Unravelling complexities in human malaria transmission dynamics in Africa through a comprehensive knowledge of vector populations

Distribution and insecticide resistance profile of the major malaria vector Anopheles funestus group across the African continent

There has been significant progress in malaria control in the last 2 decades, with a decline in mortality and morbidity. However, these gains are jeopardised by insecticide resistance, which negatively impacts the core interventions, such as insecticide-treated nets (ITN) and indoor residual spraying (IRS). While most malaria control and research efforts are still focused on Anopheles gambiae complex mosquitoes, Anopheles funestus remains an important vector in many countries and, in some cases, contributes to most of the local transmission. As countries move towards malaria elimination, it is important to ensure that all dominant vector species, including An. funestus, an important vector in some countries, are targeted. The objective of this review is to compile and discuss information related to A. funestus populations' resistance to insecticides and the mechanisms involved across Africa, emphasising the sibling species and their resistance profiles in relation to malaria elimination goals. Data on insecticide resistance in An. funestus malaria vectors in Africa were extracted from published studies. Online bibliographic databases, including Google Scholar and PubMed, were used to search for relevant studies. Articles published between 2000 and May 2023 reporting resistance of An. funestus to insecticides and associated mechanisms were included. Those reporting only bionomics were excluded. Spatial variation in species distribution and resistance to insecticides was recorded from 174 articles that met the selection criteria. It was found that An. funestus was increasingly resistant to the four classes of insecticides recommended by the World Health Organisation for malaria vector control; however, this varied by country. Insecticide resistance appears to reduce the effectiveness of vector control methods, particularly IRS and ITN. Biochemical resistance due to detoxification enzymes (P450s and glutathione-S-transferases [GSTs]) in An. funestus was widely recorded. However, An. funestus in Africa remains susceptible to other insecticide classes, such as organophosphates and neonicotinoids. This review highlights the increasing insecticide resistance of An. funestus mosquitoes, which are important malaria vectors in Africa, posing a significant challenge to malaria control efforts. While An. funestus has shown resistance to the recommended insecticide classes, notably pyrethroids and, in some cases, organochlorides and carbamates, it remains susceptible to other classes of insecticides such as organophosphates and neonicotinoids, providing potential alternative options for vector control strategies. The study underscores the need for targeted interventions that consider the population structure and geographical distribution of An. funestus, including its sibling species and their insecticide resistance profiles, to effectively achieve malaria elimination goals.

Limited association between Wolbachia and Plasmodium falciparum infections in natural populations of the major malaria mosquito Anopheles moucheti

Since the discovery of natural malaria vector populations infected by the endosymbiont bacterium Wolbachia, a renewed interest has arisen for using this bacterium as an alternative for malaria control. Among naturally infected mosquitoes, Anopheles moucheti, a major malaria mosquito in Central Africa, exhibits one of the highest prevalences of Wolbachia infection. To better understand whether this maternally inherited bacterium could be used for malaria control, we investigated Wolbachia influence in An. moucheti populations naturally infected by the malaria parasite Plasmodium falciparum. To this end, we collected mosquitoes in a village from Cameroon, Central Africa, where this mosquito is the main malaria vector. We found that the prevalence of Wolbachia bacterium was almost fixed in the studied mosquito population, and was higher than previously recorded. We also quantified Wolbachia in whole mosquitoes and dissected abdomens, confirming that the bacterium is also elsewhere than in the abdomen, but at lower density. Finally, we analyzed the association of Wolbachia presence and density on P. falciparum infection. Wolbachia density was slightly higher in mosquitoes infected with the malaria parasite than in uninfected mosquitoes. However, we observed no correlation between the P. falciparum and Wolbachia densities. In conclusion, our study indicates that naturally occurring Wolbachia infection is not associated to P. falciparum development within An. moucheti mosquitoes.

Deep learning and wing interferential patterns identify Anopheles species and discriminate amongst Gambiae complex species

We present a new and innovative identification method based on deep learning of the wing interferential patterns carried by mosquitoes of the Anopheles genus to classify and assign 20 Anopheles species, including 13 malaria vectors. We provide additional evidence that this approach can identify Anopheles spp. with an accuracy of up to 100% for ten out of 20 species. Although, this accuracy was moderate (> 65%) or weak (50%) for three and seven species. The accuracy of the process to discriminate cryptic or sibling species is also assessed on three species belonging to the Gambiae complex. Strikingly, An. gambiae, An. arabiensis and An. coluzzii, morphologically indistinguishable species belonging to the Gambiae complex, were distinguished with 100%, 100%, and 88% accuracy respectively. Therefore, this tool would help entomological surveys of malaria vectors and vector control implementation. In the future, we anticipate our method can be applied to other arthropod vector-borne diseases.

Advances in the genetic characterization of the malaria vector, Anopheles funestus, and implications for improved surveillance and control

Anopheles mosquitoes present a major public health challenge in sub-Saharan Africa; notably, as vectors of malaria that kill over half a million people annually. In parts of the east and southern Africa region, one species in the Funestus group, Anopheles funestus, has established itself as an exceptionally dominant vector in some areas, it is responsible for more than 90% of all malaria transmission events. However, compared to other malaria vectors, the species is far less studied, partly due to difficulties in laboratory colonization and the unresolved aspects of its taxonomy and systematics. Control of An. funestus is also increasingly difficult because it has developed widespread resistance to public health insecticides. Fortunately, recent advances in molecular techniques are enabling greater insights into species identity, gene flow patterns, population structure, and the spread of resistance in mosquitoes. These advances and their potential applications are reviewed with a focus on four research themes relevant to the biology and control of An. funestus in Africa, namely: (i) the taxonomic characterization of different vector species within the Funestus group and their role in malaria transmission; (ii) insecticide resistance profile; (iii) population genetic diversity and gene flow, and (iv) applications of genetic technologies for surveillance and control. The research gaps and opportunities identified in this review will provide a basis for improving the surveillance and control of An. funestus and malaria transmission in Africa.

Entomological Characteristics of Malaria Transmission across Benin: An Essential Element for Improved Deployment of Vector Control Interventions

Entomological surveillance in Benin has historically been limited to zones where indoor residual spraying was performed or where long-standing sentinel surveillance sites existed. However, there are significant country-wide gaps in entomological knowledge. The National Malaria Control Program (NMCP) assessed population dynamics of Anopheles vectors and malaria transmission in each of Benin’s 12 departments to create an entomological risk profile. Two communes per department (24/77 communes) were chosen to reflect diverse geographies, ecologies and malaria prevalence. Two villages per commune were selected from which four households (HH) per village were used for human landing catches (HLCs). In each HH, an indoor and outdoor HLC occurred between 7 p.m. and 7 a.m. on two consecutive nights between July−September 2017. Captured Anopheles were identified, and ovaries were dissected to determine parous rate. Heads and thoraces were tested for Plasmodium falciparum sporozoites by ELISA. The Entomological Inoculation Rate (EIR) was calculated as the product of mosquito bite rate and sporozoite index. Bite rates from An. gambiae s.l., the primary vector species complex, differed considerably between communes; average sporozoite infection index was 3.5%. The EIR ranged from 0.02 infectious bites (ib) per human per night in the departments of Ouémé and Plateau to 1.66 ib/human/night in Collines. Based on transmission risk scales, Avrankou, Sakété and Nikki are areas of low transmission (0 < EIR < 3 ib/human/year), Adjarra, Adja Ouèrè, Zè, Toffo, Bopa, Pehunco, Pèrèrè and Kandi are of medium transmission (3 < EIR < 30 ib/human/year), and the other remaining districts are high transmission (EIR > 30 ib/human/year). The heterogeneous and diverse nature of malaria transmission in Benin was not readily apparent when only assessing entomological surveillance from sentinel sites. Prospectively, the NMCP will use study results to stratify and deploy targeted vector control interventions in districts with high EIRs to better protect populations most at-risk.

Genomic approaches for monitoring transmission dynamics of malaria: A case for malaria molecular surveillance in Sub-Saharan Africa

Transmission dynamics is an important indicator for malaria control and elimination. As we move closer to eliminating malaria in Sub-Saharan Africa (sSA), transmission indices with higher resolution (genomic approaches) will complement our current measurements of transmission. Most of the present programmatic knowledge of malaria transmission patterns are derived from assessments of epidemiologic and clinical data, such as case counts, parasitological estimates of parasite prevalence, and Entomological Inoculation Rates (EIR). However, to eliminate malaria from endemic areas, we need to track changes in the parasite population and how they will impact transmission. This is made possible through the evolving field of genomics and genetics, as well as the development of tools for more in-depth studies on the diversity of parasites and the complexity of infections, among other topics. If malaria elimination is to be achieved globally, country-specific elimination activities should be supported by parasite genomic data from regularly collected blood samples for diagnosis, surveillance and possibly from other programmatic interventions. This presents a unique opportunity to track the spread of malaria parasites and shed additional light on intervention efficacy. In this review, various genetic techniques are highlighted along with their significance for an enhanced understanding of transmission patterns in distinct topological settings throughout Sub-Saharan Africa. The importance of these methods and their limitations in malaria surveillance to guide control and elimination strategies, are explored.

Diversity and behavioral activity of Anopheles mosquitoes on the slopes of Mount Cameroon

BACKGROUND

Malaria remains endemic in Cameroon, with heterogeneous transmission related to eco-climatic variations, vector diversity and spatial distribution. The intensification of malaria prevention and control through the free distribution of insecticide-treated nets in recent years may have altered the composition, geographic distribution and natural infection rate of Anopheles species, with implications for malaria transmission dynamics. The present study seeks to assess the vectorial diversity, dynamics and infectivity across different seasons and altitudes in relationship to parasite prevalence around the slopes of Mount Cameroon, southwestern region.

METHOD

Mosquitoes were sampled (indoors and outdoors) in 11 eco-epidemiological settings at low (18-197 m), intermediate (371-584 m) and high (740-1067 m) altitude by nightly human landing catches. The mosquitoes were identified morphologically and Anopheles gambiae sibling species identified by PCR. Parity status was ascertained by examining the ovaries and the entomological inoculation rates (EIR) determined by Plasmodium falciparum circumsporozoite antigen ELISA of the head-thorax. The prevalence of Plasmodium infection across target communities was assessed using rapid diagnostic tests.

