How the yellow fever mosquito found its first human victim

Aedes aegypti developed a taste for humans at the birth of the Sahara, a new analysis suggests.

Influence of landscape composition and configuration on the richness and abundance of potential sylvatic yellow fever vectors in a remnant of Atlantic Forest in the city of São Paulo, Brazil

The landscape's structure can play a relevant role in epidemic patterns of arboviruses, influencing factors such as abundance, movement, and dispersal ability in arthropod vectors and vertebrate hosts, besides promoting alterations in the rate of potential infectious contacts between these organisms. In the Americas, yellow fever (YF) exhibits only the sylvatic cycle, in which the virus circulates in sylvatic areas among non-human primates, being transmitted by mosquitoes of the Haemagogus and Sabethes genera. In this study, we investigate some aspects of the landscape in relation to diversity and abundance of culicid species associated with YF transmission. Studies were performed in the Cantareira State Park, a remnant of the Atlantic Forest located in Greater Metropolitan São Paulo, Brazil, where the YF virus circulated recently with dozens of deaths in howler monkeys (Alouatta guariba), in addition to reported human cases. Mosquito collections were carried out monthly from February 2015 to April 2017. Mosquitoes were collected from three sites using battery-powered aspirator (12-volt battery), CDC, and Shannon traps for adults, and suction samplers and entomological spoons in breeding sites to collect immature forms. 703 mosquitoes belonging to 12 species of the Aedini and Sabethini tribes were collected. Aedes scapularis and Psorophora ferox exhibited higher abundance, while Haemagogus leucocelaenus, the main vector of YF in São Paulo state, showed lower abundance in all sampled areas. The site with longer edge between forest area and anthropic area presented more richness and abundance of YF vector species, while the site with larger forest cover area and shorter edges between forest and anthropic areas exhibited an inverse pattern. Statistically significant differences were observed between the composition of potential YF vector species among the investigated sites. Although Hg. leucocelaenus occurred in all sampled sites, the different patterns of distribution and abundance of other mosquitoes such as Aedes scapularis and Psorophora ferox suggest that these species may be involved in the transmission of sylvatic YF in the study area.

Complete NMR chemical shift assignments of odorant binding protein 22 from the yellow fever mosquito, Aedes aegypti, bound to arachidonic acid

Aedes aegypti mosquitoes are the vector for transmission of Dengue, Zika and chikungunya viruses. These mosquitos feed exclusively on human hosts for a blood meal. Previous studies have established that Dengue virus infection of the mosquito results in increased expression of the odorant binding proteins 22 and 10 within the mosquito salivary gland and silencing of these genes dramatically reduces blood-feeding behaviors. Odorant binding proteins are implicated in modulating the chemosensory perception of external stimuli that regulate behaviors such as host location, feeding and reproduction. However, the role that AeOBP22 plays in the salivary gland is unclear. Here, as a first step to a more complete understanding of the function of AeOBP22, we present the complete backbone and side chain chemical shift assignments of the protein in the complex it forms with arachidonic acid. These assignments reveal that the protein consists of seven α-helices, and that the arachidonic acid is bound tightly to the protein. Comparison with the chemical shift assignments of the apo-form of the protein reveals that binding of the fatty acid is accompanied by a large conformational change in the C-terminal helix, which appears disordered in the absence of lipid. This NMR data provides the basis for determining the structure of AeOBP22 and understanding the nature of the conformational changes that occur upon ligand binding. This information will provide a path to discover novel compounds that can interfere with AeOBP22 function and impact blood feeding by this mosquito.

Feeding-induced changes in allatostatin-A and short neuropeptide F in the antennal lobes affect odor-mediated host seeking in the yellow fever mosquito, Aedes aegypti

Aedes aegypti is a model species in which the endogenous regulation of odor-mediated host seeking behavior has received some attention. Sugar feeding and host seeking in female A. aegypti are transiently inhibited following a blood meal. This inhibition is partially mediated by short neuropeptide F (sNPF). The paired antennal lobes (ALs), as the first processing centers for olfactory information, has been shown to play a significant role in the neuropeptidergic regulation of odor-mediated behaviors in insects. The expression of sNPF, along with other peptides in the ALs of A. aegypti, indicate parallel neuromodulatory systems that may affect olfactory processing. To identify neuropeptides involved in regulating the odor-mediated host seeking behavior in A. aegypti, we use a semi-quantitative neuropeptidomic analysis of single ALs to analyze changes in the levels of five individual neuropeptides in response to different feeding regimes. Our results show that the level of sNPF-2, allatostatin-A-5 (AstA-5) and neuropeptide-like precursor-1-5 (NPLP-1-5), but not of tachykinin-related-peptides and SIFamide (SIFa), in the AL of female mosquitoes, changes 24 h and 48 h post-blood meal, and are dependent on prior access to sugar. To assess the role of these neuropeptides in modulating host seeking behavior, when systemically injected individually, sNPF-2 and AstA-5 significantly reduced host seeking behavior. However, only the injection of the binary mixture of the two neuropeptides lead to a host seeking inhibition similar to that observed in blood fed females. We conclude that modulation of the odor mediated host seeking behavior of A. aegypti is likely regulated by a dual neuropeptidergic pathway acting in concert in the ALs.

