Susceptibility of Anopheles quadriannulatus Theobald (Diptera: Culicidae) to Plasmodium falciparum

Comparison of the effect of bacterial stimulation on the global epigenetic landscape and transcription of immune genes in primarily zoophilic members of the Anopheles gambiae complex (Diptera: Culicidae)

Members of the Anopheles gambiae complex vary in their vector competence, and this is often attributed to behavioural differences. Similarly, there are differences in transmission capabilities of the zoophilic members of this complex despite exhibiting similar behaviours. Therefore, behavioural differences alone cannot fully explain vector competence variation within members of the An. gambiae complex. The immune system of mosquitoes plays a key role in determining susceptibility to parasite infection and consequently transmission capacity. This study aimed to examine variations in the immune response of An. arabiensis, An. merus and An. quadriannulatus, a major, minor, and non-vector respectively. The global epigenetic landscape was characterised and the expression of Defensin-1 and Gambicin was assessed in response to Gram-positive (Streptococcus pyogenes) and Gram-negative (Escherichia coli) bacterial infections. The effect of insecticide resistance in An. arabiensis on these aspects was also assessed. The immune system was stimulated by a blood-borne bacterial supplementation. The 5mC, 5hmC, m6A methylation levels and Histone Acetyl Transferase activity were assessed with commercial ELISA kits. The transcript levels of Defensin-1 and Gambicin were assessed by quantitative Real-Time Polymerase Chain Reaction. Species-specific differences in 5mC and m6A methylation existed both constitutively as well as post immune stimulation. The epigenetic patterns observed in the laboratory strains were largely conserved in F1 offspring of wild-caught adults. The methylation patterns in the major vector typically differed from that of the minor/non-vectors. The differences between insecticide susceptible and resistant An. arabiensis were more reflected in the expression of Defensin-1 and Gambicin. The expression of these peptides differed in the strains only after bacterial stimulation. Anopheles merus and An. quadriannulatus expressed significantly higher levels of antimicrobial peptides, both constitutively and after immune stimulation. These findings suggest molecular variations in the immune response of members of the An. gambiae complex.

Species Composition, Seasonal Abundance, and Biting Behavior of Malaria Vectors in Rural Conhane Village, Southern Mozambique

Malaria vector surveillance provides important data to inform the effective planning of vector control interventions at a local level. The aim of this study was to determine the species diversity and abundance, biting activity, and Plasmodium infectivity of Anopheles mosquitoes from a rural village in southern Mozambique. Human landing catches were performed monthly between December 2020 and August 2021. All collected Anopheles were identified to the species level and tested for the presence of malaria parasites. Eight Anopheles species were identified among the 1802 collected anophelines. Anopheles gambiae sensu lato (s.l.) were the most abundant (51.9%) and were represented by Anopheles quadriannulatus and Anopheles arabiensis. Anopheles funestus s.l. represented 4.5%. The biting activity of An. arabiensis was more pronounced early in the evening and outdoors, whereas that of An. funestus sensu stricto (s.s.) was more intense late in the night, with no significant differences in location. One An. funestus s.s. and one An. arabiensis, both collected outdoors, were infected with Plasmodium falciparum. The overall entomologic inoculation rate was estimated at 0.015 infective bites per person per night. The significant outdoor and early evening biting activity of An. arabiensis and An. funestus found in this village may negatively impact the effectiveness of current vector control interventions. Additional vector control tools that can target these mosquitoes are needed.

The mosquito vectors that sustained malaria transmission during the Magude project despite the combined deployment of indoor residual spraying, insecticide-treated nets and mass-drug administration