RESULTS

A total of 7327 (18.0 mosquitoes/trap/night) mosquitoes were trapped, mainly during the rainy season (5678, 77.5%) and at low altitude (3669, 50.1%). Anopheles spp. (5079, 69.3%) was the most abundant genera and An. gambiae complex (2691, 36.7%) the major vector, varying with altitude (χ² = 183.87, df = 8, P < 0.001) and season (χ² = 28.14, df = 4, P < 0.001). Only An. gambiae (s.s.) was identified following molecular analysis of An. gambiae complex siblings. The overall biting peak for An. gambiae complex was 2-3 a.m. Anopheles cinctus was the most abundant secondary vector in the area. The average EIR in the area was 2.08 infective bites per person per night (ib/p/n), higher at low (2.45 ib/p/n) than at intermediate altitude (1.39 ib/p/n) and during the rainy (1.76 ib/p/n) compared to the dry season (0.34 ib/p/n). Anopheles funestus was most infectious overall (28.1%, 16/57) while An. gambiae had the highest inoculation rates averaging 1.33 ib/p/n. Most Anopheles species across all altitudes and seasons were parous, highest in communities with the highest proportion of malaria parasite infections.

CONCLUSION

Anopheles gambiae (s.s.) remains the major malaria vector in the area and An. cinctus possibly a secondary vector of the disease in the slopes of Mt. Cameroon. The seasonal and altitudinal effects on the distribution of these mosquitoes may have implications for the transmission of malaria and its control strategies in the area. Regular monitoring of the bionomics of local Anopheles vector species and targeted control interventions in the 'hotspots' is necessary to curb the prevalence of the infection and incidence of disease.

Epidemiological and entomological studies of malaria transmission in Tibati, Adamawa region of Cameroon 6 years following the introduction of long-lasting insecticide nets

BACKGROUND

Malaria remains a serious public health problem in Cameroon. Implementation of control interventions requires prior knowledge of the local epidemiological situation. Here we report the results of epidemiological and entomological surveys carried out in Tibati, Adamawa Region, Cameroon, an area where malaria transmission is seasonal, 6 years after the introduction of long-lasting insecticidal bed nets.

METHODS

Cross-sectional studies were carried out in July 2015 and 2017 in Tibati. Thick blood smears and dried blood spots were collected from asymptomatic and symptomatic individuals in the community and at health centers, respectively, and used for the molecular diagnosis of Plasmodium species. Adult mosquitoes were collected by indoor residual spraying and identified morphologically and molecularly. The infection status of Plasmodium spp. was determined by quantitative PCR, and positivity of PCR-positive samples was confirmed by Sanger sequencing.

RESULTS

Overall malaria prevalence in our study population was 55.0% (752/1367) and Plasmodium falciparum was the most prevalent parasite species (94.3%), followed by P. malariae (17.7%) and P. ovale (0.8%); 92 (12.7%) infections were mixed infections. Infection parameters varied according to clinical status (symptomatic/asymptomatic) and age of the sampled population and the collection sites. Infection prevalence was higher in asymptomatic carriers (60.8%), but asexual and sexual parasite densities were lower. Prevalence and intensity of infection decreased with age in both the symptomatic and asymptomatic groups. Heterogeneity in infections was observed at the neighborhood level, revealing hotspots of transmission. Among the 592 Anopheles mosquitoes collected, 212 (35.8%) were An. gambiae, 172 (29.1%) were An. coluzzii and 208 (35.1%) were An. funestus (s.s.). A total of 26 (4.39%) mosquito specimens were infected by Plasmodium sp. and the three Anopheles mosquitoes transmitted Plasmodium at equal efficiency. Surprisingly, we found an An. coluzzii specimen infected by Plasmodium vivax, which confirms circulation of this species in Cameroon. The positivity of all 26 PCR-positive Plasmodium-infected mosquitoes was successively confirmed by sequencing analysis.

CONCLUSION

Our study presents the baseline malaria parasite burden in Tibati, Adamawa Region, Cameroon. Our results highlight the high malaria endemicity in the area, and hotspots of disease transmission are identified. Parasitological indices suggest low bednet usage and that implementation of control interventions in the area is needed to reduce malaria burden. We also report for the first time a mosquito vector with naturally acquired P. vivax infection in Cameroon.

Review of malaria situation in Cameroon: technical viewpoint on challenges and prospects for disease elimination

Malaria still has a devastating impact on public health and welfare in Cameroon. Despite the increasing number of studies conducted on disease prevalence, transmission patterns or treatment, there are to date, not enough studies summarising findings from previous works in order to identify gaps in knowledge and areas of interest where further evidence is needed to drive malaria elimination efforts. The present study seeks to address these gaps by providing a review of studies conducted so far on malaria in Cameroon since the 1940s to date. Over 250 scientific publications were consulted for this purpose. Although there has been increased scale-up of vector control interventions which significantly reduced the morbidity and mortality to malaria across the country from a prevalence of 41% of the population reporting at least one malaria case episode in 2000 to a prevalence of 24% in 2017, the situation is not yet under control. There is a high variability in disease endemicity between epidemiological settings with prevalence of Plasmodium parasitaemia varying from 7 to 85% in children aged 6 months to 15 years after long-lasting insecticidal nets (LLINs) scale-up. Four species of Plasmodium have been recorded across the country: Plasmodium falciparum, P. malariae, P. ovale and P. vivax. Several primate-infecting Plasmodium spp. are also circulating in Cameroon. A decline of artemisinin-based combinations therapeutic efficacy from 97% in 2006 to 90% in 2016 have been reported. Several mutations in the P. falciparum chloroquine resistance (Pfcrt) and P. falciparum multidrug resistance 1 (Pfmdr1) genes conferring resistance to either 4-amino-quinoleine, mefloquine, halofanthrine and quinine have been documented. Mutations in the Pfdhfr and Pfdhps genes involved in sulfadoxine-pyrimethamine are also on the rise. No mutation associated with artemisinin resistance has been recorded. Sixteen anopheline species contribute to malaria parasite transmission with six recognized as major vectors: An. gambiae, An. coluzzii, An. arabiensis, An. funestus, An. nili and An. moucheti. Studies conducted so far, indicated rapid expansion of DDT, pyrethroid and carbamate resistance in An. gambiae, An. coluzzii, An. arabiensis and An. funestus threatening the performance of LLINs. This review highlights the complex situation of malaria in Cameroon and the need to urgently implement and reinforce integrated control strategies in different epidemiological settings, as part of the substantial efforts to consolidate gains and advance towards malaria elimination in the country.

Malaria Transmission around the Memve'ele Hydroelectric Dam in South Cameroon: A Combined Retrospective and Prospective Study, 2000⁻2016

Dam constructions are considered a great concern for public health. The current study aimed to investigate malaria transmission in the Nyabessan village around the Memve'ele dam in South Cameroon. Adult mosquitoes were captured by human landing catches in Nyabessan before and during dam construction in 2000-2006 and 2014-2016 respectively, as well as in the Olama village, which was selected as a control. Malaria vectors were morphologically identified and analyzed for Plasmodium falciparum circumsporozoite protein detection and molecular identification of Anopheles (A.) gambiae species. Overall, ten malaria vector species were identified among 12,189 Anopheles specimens from Nyabessan (N = 6127) and Olama (N = 6062), including A. gambiae Giles (1902), A. coluzzii Coetzee (2013), A. moucheti Evans (1925), A. ovengensis Awono (2004), A. nili Theobald (1903), A. paludis Theobald (1900), A. zieanni, A. marshallii Theobald (1903), A. coustani Laveran (1900), and A. obscurus Grünberg (1905). In Nyabessan, A. moucheti and A. ovengensis were the main vector species before dam construction (16-50 bites/person/night-b/p/n, 0.26-0.71 infective bites/person/night-ib/p/n) that experienced a reduction of their role in disease transmission in 2016 (3-35 b/p/n, 0-0.5 ib/p/n) (p < 0.005). By contrast, the role of A. gambiae s.l. and A. paludis increased (11-38 b/p/n, 0.75-1.2 ib/p/n) (p < 0.01). In Olama, A. moucheti remained the main malaria vector species throughout the study period (p = 0.5). These findings highlight the need for a strong vector-borne disease surveillance and control system around the Memve'ele dam.

Involvement of Anopheles nili in Plasmodium falciparum transmission in North Benin

BACKGROUND

Several studies carried out in Benin have shown the involvement of Anopheles gambiae sensu lato (s.l.), the Anopheles funestus group in malaria transmission, but none of them reported the contribution of the Anopheles nili group to the transmission of this disease. The current study investigated the question through an entomological cross-sectional survey performed in Northern Benin.

METHODS

Mosquito samplings were performed in September and October 2017 in 4 villages located in two districts: Bambaba and Wodara (Kérou district) and, Péhunco 2 and Béké (Péhunco district). The collections were carried out indoors and outdoors using human landing catches (HLC) to assess the human biting rate (HBR) and pyrethrum spray catches (PSC) to evaluate the blood feeding rate and the blood meal origin using the ELISA test. All collected mosquitoes were morphologically identified and, the polymerase chain reaction (PCR) technique was used for molecular identification of sibling species of An. gambiae s.l., An. funestus group and An. nili group sporozoite index (SI) was also assessed by the ELISA test.

RESULTS

Overall, An. gambiae s.l., An. funestus group and An. nili group were the three vectors found in the study area. A significantly higher human biting rate (HBR) was recorded in An. nili group (5 bites/human/night) compared to An. funestus group (0.656 bites/human/night) in the Kérou district (p < 0.0001). Anopheles gambiae s.l. displayed the highest HBR (26.19 bites/human/night) in the same district. The entomological inoculation rate (EIR) was 1.875 infected bites/human/month in An. nili group against 13.05 infected bites/human/month in An. gambiae s.l. and 0.938 infected bites/human/month in An. funestus group in Kérou. In Péhunco, the EIR was 1.02 infected bites/human/month in An. gambiae s.l. PCR results showed that An. nili sensu stricto (s.s.) and An. funestus s.s. were the only species of the An. nili and An. funestus groups, respectively. The anthropophagic character of An. gambiae s.l. was also highlighted.

CONCLUSION

This study provides useful information on the contribution of An. nili group as secondary vector to malaria transmission in northern Benin. Broader studies must also be carried out in a larger study area to assess the involvement of other Anopheles species to malaria transmission. This will aid to better plan malaria vector control interventions.