[A case of Yellow fever in 1887]

A young Danish sailor died from yellow fever in Barbados in 1887. The Shipmaster's letter to the family with a description of the course of the disease, which has been preserved, is presented here together with a photo of the sailor and a painting of the Danish sailing-ship.

The fat body transcriptomes of the yellow fever mosquito Aedes aegypti, pre- and post- blood meal

BACKGROUND

The fat body is the main organ of intermediary metabolism in insects and the principal source of hemolymph proteins. As part of our ongoing efforts to understand mosquito fat body physiology and to identify novel targets for insect control, we have conducted a transcriptome analysis of the fat body of Aedes aegypti before and in response to blood feeding.

RESULTS

We created two fat body non-normalized EST libraries, one from mosquito fat bodies non-blood fed (NBF) and another from mosquitoes 24 hrs post-blood meal (PBM). 454 pyrosequencing of the non-normalized libraries resulted in 204,578 useable reads from the NBF sample and 323,474 useable reads from the PBM sample. Alignment of reads to the existing reference Ae. aegypti transcript libraries for analysis of differential expression between NBF and PBM samples revealed 116,912 and 115,051 matches, respectively. De novo assembly of the reads from the NBF sample resulted in 15,456 contigs, and assembly of the reads from the PBM sample resulted in 15,010 contigs. Collectively, 123 novel transcripts were identified within these contigs. Prominently expressed transcripts in the NBF fat body library were represented by transcripts encoding ribosomal proteins. Thirty-five point four percent of all reads in the PBM library were represented by transcripts that encode yolk proteins. The most highly expressed were transcripts encoding members of the cathepsin b, vitellogenin, vitellogenic carboxypeptidase, and vitelline membrane protein families.

CONCLUSION

The two fat body transcriptomes were considerably different from each other in terms of transcript expression in terms of abundances of transcripts and genes expressed. They reflect the physiological shift of the pre-feeding fat body from a resting state to vitellogenic gene expression after feeding.

Functional characterization of the octenol receptor neuron on the maxillary palps of the yellow fever mosquito, Aedes aegypti

BACKGROUND

1-Octen-3-ol (octenol) is a common attractant released by vertebrates which in combination with carbon dioxide (CO(2)) attracts hematophagous arthropods including mosquitoes. A receptor neuron contained within basiconic sensilla on the maxillary palps of adult mosquitoes responds selectively to 1-octen-3-ol. Recently, an odorant receptor (AaegOR8) known to occur on the maxillary palps was expressed in a heterologous system and demonstrated to be selectively sensitive to (R)-(-)-1-octen-3-ol, one of two enantiomeric forms. Lesser responses were elicited by stimulation with the (S)-enantiomer and various structural analogs.

METHODOLOGY/PRINCIPAL FINDINGS

Here we characterize the specificity of the octenol receptor neuron in the yellow fever mosquito, Aedes aegypti (L.), in vivo using single cell recordings. The octenol neuron is exquisitely sensitive to (R)-(-)-1-octen-3-ol; comparable responses to (S)-(+)-1-octen-3-ol were elicited only at stimulus doses over 100× that required for the (R)-enantiomer. An intermediate response closer to that elicited by the (R)-(-)-enantiomer was elicited by racemic 1-octen-3-ol. Small structural changes in (R)-(-)-1-octen-3-ol resulted in large decreases in responses. Increases in spike activity were also elicited in the octenol neuron by 2-undecanone, a known repellent; other repellents (DEET, IR3535 and picaridin) were inactive.

CONCLUSIONS/SIGNIFICANCE

The results of our electrophysiological studies of the octenol receptor neuron in vivo approximates results of a previous study of the octenol receptor (AaegOR8 with its obligate partner Aaeg\ORco) expressed heterologously in Xenopus oocytes. By comparison of our current results with those of the heterologous expression study, we conclude that specificity of the octenol receptor neuron can be explained largely by characteristics of the OR alone without other associated proteins present in vivo. Our findings show that repellents may have specific stimulatory effects on receptor neurons and support the notion of repellents as modulators of mosquito odorant receptor activity.

In vivo functional genomic studies of sterol carrier protein-2 gene in the yellow fever mosquito

A simple and efficient DNA delivery method to introduce extrachromosomal DNA into mosquito embryos would significantly aid functional genomic studies. The conventional method for delivery of DNA into insects is to inject the DNA directly into the embryos. Taking advantage of the unique aspects of mosquito reproductive physiology during vitellogenesis and an in vivo transfection reagent that mediates DNA uptake in cells via endocytosis, we have developed a new method to introduce DNA into mosquito embryos vertically via microinjection of DNA vectors in vitellogenic females without directly manipulating the embryos. Our method was able to introduce inducible gene expression vectors transiently into F0 mosquitoes to perform functional studies in vivo without transgenic lines. The high efficiency of expression knockdown was reproducible with more than 70% of the F0 individuals showed sufficient gene expression suppression (<30% of the controls' levels). At the cohort level, AeSCP-2 expression knockdown in early instar larvae resulted in detectable phenotypes of the expression deficiency such as high mortality, lowered fertility, and distorted sex ratio after induction of AeSCP-2 siRNA expression in vivo. The results further confirmed the important role of AeSCP-2 in the development and reproduction of A. aegypti. In this study, we proved that extrachromosomal transient expression of an inducible gene from a DNA vector vertically delivered via vitellogenic females can be used to manipulate gene expression in F0 generation. This new method will be a simple and efficient tool for in vivo functional genomic studies in mosquitoes.