The "Magude project" aimed but failed to interrupt local malaria transmission in Magude district, southern Mozambique, by using a comprehensive package of interventions, including indoor residual spraying (IRS), pyrethroid-only long-lasting insecticide treated nets (LLINs) and mass-drug administration (MDA). Here we present detailed information on the vector species that sustained malaria transmission, their association with malaria incidence and behaviors, and their amenability to the implemented control interventions. Mosquitoes were collected monthly between May 2015 and October 2017 in six sentinel sites in Magude district, using CDC light traps both indoors and outdoors. Anopheles arabiensis was the main vector during the project, while An. funestus s.s., An. merus, An. parensis and An. squamosus likely played a secondary role. The latter two species have never previously been found positive for Plasmodium falciparum in southern Mozambique. The intervention package successfully reduced vector sporozoite rates in all species throughout the project. IRS was effective in controlling An. funestus s.s. and An. parensis, which virtually disappeared after its first implementation, but less effective at controlling An. arabiensis. Despite suboptimal use, LLINs likely provided significant protection against An. arabiensis and An. merus that sought their host largely indoors when people where in bed. Adding IRS on top of LLINs and MDA likely added value to the control of malaria vectors during the Magude project. Future malaria elimination attempts in the area could benefit from i) increasing the use of LLINs, ii) using longer-lasting IRS products to counteract the increase in vector densities observed towards the end of the high transmission season, and iii) a higher coverage with MDA to reduce the likelihood of human infection. However, additional interventions targeting vectors that survive IRS and LLINs by biting outdoors or indoors before people go to bed, will be likely needed to achieve local malaria elimination.

The dynamic gut microbiota of zoophilic members of the Anopheles gambiae complex (Diptera: Culicidae)

The gut microbiota of mosquitoes plays a critical role in the life history of the animal. There is a growing body of research characterising the gut microbiota of a range of mosquito species, but there is still a paucity of information on some members of the Anopheles gambiae complex. In this study, the gut microbiota of four laboratory strains were characterised. SENN (Anopheles arabiensis-insecticide susceptible major vector), SENN DDT (Anopheles arabiensis-insecticide resistant major vector), MAFUS (Anopheles merus-minor vector) and SANGWE (Anopheles quadriannulatus-non-vector) were used in this study. The microbiota of fourth instar larvae, 3-day old, 15-day old non-blood fed and 15-day old blood fed females were characterised by MALDI-TOF mass spectroscopy and 16 s rRNA gene sequencing by next generation sequencing. The four strains differed in species richness but not diversity. The major vectors differ in β-diversity from that of the minor and non-vectors. There was no difference in α- or β-diversity in 15 non-blood fed females and 15-day old females that had 3 blood meals before day 15. These differences may be related to a mixture of the effect of insecticide resistance phenotype as well as a potential relationship to vector competence to a limited extent. Bacterial diversity is affected by species and age. There is also a potential relationship between the differences in gut microbiota and capacity to transmit parasites. This genetic background of the mosquitoes, however, play a major role, and must be considered in this relationship.

Beyond taxonomy: species complexes in New World phlebotomine sand flies

A species complex (= species group, species series) is an assemblage of species, which are related morphologically and phylogenetically. Recent research has revealed several arthropod vector species that were believed to be a single nominal species actually representing a group of closely related species, which are sometimes morphologically indistinguishable at one or more developmental stages. In some instances, differences in terms of vector competence, capacity, or both have been recorded. It highlights the importance of detecting and studying species complexes to improve our understanding of pathogen transmission patterns, which may be vectored more or less efficiently by different species within the complex. Considering more than 540 species, about one-third of the phlebotomine sand flies in the New World present males and/or females morphologically indistinguishable to one or more species. Remarkably, several of these species may act in transmission of pathogenic agents. In this article, we review recent research on species complexes in phlebotomine sand flies from the Americas. Possible practical implications of recently acquired knowledge and future research needs are also discussed.

Implications of increasing temperature stress for predatory biocontrol of vector mosquitoes

BACKGROUND

Predators play a critical role in regulating larval mosquito prey populations in aquatic habitats. Understanding predator-prey responses to climate change-induced environmental perturbations may foster optimal efficacy in vector reduction. However, organisms may differentially respond to heterogeneous thermal environments, potentially destabilizing predator-prey trophic systems.

METHODS

Here, we explored the critical thermal limits of activity (CTLs; critical thermal-maxima [CTmax] and minima [CTmin]) of key predator-prey species. We concurrently examined CTL asynchrony of two notonectid predators (Anisops sardea and Enithares chinai) and one copepod predator (Lovenula falcifera) as well as larvae of three vector mosquito species, Aedes aegypti, Anopheles quadriannulatus and Culex pipiens, across instar stages (early, 1st; intermediate, 2nd/3rd; late, 4th).

