Population structure of the mosquito Aedes aegypti (Stegomyia aegypti) in Pakistan

Eleven microsatellite markers were used to determine the genetic population structure and spread of Aedes aegypti (Stegomyia aegypti) (Diptera: Culicidae) in Pakistan using mosquitoes collected from 13 different cities. There is a single genetic cluster of Ae. aegypti in Pakistan with a pattern of isolation by distance within the population. The low level of isolation by distance suggests the long‐range passive dispersal of this mosquito, which may be facilitated by the tyre trade in Pakistan. A decrease in genetic diversity from south to north suggests a recent spread of this mosquito from Karachi. A strong negative correlation between genetic distance and the quality of road connections shows that populations in cities connected by better road networks are less differentiated, which suggests the human‐aided passive dispersal of Ae. aegypti in Pakistan. Dispersal on a large spatial scale may facilitate the strategy of introducing transgenic Ae. aegypti or intracellular bacteria such as Wolbachia to control the spread of dengue disease in Pakistan, but it also emphasizes the need for simple measures to control container breeding sites.     

Population genetic structure of the dengue mosquito Aedes aegypti in Venezuela

The mosquito Aedes aegypti is the main vector of dengue in Venezuela. The genetic structure of this vector was investigated in 24 samples collected from eight geographic regions separated by up to 1160 km. We examined the distribution of a 359-basepair region of the NADH dehydrogenase subunit 4 mitochondrial gene among 1144 Ae. aegypti from eight collections. This gene was amplified by the polymerase chain reaction and tested for variation using single strand conformation polymorphism analysis. Seven haplotypes were detected throughout Venezuela and these were sorted into two clades. Significant differentiation was detected among collections and these were genetically isolated by distance.     

Dynamics and Characterization of Aedes aegypti (L.) (Diptera: Culicidae) Key Breeding Sites

The present study aimed to analyze the dynamics of containers used as breeding sites by Aedes aegypti (L.) in the city of Aracaju, SE, one of the Northeast Brazilian states. A total of three entomological surveys were performed during different precipitation levels. Breeding sites were categorized according to their function into storage, disposable containers, and reusable containers. “Mean number of pupae” and “frequency of each type of breeding site” were the criteria considered to identify key breeding sites. House index and Breteau index were calculated in each survey. A total of 3,647 water reservoirs were found, of which 220 were breeding sites, where 22,880 immature forms were identified. There were no differences in the mean number of larvae of several types of breeding sites and in the number of larvae among surveys. Larval indices showed a reduction in the second visit, but with no effect on adult occurrence when the number of pupae was considered. Key breeding sites resulted from containers used for water storage. The area studied showed conditions favorable to a new epidemic of dengue fever.     

Dynamics of vitellogenin uptake in Aedes aegypti as demonstrated by trypan blue

Trypan blue has been shown to be a reliable indicator of the micropinocytotic uptake of vitellogenin by developing oöcytes. Trypan blue was injected into the mosquito Aedes aegypti to determine at what times after the blood meal vitellogenin was taken up. Histological sections examined by light microscopy showed that trypan blue began to be sequestered from 2 to 5 hr after the blood meal. Any association between dye and ovariole ended from 39 to 42 hr after the blood meal, in which period no dye was incorporated into spheres of yolk protein. Of the times investigated in this experiment, the greatest amount of dye was seen in the oöcyte at 24 hr after the blood meal. The onset and conclusion of trypan blue uptake correspond with the related events in the synthesis of vitellogenin by the fat body. The presence of trypan blue in occasional interfollicular spaces suggests that the route of entry of vitellogenin in Aedes aegypti is indeed an interfollicular one.     

Temporal genetic variation in Aedes aegypti populations in Ho Chi Minh City (Vietnam)

Aedes aegypti, the main vector of dengue viruses in Asia, displays variation in population density over time. The larval habitats of this species being unevenly distributed and transient (depending on cycles of drought and flood), the forces generating temporal variation in gene frequencies in populations are studied. We sampled seven mosquito populations from Ho Chi Minh City (Vietnam) and its suburbs on five occasions between April 1999 and August 2000. We investigated genetic variation by studying isoenzyme and microsatellite polymorphism and susceptibility to a dengue 2 virus strain. Ae. aegypti populations collected during the dry season (January-April) showed genetic differentiation (F(ST) = 0.016, P < 10(-6) for isoenzymes) and showed more differentiated infection rates of the dengue 2 virus. The genetic structure of the population is less marked during the rainy season (F(ST) = 0.081, P < 10(-6)). Thus, environmental factors, such as rainfall and factors related to human activity, such as breeding site density and insecticide treatment, control the genetic structure of Ae. aegypti populations in the short term. The implications of studies of this kind for the design of future control programmes are discussed.     