[Next-generation vector control]

Vector control is a cornerstone of vector-borne infectious disease control, a group of emerging and re-emerging diseases of major public health concern at a global scale. The history and evolution of mosquito disease vectors control, mainly based on the use of chemical insecticides, is emblematic of the successes, failures, lessons learned and experiences gained in setting-up and implementing vector control, and of the challenges that pave the way to sustainable disease vector management. This paper provides a non-exhaustive and non-exclusive overview of some of the most promising cutting-edge technical and strategic innovations that are committed to this endeavour, assessing the strength of scientific evidences for proof of concept, perspectives for scaling-up, and expected impact and outcomes in a rapidly changing world.

Exposure to the insecticide-treated bednet PermaNet 2.0 reduces the longevity of the wild African malaria vector Anopheles funestus but GSTe2-resistant mosquitoes live longer

BACKGROUND

Despite the increased report of insecticide resistance in malaria vectors, its impact on mosquito's life-traits after exposure to insecticide-treated nets remains under investigated. Here, we assessed the effects of exposure to PermaNet 2.0 on several life traits of An. gambiae s.l. and An. funestus s.l. field mosquitoes in Cameroon.

METHODOLOGY

Female Anopheles mosquitoes were collected indoor using electric aspirators in southern Cameroon (Obout) in 2016. After assessing the resistance status of F1 from the field collected-mosquitoes, progeny of the first generation (An. funestus s.l.) and seventh generation (An. gambiae s.l.) were used to assess the long-term effect of exposure to PermaNet 2.0 on several life-traits of these vectors (longevity, blood feeding ability, fecundity and fertility) in comparison to untreated net. In addition, the L119F-GSTe2 mutation associated with DDT/pyrethroids resistance in An. funestus was genotyped to assess its association with increased life-span post-exposure.

PRINCIPAL FINDINGS

Both An. funestus and An. gambiae were resistant to pyrethroids and DDT with a greater level in the latter. Pyrethroid-only nets PermaNet 2.0 (17.5% mortality) and Olyset (0% mortality) exhibited a significantly reduced efficacy against An. funestus in contrast to a greater efficacy for PBO-based Nets Olyset Plus (65% mortality), PermaNet 3.0 top (100% mortality). In both species, mosquitoes that survived exposure to PermaNet 2.0 exhibited a significantly reduced longevity than those non-exposed (6.95 days vs 12.46 for An. funestus P<0.001; 8.87 vs 11.25 days for An. gambiae; P<0.001). However, no significant difference was observed for blood feeding and fecundity in both species. In addition, molecular analysis of the L119F-GSTe2 mutation revealed that this mutation is associated with an increase in the chance of surviving after exposure to this net in An. funestus.

CONCLUSIONS

These results show that although the PermaNet 2.0 presents a reduced efficacy against resistant populations, it remains efficient after exposure by reducing the life expectancy of the vectors which could contribute in the reduction of malaria incidence.

Biodiversity Pattern of Mosquitoes in Southeastern Senegal, Epidemiological Implication in Arbovirus and Malaria Transmission

The composition, density, diversity, and temporal distribution of mosquito species and the influence of temperature, relative humidity, and rainfall on these data were investigated in 50 sites across five land cover classes (forest, savannah, barren, village, and agriculture) in southeastern Senegal. Mosquitoes were collected monthly in each site between June 2009 and March 2011, with three people collecting mosquitoes landing on their legs for one to four consecutive days. In total, 81,219 specimens, belonging to 60 species and 7 genera, were collected. The most abundant species were Aedes furcifer (Edwards) (Diptera: Culicidae) (20.7%), Ae. vittatus (Bigot) (19.5%), Ae. dalzieli (Theobald) (14.7%), and Ae. luteocephalus (Newstead) (13.7%). Ae. dalzieli, Ae. furcifer, Ae. vittatus, Ae. luteocephalus, Ae. taylori Edwards, Ae. africanus (Theobald), Ae. minutus (Theobald), Anopheles coustani Laveran, Culex quinquefasciatus Say, and Mansonia uniformis (Theobald) comprised ≥10% of the total collection, in at least one land cover. The lowest species richness and Brillouin diversity index (HB = 1.55) were observed in the forest-canopy. The urban-indoor fauna showed the highest dissimilarity with other land covers and was most similar to the urban-outdoor fauna following Jaccard and Morisita index. Mosquito abundance peaked in June and October 2009 and July and October 2010. The highest species density was recorded in October. The maximum temperature was correlated positively with mean temperature and negatively with rainfall and relative humidity. Rainfall showed a positive correlation with mosquito abundance and species density. These data will be useful for understanding the transmission of arboviruses and human malaria in the region.

Population genetics of Anopheles funestus, the African malaria vector, Kenya

Background

Anopheles funestus is among the major malaria vectors in Kenya and sub-Saharan Africa and has been recently implicated in persistent malaria transmission. However, its ecology and genetic diversity remain poorly understood in Kenya.

Methods

Using 16 microsatellite loci, we examined the genetic structure of An. funestus sampled from 11 locations (n = 426 individuals) across a wide geographical range in Kenya spanning coastal, western and Rift Valley areas.

Results

Kenyan An. funestus resolved as three genetically distinct clusters. The largest cluster (FUN1) broadly included samples from western and Rift Valley areas of Kenya with two clusters identified from coastal Kenya (FUN2 and FUN3), not previously reported. Geographical distance had no effect on population differentiation of An. funestus. We found a significant variation in the mean Plasmodium infectivity between the clusters (χ² = 12.1, df = 2, P = 0.002) and proportional to the malaria prevalence in the different risk zones of Kenya. Notably, there was variation in estimated effective population sizes between the clusters, suggesting possible differential impact of anti-vector interventions in represented areas.

Conclusions

Heterogeneity among Kenyan populations of An. funestus will impact malaria vector control with practical implications for the development of gene-drive technologies. The difference in Plasmodium infectivity and effective population size between the clusters could suggest potential variation in phenotypic characteristics relating to competence or insecticide resistance. This is worth examining in future studies.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3252-3) contains supplementary material, which is available to authorized users.

Molecular basis of permethrin and DDT resistance in an Anopheles funestus population from Benin

BACKGROUND

Insecticide resistance in Anopheles mosquitoes is threatening the success of malaria control programmes. In order to implement suitable insecticide resistance management strategies, it is necessary to understand the underlying mechanisms involved. To achieve this, the molecular basis of permethrin and DDT resistance in the principal malaria vector, Anopheles funestus from inland Benin (Kpome), was investigated.

RESULTS

Here, using a microarray-based genome-wide transcription and qRT-PCR analysis, we showed that metabolic resistance mechanisms through over-expression of cytochrome P450 and glutathione S-transferase genes (GSTs) are a major contributor to DDT and permethrin resistance in Anopheles funestus from Kpome. The GSTe2 gene was the most upregulated detoxification gene in both DDT- [fold-change (FC: 16.0)] and permethrin-resistant (FC: 18.1) mosquitoes suggesting that upregulation of this gene could contribute to DDT resistance and cross-resistance to permethrin. CYP6P9a and CYP6P9b genes that have been previously associated with pyrethroid resistance were also significantly overexpressed with FC 5.4 and 4.8, respectively, in a permethrin resistant population. Noticeably, the GSTs, GSTd1-5 and GSTd3, were more upregulated in DDT-resistant than in permethrin-resistant Anopheles funestus suggesting these genes are more implicated in DDT resistance. The absence of the L1014F or L1014S kdr mutations in the voltage-gated sodium channel gene coupled with the lack of directional selection at the gene further supported that knockdown resistance plays little role in this resistance.

CONCLUSIONS

The major role played by metabolic resistance to pyrethroids in this An. funestus population in Benin suggests that using novel control tools combining the P450 synergist piperonyl butoxide (PBO), such as PBO-based bednets, could help manage the growing pyrethroid resistance in this malaria vector in Benin.

Baseline entomologic data on malaria transmission in prelude to an indoor residual spraying intervention in the regions of Alibori and Donga, Northern Benin, West Africa

Background

Despite the success of indoor residual insecticide spraying (IRS) in Africa, particularly in Benin, some gaps of information need to be filled to optimize the effectiveness of this intervention in the perspective of the country’s effort to eliminate malaria. In anticipation to the 2018 IRS campaign in two targeted regions of northern Benin, this study aimed, to collect baseline information on vector composition, spatio-temporal variation and peak malaria transmission in the Alibori and Donga, two targeted regions of northern Benin. Information collected will help to better plan the implementation and later on the impact assessment of this IRS campaign.

Methods

The study was carried out in four districts of the two IRS targeted regions of northern Benin. Human landing catches and pyrethrum spray catches protocols were used to assess the biting rate (HBR) and, biting/resting behaviour of malaria vector populations. After morphological identification of collected Anopheles, the heads and thoraxes of Anopheles gambiae sensu lato (s.l.) were analysed by the ELISA CSP tests to estimate the sporozoite index (SI). The entomological inoculation rate was calculated as the product of mosquito biting rate (HBR) and the SI.

Results

The biting rates of An. gambiae s.l., the major vector in this study sites, varied significantly from region to region. It was higher: in rural than in urban areas, in rainy season than in dry season, indoors than outdoors. Overall, SI was comparable between sites. The highest EIRs were observed in the Donga region (16.84 infectious bites/man/month in Djougou district and 17.64 infectious bites/man/month in Copargo district) and the lowest in the Alibori region (10.74 infectious bites/man/month at Kandi district and 11.04 infectious bites/man/month at Gogounou district).

Conclusion

This study showed the heterogeneous and various nature of malaria epidemiology in Northern Benin. Indeed, the epidemiological profile of malaria transmission in the Alibori and Donga regions is made of a single season of transmission interrupted by a dry season. This period of transmission is relatively longer in Donga region than in Alibori. This information can be used to guide the extension of IRS in the Alibori and in the Donga, by primarily targeting areas with short periods of transmission, and easy to cover.

Evidence of Insecticide Resistance to Pyrethroids and Bendiocarb in Anopheles funestus from Tsararano, Marovoay District, Madagascar

Introduction

In Madagascar, malaria control relies on the countrywide use of long lasting insecticide treated bed nets (LLINs) and on indoor residual spraying (IRS) in the central highland area as well as a small area on the eastern coast. We tested insecticide resistance mechanisms of Anopheles funestus from Tsararano, a malaria endemic village in the coastal health district of Marovoay.

