chromosome only in the La Lopé populations.

Conclusion

Collectively, these results demonstrated that there is little genetic isolation between forest and village habitats, which suggests possible extensive gene flow between them. From an epidemiological perspective, the forest habitat could act as a refuge for mosquitoes against vector control programmes in the domestic settings. Moreover, sylvatic populations could play a role in zoonotic pathogen transferred to humans. Therefore, future studies on disease transmission and vector control planning in the study area should take natural populations into consideration.

Population genomics of two invasive mosquitoes (Aedes aegypti and Aedes albopictus) from the Indo-Pacific

The arbovirus vectors Aedes aegypti (yellow fever mosquito) and Ae. albopictus (Asian tiger mosquito) are both common throughout the Indo-Pacific region, where 70% of global dengue transmission occurs. For Ae. aegypti all Indo-Pacific populations are invasive, having spread from an initial native range of Africa, while for Ae. albopictus the Indo-Pacific includes invasive populations and those from the native range: putatively, India to Japan to Southeast Asia. This study analyses the population genomics of 480 of these mosquitoes sampled from 27 locations in the Indo-Pacific. We investigated patterns of genome-wide genetic differentiation to compare pathways of invasion and ongoing gene flow in both species, and to compare invasive and native-range populations of Ae. albopictus. We also tested landscape genomic hypotheses that genetic differentiation would increase with geographical distance and be lower between locations with high connectivity to human transportation routes, the primary means of dispersal at these scales. We found that genetic distances were generally higher in Ae. aegypti, with Pacific populations the most highly differentiated. The most differentiated Ae. albopictus populations were in Vanuatu, Indonesia and Sri Lanka, the latter two representing potential native-range populations and potential cryptic subspeciation respectively. Genetic distances in Ae. aegypti increased with geographical distance, while in Ae. albopictus they decreased with higher connectivity to human transportation routes. Contrary to the situation in Ae. aegypti, we found evidence of long-distance Ae. albopictus colonisation events, including colonisation of Mauritius from East Asia and of Fiji from Southeast Asia. These direct genomic comparisons indicate likely differences in dispersal ecology in these species, despite their broadly sympatric distributions and similar use of human transport to disperse. Our findings will assist biosecurity operations to trace the source of invasive material and for biocontrol operations that benefit from matching genetic backgrounds of released and local populations.

Mosquito surveillance and disease outbreak risk models to inform mosquito-control operations in Europe

Surveillance programs are needed to guide mosquito-control operations to reduce both nuisance and the spread of mosquito-borne diseases. Understanding the thresholds for action to reduce both nuisance and the risk of arbovirus transmission is becoming critical. To date, mosquito surveillance is mainly implemented to inform about pathogen transmission risks rather than to reduce mosquito nuisance even though lots of control efforts are aimed at the latter. Passive surveillance, such as digital monitoring (validated by entomological trapping), is a powerful tool to record biting rates in real time. High-quality data are essential to model the risk of arbovirus diseases. For invasive pathogens, efforts are needed to predict the arrival of infected hosts linked to the small-scale vector to host contact ratio, while for endemic pathogens efforts are needed to set up region-wide highly structured surveillance measures to understand seasonal re-activation and pathogen transmission in order to carry out effective control operations.

Accelerating invasion potential of disease vector Aedes aegypti under climate change

Vector-borne diseases remain a major contributor to the global burden of disease, while climate change is expected to exacerbate their risk. Characterising vector development rate and its spatio-temporal variation under climate change is central to assessing the changing basis of human disease risk. We develop a mechanistic phenology model and apply it to Aedes aegypti, an invasive mosquito vector for arboviruses (e.g. dengue, zika and yellow fever). The model predicts the number of life-cycle completions (LCC) for a given location per unit time based on empirically derived biophysical responses to environmental conditions. Results suggest that the world became ~1.5% more suitable per decade for the development of Ae. aegypti during 1950–2000, while this trend is predicted to accelerate to 3.2–4.4% per decade by 2050. Invasion fronts in North America and China are projected to accelerate from ~2 to 6 km/yr by 2050. An increase in peak LCC combined with extended periods suitable for mosquito development is simulated to accelerate the vector’s global invasion potential.