RESULTS

Overall, predators and prey differed significantly in CTmax and CTmin. Predators generally had lower CTLs than mosquito prey, dependent on prey instar stage and species, with first instars having the lowest CTmax (lowest warm tolerance), but also the lowest CTmin (highest cold tolerance). For predators, L. falcifera exhibited the narrowest CTLs overall, with E. chinai having the widest and A. sardea intermediate CTLs, respectively. Among prey species, the global invader Ae. aegypti consistently exhibited the highest CTmax, whilst differences among CTmin were inconsistent among prey species according to instar stage.

CONCLUSION

These results point to significant predator-prey mismatches under environmental change, potentially adversely affecting natural mosquito biocontrol given projected shifts in temperature fluctuations in the study region. The overall narrower thermal breadth of native predators relative to larval mosquito prey may reduce natural biotic resistance to pests and harmful mosquito species, with implications for population success and potentially vector capacity under global change.

Malaria Vectors and Vector Surveillance in Limpopo Province (South Africa): 1927 to 2018

Despite the annual implementation of a robust and extensive indoor residual spraying programme against malaria vectors in Limpopo Province (South Africa), significant transmission continues and is a serious impediment to South Africa’s malaria elimination objectives. In order to gain a better understanding regarding possible causes of this residual malaria, we conducted a literature review of the historical species composition and abundance of malaria vector mosquitoes in the Limpopo River Valley region of the Vhembe District, northern Limpopo Province, the region with the highest remaining annual malaria cases in South Africa. In addition, mosquito surveys were carried out in the same region between October 2017 and October 2018. A total of 2225 adult mosquitoes were collected using CO₂-baited tent and light traps, human landing catches and cow-baited traps. Of the 1443 Anopheles collected, 516 were members of the An. gambiae complex and 511 An. funestus group. In the malaria endemic rural areas outside the Kruger National Park, one specimen each of An. gambiae s.s. and An. funestus and only three of An. arabiensis were collected. The latter species was abundant at a remote hot spring in the neighboring Kruger National Park. Eighteen other species of Anopheles were collected. Our survey results support the historical findings that An. arabiensis, the species widely held to be the prime malaria vector in South Africa, is a rare species in the malaria endemic Limpopo River Valley. The implications of the mosquito surveys for malaria transmission, elimination and vector control in northern Limpopo Province and neighboring regions are discussed.

Gene copy number and function of the APL1 immune factor changed during Anopheles evolution

Background

The recent reference genome assembly and annotation of the Asian malaria vector Anopheles stephensi detected only one gene encoding the leucine-rich repeat immune factor APL1, while in the Anopheles gambiae and sibling Anopheles coluzzii, APL1 factors are encoded by a family of three paralogs. The phylogeny and biological function of the unique APL1 gene in An. stephensi have not yet been specifically examined.

Methods

The APL1 locus was manually annotated to confirm the computationally predicted single APL1 gene in An. stephensi. APL1 evolution within Anopheles was explored by phylogenomic analysis. The single or paralogous APL1 genes were silenced in An. stephensi and An. coluzzii, respectively, followed by mosquito survival analysis, experimental infection with Plasmodium and expression analysis.

Results

APL1 is present as a single ancestral gene in most Anopheles including An. stephensi but has expanded to three paralogs in an African lineage that includes only the Anopheles gambiae species complex and Anopheles christyi. Silencing of the unique APL1 copy in An. stephensi results in significant mosquito mortality. Elevated mortality of APL1-depleted An. stephensi is rescued by antibiotic treatment, suggesting that pathology due to bacteria is the cause of mortality, and indicating that the unique APL1 gene is essential for host survival. Successful Plasmodium development in An. stephensi depends upon APL1 activity for protection from high host mortality due to bacteria. In contrast, silencing of all three APL1 paralogs in An. coluzzii does not result in elevated mortality, either with or without Plasmodium infection. Expression of the single An. stephensi APL1 gene is regulated by both the Imd and Toll immune pathways, while the two signaling pathways regulate different APL1 paralogs in the expanded APL1 locus.