Household and climate factors influence Aedes aegypti presence in the arid city of Huaquillas,Ecuador

Arboviruses transmitted by Aedes aegypti (e.g., dengue, chikungunya, Zika) are of major public health concern on the arid coastal border of Ecuador and Peru. This high transit border is a critical disease surveillance site due to human movement-associated risk of transmission. Local level studies are thus integral to capturing the dynamics and distribution of vector populations and social-ecological drivers of risk, to inform targeted public health interventions. Our study examines factors associated with household-level Ae. aegypti presence in Huaquillas, Ecuador, while accounting for spatial and temporal effects. From January to May of 2017, adult mosquitoes were collected from a cohort of households (n = 63) in clusters (n = 10), across the city of Huaquillas, using aspirator backpacks. Household surveys describing housing conditions, demographics, economics, travel, disease prevention, and city services were conducted by local enumerators. This study was conducted during the normal arbovirus transmission season (January—May), but during an exceptionally dry year. Household level Ae. aegypti presence peaked in February, and counts were highest in weeks with high temperatures and a week after increased rainfall. Univariate analyses with proportional odds logistic regression were used to explore household social-ecological variables and female Ae. aegypti presence. We found that homes were more likely to have Ae. aegypti when households had interruptions in piped water service. Ae. aegypti presence was less likely in households with septic systems. Based on our findings, infrastructure access and seasonal climate are important considerations for vector control in this city, and even in dry years, the arid environment of Huaquillas supports Ae. aegypti breeding habitat.     

Skeeter Buster: A Stochastic, Spatially Explicit Modeling Tool for Studying Aedes aegypti Population Replacement and Population Suppression Strategies

Background

Dengue is the most important mosquito-borne viral disease affecting humans. The only prevention measure currently available is the control of its vectors, primarily Aedes aegypti. Recent advances in genetic engineering have opened the possibility for a new range of control strategies based on genetically modified mosquitoes. Assessing the potential efficacy of genetic (and conventional) strategies requires the availability of modeling tools that accurately describe the dynamics and genetics of Ae. aegypti populations.

Methodology/Principal findings

We describe in this paper a new modeling tool of Ae. aegypti population dynamics and genetics named Skeeter Buster. This model operates at the scale of individual water-filled containers for immature stages and individual properties (houses) for adults. The biology of cohorts of mosquitoes is modeled based on the algorithms used in the non-spatial Container Inhabiting Mosquitoes Simulation Model (CIMSiM). Additional features incorporated into Skeeter Buster include stochasticity, spatial structure and detailed population genetics. We observe that the stochastic modeling of individual containers in Skeeter Buster is associated with a strongly reduced temporal variation in stage-specific population densities. We show that heterogeneity in container composition of individual properties has a major impact on spatial heterogeneity in population density between properties. We detail how adult dispersal reduces this spatial heterogeneity. Finally, we present the predicted genetic structure of the population by calculating F_ST values and isolation by distance patterns, and examine the effects of adult dispersal and container movement between properties.

Conclusions/Significance

We demonstrate that the incorporated stochasticity and level of spatial detail have major impacts on the simulated population dynamics, which could potentially impact predictions in terms of control measures. The capacity to describe population genetics confers the ability to model the outcome of genetic control methods. Skeeter Buster is therefore an important tool to model Ae. aegypti populations and the outcome of vector control measures.     

Water channel activity of putative aquaporin‐6 present in Aedes aegypti

Aquaporins (AQPs) are integral membrane channels that facilitate the bidirectional transport of water and sometimes other small solutes across biological membranes. AQPs are important in mediating environmental adaptations in mosquitoes and are considered as a novel target for the development of effective insecticides against mosquitoes. Here, we expressed Aedes aegypti AQP6 ( AaAQP6) in human embryonic kidney (HEK) 293 cells and analyzed the water permeability by a conventional swelling assay, that is, a real‐time change in cell size corresponding to the cell swelling induced by hyposmotic solution. The swelling assay revealed that AaAQP6 is a mercury‐sensitive water channel. Gene expression studies showed that AaAQP6 is highly expressed in the pupae than other developmental stages. Heterologous expression of AaAQP6 in HEK cell was mainly observed intracellularly suggesting AaAQP6 possibly could be a subcellular water channel and may play an osmoregulatory function in the pupae of A. aegypti.     

A study on the fecundity of male Aedes aegypti

After inseminating about 5 females, the males of Aedes aegypti are sexually depleted and most do not then re-inseminate additional females for the rest of their lives. Sexually depleted males generally replenish spermatozoa in their seminal vesicles and renew the secretion in their accessory glands, i.e. they again become fecund. Many replenished males do not chase or attempt to copulate with virgin females. Those males which do copulate do not ejaculate. Thus, celibate behaviour and failure to ejaculate rather than lack of fecundity are responsible for the inability of sexually depleted males to impregnate females.     