Understanding how life cycles of vectors respond to climatic factors is important to predict potential shifts in vector-borne disease risk in the coming decades. Here the authors develop a mechanistic phenological model for the invasive mosquito Aedes aegypti and apply it to project shifts under climate change scenarios.

Novel genome sequences of cell-fusing agent virus allow comparison of virus phylogeny with the genetic structure of Aedes aegypti populations

Flaviviruses encompass not only medically relevant arthropod-borne viruses (arboviruses) but also insect-specific flaviviruses (ISFs) that are presumably maintained primarily through vertical transmission in the insect host. Interestingly, ISFs are commonly found infecting important arbovirus vectors such as the mosquito Aedes aegypti. Cell-fusing agent virus (CFAV) was the first described ISF of mosquitoes more than four decades ago. Despite evidence for widespread CFAV infections in A.aegypti populations and for CFAV potential to interfere with arbovirus transmission, little is known about CFAV evolutionary history. Here, we generated six novel CFAV genome sequences by sequencing three new virus isolates and subjecting three mosquito samples to untargeted viral metagenomics. We used these new genome sequences together with published ones to perform a global phylogenetic analysis of CFAV genetic diversity. Although there was some degree of geographical clustering among CFAV sequences, there were also notable discrepancies between geography and phylogeny. In particular, CFAV sequences from Cambodia and Thailand diverged significantly, despite confirmation that A.aegypti populations from both locations are genetically close. The apparent phylogenetic discrepancy between CFAV and its A.aegypti host in Southeast Asia indicates that other factors than host population structure shape CFAV genetic diversity.

The challenge of invasive mosquito vectors in the U.K. during 2016-2018: a summary of the surveillance and control of Aedes albopictus

Mosquito-borne diseases resulting from the expansion of two key vectors, Aedes aegypti and Aedes albopictus (Diptera: Culicidae), continue to challenge whole regions and continents around the globe. In recent years there have been human cases of disease associated with Chikungunya, dengue and Zika viruses. In Europe, the expansion of Ae. albopictus has resulted in local transmission of Chikungunya and dengue viruses. This paper considers the risk that Ae. aegypti and Ae. albopictus represent for the U.K. and details the results of mosquito surveillance activities. Surveillance was conducted at 34 points of entry, 12 sites serving vehicular traffic and two sites of used tyre importers. The most common native mosquito recorded was Culex pipiens s.l. (Diptera: Culicidae). The invasive mosquito Ae. albopictus was detected on three occasions in southern England (September 2016, July 2017 and July 2018) and subsequent control strategies were conducted. These latest surveillance results demonstrate ongoing incursions of Ae. albopictus into the U.K. via ground vehicular traffic, which can be expected to continue and increase as populations in nearby countries expand, particularly in France, which is the main source of ex-continental traffic.

Genetic diversity of laboratory strains and implications for research: The case of Aedes aegypti

The yellow fever mosquito (Aedes aegypti), is the primary vector of dengue, Zika, and chikungunya fever, among other arboviral diseases. It is also a popular laboratory model in vector biology due to its ease of rearing and manipulation in the lab. Established laboratory strains have been used worldwide in thousands of studies for decades. Laboratory evolution of reference strains and contamination among strains are potential severe problems that could dramatically change experimental outcomes and thus is a concern in vector biology. We analyzed laboratory and field colonies of Ae. aegypti and an Ae. aegypti-derived cell line (Aag2) using 12 microsatellites and ~20,000 SNPs to determine the extent of divergence among laboratory strains and relationships to their wild relatives. We found that 1) laboratory populations are less genetically variable than their field counterparts; 2) colonies bearing the same name obtained from different laboratories may be highly divergent; 3) present genetic composition of the LVP strain used as the genome reference is incompatible with its presumed origin; 4) we document changes in two wild caught colonies over ~16 generations of colonization; and 5) the Aag2 Ae. aegypti cell line has experienced minimal genetic changes within and across laboratories. These results illustrate the degree of variability within and among strains of Ae. aegypti, with implications for cross-study comparisons, and highlight the need of a common mosquito repository and the implementation of strain validation tools.