Conclusions

APL1 underwent loss and gain of functions concomitant with expansion from a single ancestral gene to three paralogs in one lineage of African Anopheles. We infer that activity of the unique APL1 gene promotes longevity in An. stephensi by conferring protection from or tolerance to an effect of bacterial pathology. The evolution of an expanded APL1 gene family could be a factor contributing to the exceptional levels of malaria transmission mediated by human-feeding members of the An. gambiae species complex in Africa.

Evolution of sexually-transferred steroids and mating-induced phenotypes in Anopheles mosquitoes

Human malaria, which remains a major public health problem, is transmitted by a subset of Anopheles mosquitoes belonging to only three out of eight subgenera: Anopheles, Cellia and Nyssorhynchus. Unlike almost every other insect species, males of some Anopheles species produce steroid hormones which are transferred to females during copulation to influence their reproduction. Steroids are consequently a potential target for malaria vector control. Here, we analysed the evolution of sexually-transferred steroids and their effects on female reproductive traits across Anopheles by using a set of 16 mosquito species (five Anopheles, eight Cellia, and three Nyssorhynchus), including malaria vector and non-vector species. We show that male steroid production and transfer are specific to the Cellia and therefore represent a synapomorphy of this subgenus. Furthermore, we show that mating-induced effects in females are variable across species and differences are not correlated with sexually-transferred steroids or with Anopheles ability to transmit human malaria. Overall, our findings highlight that Anopheles mosquitoes have evolved different reproductive strategies, independently of being a malaria vector or not.

Chemosensory gene expression in olfactory organs of the anthropophilic Anopheles coluzzii and zoophilic Anopheles quadriannulatus

BACKGROUND

Anopheles (An.) coluzzii, one of Africa's primary malaria vectors, is highly anthropophilic. This human host preference contributes greatly to its ability to transmit malaria. In contrast, the closely related An. quadriannulatus prefers to feed on bovids and is not thought to contribute to malaria transmission. The diverged preference for host odor profiles between these sibling species is likely reflected in chemosensory gene expression levels in the olfactory organs. Therefore, we compared the transcriptomes of the antennae and maxillary palps between An. coluzzii and An. quadriannulatus, focusing on the major chemosensory gene families.

RESULTS

While chemosensory gene expression is strongly correlated between the two species, various chemosensory genes show significantly enhanced expression in one of the species. In the antennae of An. coluzzii the expression of six olfactory receptors (Ors) and seven ionotropic receptors (Irs) is considerably enhanced, whereas 11 Ors and 3 Irs are upregulated in An. quadriannulatus. In the maxillary palps, leaving aside Irs with very low level of expression, one Ir is strongly enhanced in each species. In addition, we find divergence in odorant binding protein (Obp) gene expression, with several highly expressed Obps being enhanced in the antennae and palps of An. coluzzii. Finally, the expression of several gustatory receptors (Grs) in the palps appears to be species-specific, including a homolog of a sugar-sensing Drosophila Gr.

CONCLUSIONS

A considerable number of Ors and Irs are differentially expressed between these two closely related species with diverging host preference. These chemosensory genes could play a role in the human host preference of the malaria vector An. coluzzii. Additionally, divergence in Obp expression between the two species suggests a possible role of these odor carrier proteins in determining host preference. Finally, divergence in chemosensory expression in the palps may point towards a possible role for the maxillary palps in host differentiation.

Profiles of soluble proteins in chemosensory organs of three members of the afro-tropical Anopheles gambiae complex

In female mosquitoes, host-seeking and preference as well as several other important behaviors are largely driven by olfaction. Species of the Afrotropical Anopheles gambiae complex display divergent host-preference that are associated with significant differences in their vectorial capacity for human malaria. Olfactory sensitivity begins with signal transduction and activation of peripheral sensory neurons that populate the antennae, maxillary palps and other appendages. We have used shotgun proteomics to characterize the profile of soluble proteins of antennae and maxillary palps of three different species: An. coluzzii, An. arabiensis and An. quadriannulatus that display remarkable differences in anthropophilic behavior. This analysis revealed interspecific differences in the abundance of several proteins that comprise cuticular components, glutathione S-transferase and odorant binding proteins, the latter of which known to be directly involved in odor recognition.