Methods

Insecticide susceptibility bioassays were done in July 2017 on first-generation Anopheles funestus (F1) to assess (i) the susceptibility to permethrin (0.05%), deltamethrin (0.05%), DDT (4%), malathion (5%), fenitrothion (1%), and bendiocarb (0.1%); (ii) the effect of preexposure to the piperonyl butoxide (PBO) synergist; and (iii) the enzymatic activities of cytochrome P450, esterases, and glutathione S-transferases (GST).

Results

Our results demonstrated that An. funestus was phenotypically resistant to pyrethroids and bendiocarb, with a mortality rate (MR) of 33.6% (95%CI: 24.5-43.7%) and 86% (95%CI: 77.6-92.1%), respectively. In contrast, An. funestus were 100% susceptible to DDT and organophosphates (malathion and fenitrothion). Preexposure of An. funestus to PBO synergist significantly restored the susceptibility to bendiocarb (MR=100%) and increased the MR in the pyrethroid group, from 96% (95%CI: 90.0-98.9%) to 100% for deltamethrin and permethrin, respectively (χ² = 43, df = 3, P< 0.0001). Enzymatic activities of cytochrome P450 and α-esterases were significantly elevated among An. funestus compared with the IPM reference strain (Mann-Whitney U= 30, P<0.0001; U = 145.5, P <0.0001, respectively). No significant differences of β-esterases activities compared to the IPM reference strain were observed (Mann-Whitney U = 392.5, P = 0.08).

Conclusion

In Tsararano, despite the absence of an IRS programme, there is evidence of high levels of insecticide resistance to pyrethroids and bendiocarb in An.

Bionomics and vectorial role of anophelines in wetlands along the volcanic chain of Cameroon

BACKGROUND

The epidemiological profiles of vector-borne diseases, such as malaria, are strongly associated with landscape components. The reduction of malaria burden in endemic and epidemic regions mainly depends on knowledge of the malaria-transmitting mosquito species, populations and behavioural characteristics, as well as malaria exposure risks. This work aimed at carrying out a holistic study in order to characterise Anopheles species in relation to human malaria in seven wetlands along the lower section of the volcanic chain of Cameroon.

RESULTS

Eight malaria vectors: Anopheles arabiensis, An. coluzzii, An. funestus (s.s.), An. gambiae, An. hancocki, An. melas, An. nili and An. ziemanni, were found biting humans. Anopheles gambiae was widespread; however, it played a secondary role in the Ndop plain where An. ziemmani was the primary vector species (79.2%). Anophelines were more exophagic (73.6%) than endophagic (26.4%), showing a marked nocturnal activity (22:00-4:00 h) for An. coluzzii and An. gambiae while An. funestus (s.s.) was mostly caught between 1:00 and 6:00 h and An. ziemanni having an early evening biting behaviour (18:00-00:00 h). Female Anopheles were mostly observed to have relative high parity rates (≥ 70%), with the exception of the Meanja site where species parity varies from 46 to 55%. Overall, the transmission level was low with entomological inoculation rates estimated to 0.7 infected bites per person per month (ib/p/mth) in Tiko and Ndop, 1.4 ib/p/mth in Mamfe and 2.24 ib/p/mth in Santchou.

CONCLUSIONS

The present study represents detailed Anopheles vector characterisation from an understudied area along the volcanic chain of Cameroon with endemic malaria transmission. The significant differences in bionomics and Anopheles species distribution within the studied wetlands, highlights the importance of providing baseline data and an opportunity to assess the outcome of ongoing malaria control interventions in the country.

The transcription factor Maf-S regulates metabolic resistance to insecticides in the malaria vector Anopheles gambiae

Background

Malaria control in Africa is dependent upon the use insecticides but intensive use of a limited number of chemicals has led to resistance in mosquito populations. Increased production of enzymes that detoxify insecticides is one of the most potent resistance mechanisms. Several metabolic enzymes have been implicated in insecticide resistance but the processes controlling their expression have remained largely elusive.

Results

Here, we show that the transcription factor Maf-S regulates expression of multiple detoxification genes, including the key insecticide metabolisers CYP6M2 and GSTD1 in the African malaria vector Anopheles gambiae. Attenuation of this transcription factor through RNAi induced knockdown reduced transcript levels of these effectors and significantly increased mortality after exposure to the pyrethroid insecticides and DDT (permethrin: 9.2% to 19.2% (p = 0.015), deltamethrin: 3.9% to 21.6% (p = 0.036) and DDT: 1% to 11.7% (p = <0.01), whilst dramatically decreasing mortality induced by the organophosphate malathion (79.6% to 8.0% (p = <0.01)). Additional genes regulated by Maf-S were also identified providing new insight into the role of this transcription factor in insects.

Conclusion

Maf-S is a key regulator of detoxification genes in Anopheles mosquitoes. Disrupting this transcription factor has opposing effects on the mosquito’s response to different insecticide classes providing a mechanistic explanation to the negative cross resistance that has been reported between pyrethroids and organophosphates.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4086-7) contains supplementary material, which is available to authorized users.

Larvicidal Activity of Essential Oil Constituents against Malaria Vector, Anopheles gambiae (Diptera: Culicidae)

Malaria is one of the most important public health problems worldwide. This illness is controlled, mainly, by combating the vector mosquitoes using chemical insecticides, but this use has caused environmental impact and the emergence of tolerance in adult mosquitoes. Herein, we report the larvicidal activity of nine chemical constituents found in essential oils against third-instar larvae of Anopheles gambiae. All the tested compounds showed larval toxicity. Among the nine effective components, citronellol exhibited a pronounced larvicidal effect against the larvae of An. gambiae, with LC50 values of 96.1 and 55.6 ppm after 12 and 24 hours of treatment, respectively; however, hydroxycitronellal was the most potent compound after 48 and 72 h of exposure (LC50=3.7 and 2.3 ppm, respectively), followed by citronellol (LC50=16.7 and 6.3 ppm, respectively). Moreover, larval mortality was concentration- and time-dependent.

Shift in species composition in the Anopheles gambiae complex after implementation of long-lasting insecticidal nets in Dielmo, Senegal

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are the cornerstones of malaria vector control. However, the effectiveness of these control tools depends on vector ecology and behaviour, which also largely determine the efficacy of certain Anopheles mosquitoes (Diptera: Culicidae) as vectors. Malaria vectors in sub-Saharan Africa are primarily species of the Anopheles gambiae complex, which present intraspecific differences in behaviour that affect how they respond to vector control tools. The focus of this study is the change in species composition in the An. gambiae complex after the implementation of LLINs in Dielmo, Senegal. The main findings referred to dramatic decreases in the proportions of Anopheles coluzzii and An. gambiae after the introduction of LLINs, and an increase in the proportion of Anopheles arabiensis. Two years after LLINs were first introduced, An. arabiensis remained the most prevalent species and An. gambiae had begun to rebound. This indicated a need to develop additional vector control tools that can target the full range of malaria vectors.

High susceptibility of wild Anopheles funestus to infection with natural Plasmodium falciparum gametocytes using membrane feeding assays

Background

Anopheles funestus is a major vector of malaria in sub-Saharan Africa. However, because it is difficult to colonize, research on this mosquito species has lagged behind other vectors, particularly the understanding of its susceptibility and interactions with the Plasmodium parasite. The present study reports one of the first experimental infections of progeny from wild-caught An. funestus with the P. falciparum parasite providing a realistic avenue for the characterisation of immune responses associated with this infection.

Methods

Wild-fed resting An. funestus females were collected using electric aspirators and kept in cages for four days until they were fully gravid and ready to oviposit. The resulting eggs were reared to adults F1 mosquitoes under insectary conditions. Three to five day-old An. funestus F1 females were fed with infected blood taken from gametocyte carriers using an artificial glass-parafilm feeding system. Feeding rate was recorded and fed mosquitoes were dissected at day 7 to count oocysts in midguts. Parallel experiments were performed with the known Plasmodium-susceptible An. coluzzii Ngousso laboratory strain, to monitor our blood handling procedures and infectivity of gametocytes.

Results

The results revealed that An. funestus displays high and similar level of susceptibility to Plasmodium infection compared to An. coluzzii, and suggest that our methodology produces robust feeding and infection rates in wild An. funestus progeny. The prevalence of infection in An. funestus mosquitoes was 38.52 % (range 6.25–100 %) and the median oocyst number was 12.5 (range 1–139). In parallel, the prevalence in An. coluzzii was 39.92 % (range 6.85–97.5 %), while the median oocyst number was 32.1 (range 1–351).

Conclusions

Overall, our observations are in line with the fact that both species are readily infected with P. falciparum, the most common and dangerous malaria parasite in sub-Saharan Africa, and since An. funestus is widespread throughout Africa, malaria vector control research and implementation needs to seriously address this vector species too. Additionally, the present work indicates that it is feasible to generate large number of wild F1 infected An. funestus mosquitoes using membrane feeding assays, which can be used for comprehensive study of interactions with the Plasmodium parasite.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1626-y) contains supplementary material, which is available to authorized users.

Population genetics of the Asian tiger mosquito Aedes albopictus, an invasive vector of human diseases

The Asian tiger mosquito Aedes albopictus is currently one of the most threatening invasive species in the world. Native to Southeast Asia, the species has spread throughout the world in the past 30 years and is now present in every continent but Antarctica. Because it was the main vector of recent Dengue and Chikungunya outbreaks, and because of its competency for numerous other viruses and pathogens such as the Zika virus, A. albopictus stands out as a model species for invasive diseases vector studies. A synthesis of the current knowledge about the genetic diversity of A. albopictus is needed, knowing the interplays between the vector, the pathogens, the environment and their epidemiological consequences. Such resources are also valuable for assessing the role of genetic diversity in the invasive success. We review here the large but sometimes dispersed literature about the population genetics of A. albopictus. We first debate about the experimental design of these studies and present an up-to-date assessment of the available molecular markers. We then summarize the main genetic characteristics of natural populations and synthesize the available data regarding the worldwide structuring of the vector. Finally, we pinpoint the gaps that remain to be addressed and suggest possible research directions.

