Evidence of carbamate resistance in urban populations of Anopheles gambiae s.s. mosquitoes resistant to DDT and deltamethrin insecticides in Lagos, South-Western Nigeria

Plant-based nanoparticles targeting malaria management

Malaria is one of the most devastating diseases across the globe, particularly in low-income countries in Sub-Saharan Africa. The increasing incidence of malaria morbidity is mainly due to the shortcomings of preventative measures such as the lack of vaccines and inappropriate control over the parasite vector. Additionally, high mortality rates arise from therapeutic failures due to poor patient adherence and drug resistance development. Although the causative pathogen (Plasmodium spp.) is an intracellular parasite, the recommended antimalarial drugs show large volumes of distribution and low-to no-specificity towards the host cell. This leads to severe side effects that hamper patient compliance and promote the emergence of drug-resistant strains. Recent research efforts are promising to enable the discovery of new antimalarial agents; however, the lack of efficient means to achieve targeted delivery remains a concern, given the risk of further resistance development. New strategies based on green nanotechnologies are a promising avenue for malaria management due to their potential to eliminate malaria vectors (Anopheles sp.) and to encapsulate existing and emerging antimalarial agents and deliver them to different target sites. In this review we summarized studies on the use of plant-derived nanoparticles as cost-effective preventative measures against malaria parasites, starting from the vector stage. We also reviewed plant-based nanoengineering strategies to target malaria parasites, and further discussed the site-specific delivery of natural products using ligand-decorated nanoparticles that act through receptors on the host cells or malaria parasites. The exploration of traditionally established plant medicines, surface-engineered nanoparticles and the molecular targets of parasite/host cells may provide valuable insights for future discovery of antimalarial drugs and open new avenues for advancing science toward the goal of malaria eradication.

Development of terpenoid repellents against Aedes albopictus: a combined study of biological activity evaluation and computational modelling

To explore novel terpenoid repellents, 22 candidate terpenoid derivatives were synthesized and tested for their electroantennogram (EAG) responses and repellent activities against Aedes albopictus. The results from the EAG experiments revealed that 5-(2-hydroxypropan-2-yl)-2-methylcyclohex-2-en-1-yl formate (compound 1) induced distinct EAG responses in female Aedes albopictus. At concentrations of 0.1, 1, 10, 100, and 1000 mg/L, the EAG response values for compound 1 were 179.59, 183.99, 190.38, 193.80, and 196.66 mV, demonstrating comparable or superior effectiveness to DEET. Repellent activity analysis indicated significant repellent activity for compound 1, closest to the positive control DEET. The in silico assessment of the ADMET profile of compound 1 indicates that it successfully passed the ADMET evaluation. Molecular docking studies exhibited favourable binding of compound 1 to the active site of the odorant binding protein (OBP) of Aedes albopictus, involving hydrophobic forces and hydrogen bond interactions with residues in the OBP pocket. The QSAR model highlighted the influential role of hydrogen-bonding receptors, positively charged surface area of weighted atoms, polarity parameters of molecules, and maximum nuclear-nuclear repulsion force of carbon-carbon bonds on the relative EAG response values of the tested compounds. This study holds substantial significance for the advancement of new terpenoid repellents.

Entomological surveillance reveals transmission of malaria but not lymphatic filariasis in two communities in North-West Nigeria

Malaria and lymphatic filariasis (LF) are two serious public health challenges in sub-Saharan Africa, and both diseases are transmitted by Anopheles mosquitoes. Successful control of both diseases requires detailed information on transmission dynamics; thus, this study investigated malaria and LF transmission indices in two (2) communities (Jidawa and Kargo) in North-West Nigeria. Anopheles mosquitoes were sampled from twenty-five (25) randomly selected houses from each of the two communities using pyrethrum spray collection (PSC). The samples were identified morphologically and molecularly characterised using polymerase chain reaction (PCR). Human biting rate (HBR), indoor resting density (IRD), sporozoite rate (SR) and entomological inoculation rate (EIR) were calculated using standard formulae. The thorax region of the collected samples were dissected and smeared; then, Plasmodium and Wuchereria bancrofti parasites were identified using microscopy. A total of 2417 Anopheles mosquitoes were collected, and all were identified morphologically as An. gambiae s.l. Further molecular identification of sibling species revealed that An. gambiae and An. arabiensis were the only sibling species present. A total of 818 Anopheles mosquitoes were screened for Plasmodium and Wuchereria bancrofti parasites. A total of 180 samples were positive for Plasmodium parasites (Jidawa = 151; Kargo = 29), and none was positive for W. bancrofti (0%). Result of entomological indices for malaria transmission showed that indoor resting density was higher in Jidawa (10 mosquitoes/room/night) while human biting rate (2.07 bites/person/night), sporozoite rate (29.3%) and entomological inoculation rate (0.61) were higher in Kargo. In total, 35.2% of the samples were blood-fed while 67.4% were parous. There is active transmission of malaria in the two communities but not LF, suggesting the effectiveness of mass drug administration for LF. Concerted efforts should be focused on malaria control as transmission of the disease persists.

Multiple insecticide resistance mechanisms in urban population of Anopheles coluzzii (Diptera: culicidae) from Lagos, South-West Nigeria

Malaria is a major public health challenge in Africa with Nigeria accounting for the highest burden of the disease in the world. Vector control has proved to be a highly effective component of malaria control, however, the development and spread of insecticide resistance in major vectors of malaria have been a major challenge. This study assessed resistance mechanisms in Anopheles coluzzii populations from Kosofe, Lagos mainland and Ojo Local Government Areas in Lagos, Nigeria where An. gambiae s.l is resistant to DDT and Permethrin. WHO susceptibility bioassay test was used in determining resistance status of An. coluzzii to discriminating doses of DDT and Permethrin while synergist assay was used to assess the involvement of monooxygenases in resistance development. Sub-species of An. gambiae s.l (An. gambiae and An. coluzzii) were identified using polymerase chain reaction (PCR) and Restriction Fragment Length Polymorphism (PCR-RFLP) while Allele-Specific Polymerase Chain Reaction (AS-PCR) assay was used to detect knockdown mutation (kdr-West; L1014F). Biochemical assays were used in determining the activities of metabolic enzymes. High DDT resistance was recorded in An. coluzzii populations from the three sites. Mortality rate of mosquitoes exposed confirmed Permethrin resistance in Kosofe (50%) and Lagos mainland (48%) but resistance was suspected in Ojo (96%). All specimens tested were confirmed as An. coluzzii with low kdr frequency; 11.6%, 16.4% and 6.7% in Kosofe, Lagos mainland and Ojo respectively. Pre-exposure to synergist (PBO) before exposure to Permethrin led to increased mortality in all populations. Esterase activity was insignificantly overexpressed in Kosofe (p = 0.849) and Lagos mainland (p = 0.229) populations. In contrast, GST activity was significantly lower in populations from Lagos mainland (63.650 ± 9.861; p = 0.007) and Ojo (91.765 ± 4.959; p = 0.042) than Kisumu susceptible strains (120.250 ± 13.972). Monooxygenase activity was higher in Lagos mainland (2.371 ± 0.261) and Ojo (1.361 ± 0.067) populations, albeit significantly in Lagos mainland (p = 0.007) only. Presence of target-site mutation in all populations, increased mortality with pre-exposure to PBO and elevated monooxygenase in Lagos mainland population were confirmed. Multiple resistance mechanisms in some urban populations of An. coluzzii from Lagos, Nigeria calls for appropriate resistance management strategies.