Immune resistance and tolerance strategies in malaria vector and non-vector mosquitoes

Background

The Anopheles gambiae complex consists of species that vary greatly in their capacity to transmit malaria. The mosquito immune system has been identified as a key factor that can influence whether Plasmodium infection establishes within the mosquito vector. This study was designed to investigate the immune responses of An. coluzzii, An. arabiensis and An. quadriannulatus mosquitoes. The first two mosquito species are major vectors of malaria in sub-Saharan Africa, while the third is thought to be a non-vector.

Methods

All three mosquito species were reared in mixed cultures. Their capacity to eliminate P. berghei and regulate midgut bacteria was examined.

Results

Our results revealed large differences in mosquito resistance to P. berghei. In all three mosquito species, immune reactions involving the complement system were triggered when the number of parasites that mosquitoes were challenged with exceeded a certain level, i.e. immune tolerance threshold. This threshold was markedly lower in An. quadriannulatus compared to An. coluzzii and An. arabiensis. We also demonstrated that the level of immune tolerance to P. berghei infection in the haemolymph is inversely correlated with the level of immune tolerance to microbiota observed in the midgut lumen after a blood meal. The malaria non-vector mosquito species, An. quadriannulatus was shown to have a much higher level of tolerance to microbiota in the midgut than An. coluzzii.

Conclusions

We propose a model whereby an increased tolerance to microbiota in the mosquito midgut results in lower tolerance to Plasmodium infection. In this model, malaria non-vector mosquito species are expected to have increased immune resistance in the haemocoel, possibly due to complement priming by microbiota elicitors. We propose that this strategy is employed by the malaria non-vector mosquito, An. quadriannulatus, while An. coluzzii has reduced tolerance to bacterial infection in the midgut and consequently reduced immune resistance to Plasmodium infection at the haemocoel level. An in-depth understanding of the molecular mechanisms regulating immune tolerance versus resistance in different mosquito vectors of malaria could guide the design of new vector and disease control strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-017-2109-5) contains supplementary material, which is available to authorized users.

Species-specific chemosensory gene expression in the olfactory organs of the malaria vector Anopheles gambiae

Background

The malaria mosquito Anopheles gambiae has a high preference for human hosts, a characteristic that contributes greatly to its capacity for transmitting human malaria. A sibling species, An. quadriannulatus, has a quite different host preference and feeds mostly on bovids. For this reason it does not contribute to human malaria transmission. Host seeking in mosquitoes is modulated by the olfactory system, which is primarily housed in the antennae and maxillary palps. Therefore, the detection of differing host odors by sibling species may be reflected in the expression level of the olfactory genes involved. Accordingly, we compared the transcriptomes of the antennae and maxillary palps of An. gambiae and An. quadriannulatus.

Results

We identified seven relatively abundant olfactory receptors, nine ionotropic receptors and three odorant binding proteins that are substantially up-regulated in An. gambiae antennae. Interestingly, we find that the maxillary palps of An. gambiae contain a species-specific olfactory receptor, Or52, and five An. gambiae-specific gustatory receptors (AgGr48-52) that are relatively abundant. These five gustatory receptors are also expressed in An. gambiae antennae, although at lower level, indicating a likely role in olfaction, rather than gustation. We also document an approximately three-fold higher overall expression of olfaction genes in the maxillary palps of An. quadriannulatus, indicating an important role of this organ in the olfaction system of this species. Finally, the expression of the CO₂ receptor genes is five to six-fold higher in the zoophilic An. quadriannulatus, implying a much higher sensitivity for detecting CO₂.

Conclusions

These results identify potential human host preference genes in the malaria vector An. gambiae. Interestingly, species-specific expression of several gustatory receptors in the olfactory organs indicate a role in olfaction rather than gustation. Additionally, a more expansive role for maxillary palps in olfaction is implicated than previously thought, albeit more so in the zoophilic An. quadriannulatus.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-1089) contains supplementary material, which is available to authorized users.