A yellow flag on the horizon: The looming threat of yellow fever to North America

OBJECTIVES

Yellow fever virus historically was a frequent threat to American and European coasts. Medical milestones such as the discovery of mosquitoes as vectors and subsequently an effective vaccine significantly reduced its incidence, in spite of which, thousands of cases of this deathly disease still occur regularly in Sub-Saharan Africa and the Amazonian basin in South America, which are usually not reported. An urban outbreak in Angola, consecutive years of increasing incidence near major Brazilian cities, and imported cases in China, South America and Europe, have brought this virus back to the global spotlight. The aim of this article is to underline that the preventive YFV measures, such as vaccination, need to be carefully revised in order to minimize the risks of new YFV outbreaks, especially in urban or immunologically vulnerable places. Furthermore, this article highlights the diverse factors that have favored the spread of other Aedes spp.-associated arboviral diseases like Dengue, Chikungunya and Zika, to northern latitudes causing epidemics in the United States and Europe, emphasizing the possibility that YFV might follow the path of these viruses unless enhanced surveillance and efficient control systems are urgently initiated.

Towards harmonisation of entomological surveillance in the Mediterranean area

BACKGROUND

The Mediterranean Basin is historically a hotspot for trade, transport, and migration. As a result, countries surrounding the Mediterranean Sea share common public health threats. Among them are vector-borne diseases, and in particular, mosquito-borne viral diseases are prime candidates as (re)emerging diseases and are likely to spread across the area. Improving preparedness and response capacities to these threats at the regional level is therefore a major issue. The implementation of entomological surveillance is, in particular, of utmost importance. Guidance in designing entomological surveillance systems is critical, and these systems may pursue different specific objectives depending on the disease. The purpose of the proposed review is to draw up guidelines for designing effective and sustainable entomological surveillance systems in order to improve preparedness and response. However, we make it clear that there is no universal surveillance system, so the thinking behind harmonisation is to define evidence-based standards in order to promote best practises, identify the most appropriate surveillance activities, and optimise the use of resources. Such guidance is aimed at policymakers and diverse stakeholders and is intended to be used as a framework for the implementation of entomological surveillance programmes. It will also be useful to collaborate and share information with health professionals involved in other areas of disease surveillance. Medical entomologists and vector control professionals will be able to refer to this report to advocate for tailored entomological surveillance strategies. The main threats targeted in this review are the vectors of dengue virus, chikungunya virus, Zika virus, West Nile virus, and Rift Valley fever virus. The vectors of all these arboviruses are mosquitoes.

METHODS

Current knowledge on vector surveillance in the Mediterranean area is reviewed. The analysis was carried out by a collaboration of the medical entomology experts in the region, all of whom belong to the MediLabSecure network, which is currently funded by the European Union and represents an international effort encompassing 19 countries in the Mediterranean and Black Sea region.

FINDINGS

Robust surveillance systems are required to address the globalisation of emerging arboviruses. The prevention and management of mosquito-borne viral diseases must be addressed in the prism of a One Health strategy that includes entomological surveillance as an integral part of the policy. Entomological surveillance systems should be designed according to the entomological and epidemiological context and must have well-defined objectives in order to effect a tailored and graduated response. We therefore rely on different scenarios according to different entomological and epidemiological contexts and set out detailed objectives of surveillance. The development of multidisciplinary networks involving both academics and public authorities will provide resources to address these health challenges by promoting good practises in surveillance (identification of surveillance aims, design of surveillance systems, data collection, dissemination of surveillance results, evaluation of surveillance activities) and through the sharing of effective knowledge and information. These networks will also contribute to capacity building and stronger collaborations between sectors at both the local and regional levels. Finally, concrete guidance is offered on the vector of the main arbovirus based on the current situation in the area.

Arboviral screening of invasive Aedes species in northeastern Turkey: West Nile virus circulation and detection of insect-only viruses

BACKGROUND

The recent reports of Aedes aegypti and Ae. albopictus populations in Turkey, in parallel with the territorial expansion identified in several surrounding countries, have raised concerns about the establishment and re-establishment of these invasive Aedes mosquitoes in Turkey. This cross-sectional study was performed to detect Aedes aegypti and Ae. albopictus in regions of recent incursions, and screen for viral pathogens known to be transmitted elsewhere by these species.