Current Status of Insecticide Susceptibility in the Principal Malaria Vector, Anopheles gambiae in Three Northern States of Nigeria

Background:

Malaria is a major public health problem in Nigeria with 97% of its population with high morbidity and mortality. Mosquitoes play an important role in the transmission of malaria parasites. This study was conducted to evaluate the current resistance status of Anopheles gambiae to insecticides.

Methods:

Larvae of An. gambiae was collected from three zones; A, B and C differentiated on the basis of variation in agricultural ecosystems between August and November, 2018 in the northeast and northwestern parts of Nigeria. They were carefully reared to adult stage and insecticidal susceptibility tests were conducted.

Results:

The mosquitoes tested showed high levels of resistance to all the insecticides used with the exception of malathion. Study zone A, recorded 74% mortality after 24h to deltamethrin compared to 81% from zone B and 82% from zone C, respectively. Mosquitoes from zone B exposed to DDT had the highest level of resistance at 37% compared to 40% and 53% from zones A and C, respectively. Resistant to bendiocarb was also observed, with zone A having the lowest mortality of 44% compared to 48% from zone C and 55% from Zone B, respectively. According to the results of knockdown tests, mosquitoes from Zone A exposed to deltamethrin recorded the lowest knockdown across the study locations while zone B recorded the lowest knockdown for DDT.

Conclusion:

The results of the study provide an insight into the current status of An. gambiae to four major insecticides in northern Nigeria as guideline for mosquitocontrol.

Urban malaria in sub-Saharan Africa: dynamic of the vectorial system and the entomological inoculation rate

Sub-Saharan Africa is registering one of the highest urban population growth across the world. It is estimated that over 75% of the population in this region will be living in urban settings by 2050. However, it is not known how this rapid urbanization will affect vector populations and disease transmission. The present study summarizes findings from studies conducted in urban settings between the 1970s and 2020 to assess the effects of urbanization on the entomological inoculation rate pattern and anopheline species distribution. Different online databases such as PubMed, ResearchGate, Google Scholar, Google were screened. A total of 90 publications were selected out of 1527. Besides, over 200 additional publications were consulted to collate information on anopheline breeding habitats and species distribution in urban settings. The study confirms high malaria transmission in rural compared to urban settings. The study also suggests that there had been an increase in malaria transmission in most cities after 2003, which could also be associated with an increase in sampling, resources and reporting. Species of the Anopheles gambiae complex were the predominant vectors in most urban settings. Anopheline larvae were reported to have adapted to different aquatic habitats. The study provides updated information on the distribution of the vector population and the dynamic of malaria transmission in urban settings. The study also highlights the need for implementing integrated control strategies in urban settings.

First Report of Target Site Insensitivity in Pyrethroid Resistant Anopheles gambiae from Southern Guinea Savanna, Northern-Nigeria

Background:

Malaria is a major public health problem and life threatening parasitic vector-borne disease. For the first time, we established and report the molecular mechanism responsible for Anopheles gambiae s.l. resistance to pyrethroids and DDT from Yamaltu Deba, Southern Guinea Savanna, Northern-Nigeria.

Methods:

The susceptibility profile of An. gambiae s.l. to four insecticides (DDT 4%, bendiocarb 0.1%, malathion 5% and deltamethrin 0.05%) using 2–3 days old females from larvae collected from study area between August and November, 2018 was first established. Genomic DNA was then extracted from 318 mosquitoes using Livak DNA extraction protocol for specie identification and kdr genotyping. The mosquitoes were identified to species level and then 96 genotyped for L1014F and L1014S kdr target site mutations.

Results:

The mosquitoes were all resistant to DDT, bendiocarb and deltamethrin but fully susceptible to malathion. An. coluzzii was found to be the dominant sibling species (97.8%) followed by An. arabiensis (1.9%) and An. gambiae s.s (0.3%). The frequency of the L1014F kdr mutation was relatively higher (83.3%) than the L1014S (39%) in the three species studied. The L1014F showed a genotypic frequency of 75% resistance (RR), 17% heterozygous (RS) and 8% susceptible (SS) with an allelic frequency of 87% RR and 13% SS while the L1014S showed a genotypic frequency of RR (16%), RS (38%) and SS (46%) with an allelic frequency of 40% RR and 60% SS, respectively.

Conclusion:

This study reveals that both kdr mutations present simultaneously in Northern-Nigeria, however contribution of L1014F which is common in West Africa was more than twice of L1014S mutation found in East Africa.

Biochemical Mechanism of Insecticide Resistance in Malaria Vector, Anopheles gambiae s.l in Nigeria

Background

Malaria is a parasitic vector-borne disease endemic in the tropical and subtropical countries of the world. The aim of this study was to investigate the current activities of the detoxification enzymes in resistant and susceptible Anopheles gambiae s.l. in northern Nigeria.

Methods

Anopheles larvae were collected from northeast and northwestern Nigeria between Aug and Nov 2018. Biochemical analyses was carried out on the mosquitoes exposed to various insecticides (deltamethrin, DDT, bendiocarb, malathion) to measure and compare the enzymatic activities of the major detoxification enzymes (P₄₅₀, GSTs, Esterase).