Antennal transcriptome profiles of anopheline mosquitoes reveal human host olfactory specialization in Anopheles gambiae

BACKGROUND

Two sibling members of the Anopheles gambiae species complex display notable differences in female blood meal preferences. An. gambiae s.s. has a well-documented preference for feeding upon human hosts, whereas An. quadriannulatus feeds on vertebrate/mammalian hosts, with only opportunistic feeding upon humans. Because mosquito host-seeking behaviors are largely driven by the sensory modality of olfaction, we hypothesized that hallmarks of these divergent host seeking phenotypes will be in evidence within the transcriptome profiles of the antennae, the mosquito's principal chemosensory appendage.

RESULTS

To test this hypothesis, we have sequenced antennal mRNA of non-bloodfed females from each species and observed a number of distinct quantitative and qualitative differences in their chemosensory gene repertoires. In both species, these gene families show higher rates of sequence polymorphisms than the overall rates in their respective transcriptomes, with potentially important divergences between the two species. Moreover, quantitative differences in odorant receptor transcript abundances have been used to model potential distinctions in volatile odor receptivity between the two sibling species of anophelines.

CONCLUSION

This analysis suggests that the anthropophagic behavior of An. gambiae s.s. reflects the differential distribution of olfactory receptors in the antenna, likely resulting from a co-option and refinement of molecular components common to both species. This study improves our understanding of the molecular evolution of chemoreceptors in closely related anophelines and suggests possible mechanisms that underlie the behavioral distinctions in host seeking that, in part, account for the differential vectorial capacity of these mosquitoes.

A new chromosomal phylogeny supports the repeated origin of vectorial capacity in malaria mosquitoes of the Anopheles gambiae complex

Understanding phylogenetic relationships within species complexes of disease vectors is crucial for identifying genomic changes associated with the evolution of epidemiologically important traits. However, the high degree of genetic similarity among sibling species confounds the ability to determine phylogenetic relationships using molecular markers. The goal of this study was to infer the ancestral–descendant relationships among malaria vectors and nonvectors of the Anopheles gambiae species complex by analyzing breakpoints of fixed chromosomal inversions in ingroup and several outgroup species. We identified genes at breakpoints of fixed overlapping chromosomal inversions 2Ro and 2Rp of An. merus using fluorescence in situ hybridization, a whole-genome mate-paired sequencing, and clone sequencing. We also mapped breakpoints of a chromosomal inversion 2La (common to An. merus, An. gambiae, and An. arabiensis) in outgroup species using a bioinformatics approach. We demonstrated that the “standard” 2R+^p arrangement and “inverted” 2Ro and 2La arrangements are present in outgroup species Anopheles stephensi, Aedes aegypti, and Culex quinquefasciatus. The data indicate that the ancestral species of the An. gambiae complex had the 2Ro, 2R+^p, and 2La chromosomal arrangements. The “inverted” 2Ro arrangement uniquely characterizes a malaria vector An. merus as the basal species in the complex. The rooted chromosomal phylogeny implies that An. merus acquired the 2Rp inversion and that its sister species An. gambiae acquired the 2R+^o inversion from the ancestral species. The karyotype of nonvectors An. quadriannulatus A and B was derived from the karyotype of the major malaria vector An. gambiae. We conclude that the ability to effectively transmit human malaria had originated repeatedly in the complex. Our findings also suggest that saltwater tolerance originated first in An. merus and then independently in An. melas. The new chromosomal phylogeny will facilitate identifying the association of evolutionary genomic changes with epidemiologically important phenotypes.

Malaria causes more than one million deaths every year, mostly among children in Sub-Saharan Africa. Anopheles mosquitoes are exclusive vectors of human malaria. Many malaria vectors belong to species complexes, and members within these complexes can vary significantly in their ecological adaptations and ability to transmit the parasite. To better understand evolution of epidemiologically important traits, we studied relationships among nonvector and vector species of the African Anopheles gambiae complex. We analyzed gene orders at genomic regions where evolutionary breaks of chromosomal inversions occurred in members of the complex and compared them with gene orders in species outside the complex. This approach allowed us to identify ancient and recent gene orders for three chromosomal inversions. Surprisingly, the more ancestral chromosomal arrangements were found in mosquito species that are vectors of human malaria, while the more derived arrangements were found in both nonvectors and vectors. Our finding strongly suggests that the increased ability to transmit human malaria originated repeatedly during the recent evolution of these African mosquitoes. This knowledge can be used to identify specific genetic changes associated with the human blood choice and ecological adaptations.