Hemocyte-mediated melanization of microfilariae in Aedes aegypti

The melanization response of adult female Aedes aegypti (black-eyed Liverpool strain) against intrathoracically inoculated Dirofilaria immitis microfilariae (mff) was assessed with transmission electron microscopy. The initial reaction involved the lysis of hemocytes at or near the surface of the parasite prior to the deposition of pigment. Subsequently, melanin formation was noted in the area of lysed cells and appeared to cascade onto the parasite surface. Observations suggest that melanin may be synthesized within certain hemocytes and released by exocytosis or upon cell lysis. Intact and disrupted nuclei and cytoplasmic elements from lysed hemocytes became numerous as mff became completely coated with melanin. A double membrane-like structure formed around the melanized mff and cellular debris during the later stages of the reaction, which eventually isolated the melanin capsule from hemolymph components. Results obtained are discussed in relation to the melanization response previously described for Aedes trivittatus.     

Genetics of an esterase in Aedes aegypti

Zymograms of single individuals of Aedes aegypti were obtained by means of starch gel electrophoresis, using alpha-naphthyl acetate as substrate. Inbred lines gave consistently homogeneous patterns; earlier results from random-breeding laboratory strains had shown considerable variability. Six distinct bands were observed. The furthest moving band, designated Esterase 6, showed differential migration in two inbred lines. Reciprocal crosses between these lines gave F₁ progeny showing both bands. Backcrosses of F₁ to either parental line gave a 1:1 segregation. These results are consistent with the hypothesis that the two forms of Esterase 6 are controlled by a single pair of codominant alleles at a single gene locus (Est 6 ^a and Est 6 ^b). Linkage tests with marker genes have demonstrated that Est 6 is on linkage group 2, with the following alignment: spot-abdomen (9.0±1.0) yellow-larva (17.4±1.3) Est 6. Crosses with another inbred line demonstrated a third band with intermediate mobility, designated Est 6 ^c. An additional electrophoretic variant which seems to have a simple Mendelian basis was found in esterase band 1.This work was supported by NIH Research Grant No. A1-02753.     

Lessons of Aedes aegypti control in Thailand

Abstract. The incidence of dengue haemorrhagic fever (DHF) in Thailand has increased cyclically since the first recognized outbreak in 1958. Without an effective vaccine against dengue, and considering the clinical difficulty of treating DHF cases, vector control is needed to prevent dengue transmission. Since the establishment of the WHO Aedes Research Unit in 1964, continued since 1973 as the WHO Collaborating Centre at the Department of Medical Research in Bangkok, much operational research has been carried out in Thailand on the bionomics and control of dengue vectors: Aedes aegypti and Ae.albopictus. This review shows that, as in most other countries, dengue vector control programmes in Thailand make little use of the procedures arising from research, nor have they reduced the upward trend of dengue or prevented DHF outbreaks. Implications of the reluctance to use results of operational research on vector control are considered and remedial suggestions made.     

Modeling Dengue Vector Dynamics under Imperfect Detection: Three Years of Site-Occupancy by Aedes aegypti and Aedes albopictus in Urban Amazonia

Aedes aegypti and Ae. albopictus are the vectors of dengue, the most important arboviral disease of humans. To date, Aedes ecology studies have assumed that the vectors are truly absent from sites where they are not detected; since no perfect detection method exists, this assumption is questionable. Imperfect detection may bias estimates of key vector surveillance/control parameters, including site-occupancy (infestation) rates and control intervention effects. We used a modeling approach that explicitly accounts for imperfect detection and a 38-month, 55-site detection/non-detection dataset to quantify the effects of municipality/state control interventions on Aedes site-occupancy dynamics, considering meteorological and dwelling-level covariates. Ae. aegypti site-occupancy estimates (mean 0.91; range 0.79–0.97) were much higher than reported by routine surveillance based on ‘rapid larval surveys’ (0.03; 0.02–0.11) and moderately higher than directly ascertained with oviposition traps (0.68; 0.50–0.91). Regular control campaigns based on breeding-site elimination had no measurable effects on the probabilities of dwelling infestation by dengue vectors. Site-occupancy fluctuated seasonally, mainly due to the negative effects of high maximum (Ae. aegypti) and minimum (Ae. albopictus) summer temperatures (June-September). Rainfall and dwelling-level covariates were poor predictors of occupancy. The marked contrast between our estimates of adult vector presence and the results from ‘rapid larval surveys’ suggests, together with the lack of effect of local control campaigns on infestation, that many Aedes breeding sites were overlooked by vector control agents in our study setting. Better sampling strategies are urgently needed, particularly for the reliable assessment of infestation rates in the context of control program management. The approach we present here, combining oviposition traps and site-occupancy models, could greatly contribute to that crucial aim.     

A new sex-linked mutant short wing in Aedes aegypti.

A spontaneous recessive sex-linked mutant short wing has been discovered in the mosquito Aedes aegypti. It is situated less than one cross-over unit from the sex determining locus. In homozygous females, flight is impaired and the survival and fecundity is markedly subnormal. Two possible uses of this gene for genetic control operations are envisaged: (a) to provide automatic sexing of males for release and (b) enhancement of the population control potential of other available genetics systems.