Results

High levels of resistance was observed; DDT 37%-53% (95%, CI: 29-61), bendiocarb 44%-55% (CI: 39-60) and deltamethrin 74%-82% (CI: 70-86). However, these mosquitoes were found to be susceptible to malathion 99%-100% (CI: 98-100). The P450 and GSTs enzymes were found to be elevated in the resistant mosquitoes exposed to deltamethrin (1.0240±0.1902); (1.3088±1.2478), DDT (1.7703±1.4528); (1.7462±0.9418) and bendiocarb (1.1814±0.0918); (1.4479±1.0083) compared to the Kisumu strain (0.764±0.4226); (0.6508±0.6542), (0.3875±0.3482); (0.4072±0.4916) and (0.6672±0.3949); (0.7126±0.7259) at P<0.05. Similarly, the resistant mosquitoes expressed increased activity to esterase (0.7606±1.1477), (0.3269±1.1957) and (2.8203±0.6488) compared to their susceptible counterpart (0.6841±0.7597), (0.7032±0.5380) and (0.6398±0.4159) at P<0.05. The enzyme ratio was found to be: P450 (1.341, 4.568 and 1.77); GSTs (2.011, 4.288 and 2.031); Esterases (1.111, 0.469 and 4.408). One way Anova and single sample t-test were also conducted to determine the effect of the enzymes on the resistant and susceptible strains.

Conclusion

High level of insecticide resistance was observed with significant elevation of detoxification enzymes activities in the resistant mosquitoes.

Acetylcholinesterase (ace-1R) target site mutation G119S and resistance to carbamates in Anopheles gambiae (sensu lato) populations from Mali

BACKGROUND

The long-lasting insecticidal nets (LLINs) and indoor residual spraying of insecticide (IRS) are major malaria vector control strategies in Mali. The success of control strategies depends on a better understanding of the status of malaria vectors with respect to the insecticides used. In this study we evaluate the level of resistance of Anopheles gambiae (sensu lato) to bendiocarb and the molecular mechanism that underlies it.

METHODS

Larvae of An. gambiae (s.l.) were collected from breeding habitats encountered in the three study sites and bioassayed with bendiocarb. The ace-1 target site substitution G119S was genotyped using a TaqMan assay.

RESULTS

The three species of the An. gambiae complex in Mali, i.e. An. arabiensis, An. coluzzii and An. gambiae (s.s.) were found in sympatry in the three surveyed localities with different frequencies. We observed a resistance and suspicious resistance of the three species to bendiocarb with a mortality rate ranging from 37% to 86%. The allelic frequency of the G119S mutation was higher in An. gambiae (s.s.) compared to the other two species; 42.86%, 25.61% and 16.67% respectively in Dangassa, Koula, and Karadié. The allelic frequency of G119S in An. coluzzii ranged from 4.5% to 8.33% and from 1.43% to 21.15% for An. arabiensis. After exposure to bendiocarb, the G119S mutation was found only in survivors. The survival of Anopheles gambiae (s.l) populations from the three surveyed localities was associated with the presence of the mutation.

CONCLUSIONS

The study highlights the implication of G119S mutation in bendiocarb resistance in An. gambiae (s.s.), An. arabiensis and An. coluzzii populations from the three surveyed localities.

First report of AChE1 (G119S) mutation and multiple resistance mechanisms in Anopheles gambiae s.s. in Nigeria

Susceptibility and PBO synergist bioassays were done using 3-5 days old female Anopheles mosquito collected from Lagos State, Nigeria with WHO test papers DDT (4%), permethrin (0.75%), Bendiocarb (1%) and PBO (4%) according to standard procedures. The activities of cytochrome P450s, glutathione S-transferase and carboxylesterases were determined using biochemical assays. The presence of kdr-w, kdr-e and Ace-1R mutations were examined using molecular assays. Resistance to DDT and permethrin in An gambiae s.s from the four Local Government Areas (LGAs) was recorded while suspected resistance to bendiocarb was recorded in mosquitoes from Alimosho and Kosofe LGAs. PBO synergist reduced the knockdown time and also recorded significantly (P < 0.05) higher 24 hrs percentage mortality compared to non-synergized bioassays. Increased activities of detoxifying enzymes was recorded in wild mosquito compared to the insecticides susceptible laboratory strain and this was significant (P < 0.05) in P450s, esterase α and β. Kdr-w was detected in An. gambiae s.s from all the LGAs, kdr-e (L1014S) was detected in Alimosho, Kosofe and Ibeju-Lekki, while the Ace-1R gene was detected in Alimosho and Kosofe. Results from this study provide evidence for resistance of An. gambiae from Lagos State to multiple classes of neurotoxic insecticides with multiple resistance mechanisms to these insecticides.

Dynamic of resistance alleles of two major insecticide targets in Anopheles gambiae (s.l.) populations from Benin, West Africa

BACKGROUND

Insecticide resistance is a growing concern for malaria control and vector control effectiveness relies on assessing it distribution and understanding its evolution.

METHODS

We assessed resistance levels and the frequencies of two major target-site mutations, L1014F-VGSC and G119S-ace-1, conferring resistance to pyrethroids (PYRs) and carbamates/organophosphates (CXs/OPs) insecticides. These data were compared to those acquired between 2006 and 2010 to follow resistance evolutionary trends over ten years.

RESULTS

We report the results of a 3-year survey (2013-2015) of insecticide resistance in 13 localities across the whole country of Benin. Permethrin (PYR) resistance was found in all populations tested, L1014F-VGSC being almost fixed everywhere, while bendiocarb resistance was limited to a few localities, G119S-ace-1 remaining rare, with very limited variations during surveyed period. Interestingly, we found no effect of the type of insecticide pressure on the dynamics of these mutations.

CONCLUSIONS

These results confirm both the high prevalence of PYR resistance and the potential of CXs/OPs as short- to medium-term alternatives in Benin. They also underline the need for regular resistance monitoring and informed management in their usage, as the G119S-ace-1 mutation is already present in Benin and surrounding countries. Their unwise usage would rapidly lead to its spread, which would jeopardize PYR-resistant Anopheles control.