Simplified models of vector control impact upon malaria transmission by zoophagic mosquitoes

BACKGROUND

High coverage of personal protection measures that kill mosquitoes dramatically reduce malaria transmission where vector populations depend upon human blood. However, most primary malaria vectors outside of sub-Saharan Africa can be classified as "very zoophagic," meaning they feed occasionally (<10% of blood meals) upon humans, so personal protection interventions have negligible impact upon their survival.

METHODS AND FINDINGS

We extended a published malaria transmission model to examine the relationship between transmission, control, and the baseline proportion of bloodmeals obtained from humans (human blood index). The lower limit of the human blood index enables derivation of simplified models for zoophagic vectors that (1) Rely on only three field-measurable parameters. (2) Predict immediate and delayed (with and without assuming reduced human infectivity, respectively) impacts of personal protection measures upon transmission. (3) Illustrate how appreciable indirect communal-level protection for non-users can be accrued through direct personal protection of users. (4) Suggest the coverage and efficacy thresholds required to attain epidemiological impact. The findings suggest that immediate, indirect, community-wide protection of users and non-users alike may linearly relate to the efficacy of a user's direct personal protection, regardless of whether that is achieved by killing or repelling mosquitoes. High protective coverage and efficacy (≥80%) are important to achieve epidemiologically meaningful impact. Non-users are indirectly protected because the two most common species of human malaria are strict anthroponoses. Therefore, the small proportion of mosquitoes that are killed or diverted while attacking humans can represent a large proportion of those actually transmitting malaria.

CONCLUSIONS

Simplified models of malaria transmission by very zoophagic vectors may be used by control practitioners to predict intervention impact interventions using three field-measurable parameters; the proportion of human exposure to mosquitoes occurring when an intervention can be practically used, its protective efficacy when used, and the proportion of people using it.

Human exposure to anopheline mosquitoes occurs primarily indoors, even for users of insecticide-treated nets in Luangwa Valley, South-east Zambia

BACKGROUND

Current front line malaria vector control methods such as indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs), rely upon the preference of many primary vectors to feed and/or rest inside human habitations where they can be targeted with domestically-applied insecticidal products. We studied the human biting behaviour of the malaria vector Anopheles funestus Giles and the potential malaria vector Anopheles quadriannulatus Theobald in Luangwa valley, south-east Zambia.

METHODS

Mosquitoes were collected by human landing catch in blocks of houses with either combined use of deltamethrin-based IRS and LLINs or LLINs alone. Human behaviour data were collected to estimate how much exposure to mosquito bites indoors and outdoors occurred at various times of the night for LLIN users and non-users.

RESULTS

Anopheles funestus and An. quadriannulatus did not show preference to bite either indoors or outdoors: the proportions [95% confidence interval] caught indoors were 0.586 [0.303, 0.821] and 0.624 [0.324, 0.852], respectively. However, the overwhelming majority of both species were caught at times when most people are indoors. The proportion of mosquitoes caught at a time when most people are indoors were 0.981 [0.881, 0.997] and 0.897 [0.731, 0.965], respectively, so the proportion of human exposure to both species occuring indoors was high for individuals lacking LLINs (An. funestus: 0.983 and An. quadriannulatus: 0.970, respectively). While LLIN users were better protected, more than half of their exposure was nevertheless estimated to occur indoors (An. funestus: 0.570 and An. quadriannulatus: 0.584).

CONCLUSIONS

The proportion of human exposure to both An. funestus and An. quadriannulatus occuring indoors was high in the area and hence both species might be responsive to further peri-domestic measures if these mosquitoes are susceptible to insecticidal products.

Evolution of Anopheles gambiae in Relation to Humans and Malaria

The closely related and morphologically indistinguishable mosquito species in the Afrotropical Anopheles gambiae complex differ dramatically in their contribution to malaria transmission, ranging from major vectors through minor or locally important vectors and nonvectors. Radiation of the A. gambiae complex and ongoing diversification within its nominal species appears to be a product of recent and rapid adaptation to environmental heterogeneities, notably those of anthropogenic origin. Polytene chromosome and genomic analyses suggest that paracentric chromosomal inversions and possibly other low-recombination regions have played instrumental roles in this process by facilitating ecotypic differentiation both within and across semipermeable species boundaries. Forthcoming complete genome sequences from several members of the A. gambiae complex will provide powerful tools to accelerate ongoing investigation of how genetic diversification of populations and species has shaped behavioral and physiological traits, such as vector competence, that bear on vectorial importance.