Insecticide resistance status of the Anopheles funestus population in Central African Republic: a challenge in the war

Background

In the Central African Republic, malaria is a major public health problem and the leading cause of death among children. This disease appears to be hyperendemic but no substantial entomological data, including data on Anopheles spp. susceptibility to insecticides, is available. This study evaluates, for the first time in the CAR, the status of insecticide resistance in the Anopheles funestus population, the second major vector of malaria in Africa.

Methods

WHO standard bioassay susceptibility tests were performed on the An. funestus population using F1 generation from gravid females mosquitoes (F0) collected by manual aspirator sampling of households in Gbanikola, Bangui in October 2014 to assess: (i) An. funestus susceptibility to bendiocarb, malathion, permethrin, lamda-cyhalothrin, deltamethrin and DDT, and (ii) the effect of pre-exposure to the piperonyl butoxide (PBO) synergist on insecticide susceptibility. Additional tests were conducted to investigate metabolic resistance status (cytochrome P450 monooxygenases, glutathione S-transferases, and esterases).

Results

A high phenotypic resistance of An. funestus population to malathion, DDT and pyrethroids was observed with a mortality rate ranging from 23 to 74 %. For the pyrethroid groups, the mortality rate was 35, 31 and 23 % for lambda-cyhalothrin, deltamethrin, and permethrin, respectively. In contrast a 100 % mortality rate to bendiocarb was recorded. Knockdown time (KDT) was long for all pyrethroids, DDT and malathion with KDT₅₀ higher than 50 min. Pre-exposure of An. funestus to PBO synergist significantly restored susceptibility to all pyrethroids (Fisher's exact test P <0.0001) but not in DDT (Fisher's exact test P = 0.724). Data from biochemical tests suggest the involvement of cytochrome P450 monooxygenases, esterases and glutatione S-transferases in the resistance of An. funestus population from Gbanikola (Wilcoxon test P <0.05).

Conclusion

Evidence of biochemical resistance to insecticide was detected in An. funestus population from the district of Gbanikola, Bangui. This study suggests that detoxifying enzymes are involved in insecticide resistance of An. funestus. However, despite disruptive violence, further research is urgently needed to assess the insecticide susceptibility status of An. funestus population in all CAR regions; insecticide resistance could rapidly compromise the success of malaria control programs.

Reusing larval rearing water and its effect on development and quality of Anopheles arabiensis mosquitoes

Background

There is growing interest in applying the sterile insect technique (SIT) against mosquitoes. Mass production of mosquitoes for large-scale releases demands a huge amount of water. Yet, many arid and/or seasonally arid countries face the difficulties of acute water shortage, deterioration of water quality and environmental constraints. The re-use of water to rear successive generations of larvae is attractive as a way to reduce water usage and running costs, and help to make this control method viable.

Methods

To determine whether dirty larval water was a suitable rearing medium for Anopheles arabiensis, in place of the ‘clean’ dechlorinated water routinely used, a series of three experiments was carried out to evaluate the effect of dirty water or mixed clean and dirty water on several parameters of insect quality. Batches of 100 fresh eggs were distributed in dirty water or added to clean water to test the effect of dirty water on egg hatching, whereas first-instar larvae were used to determine the effect on immature development time, pupation, adult emergence, body size, and longevity. Moreover, to assess the effect of dirty water on larval mortality, pupation rate, adult emergence, and longevity, L4 larvae collected after the tilting or larvae/pupae separation events were returned either to the dirty water or added to clean water.

Results

Results indicated that reusing dirty water or using a 50:50 mix of clean and dirty water did not affect egg hatching. Moreover, no difference was found in time to pupation, larval mortality or sex ratio when first-instar larvae were added to clean water, dirty water, or a 75:25, 50:50 or 25:75 mix of clean and dirty water and reared until emergence. When late-instar larvae were put back into their own rearing water, there was no effect on pupation rate, emergence rate or female longevity, though male longevity was reduced. When reared from first-instar larvae, however, dirty water decreased pupation rate, emergence rate, body size, and adult longevity.

Conclusions

Re-used larval-rearing water has no impact on egg hatching, development time or mortality of the immature stages of An. arabiensis. However, dirty water is not suitable for the production of high quality adult mosquitoes. Recycling processes to improve water quality and increase insect quality will be investigated, since it may have important implications for the implementation of the SIT in areas where clean water is a scarce or costly resource.

Modified mosquito landing boxes dispensing transfluthrin provide effective protection against Anopheles arabiensis mosquitoes under simulated outdoor conditions in a semi-field system

BACKGROUND

Efforts to control malaria vectors have primarily focused on scaling-up of long-lasting insecticidal nets (LLINs) and indoor residual spraying. Although highly efficient against indoor-biting and indoor-resting vectors, these interventions have lower impact on outdoor-biting mosquitoes. Innovative vector control tools are required to prevent outdoor human-mosquito contacts. In this work, the potential of spatial repellents, delivered in an active system that requires minimal user compliance, to provide personal protection against exophagic mosquitoes active in the early evening was explored.

METHODS

A device previously used as an odour-baited lure and kill apparatus, the mosquito landing box (MLB), was modified to dispense the volatile synthetic pyrethroid, transfluthrin, as a spatial repellent. The MLB has an active odour-dispensing mechanism that uses a solar-powered fan and switches on at dusk to provide long duration dispensing of volatile compounds without the need for the user to remember to employ it. Two MLBs were located 5 m from a human volunteer to investigate the repellent effects of a transfluthrin 'bubble' created between the MLBs. Transfluthrin was emanated from polyester strips, hanging inside the MLB odour-dispensing unit. A fully randomized cross-over design was performed in a large, semi-field, screened cage to assess the effect of the repellent against laboratory-reared Anopheles arabiensis mosquitoes under ambient outdoor conditions. The knock-down capacity of the transfluthrin-treated strips was also evaluated at different time points up to 3 weeks after being impregnated to measure duration of efficacy.

RESULTS

The protective transfluthrin bubble provided 68.9% protection against An. arabiensis bites under these simulated outdoor conditions. Volatile transfluthrin caused low mortality among mosquitoes in the semi-field system. Transfluthrin-treated strips continued to knock down mosquitoes in laboratory tests, 3 weeks after impregnation, although this effect diminished with time.

CONCLUSION

Modified MLBs can be used as efficient and long-lasting dispensers of volatile spatial repellents such as transfluthrin, thereby providing high levels of protection against outdoor-biting mosquitoes in the peri-domestic space. They have a potential role in combatting outdoor malaria transmission without interfering with effective indoor interventions such as LLINs.

Does malaria epidemiology project Cameroon as 'Africa in miniature'?

Cameroon, a west-central African country with a ~ 20 million population, is commonly regarded as 'Africa in miniature' due to the extensive biological and cultural diversities of whole Africa being present in a single-country setting. This country is inhabited by ancestral human lineages in unique eco-climatic conditions and diverse topography. Over 90 percent Cameroonians are at risk of malaria infection, and ~ 41 percent have at least one episode of malaria each year. Historically, the rate of malaria infection in Cameroon has fluctuated over the years; the number of cases was about 2 million in 2010 and 2011. The Cameroonian malaria control programme faces an uphill task due to high prevalence of multidrug-resistant parasites and insecticide-resistant malaria vectors. Above all, continued human migration from the rural to urban areas as well as population exchange with adjoining countries, high rate of ecological instabilities caused by deforestation, poor housing, lack of proper sanitation and drainage system might have resulted in the recent increase in incidences of malaria and other vector-borne diseases in Cameroon. The available data on eco-environmental variability and intricate malaria epidemiology in Cameroon reflect the situation in the whole of Africa, and warrant the need for in-depth study by using modern surveillance tools for meaningful basic understanding of the malaria triangle (host-parasite-vector-environment).

Capacity of mosquitoes to transmit malaria depends on larval environment

BACKGROUND

Adult traits of holometabolous insects such as reproduction and survival can be shaped by conditions experienced during larval development. These "carry-over" effects influence not only individual life history and fitness, but can also impact interactions between insect hosts and parasites. Despite this, the implications of larval conditions for the transmission of human, wildlife and plant diseases that are vectored by insects remain poorly understood.

METHODS

We used Anopheles stephensi mosquitoes and the rodent malaria, Plasmodium yoelii yoelii, to investigate whether quality of larval habitat influenced vectorial capacity of adult mosquitoes. Larvae were reared under two dietary conditions; one group received a diet commonly used for colony maintenance (0.3 mg/individual/day of Tetrafin fish food) while the other group received a reduced food diet (0.1 mg/individual/day). Upon emergence, adults were provided an infectious blood feed. We assessed the effects of diet on a range of larval and adult traits including larval development times and survival, number of emerging adults, adult body size and survival, gonotrophic cycle length, and mating success. We also estimated the effects of larval diet on parasite infection rates and growth kinetics within the adult mosquitoes.

RESULTS

Larval dietary regime affected larval survival and development, as well as size, reproductive success and survival of adult mosquitoes. Larval diet also affected the intensity of initial Plasmodium infection (oocyst stage) and parasite replication, but without differences in overall infection prevalence at either the oocyst or sporozoite stage.

CONCLUSIONS

Together, the combined effects led to a relative reduction in vectorial capacity (a measure of the transmission potential of a mosquito population) in the low food treatment of 70%. This study highlights the need to consider environmental variation at the larval stages to better understand transmission dynamics and control of vector-borne diseases.

Novel insights into the metabolic and biochemical underpinnings assisting dry-season survival in female malaria mosquitoes of the Anopheles gambiae complex

The mechanisms by which Anopheles gambiae mosquitoes survive the desiccating conditions of the dry season in Africa and are able to readily transmit malaria soon after the rains start remain largely unknown. The desiccation tolerance and resistance of female An. gambiae M and S reared in contrasting environmental conditions reflecting the onset of dry season ("ods") and the rainy season ("rs") was determined by monitoring their survival and body water loss in response to low relative humidity. Furthermore, we investigated the degree to which the physiology of 1-h and 24-h-old females is altered at "ods" by examining and comparing their quantitative metabotypes and proteotypes with conspecifics exposed to "rs" conditions. Results showed that distinct biochemical rearrangements occurred soon after emergence in female mosquitoes that enhance survival and limit water loss under dry conditions. In particular, three amino acids (phenylalanine, tyrosine, and valine) playing a pivotal role in cuticle permeability decreased significantly from the 1-h to 24-h-old females, regardless of the experimental conditions. However, these amino acids were present in higher amounts in 1-h-old female An. gambiae M reared under "ods" whereas no such seasonal difference was reported in S ones. Together with the 1.28- to 2.84-fold increased expression of cuticular proteins 70 and 117, our data suggests that cuticle composition, rigidity and permeability were adjusted at "ods". Increased expression of enzymes involved in glycogenolytic and proteolytic processes were found in both forms at "ods". Moreover, 1-h-old S forms were characterised by elevated amounts of glycogen phosphorylase, isocitrate dehydrogenase, and citrate synthase, suggesting an increase of energetic demand in these females at "ods".