Temporal escalation of Pyrethroid Resistance in the major malaria vector Anopheles coluzzii from Sahelo-Sudanian Region of northern Nigeria

Despite the highest global burden of malaria, information on bionomics and insecticide resistance status of malaria vectors is grossly lacking in the densely populated Sahelo-Sudanian region of Nigeria. To support evidence-based vector control we characterised transmission and resistance profiles of Anopheles coluzzii populations from three sites in northern Nigeria. High sporozoite infection (~19.51%) was found in the An. coluzzii populations. A high pyrethroid resistance was observed with only 1% mortality against deltamethrin, a high LD₅₀ (96.57 µg/ml), and a high LT₅₀ (170.27 min, resistance ratio of ~51 compared with the fully susceptible Ngoussou colony). Moderate carbamate resistance was observed. Synergist bioassays significantly recovered deltamethrin susceptibility implicating CYP450s (mortality = 85%, χ² = 134.04, p < 0.0001) and esterases (mortality = 56%, χ² = 47.31, p < 0.0001). Reduced bed net efficacy was also observed, with mortalities on exposure to the roof of PermaNet3.0 (PBO + deltamethrin) more than 22 times compared to the side panel (deltamethrin). TaqMan genotyping revealed a high frequency of 1014F kdr mutation (82%) with significant difference in genotype distribution associated with permethrin resistance [OR = 4.69 (CI:1.53–14.35, χ² = 8.22 p = 0.004]. Sequencing of exons 18–21 of the VGSC led to detection of two additional nonsynonymous mutations, Ile10148Asn and Ser1156Gly. These findings highlight the threats posed by the highly resistant An. coluzzii to malaria control in Nigeria.

Malaria vectors in the Democratic Republic of the Congo: the mechanisms that confer insecticide resistance in Anopheles gambiae and Anopheles funestus

Background

The Democratic Republic of the Congo (DRC) is characterized as a holoendemic malaria area with the main vectors being Anopheles funestus and members of the Anopheles gambiae complex. Due to political instability and socio-economic challenges in the region, knowledge of insecticide resistance status and resistance mechanisms in these vectors is limited. Mosquitoes were collected from a mining site in the north-eastern part of the country and, following identification, were subjected to extensive testing for the target-site and biochemical basis of resistance. Quantitative real-time PCR was used to assess a suite of 10 genes frequently involved in pyrethroid and dichlorodiphenyltrichloroethane (DDT) resistance in An. gambiae females and males. In An. funestus, gene expression microarray analysis was carried out on female mosquitoes.

Results

In both species, deltamethrin resistance was recorded along with high resistance and suspected resistance to DDT in An. gambiae and An. funestus, respectively. A total of 85% of An. gambiae carried the kdr mutations as either homozygous resistant (RR) (L1014S, L1014F or both) or heterozygous (RS), however only 3% carried the rdl mutant allele (RS) and no ace-1 mutations were recorded. Synergist assays indicated a strong role for P450s in deltamethrin resistance in both species. In An. gambiae, analysis of transcription levels showed that the glutathione-S-transferase, GSTS1-2, produced the highest fold change in expression (7.6-fold in females and 31-fold in males) followed by GSTE2, thioredoxin peroxidase (TPX2), and cytochrome oxidases (CYP6M2 and CYP6P1). All other genes tested produced fold change values below 2. Microarray analysis revealed significant over-transcription of cuticular proteins as well as CYP6M7, CYP6P9a and CYP6P9b in insecticide resistant An. funestus.

Conclusions

These data show that high levels of deltamethrin resistance in the main malaria vector species, conferred by enzymatic detoxification, are present in the DRC.

Electronic supplementary material

The online version of this article (10.1186/s12936-017-2099-y) contains supplementary material, which is available to authorized users.

Abuse of Rachel Carson and Misuse of DDT Science in the Service of Environmental Deregulation

Fake news?? The contact insecticide DDT has been reappraised as a safe, life-saving compound by special interest groups committed to repealing environmental regulations. It is shown in this essay how some specific toxicological data has been misused by those aiming to disingenuously influence public policy. Graphic: Pestroy, a DDT-laced coating marketed in 1946 by Sherwin-Williams Research Laboratories.

Evaluation of Larvicidal Activity of Essential Oil from Leaves of Coccinia grandis against Three Mosquito Species

BACKGROUND

To study the chemical constituents and larvicidal activity of essential oil extracted from the leaves of Coccinia grandis against three mosquito species.

METHODS

Essential oil was extracted by hydro distillation using clevenger apparatus and was analyzed for chemical constituents by gas chromatography-mass spectrophotometry (GC-MS). Larvicidal activity was recorded after 12 and 24h of post-exposure against three mosquito species, Anopheles stephensi, Aedes aegypti and Culex quinquefasciatus. Dead larvae were identified when they failed to move after probing with a needle in the siphon or cervical region. The LC₅₀ and LC₉₀ values for three mosquito larvae were calculated by Probit analysis.

RESULTS

The GC-MS analysis revealed that essential oil contains 23 different constituents. Out of these 23 constituents, major constituents identified were n-tetracosane (39.18%), n-eicosane (30.04%), tetratriacotane (2.97%), 7-octadecanal (2.81%), and tricosane (2.31%). Essential oil from leaves of Coccinia grandis exhibited significant larvicidal activity against An. stephensi with LC₅₀ and LC₉₀ values 39.41ppm and 123.24ppm, respectively. This was followed by Ae. aegypti and Cx. quinquefasciatus with LC₅₀ and LC₉₀ values of 48.20ppm, 131.84ppm and 52.80ppm, 135.48ppm, respectively after 24h of exposure.

CONCLUSION

The results could be useful in developing a cost effective, ecofriendly, region specific and practical strategy for the control of mosquito vectors.

Evidence of multiple insecticide resistance mechanisms in Anopheles gambiae populations in Bangui, Central African Republic

Background

Knowledge of insecticide resistance status in the main malaria vectors is an essential component of effective malaria vector control. This study presents the first evaluation of the status of insecticide resistance in Anopheles gambiae populations from Bangui, the Central African Republic.

Methods

Anopheles mosquitoes were reared from larvae collected in seven districts of Bangui between September to November 2014. The World Health Organisation’s bioassay susceptibility tests to lambda-cyhalothrin (0.05%), deltamethrin (0.05%), DDT (4%), malathion (5%), fenitrothion (1%) and bendiocarb (0.1%) were performed on adult females. Species and molecular forms as well as the presence of L1014F kdr and Ace-1^R mutations were assessed by PCR. Additional tests were conducted to assess metabolic resistance status.