Transmission blocking immunity in the malaria non-vector mosquito Anopheles quadriannulatus species A

Despite being phylogenetically very close to Anopheles gambiae, the major mosquito vector of human malaria in Africa, Anopheles quadriannulatus is thought to be a non-vector. Understanding the difference between vector and non-vector mosquitoes can facilitate development of novel malaria control strategies. We demonstrate that An. quadriannulatus is largely resistant to infections by the human parasite Plasmodium falciparum, as well as by the rodent parasite Plasmodium berghei. By using genetics and reverse genetics, we show that resistance is controlled by quantitative heritable traits and manifested by lysis or melanization of ookinetes in the mosquito midgut, as well as by killing of parasites at subsequent stages of their development in the mosquito. Genes encoding two leucine-rich repeat proteins, LRIM1 and LRIM2, and the thioester-containing protein, TEP1, are identified as essential in these immune reactions. Their silencing completely abolishes P. berghei melanization and dramatically increases the number of oocysts, thus transforming An. quadriannulatus into a highly permissive parasite host. We hypothesize that the mosquito immune system is an important cause of natural refractoriness to malaria and that utilization of this innate capacity of mosquitoes could lead to new methods to control transmission of the disease.

Malaria is a mosquito-borne infectious disease that threatens almost half of the human population and kills 1 to 3 million people every year. In sub-Saharan Africa, where the vast majority of deaths occur, the capacity of mosquitoes to transmit malaria varies greatly even between closely related species. We compared the ability of malaria parasites to develop in two very closely related mosquitoes, one vector and one non-vector, and found that non-vector mosquitoes kill parasites at various stages, predominantly when they invade the mosquito midgut. This is achieved by parasite clearance, possibly by lysis in the midgut cells and by melanization, both of which are reactions of the mosquito immune system. This phenotype depends on heritable and dominant traits that can be passed on to vector/non-vector mosquito hybrids. We examined whether specific components of the mosquito immune system affect the resistance of these mosquitoes to infection. By silencing the activity of three immunity genes, we transformed mosquitoes of the resistant species into highly susceptible. Our results suggest that the mosquito immune system may affect refractoriness to malaria in non-vector mosquitoes. This innate capacity of mosquitoes to kill malaria parasites could be utilized in future integrated efforts to control and ultimately eradicate the disease.

Unexpected anthropophagic behaviour in Anopheles quadriannulatus

The strongly anthropophilic behaviour of Anopheles gambiae Giles sensu stricto (Diptera: Culicidae), the most important malaria vector in Africa, has been demonstrated by field and laboratory studies. Other members of the An. gambiae complex express varied degrees of anthropophily. Anopheles quadriannulatus (Theobald) species A and B are more zoophilic members of the complex and hence are considered to be of no medical importance. Olfactometer experiments with An. quadriannulatus species A have demonstrated attraction to both human and cow odour. To extend these olfactometer observations a choice experiment was conducted in an outdoor cage with a human and a calf as baits, using laboratory-reared mosquitoes. Anopheles gambiae s.s. (from Liberia) and two strains of An. quadriannulatus species A (SKUQUA from South Africa, SANGQUA from Zimbabwe), marked with different coloured fluorescent powders for identification purposes, were released simultaneously and given an equal opportunity to feed on either host. The experiment was repeated six times. Bloodmeals were identified using the precipitin technique. Anopheles gambiae s.s. showed highly anthropophagic behaviour, taking 88% of bloodmeals from the human host. In contrast, both strains of An. quadriannulatus fed with equal frequency on the human or the calf; the response to either host was not significantly different. These results confirm the olfactometer findings and demonstrate anthropophagic behaviour not previously recorded in this species. This finding has implications for prospective manipulation of host preference for genetic control purposes.