The niche reduction approach: an opportunity for optimal control of infectious diseases in low-income countries?

BACKGROUND

During the last century, WHO led public health interventions that resulted in spectacular achievements such as the worldwide eradication of smallpox and the elimination of malaria from the Western world. However, besides major successes achieved worldwide in infectious diseases control, most elimination/control programs remain frustrating in many tropical countries where specific biological and socio-economical features prevented implementation of disease control over broad spatial and temporal scales. Emblematic examples include malaria, yellow fever, measles and HIV. There is consequently an urgent need to develop affordable and sustainable disease control strategies that can target the core of infectious diseases transmission in highly endemic areas.

DISCUSSION

Meanwhile, although most pathogens appear so difficult to eradicate, it is surprising to realize that human activities are major drivers of the current high rate of extinction among upper organisms through alteration of their ecology and evolution, i.e., their "niche". During the last decades, the accumulation of ecological and evolutionary studies focused on infectious diseases has shown that the niche of a pathogen holds more dimensions than just the immune system targeted by vaccination and treatment. Indeed, it is situated at various intra- and inter- host levels involved on very different spatial and temporal scales. After developing a precise definition of the niche of a pathogen, we detail how major advances in the field of ecology and evolutionary biology of infectious diseases can enlighten the planning and implementation of infectious diseases control in tropical countries with challenging economic constraints.

SUMMARY

We develop how the approach could translate into applied cases, explore its expected benefits and constraints, and we conclude on the necessity of such approach for pathogen control in low-income countries.

Anopheles ziemanni a locally important malaria vector in Ndop health district, north west region of Cameroon

BACKGROUND

Malaria transmission in Cameroon is mediated by a plethora of vectors that are heterogeneously distributed across the country depending on the biotope. To effectively guide malaria control operations, regular update on the role of local Anopheles species is essential. Therefore, an entomological survey was conducted between August 2010 and May 2011 to evaluate the role of the local anopheline population in malaria transmission in three villages of the Ndop health district in the northwest region of Cameroon where malaria is holoendemic, as a means to acquiring evidence based data for improved vector intervention.

METHODS

Mosquitoes were sampled both indoor and outdoor for four consecutive nights in each locality during each month of survey. Sampling was done by the human landing catch method on volunteers. Anopheles species were identified morphologically and their ovaries randomly dissected for parity determination. Infection with Plasmodium falciparum was detected by Circumsporozoite protein ELISA. Members of An. gambiae complex were further identified to molecular level by PCR and RFLP PCR.

RESULTS

An. ziemanni was the main malaria vector and whether outdoor or indoor. The man biting rate for the vectors ranged from 6.75 to 8.29 bites per person per night (b/p/n). The entomological inoculation rate for this vector species was 0.0278 infectious bites per person per night (ib/p/n) in Mbapishi, 0.034 ib/p/n in Mbafuh, and 0.063 ib/p/n in Backyit. These were by far greater than that for An. gambiae. No difference was observed in the parity rate of these two vectors. PCR analysis revealed the presence of only An. colluzzi (M- form).

CONCLUSIONS

An. ziemanni is an important local malaria vector in Ndop health district. The findings provide useful baseline information on the anopheles species composition, their distribution and role in malaria transmission that would guide the implementation of integrated vector management strategies in the locality.

Multigene phylogenetics reveals temporal diversification of major African malaria vectors

The major vectors of malaria in sub-Saharan Africa belong to subgenus Cellia. Yet, phylogenetic relationships and temporal diversification among African mosquito species have not been unambiguously determined. Knowledge about vector evolutionary history is crucial for correct interpretation of genetic changes identified through comparative genomics analyses. In this study, we estimated a molecular phylogeny using 49 gene sequences for the African malaria vectors An. gambiae, An. funestus, An. nili, the Asian malaria mosquito An. stephensi, and the outgroup species Culex quinquefasciatus and Aedes aegypti. To infer the phylogeny, we identified orthologous sequences uniformly distributed approximately every 5 Mb in the five chromosomal arms. The sequences were aligned and the phylogenetic trees were inferred using maximum likelihood and neighbor-joining methods. Bayesian molecular dating using a relaxed log normal model was used to infer divergence times. Trees from individual genes agreed with each other, placing An. nili as a basal clade that diversified from the studied malaria mosquito species 47.6 million years ago (mya). Other African malaria vectors originated more recently, and independently acquired traits related to vectorial capacity. The lineage leading to An. gambiae diverged 30.4 mya, while the African vector An. funestus and the Asian vector An. stephensi were the most closely related sister taxa that split 20.8 mya. These results were supported by consistently high bootstrap values in concatenated phylogenetic trees generated individually for each chromosomal arm. Genome-wide multigene phylogenetic analysis is a useful approach for discerning historic relationships among malaria vectors, providing a framework for the correct interpretation of genomic changes across species, and comprehending the evolutionary origins of this ubiquitous and deadly insect-borne disease.

Cytogenetic analysis of Anopheles ovengensis revealed high structural divergence of chromosomes in the Anopheles nili group

Cytogenetic analysis is an informative classical approach to understanding the relationships among members in a group of closely related species of mosquitoes. Anopheles ovengensis is a recently discovered species of the Anopheles nili group and is one of the important malaria vectors in the African equatorial forest. This study characterized polytene chromosomes of An. ovengensis and compared them with polytene chromosomes of An. nili. Using fluorescent in situ hybridization and chromosome banding pattern comparison we have established correspondence between chromosomal arms of An. ovengensis and An. nili. Analysis of chromosome morphology in the two species revealed a limited similarity in the banding patterns. The most extensive reorganization occurs in pericentromeric and intercalary heterochromatin. Chromosomes of An. ovengensis are joined together by a diffuse chromocenter and they have two large regions of intercalary heterochromatin in arms 2L and 3R. In contrast, the chromocenter and intercalary heterochromatin are not seen in An. nili chromosomes. Comparative analysis of the arm association suggests the occurrence of a whole-arm translocation between the two members of the group. The observed, substantial reorganizations of chromosome structure implies either a rapid rate of chromosome evolution in the An. nili group, or that the two species belong to different taxonomic groups within subgenus Cellia.

Cryptic genetic diversity within the Anopheles nili group of malaria vectors in the equatorial forest area of Cameroon (Central Africa)

BACKGROUND

The Anopheles nili group of mosquitoes includes important vectors of human malaria in equatorial forest and humid savannah regions of sub-Saharan Africa. However, it remains largely understudied, and data on its populations' bionomics and genetic structure are crucially lacking. Here, we used a combination of nuclear (i.e. microsatellite and ribosomal DNA) and mitochondrial DNA markers to explore and compare the level of genetic polymorphism and divergence among populations and species of the group in the savannah and forested areas of Cameroon, Central Africa.

PRINCIPAL FINDINGS

All the markers provided support for the current classification within the An. nili group. However, they revealed high genetic heterogeneity within An. nili s.s. in deep equatorial forest environment. Nuclear markers showed the species to be composed of five highly divergent genetic lineages that differed by 1.8 to 12.9% of their Internal Transcribed Spacer 2 (ITS2) sequences, implying approximate divergence time of 0.82 to 5.86 million years. However, mitochondrial data only detected three major subdivisions, suggesting different evolutionary histories of the markers.

CONCLUSIONS/SIGNIFICANCE

This study enlightened additional cryptic genetic diversity within An. nili s.s. in the deep equatorial forest environment of South Cameroon, reflecting a complex demographic history for this major vector of malaria in this environment. These preliminary results should be complemented by further studies which will shed light on the distribution, epidemiological importance and evolutionary history of this species group in the African rainforest, providing opportunities for in-depth comparative studies of local adaptation and speciation in major African malaria vectors.

Anopheles moucheti and Anopheles vinckei are candidate vectors of ape Plasmodium parasites, including Plasmodium praefalciparum in Gabon

During the last four years, knowledge about the diversity of Plasmodium species in African great apes has considerably increased. Several new species were described in chimpanzees and gorillas, and some species that were previously considered as strictly of human interest were found to be infecting African apes. The description in gorillas of P. praefalciparum, the closest relative of P. falciparum which is the main malignant agent of human malaria, definitively changed the way we understand the evolution and origin of P. falciparum. This parasite is now considered to have appeared recently, following a cross-species transfer from gorillas to humans. However, the Plasmodium vector mosquito species that have served as bridge between these two host species remain unknown. In order to identify the vectors that ensure ape Plasmodium transmission and evaluate the risk of transfer of these parasites to humans, we carried out a field study in Gabon to capture Anopheles in areas where wild and semi-wild ape populations live. We collected 1070 Anopheles females belonging to 15 species, among which An. carnevalei, An. moucheti and An. marshallii were the most common species. Using mtDNA-based PCR tools, we discovered that An. moucheti, a major human malaria vector in Central Africa, could also ensure the natural transmission of P. praefalciparum among great apes. We also showed that, together with An. vinckei, An. moucheti was infected with P. vivax-like parasites. An. moucheti constitutes, therefore, a major candidate for the transfer of Plasmodium parasites from apes to humans.

Impact of cyfluthrin (Solfac EW050) impregnated bed nets on malaria transmission in the city of Mbandjock : lessons for the nationwide distribution of long-lasting insecticidal nets (LLINs) in Cameroon

Background

Insecticide treated materials remain the mainstay for malaria prevention. The current study reports on the entomological impact of cyfluthrin impregnated bed nets on malaria transmission in Mbandjock, a semi urban locality in southern Cameroon. Several findings pertaining to the recent distribution of LLINs across Cameroon are discussed.