Results

After 1 h exposure, a significant difference of knockdown effect was observed between districts in all insecticides tested except deltamethrin and malathion. The mortality rate (MR) of pyrethroids group ranging from 27% (CI: 19–37.5) in Petevo to 86% (CI: 77.6–92.1) in Gbanikola; while for DDT, MR ranged from 5% (CI: 1.6–11.3) in Centre-ville to 39% (CI: 29.4–49.3) in Ouango. For the organophosphate group a MR of 100% was observed in all districts except Gbanikola where a MR of 96% (CI: 90–98.9) was recorded. The mortality induced by bendiocarb was very heterogeneous, ranging from 75% (CI: 62.8–82.8) in Yapele to 99% (CI: 84.5–100) in Centre-ville. A high level of kdr-w (L1014F) frequency was observed in all districts ranging from 93 to 100%; however, no kdr-e (L1014S) and Ace-1^R mutation were found in all tested mosquitoes. Data of biochemical analysis showed significant overexpression activities of cytochrome P450, GST and esterases in Gbanikola and Yapele (χ² = 31.85, df = 2, P < 0.001). By contrast, esterases activities using α and β-naphthyl acetate were significantly low in mosquitoes from PK10 and Ouango in comparison to Kisumu strain (χ² = 17.34, df = 2, P < 0.005).

Conclusions

Evidence of resistance to DDT and pyrethroids as well as precocious emergence of resistance to carbamates were detected among A. gambiae mosquitoes from Bangui, including target-site mutations and metabolic mechanisms. The co-existence of these resistance mechanisms in A. gambiae may be a serious obstacle for the future success of malaria control programmes in this region.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1965-8) contains supplementary material, which is available to authorized users.

Evidence of a multiple insecticide resistance in the malaria vector Anopheles funestus in South West Nigeria

Background

Knowing the extent and spread of insecticide resistance in malaria vectors is vital to successfully manage insecticide resistance in Africa. This information in the main malaria vector, Anopheles funestus sensu stricto, is completely lacking in the most populous country in Africa, Nigeria. This study reports the insecticide susceptibility status and the molecular basis of resistance of An. funestus as well as its involvement in malaria transmission in Akaka-Remo, a farm settlement village in southwest Nigeria.

Results

Plasmodium infection analysis using TaqMan protocol coupled with a nested PCR revealed an infection rate of 8% in An. funestus s.s. from Akaka-Remo. WHO susceptibility tests showed this species has developed multiple resistance to insecticides in the study area. Anopheles funestus s.s. population in Akaka-Remo is highly resistant to organochlorines: dieldrin (8%) and DDT (10%). Resistance was also observed against pyrethroids: permethrin (68%) and deltamethrin (87%), and the carbamate bendiocarb (84%). Mortality rate with DDT slightly increased (from 10 to 30%, n = 45) after PBO pre-exposure indicating that cytochrome P450s play little role in DDT resistance while high mortalities were recorded after PBO pre-exposure with permethrin (from 68 to 100%, n = 70) and dieldrin (from 8 to 100%, n = 48) suggesting the implication of P450s in the observed permethrin and dieldrin resistance. High frequencies of resistant allele, 119F in F₀ (77%) and F₁ (80% in resistant and 72% in susceptible) populations with an odd ratio of 1.56 (P = 0.1859) show that L119F-GSTe2 mutation is almost fixed in the population. Genotyping of the A296S-RDL mutation in both F₀ and F₁ samples shows an association with dieldrin resistance with an odd ratio of 81 (P < 0.0001) (allelic frequency (R) = 76% for F₀; for F₁, 90 and 10% were observed in resistant and susceptible populations, respectively) as this mutation is not yet fixed in the population.

Conclusion

The study reports multiple insecticide resistance in An. funestus from Akaka Remo. It is, therefore, necessary to pay more attention to this major malaria vector for effective malaria control in Nigeria.

Increasing insecticide resistance in Anopheles funestus and Anopheles arabiensis in Malawi, 2011-2015

BACKGROUND

Susceptibility of principal Anopheles malaria vectors to common insecticides was monitored over a 5-year period across Malawi to inform and guide the national malaria control programme.

METHODS

Adult blood-fed Anopheles spp. and larvae were collected from multiple sites in sixteen districts across the country between 2011 and 2015. First generation (F₁) progeny aged 2-5 days old were tested for susceptibility, using standard WHO procedures, against pyrethroids (permethrin and deltamethrin), carbamates (bendiocarb and propoxur), organophosphates (malathion and pirimiphos-methyl) and an organochlorine (DDT).

RESULTS

Mortality of Anopheles funestus to deltamethrin, permethrin, bendiocarb and propoxur declined significantly over the 5-year (2011-2015) monitoring period. There was wide variation in susceptibility to DDT but it was not associated with time. In contrast, An. funestus exhibited 100% mortality to the organophosphates (malathion and pirimiphos-methyl) at all sites tested. There was reduced mortality of Anopheles arabiensis to deltamethrin over time though this was not statistically significant. However, mortality of An. arabiensis exposed to permethrin declined significantly over time. Anopheles arabiensis exposed to DDT were more likely to be killed if there was high ITN coverage in the mosquito collection area the previous year. There were no other associations between mosquito mortality in a bioassay and ITN coverage or IRS implementation. Mortality of An. funestus from four sites exposed to deltamethrin alone ranged from 2 to 31% and from 41 to 94% when pre-exposed to the synergist piperonyl butoxide followed by deltamethrin. For permethrin alone, mortality ranged from 2 to 13% while mortality ranged from 63 to 100% when pre-exposed to PBO.

CONCLUSION

Pyrethroid resistance was detected in An. funestus and An. arabiensis populations across Malawi and has worsened over the last 5 years. New insecticides and control strategies are urgently needed to reduce the burden of malaria in Malawi.

Investigation of mechanisms of bendiocarb resistance in Anopheles gambiae populations from the city of Yaoundé, Cameroon

Background

Resistance to the carbamate insecticide bendiocarb is emerging in Anopheles gambiae populations from the city of Yaoundé in Cameroon. However, the molecular basis of this resistance remains uncharacterized. The present study objective is to investigate mechanisms promoting resistance to bendiocarb in An. gambiae populations from Yaoundé.

Methods

The level of susceptibility of An. gambiae s.l. to bendiocarb 0.1 % was assessed from 2010 to 2013 using bioassays. Mosquitoes resistant to bendiocarb, unexposed and susceptible mosquitoes were screened for the presence of the Ace-1^R mutation using TaqMan assays. Microarray analyses were performed to assess the pattern of genes differentially expressed between resistant, unexposed and susceptible.