Aedes FADD: a novel death domain-containing protein required for antibacterial immunity in the yellow fever mosquito, Aedes aegypti

Microbial infections in insects activate a series of immune responses that culminate in the production of antimicrobial peptides (AMPs). In Drosophila, two signaling pathways, govern the challenge-dependent expression of AMPs; the Toll and IMD pathways. While AMPs have been the subject of much research in mosquitoes, the regulation of the pathways required for AMP expression remains largely unknown. We report here the identification of Aedes FADD (AeFADD), a death domain protein in Aedes aegypti. AeFadd is expressed in all immune-competent tissues and all developmental stages examined. At the transcriptional level, AeFadd transcripts increased when challenged with Escherichia coli but not Micrococcus luteus. In both cases, we observed the induction of two AMP genes; cecropin and defensin. Loss of AeFadd function by dsRNA interference impaired the inducible expression of both AMPs, and rendered adult mosquitoes susceptible to both types of bacteria. Identifying molecules that regulate mosquito immunity may help elucidate the factors that contribute to the vectorial capacity and provide insights into general mechanisms that regulate innate immunity.

Regulation of the ribonucleotide reductase small subunit (R2) in the yellow fever mosquito, Aedes aegypti

Ribonucleotide reductase (RNR) catalyzes the formation of deoxyribonucleotides, a rate limiting step in DNA synthesis. Class I RNR is a tetramer that consists of two subunits, R1 and R2; enzymatic activity requires association of R1 with R2. The R2 subunit is of special interest because it dictates the interaction with R1 that is required for enzymatic activity expression, and it is expressed only during the S phase of the cell cycle. We previously sequenced an R2 cDNA clone from the yellow fever mosquito, Aedes aegypti. We found the message was upregulated by blood feeding. We now report the sequence of an R2 genomic clone. The gene consists of 4 introns and 5 exons. Both major and minor transcriptional start sites have been identified, and their use differs in sugar-fed versus blood-fed females. The gene contains putative cis-regulatory sites for E2F, Caudal (Cdx) and Dearolf (Dfd). The mosquito R2 gene contains iron-specific regulatory elements immediately upstream of the minimal promoter region. Binding of a factor to the distal putative Cdx site in the -400 region is altered by iron treatment of cells. Further, following blood feeding, R2 message is significantly induced in mosquito ovaries (tissues that are involved in oogenesis--a process requiring DNA synthesis).

Entomological investigation of a sylvatic yellow fever area in São Paulo State, Brazil

Following reports of two autochthonous cases of sylvatic yellow fever in the State of São Paulo, Brazil, in 2000, entomological surveys were conducted with the objective of verifying the occurrence of vector species in forest environments close to or associated with riparian areas located in the western and northwestern regions of the State. Culicidae were captured in 39 sites distributed in four regions. Haemagogus leucocelaenus and Aedes albopictus were the most abundant species and were captured in all the regions studied. H. leucocelaenus was the most abundant species in the municipalities of Santa Albertina and Ouroeste, where the two cases of sylvatic yellow fever had been reported. Mosquitoes from the janthinomys/capricornii group were only found at eight sites in the São José do Rio Preto region, while Sabethes chloropterus was found at one site in Ribeirão Preto. H. leucocelaenus showed its capacity to adapt to a secondary and degraded environment. Our results indicate a wide receptive area for yellow fever transmission in the State of São Paulo, with particular emphasis on the possibility of H. leucocelaenus being involved in the maintenance of this sylvatic focus of the disease.

Vector competence of Brazilian Aedes aegypti and Ae. albopictus for a Brazilian yellow fever virus isolate

Because the potential urban yellow fever (YF) mosquito vectors Aedes aegypti and Ae. albopictus are at historical highs in Brazil, both in terms of density and geographical range, we assessed the risk of an urban YF epidemic in Brazil. We evaluated and confirmed in a laboratory setting the vector competence of Brazilian Ae. aegypti for a currently circulating strain of YF virus, and investigated the potential for Brazilian Ae. albopictus to transmit YF.

[Conditions for the laboratory survival of Haemagogus janthinomys dyar, 1921 (Diptera:Culicidae)]

A little modification was made on the classical Borel tube, used for rearing isolated females of mosquitoes. The first studies were realized with the main Yellow fever vector, Haemagogus janthinomys, in Brazil. The results are: a better survival, as far as 72 days, a greater number of eggs, up to 80, and a shorter trophogonic cycle of 7-8 days. So, one can imagine more easily further studies about the vertical transmission of the YF virus by this important neo-tropical mosquito.