Methods

Malaria transmission and vector bionomics were monitored before and after impregnated net coverage. Bed nets were distributed in Mbandjock, whereas the locality of Nkoteng was free of bed nets during the entire study period. January to June 1997 represented the period before bed net coverage and September 1997 to September 1998 was the period after bed net coverage. Adult mosquitoes were collected by human landing catches. Mosquito genus and species were identified with morphological and molecular diagnostic tools. Anopheline salivary glands and ovaries were dissected to determine female infectious status and parity rates respectively.

Results

A total of 6959 anophelines corresponding to 6029 in Mbandjock and 930 in Nkoteng were collected in the course of the study. Seven species were recorded in both cities : Anopheles coustani, An. funestus, An. gambiae sl, An. moucheti, An. ziemanni, An. nili and An. paludis. An. gambiae s.l. (>95% An. gambiae S molecular form) was the most abundant species representing 75.6% and 86.6% of the total anophelines caught in Mbandjock before and after bed net coverage respectively. The human biting rate (HBR) in Mbandjock decreased from 17 bites/human/night before bed net coverage to less than 4 bites/human/night during the first 7 months following impregnated bed net coverage. A significant decrease of mosquito parity rate was recorded when comparing the period before (52%) and after (46.5%) bed net distribution. The average infection rate of malaria vectors significantly decreased from 5.3% before to 1.8% after bed net coverage (p < 0.0001). The entomological inoculation rate in Mbandjock was reduced by 74% varying from 124.1 infected bites/human/year before bed net distribution, to 32.5 infected bites/human/year after bed net coverage. All entomological indexes were relatively stable in Nkoteng and no reduction of malaria transmission was recorded in this locality.

Conclusion

The study confirms the effectiveness of cyfluthrin impregnated nets in reducing malaria transmission. Lessons from this study could be essential to draw guidelines for the management of the recent nationwide distribution of LLINs across Cameroon in 2011.

Molecular systematics and insecticide resistance in the major African malaria vector Anopheles funestus

Anopheles funestus is one of three major African vectors of malaria. Its distribution extends over much of the tropics and subtropics wherever suitable swampy breeding habitats are present. As with members of the Anopheles gambiae complex, An. funestus shows marked genetic heterogeneity across its range. Currently, two unnamed species are recognized in the group, with molecular and cytogenetic data indicating that more may be present. The control of malaria vectors in Africa has received increased attention in the past decade with the scaling up of insecticide-treated bed nets and indoor residual house spraying. Also in the past decade, the frequency of insecticide-resistant mosquitoes has increased exponentially. Whether this increase is in response to vector control initiatives or because of insecticide use in agriculture is debatable. In this article we examine the progress made on the systematics of the An. funestus group and review research on insecticide resistance and its mechanisms.

Use of a mixture statistical model in studying malaria vectors density

Vector control is a major step in the process of malaria control and elimination. This requires vector counts and appropriate statistical analyses of these counts. However, vector counts are often overdispersed. A non-parametric mixture of Poisson model (NPMP) is proposed to allow for overdispersion and better describe vector distribution. Mosquito collections using the Human Landing Catches as well as collection of environmental and climatic data were carried out from January to December 2009 in 28 villages in Southern Benin. A NPMP regression model with “village” as random effect is used to test statistical correlations between malaria vectors density and environmental and climatic factors. Furthermore, the villages were ranked using the latent classes derived from the NPMP model. Based on this classification of the villages, the impacts of four vector control strategies implemented in the villages were compared. Vector counts were highly variable and overdispersed with important proportion of zeros (75%). The NPMP model had a good aptitude to predict the observed values and showed that: i) proximity to freshwater body, market gardening, and high levels of rain were associated with high vector density; ii) water conveyance, cattle breeding, vegetation index were associated with low vector density. The 28 villages could then be ranked according to the mean vector number as estimated by the random part of the model after adjustment on all covariates. The NPMP model made it possible to describe the distribution of the vector across the study area. The villages were ranked according to the mean vector density after taking into account the most important covariates. This study demonstrates the necessity and possibility of adapting methods of vector counting and sampling to each setting.

Characterization of malaria transmission by vector populations for improved interventions during the dry season in the Kpone-on-Sea area of coastal Ghana

Background

Malaria is a major public health problem in Ghana. We present a site-specific entomological study of malaria vectors and transmission indices as part of an effort to develop a site for the testing of improved control strategies including possible vaccine trials.

Methods

Pyrethrum spray catches (PSC), and indoor and outdoor human landing collections of adult female anopheline mosquitoes were carried out over a six-month period (November 2005 - April 2006) at Kpone-on-Sea, a fishing village in southern Ghana. These were morphologically identified to species level and sibling species of the Anopheles gambiae complex further characterized by the polymerase chain reaction (PCR). Enzyme-linked immunosorbent assay was used to detect Plasmodium falciparum mosquito infectivity and host blood meal sources. Parity rate was examined based on dilatation of ovarian tracheoles following dissection.

Results

Of the 1233 Anopheles mosquitoes collected, An. gambiae s.l. was predominant (99.5%), followed by An. funestus (0.4%) and An. pharoensis (0.1%). All An. gambiae s.l. examined (480) were identified as An. gambiae s.s. with a majority of M molecular form (98.2%) and only 1.8% S form with no record of M/S hybrid. A significantly higher proportion of anophelines were observed outdoors relative to indoors (χ² = 159.34, df = 1, p < 0.0000). Only An. gambiae M molecular form contributed to transmission with a high degree of anthropophily, parity rate and an estimated entomological inoculation rate (EIR) of 62.1 infective bites/person/year. The Majority of the infective bites occurred outdoors after 09.00 pm reaching peaks between 12.00-01.00 am and 03.00-04.00 am.

Conclusion

Anopheles gambiae M molecular form is responsible for maintaining the status quo of malaria in the surveyed site during the study period. The findings provide a baseline for evidence-based planning and implementation of improved malaria interventions. The plasticity observed in biting patterns especially the combined outdoor and early biting behavior of the vector may undermine the success of insecticide-based strategies using insecticide treated nets (ITN) and indoor residual spray (IRS). As such, novel or improved vector interventions should be informed by the local malaria epidemiology data as it relates to vector behavior.

Seasonal prevalence of malaria vectors and entomological inoculation rates in the rubber cultivated area of Niete, South Region of Cameroon

Background

Development of large scale agro-industries are subject to serious environmental modifications. In malaria endemic areas this would greatly impact on the transmission paradigm. Two cross-sectional entomological surveys to characterize the Anopheles fauna and their entomological inoculation rates were conducted during May 2010 (peak rainy season) and December 2010 (peak dry season) in the intense rubber cultivated area of Niete in southern forested Cameroon.

Methods

Mosquitoes were sampled by night collections on human volunteers, identified morphologically and members of the Anopheles gambiae complex further identified to species and molecular form. Parity status was determined following the dissection of the ovaries. Plasmodium falciparum circumsporozoite antigen indices were estimated after the identification of CS antigen by ELISA and the average entomological inoculation rates determined.

Results

A total of 1187 Anopheles was collected, 419 (35.3%) in the rainy season and 768 (64.7%) in the dry season. Species found were the M molecular form of An. gambiae s.s (66.8%), An. ziemanni (28.3%), An. paludis (4.7%), An. smithii (0.2%). An. gambiae M-form was the principal species in the dry (56.2%) and wet (86.2%) seasons. Average overall entomological inoculation rate for the malaria vectors varied between the dry season (1.09 ib/p/n) and the rainy season (2.30 ib/p/n).

Conclusions

Malaria transmission in Niete occurs both in the dry and rainy season with the intensities peaking in the dry season. This is unlike previous studies in other areas of southern forested Cameroon where transmission generally peaks in the rainy season. Environmental modifications due to agro-industrial activities might have influenced vector distribution and the dynamics of malaria transmission in this area. This necessitates the possible implementation of control strategies that are related to the eco-geography of the area.

Advances in our understanding of the epidemiology of Plasmodium and schistosome infection: informing coinfection studies

PURPOSE OF REVIEW

Schistosomes and Plasmodium parasites have complex patterns of transmission, leading to differing dynamics of host-parasite interactions across study sites and a bias of studying differing age groups in monoinfection studies. Combined, these infections lead to difficulties in conducting and interpreting human coinfection studies.

RECENT FINDINGS

Interactions between the two parasites may affect morbidity associated with either infection; both by influencing mechanisms directly associated with the development of those morbidities and by influencing mechanisms associated with resistance or susceptibility to the other infection. However, conflicting results are reported due to inherent difficulties in studying coinfections. More studies with stringent designs are required to clarify interactions between the two parasites. Recent monoinfection studies indicate that further coinfection studies may need to have a wider age range than previously studied. These studies also need to harness new techniques, both for data collection and analysis that are being developed for modern epidemiological studies. These techniques will allow an essential multidisciplinary approach to be taken.

SUMMARY

Coinfection with Plasmodium and schistosome infection has implications for the health of children of all ages in sub-Saharan Africa. It is important to gain further understanding of the interactions between the two parasites in all age groups.

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.

Larval habitat segregation between the molecular forms of the mosquito Anopheles gambiae in a rice field area of Burkina Faso, West Africa

In West Africa, lineage splitting between the M and S molecular forms of the major Afro-tropical malaria mosquito, Anopheles gambiae (Diptera: Culicidae), is thought to be driven by ecological divergence, occurring mainly at the larval stage. Here, we present evidence for habitat segregation between the two molecular forms in and around irrigated rice fields located within the humid savannahs of western Burkina Faso. Longitudinal sampling of adult mosquitoes emerging from a range of breeding sites distributed along a transect extending from the heart of the rice field area into the surrounding savannah was conducted from June to November 2009. Analysis revealed that the two molecular forms and their sibling species Anopheles arabiensis are not randomly distributed in the area. A major ecological gradient was extracted in relation to the perimeter of the rice fields. The M form was associated with larger breeding sites mostly consisting of rice paddies, whereas the S form and An. arabiensis were found to depend upon temporary, rain-filled breeding sites. These results support hypotheses about larval habitat segregation and confirm the suggestion that the forms have different larval habitat requirements. Segregation appears to be clearly linked to anthropogenic permanent habitats and the community structure they support.