Results

Bendiocarb resistance was more prevalent in mosquitoes originating from cultivated sites compared to those from polluted and unpolluted sites. Both An. gambiae and Anopheles coluzzii were found to display resistance to bendiocarb. No G119S mutation was detected suggesting that resistance was mainly metabolic. Microarray analysis revealed the over-expression of several cytochrome P450 s genes including cyp6z3, cyp6z1, cyp12f2, cyp6m3 and cyp6p4. Gene ontology (GO) enrichment analysis supported the detoxification role of cytochrome P450 s with several GO terms associated with P450 activity significantly enriched in resistant samples. Other detoxification genes included UDP-glucosyl transferases, glutathione-S transferases and ABC transporters.

Conclusion

The study highlights the probable implication of metabolic mechanisms in bendiocarb resistance in An. gambiae populations from Yaoundé and stresses the need for further studies leading to functional validation of detoxification genes involved in this resistance.

An ace-1 gene duplication resorbs the fitness cost associated with resistance in Anopheles gambiae, the main malaria mosquito

Widespread resistance to pyrethroids threatens malaria control in Africa. Consequently, several countries switched to carbamates and organophophates insecticides for indoor residual spraying. However, a mutation in the ace-1 gene conferring resistance to these compounds (ace-1(R) allele), is already present. Furthermore, a duplicated allele (ace-1(D)) recently appeared; characterizing its selective advantage is mandatory to evaluate the threat. Our data revealed that a unique duplication event, pairing a susceptible and a resistant copy of the ace-1 gene spread through West Africa. Further investigations revealed that, while ace-1(D) confers less resistance than ace-1(R), the high fitness cost associated with ace-1(R) is almost completely suppressed by the duplication for all traits studied. ace-1 duplication thus represents a permanent heterozygote phenotype, selected, and thus spreading, due to the mosaic nature of mosquito control. It provides malaria mosquito with a new evolutionary path that could hamper resistance management.

Characterizing the insecticide resistance of Anopheles gambiae in Mali

BACKGROUND

The impact of indoor residual spraying (IRS) and long-lasting insecticide nets (LLINs), key components of the national malaria control strategy of Mali, is threatened by vector insecticide resistance. The objective of this study was to assess the level of insecticide resistance in Anopheles gambiae sensu lato populations from Mali against four classes of insecticide recommended for IRS: organochlorines (OCs), pyrethroids (PYs), carbamates (CAs) and organophosphates (OPs). Characterization of resistance was done in 13 sites across southern Mali and assessed presence and distribution of physiological mechanisms that included target-site modifications: knockdown resistance (kdr) and altered acetycholinesterase (AChE), and/or metabolic mechanisms: elevated esterases, glutathione S-transferases (GSTs), and monooxygenases.

METHODS

The World Health Organization (WHO) tube test was used to determine phenotypic resistance of An. gambiae s.l. to: dichlorodiphenyltrichloroethane (DDT) (OC), deltamethrin (PY), lambda-cyhalothrin (PY), bendiocarb (CA), and fenitrothion (OP). Identification of sibling species and presence of the ace-1 (R) and Leu-Phe kdr, resistance-associated mutations, were determined using polymerase chain reaction (PCR) technology. Biochemical assays were conducted to detect increased activity of GSTs, oxidases and esterases.

RESULTS

Populations tested showed high levels of resistance to DDT in all 13 sites, as well as increased resistance to deltamethrin and lambda-cyhalothrin in 12 out of 13 sites. Resistance to fenitrothion and bendiocarb was detected in 1 and 4 out of 13 sites, respectively. Anopheles coluzzii, An. gambiae sensu stricto and Anopheles arabiensis were identified with high allelic frequencies of kdr in all sites where each of the species were found (13, 12 and 10 sites, respectively). Relatively low allelic frequencies of ace-1 (R) were detected in four sites where this assessment was conducted. Evidence of elevated insecticide metabolism, based on oxidase, GSTs and esterase detoxification, was also documented.

CONCLUSION

Multiple insecticide-resistance mechanisms have evolved in An. coluzzii, An. gambiae s.s. and An. arabiensis in Mali. These include at least two target site modifications: kdr, and ace-1 (R) , as well as elevated metabolic detoxification systems (monooxygenases and esterases). The selection pressure for resistance could have risen from the use of these insecticides in agriculture, as well as in public health. Resistance management strategies, based on routine resistance monitoring to inform insecticide-based malaria vector control in Mali, are recommended.

The Insecticide Susceptibility Status of Aedes aegypti (Diptera: Culicidae) in Farm and Nonfarm Sites of Lagos State, Nigeria

Nigeria is one of the malaria-endemic countries. In Lagos State, Nigeria, various malaria vector control programs including the use of chemical insecticides are currently being implemented. This study was designed to provide information on the susceptibility status of some nontargeted vectors such as Aedes aegypti. Adult Ae. aegypti mosquitoes from two farm sites and a nonfarm site were exposed to World Health Organization test papers impregnated with Deltamethrin (0.05%), Permethrin (0.75%), and DDT (4%) insecticides. The Knockdown time (KdT50 and KdT95) and percentage mortality after 24 h post exposure were determined. In all the exposed mosquito populations to permethrin, mortality rate > 98% (susceptibility) was recorded, whereas mortality rates < 95.8% (resistance) and > 98% (susceptibility) to deltamethrin were observed in the nonfarm site and farm sites mosquito populations, respectively. All the mosquito populations were resistant to DDT in 2 yr. The KdT50 of the populations to DDT increased (60.2-69.6) in one of the farm sites and the nonfarm site (68.9-199.96), while a decrease (243-63.4) in another farm site in 2 yr. Significant difference (P < 0.05) in KdT50 was recorded between the farm and nonfarm sites Ae. aegypti mosquitoes in the second year after exposure to deltamethrin and DDT. An increase in KdT95 after exposure to deltamethrin in the first year was recorded. Higher KdT values and lower mortality rates in Ae. aegypti populations in the nonfarm sites are indications there are existing factors selecting for insecticide resistance outside agricultural use of insecticides.