METHODOLOGY

Mosquitoes were collected at several locations in Artvin, Rize and Trabzon provinces of the Black Sea region during 2016-2017, identified morphologically, pooled and analyzed via generic or specific nucleic acid amplification assays. Viruses in positive pools were identified by product sequencing, cell culture inoculation and next generation sequencing (NGS) in selected specimens.

PRINCIPAL FINDINGS

The study group comprised 791 specimens. Aedes albopictus was the most abundant species in all locations (89.6%), followed by Ae. aegypti (7.8%) and Culex pipiens (2.5%). Mosquitoes were screened for viruses in 65 pools where fifteen (23.1%) were reactive. The infecting strains was identified as West Nile virus (WNV) in 5 pools (7.7%) with Ae. albopictus or Cx. pipiens mosquitoes. The obtained WNV sequences phylogenetically grouped with local and global lineage 1 clade 1a viruses. In 4 (6.2%) and 6 (9.2%) pools, respectively, cell fusing agent virus (CFAV) and Aedes flavivirus (AEFV) sequences were characterized. NGS provided a near-complete AEFV genome in a pool of Ae. albopictus. The strain is provisionally called "AEFV-Turkey", and functional analysis of the genome revealed several conserved motifs and regions associated with virus replication. Merida-like virus Turkey (MERDLVT), a recently-described novel rhabdovirus, was also co-detected in a Cx. pipiens pool also positive for WNV.

CONCLUSIONS/SIGNIFICANCE

Invasive Aedes mosquitoes are established in certain locations of northeastern Turkey. Herein we conclusively show the role of these species in WNV circulation in the region. Biosurveillance is imperative to monitor the spread of these species further into Asia Minor and to detect possible introduction of pathogens.

Recent History of Aedes aegypti: Vector Genomics and Epidemiology Records

Aedes aegypti bears the common name "the yellow fever mosquito," although, today, it is of more concern as the major vector of dengue, chikungunya, and, most recently, Zika viruses. In the present article, we review recent work on the population genetics of this mosquito in efforts to reconstruct its recent (approximately 600 years) history and relate these findings to epidemiological records of occurrences of diseases transmitted by this species. The two sources of information are remarkably congruent. Ae. aegypti was introduced to the New World 400-550 years ago from its ancestral home in West Africa via European slave trade. Ships from the New World returning to their European ports of origin introduced the species to the Mediterranean region around 1800, where it became established until about 1950. The Suez Canal opened in 1869 and Ae. aegypti was introduced into Asia by the 1870s, then on to Australia (1887) and the South Pacific (1904).

Genetic Variation in Insect Vectors: Death of Typology?

The issue of typological versus population thinking in biology is briefly introduced and defined. It is then emphasized how population thinking is most relevant and useful in vector biology. Three points are made: (1) Vectors, as they exist in nature, are genetically very heterogeneous. (2) Four examples of how this is relevant in vector biology research are presented: Understanding variation in vector competence, GWAS, identifying the origin of new introductions of invasive species, and resistance to inbreeding. (3) The existence of high levels of vector genetic heterogeneity can lead to failure of some approaches to vector control, e.g., use of insecticides and release of sterile males (SIT). On the other hand, vector genetic heterogeneity can be harnessed in a vector control program based on selection for refractoriness.

Population structure of a vector of human diseases: Aedes aegypti in its ancestral range, Africa

Aedes aegypti, the major vector of dengue, yellow fever, chikungunya, and Zika viruses, remains of great medical and public health concern. There is little doubt that the ancestral home of the species is Africa. This mosquito invaded the New World 400-500 years ago and later, Asia. However, little is known about the genetic structure and history of Ae. aegypti across Africa, as well as the possible origin(s) of the New World invasion. Here, we use ~17,000 genome-wide single nucleotide polymorphisms (SNPs) to characterize a heretofore undocumented complex picture of this mosquito across its ancestral range in Africa. We find signatures of human-assisted migrations, connectivity across long distances in sylvan populations, and of local admixture between domestic and sylvan populations. Finally, through a phylogenetic analysis combined with the genetic structure analyses, we suggest West Africa and especially Angola as the source of the New World's invasion, a scenario that fits well with the historic record of 16th-century slave trade between Africa and Americas.