Using infections to fight infections: paratransgenic fungi can block malaria transmission in mosquitoes

EVALUATION OF: Fang W, Vega-Rodríguez J, Ghosh AK et al. Development of transgenic fungi that kill human malaria parasites in mosquitoes. Science 331(6020), 1074-1077 (2011). Paratransgenesis is the genetic manipulation of insect endosymbiotic microorganisms such as bacteria, viruses or fungi. Paratransgenesis has been proposed as a potential method to control vector-borne diseases such as malaria. In this article, Fang and colleagues have used genetic manipulation to insert multiple antimalaria effector genes into the entomopathogenic fungus Metarhizium anisopliae. When the modified fungus was used to infect Anopheles mosquitoes, it expressed the antimalaria effector molecules in the mosquito hemolymph. When several different effector molecules were coexpressed, malaria levels in the mosquito salivary glands were inhibited by up to 98% compared with controls. Significant inhibition could be initiated by as little as seven fungal spores and was very rapid and long lasting. These data suggest that recombinant entomopathogenic fungi could be deployed as part of a strategy to control malaria.

Bionomics of sympatric chromosomal forms of Anopheles funestus (Diptera: Culicidae)

Anopheles funestus is one of the major vectors of malaria in Africa. Cytogenetic studies conducted on populations from West Africa have shown variable degrees of polymorphism with a genetic structure leading to the description of two chromosomal forms called "Folonzo" and "Kiribina" that exhibit limited gene flow. Because studies on allopatric populations showed bionomical heterogeneities, the present study was undertaken during three consecutive years (2006, 2007, and 2008) in an area of sympatry in Senegal, in order to assess their bionomical characteristics and compare their epidemiologic role in malaria transmission. Overall, the two forms coexisted in the study area; the Kiribina form being more abundant and exhibiting higher biting rates. Based on an enzyme-linked immunosorbent assay, the anthropophilic rates were statistically comparable and were, respectively, 30.7% and 28.6% for Kiribina and Folonzo. Plasmodium falciparum circumsporozoite rates were also comparable and were 2.7% for Kiribina and 3.1% for Folonzo. Both forms were involved in malaria transmission; Kiribina being responsible for 68% of transmission. Thus, due to the limited gene flow between the two forms, the introduction of transgene for Plasmodium resistance in one of the two forms could be a disadvantage for the implementation of control strategies based on the use of Plasmodium-refractory genetically modified individuals. Nevertheless, it could represent an advantage limiting the insurgence of insecticide resistance gene spread between forms and should be taken into account for the implementation of control strategies.

Comparative susceptibility to Plasmodium falciparum of the molecular forms M and S of Anopheles gambiae and Anopheles arabiensis

Background

The different taxa belonging to Anopheles gambiae complex display phenotypic differences that may impact their contribution to malaria transmission. More specifically, their susceptibility to infection, resulting from a co-evolution between parasite and vector, might be different. The aim of this study was to compare the susceptibility of M and S molecular forms of Anopheles gambiae and Anopheles arabiensis to infection by Plasmodium falciparum.

Methods

F3 progenies of Anopheles gambiae s.l. collected in Senegal were infected, using direct membrane feeding, with P. falciparum gametocyte-containing blood sampled on volunteer patients. The presence of oocysts was determined by light microscopy after 7 days, and the presence of sporozoite by ELISA after 14 days. Mosquito species and molecular forms were identified by PCR.

Results

The oocyst rate was significantly higher in the molecular S form (79.07%) than in the M form (57.81%, Fisher's exact test p < 0.001) and in Anopheles arabiensis (55.38%, Fisher's exact test vs. S group p < 0.001). Mean ± s.e.m. number of oocyst was greater in the An. gambiae S form (1.72 ± 0.26) than in the An. gambiae M form (0.64 ± 0.04, p < 0.0001) and in the An. arabiensis group (0.58 ± 0.04, vs. S group, p < 0.0001). Sporozoite rate was also higher in the molecular form S (83.52%) than in form M (50.98%, Fisher's exact test p < 0.001) and Anopheles arabiensis 50.85%, Fisher's exact test vs. S group p < 0.001).

Conclusion

Infected in the same experimental conditions, the molecular form S of An. gambiae is more susceptible to infection by P. falciparum than the molecular form M of An. gambiae and An. arabiensis.

Comparison of the CDC Backpack aspirator and the Prokopack aspirator for sampling indoor- and outdoor-resting mosquitoes in southern Tanzania

Background

Resting mosquitoes can easily be collected using an aspirating device. The most commonly used mechanical aspirator is the CDC Backpack aspirator. Recently, a simple, and low-cost aspirator called the Prokopack has been devised and proved to have comparable performance. The following study evaluates the Prokopack aspirator compared to the CDC backpack aspirator when sampling resting mosquitoes in rural Tanzania.

Methods

Mosquitoes were sampled in- and outdoors of 48 typical rural African households using both aspirators. The aspirators were rotated between collectors and households in a randomized, Latin Square design. Outdoor collections were performed using artificial resting places (large barrel and car tyre), underneath the outdoor kitchen (kibanda) roof and from a drop-net. Data were analysed with generalized linear models.

Results

The number of mosquitoes collected using the CDC Backpack and the Prokopack aspirator were not significantly different both in- and outdoors (indoors p = 0.735; large barrel p = 0.867; car tyre p = 0.418; kibanda p = 0.519). The Prokopack was superior for sampling of drop-nets due to its smaller size. The number mosquitoes collected per technician was more consistent when using the Prokopack aspirator. The Prokopack was more user-friendly: technicians preferred using the it over the CDC backpack aspirator as it weighs considerably less, retains its charge for longer and is easier to manoeuvre.

Conclusions

The Prokopack proved in the field to be more advantageous than the CDC Backpack aspirator. It can be self assembled using simple, low-cost and easily attainable materials. This device is a useful tool for researchers or vector-control surveillance programs operating in rural Africa, as it is far simpler and quicker than traditional means of sampling resting mosquitoes. Further longitudinal evaluations of the Prokopack aspirator versus the gold standard pyrethrum spray catch for indoor resting catches are recommended.

Cytogenetic map for Anopheles nili: application for population genetics and comparative physical mapping

Anopheles nili is one of the major malaria vectors in Africa with a wide geographic distribution. However, the taxonomic and population genetic studies on this species are scarce. New research tools are urgently needed to genetically characterize this important malaria vector. In this study, a high-resolution cytogenetic map was developed for An. nili polytene chromosomes. Chromosomes were straightened and subdivided into 46 numbered divisions according to the banding pattern. Population analysis of An. nili females collected in Burkina Faso revealed the presence of two highly polymorphic inversions on the 2R chromosomal arm. A statistically significant departure from Hardy-Weinberg equilibrium due to a deficit in heterozygotes was detected for inversion 2Rb. To determine chromosome homologies and gene order conservation between An. nili and other major malaria vectors, PCR probes based on the An. gambiae coding sequences were mapped to An. nili chromosomes. Comparative mapping demonstrated that An. nili chromosomes have an An. stephensi-like arm association and that whole-arm translocations and paracentric inversions were the major types of rearrangement in evolution of these mosquitoes. The minimum number of fixed inversions among An. nili, An. gambiae, and An. stephensi was calculated using the Multiple Genome Rearrangements (MGR), Genome Rearrangements In Man and Mouse (GRIMM), and Sorting Permutation by Reversals and block-INterchanGes (SPRING) programs. The data suggest that the An. nili is, at least, as diverged from An. gambiae as An. stephensi. We provide evidence that 2La/a arrangement of An. gambiae is present in outgroup species An. nili and An. stephensi confirming the ancestral status of the 2La inversion in the An. gambiae complex. Availability of the new polytene chromosome map, polymorphic inversions, and physically mapped DNA markers for An. nili will further stimulate population genetic, taxonomic, and genomic studies of this neglected malaria vector.

Exceptional diversity, maintenance of polymorphism, and recent directional selection on the APL1 malaria resistance genes of Anopheles gambiae

The three-gene APL1 locus encodes essential components of the mosquito immune defense against malaria parasites. APL1 was originally identified because it lies within a mapped QTL conferring the vector mosquito Anopheles gambiae natural resistance to the human malaria parasite, Plasmodium falciparum, and APL1 genes have subsequently been shown to be involved in defense against several species of Plasmodium. Here, we examine molecular population genetic variation at the APL1 gene cluster in spatially and temporally diverse West African collections of A. gambiae. The locus is extremely polymorphic, showing evidence of adaptive evolutionary maintenance of genetic variation. We hypothesize that this variability aids in defense against genetically diverse pathogens, including Plasmodium. Variation at APL1 is highly structured across geographic and temporal subpopulations. In particular, diversity is exceptionally high during the rainy season, when malaria transmission rates are at their peak. Much less allelic diversity is observed during the dry season when mosquito population sizes and malaria transmission rates are low. APL1 diversity is weakly stratified by the polymorphic 2La chromosomal inversion but is very strongly subdivided between the M and S “molecular forms.” We find evidence that a recent selective sweep has occurred at the APL1 locus in M form mosquitoes only. The independently reported observation of a similar M-form restricted sweep at the Tep1 locus, whose product physically interacts with APL1C, suggests that epistatic selection may act on these two loci causing them to sweep coordinately.

Immune defense genes are sometimes highly variable in host populations, reflecting selective pressure to combat diverse pathogens. In other instances, where there are only a few dominant pathogens, natural selection may favor only one or a few defense alleles. Here, we show that both adaptive strategies can occur in the same genes under different circumstances. We examined diversity in the APL1 genes of the human malaria vector mosquito Anophleles gambiae, which play a role in defense against malaria parasites. We found that the APL1 genes are exceptionally polymorphic, being 10-fold more diverse than typical A. gambiae genes. The distribution of APL1 allelic diversity, however, is strongly structured depending on whether the genes are carried by the M or S “molecular forms” of the vector, which are thought to constitute newly forming species. We show that despite the evolutionary maintenance of APL1 diversity in the S form of A. gambiae, there is evidence of strong recent directional selection on APL1 genes in the M form. Independent research has shown that Tep1, a gene which encodes a protein that physically interacts with the APL1C protein, also harbors high allelic diversity in the S form and shows evidence of recent directional selection in the M form, suggesting that the evolutionary trajectories of the Tep1 and APL1 defense loci may be correlated.