Insecticide susceptibility of Anopheles coluzzii and Anopheles gambiae mosquitoes in Ibadan, Southwest Nigeria

The emergence of insecticide resistance in Anopheles (Diptera: Culicidae) mosquitoes has great implications for malaria control in Nigeria. This study aimed to determine the dynamics of insecticide susceptibility levels and the frequency of knock-down resistance (kdr) mutations (L1014F) in wild Anopheles coluzzii Coetzee & Wilkerson sp. n. and Anopheles gambiae Giles from the Ojoo and Bodija areas of Ibadan, in southwest Nigeria. Insecticide susceptibility to pyrethroids, organophosphates, carbamates and organochlorines was assessed using World Health Organization (WHO) bioassays. A subset of the mosquitoes exposed to pyrethroids and DDT was used for species and molecular form identification; kdr genotyping was determined using the TaqMan real-time polymerase chain reaction assay. The mosquitoes were resistant to pyrethroids and DDT but completely susceptible to organophosphates and carbamates. Bodija samples (n = 186) consisted of An. gambiae (91.4%) and An. coluzzii (8.1%) and included one An. coluzzii/An. gambiae hybrid specimen. All mosquitoes screened in Ojoo (n = 26) were An. gambiae. The 1014F kdr mutation was detected at frequencies of 24.5 and 5.8% in Bodija and Ojoo, respectively. No correlation was observed between kdr genotypes and resistance phenotypes. The results indicate that metabolic resistance probably plays an important role in the development of resistance and highlight the need to implement insecticide resistance management strategies.

Evidence of multiple mechanisms providing carbamate and organophosphate resistance in field An. gambiae population from Atacora in Benin

BACKGROUND

Insecticide resistance in Anopheles gambiae s.l is a major concern to malaria vector control programmes. In West Africa, resistance is mainly due to target-site insensitivity arising from a single point mutation. Metabolic-based resistance mechanisms have also been implicated and are currently being investigated in west Africa. The aim of this study is to better understand the origins of carbamate and organophosphate resistance in An. gambiae population from Atacora, Benin in West Africa.

METHODS

Anopheles mosquitoes were reared from larvae collected in two districts (Kouandé and Tanguiéta) of the Atacora department. Mosquitoes were then exposed to WHO impregnated papers. Four impregnated papers were used: carbamates (0.1% bendiocarb, 0.1% propoxur) and organophosphates (0.25% pirimiphos methyl, 1% fenitrothion). PCR assays were run to determine the members of the An. gambiae complex, as well as phenotypes for insensitive acetylcholinesterase (AChE1). Biochemical assays were also carried out to detect any increase in the activity of enzyme typically involved in insecticide metabolism (oxidase, esterase and glutathion-S-transferase).

RESULTS

769 female of An. gambiae mosquitoes from Kouandé and Tanguiéta were exposed to bendiocarb, propoxur, pirimiphos methyl and fenitrothion. Bioassays showed resistance with low mortality to bendiocarb (78.57% to 80.17%), propoxur (77.21% to 89.77%), and fenitrothion (89.74% to 92.02%). On the other hand, the same populations of An. gambiae from Kouandé and Tanguiéta showed high susceptibility to pirimiphos methyl with recorded mortality of 99.02% and 100% respectively. The low rate of ace-1R allele frequency (3.75% among survivors and 0.48% among dead) added to the high proportion of homozygous susceptible specimens which survived the WHO bioassays (8/28), suggest that the ace-1 mutation could not entirely explain Anopheles gambiae resistance to carbamate and organophosphate. Biochemical assays suggest that resistance in this population is mediated by metabolic resistance with elevated level of GST, MFO and NSE compared to a susceptible strain An. gambiae Kisumu.

CONCLUSIONS

Anopheles gambiae populations resistance from Atacora is multifactorial and includes target-site mutation and metabolic mechanism. The co-implication of both resistance mechanisms in An. gambiae s.l may be a serious obstacle for the future success of malaria control operations based on LLINs and IRS.

Long-term trends in Anopheles gambiae insecticide resistance in Côte d'Ivoire

BACKGROUND

Malaria control is heavily dependent on the use of insecticides that target adult mosquito vectors via insecticide treated nets (ITNs) or indoor residual spraying (IRS). Four classes of insecticide are approved for IRS but only pyrethroids are available for ITNs. The rapid rise in insecticide resistance in African malaria vectors has raised alarms about the sustainability of existing malaria control activities. This problem might be particularly acute in Côte d'Ivoire where resistance to all four insecticide classes has recently been recorded. Here we investigate temporal trends in insecticide resistance across the ecological zones of Côte d'Ivoire to determine whether apparent pan-African patterns of increasing resistance are detectable and consistent across insecticides and areas.

METHODS

We combined data on insecticide resistance from a literature review, and bioassays conducted on field-caught Anopheles gambiae mosquitoes for the four WHO-approved insecticide classes for ITN/IRS. The data were then mapped using Geographical Information Systems (GIS) and the IR mapper tool to provide spatial and temporal distribution data on insecticide resistance in An. gambiae sensu lato from Côte d'Ivoire between 1993 and 2014.

RESULTS

Bioassay mortality decreased over time for all insecticide classes, though with significant spatiotemporal variation, such that stronger declines were observed in the southern ecological zone for DDT and pyrethroids than in the central zone, but with an apparently opposite effect for the carbamate and organophosphate. Variation in relative abundance of the molecular forms, coupled with dramatic increase in kdr 1014F frequency in M forms (An. coluzzii) seems likely to be a contributory factor to these patterns. Although records of resistance across insecticide classes have become more common, the number of classes tested in studies has also increased, precluding a conclusion that multiple resistance has also increased.

CONCLUSION

Our analyses attempted synthesis of 22 years of bioassay data from Côte d'Ivoire, and despite a number of caveats and potentially confounding variables, suggest significant but spatially-variable temporal trends in insecticide resistance. In the light of such spatio-temporal dynamics, regular, systematic and spatially-expanded monitoring is warranted to provide accurate information on insecticide resistance for control programme management.

Mutations in the voltage-gated sodium channel gene of anophelines and their association with resistance to pyrethroids - a review

Constant and extensive use of chemical insecticides has created a selection pressure and favored resistance development in many insect species worldwide. One of the most important pyrethroid resistance mechanisms is classified as target site insensitivity, due to conformational changes in the target site that impair a proper binding of the insecticide molecule. The voltage-gated sodium channel (NaV) is the target of pyrethroids and DDT insecticides, used to control insects of medical, agricultural and veterinary importance, such as anophelines. It has been reported that the presence of a few non-silent point mutations in the NaV gene are associated with pyrethroid resistance, termed as 'kdr' (knockdown resistance) for preventing the knockdown effect of these insecticides. The presence of these mutations, as well as their effects, has been thoroughly studied in Anopheles mosquitoes. So far, kdr mutations have already been detected in at least 13 species (Anopheles gambiae, Anopheles arabiensis, Anopheles sinensis, Anopheles stephensi, Anopheles subpictus, Anopheles sacharovi, Anopheles culicifacies, Anopheles sundaicus, Anopheles aconitus, Anopheles vagus, Anopheles paraliae, Anopheles peditaeniatus and Anopheles albimanus) from populations of African, Asian and, more recently, American continents. Seven mutational variants (L1014F, L1014S, L1014C, L1014W, N1013S, N1575Y and V1010L) were described, with the highest prevalence of L1014F, which occurs at the 1014 site in NaV IIS6 domain. The increase of frequency and distribution of kdr mutations clearly shows the importance of this mechanism in the process of pyrethroid resistance. In this sense, several species-specific and highly sensitive methods have been designed in order to genotype individual mosquitoes for kdr in large scale, which may serve as important tolls for monitoring the dynamics of pyrethroid resistance in natural populations. We also briefly discuss investigations concerning the course of Plasmodium infection in kdr individuals. Considering the limitation of insecticides available for employment in public health campaigns and the absence of a vaccine able to brake the life cycle of the malaria parasites, the use of pyrethroids is likely to remain as the main strategy against mosquitoes by either indoor residual spraying (IR) and insecticide treated nets (ITN). Therefore, monitoring insecticide resistance programs is a crucial need in malaria endemic countries.

Genetic diversity and complexity of Plasmodium falciparum infections in Lagos, Nigeria

OBJECTIVE

To analyse the genetic diversity of Plasmodium falciparum (P. falciparum) using msp-1 and msp-2 as antigenic markers.

METHODS

Parasite DNA was extracted from 100 blood samples collected from P. falciparum-positive patients confirmed by microscopy, and followed by PCR-genotyping targeting the msp-1 (block2) and msp-2 (block 3) allelic families.

RESULTS

All the families of msp-1 (K1, MAD20 and R033) and msp-2 (FC27 and 3D7) locus were observed. Results revealed that K1 (60/100) was the most predominant genotype of msp-1 allelic family followed by the genotypes of MAD20 (50/100) and R033 (45/100). In the msp-2 locus, FC27 genotype (62/100) showed higher frequency than 3D7 genotype (55/100). The allelic families were detected either alone or in combination with other families. However, no R033/MAD20 combination was observed. Multiplicity of infection (MOI) with msp-1 was higher in the locality of Ikorodu (1.50) than in Lekki (1.39). However, MOI with msp-2 was lower in the locality of Ikorodu (1.14) than in Lekki (1.76). There was no significant difference in the mean MOI between the two study areas (P=0.427).

CONCLUSIONS

The observation of limited diversity of malaria parasites may imply that the use of antigenic markers as genotyping tools for distinguishing recrudescence and re-infections with P. falciparum during drug trials is subjective.

High frequency of kdr L1014F is associated with pyrethroid resistance in Anopheles coluzzii in Sudan savannah of northern Nigeria

BACKGROUND

Malaria burden is high in Nigeria, yet information on the major mosquito vectors is lacking especially in the Sudan savannah region of the country. In order to facilitate the design of future insecticide-based control interventions in the region, this study has established the resistance profile of An. gambiae s.l. populations in two northern Nigeria locations and assessed the contribution of target site resistance mutations.

METHODS

Larval collection was conducted in two localities in Sudan savannah (Bunkure and Auyo) of northern Nigeria between 2009 and 2011, from which resulting adult, female mosquitoes were used for insecticides bioassays with deltamethrin, lambda-cyhalothrin, DDT and malathion. The mosquitoes were identified to species level and molecular forms and then genotyped for the presence of L1014F-kdr, L1014S-kdr and ace-1R mutations.

RESULTS

WHO bioassays revealed that An. gambiae s.l. from both localities were highly resistant to lambda-cyhalothrin and DDT, but only moderately resistant to deltamethrin. Full susceptibility was observed to malathion. An. gambiae, M form (now An. coluzzii), was predominant over An. arabiensis in Auyo and was more resistant to lambda-cyhalothrin than An. arabiensis. No 'S' form (An. gambiae s.s.) was detected. A high frequency of 1014 F mutation (80.1%) was found in An. coluzzii in contrast to An. arabiensis (13.5%). The presence of the 1014 F kdr allele was significantly associated with resistance to lambda-cyhalothrin in An. coluzzii (OR = 9.85; P < 0.001) but not in An. arabiensis. The L1014S-kdr mutation was detected in a single An. arabiensis mosquito while no ace-1R mutation was found in any of the mosquitoes analysed.

CONCLUSIONS

The predominance of An. coluzzii and its resistance profile to main insecticides described in this study can guide the implementation of appropriate vector control interventions in this region of Nigeria where such information was previously lacking.

Underpinning sustainable vector control through informed insecticide resistance management

BACKGROUND

There has been rapid scale-up of malaria vector control in the last ten years. Both of the primary control strategies, long-lasting pyrethroid treated nets and indoor residual spraying, rely on the use of a limited number of insecticides. Insecticide resistance, as measured by bioassay, has rapidly increased in prevalence and has come to the forefront as an issue that needs to be addressed to maintain the sustainability of malaria control and the drive to elimination. Zambia's programme reported high levels of resistance to the insecticides it used in 2010, and, as a result, increased its investment in resistance monitoring to support informed resistance management decisions.

METHODOLOGY/PRINCIPAL FINDINGS

A country-wide survey on insecticide resistance in Zambian malaria vectors was performed using WHO bioassays to detect resistant phenotypes. Molecular techniques were used to detect target-site mutations and microarray to detect metabolic resistance mechanisms. Anopheles gambiae s.s. was resistant to pyrethroids, DDT and carbamates, with potential organophosphate resistance in one population. The resistant phenotypes were conferred by both target-site and metabolic mechanisms. Anopheles funestus s.s. was largely resistant to pyrethroids and carbamates, with potential resistance to DDT in two locations. The resistant phenotypes were conferred by elevated levels of cytochrome p450s.

CONCLUSIONS/SIGNIFICANCE

Currently, the Zambia National Malaria Control Centre is using these results to inform their vector control strategy. The methods employed here can serve as a template to all malaria-endemic countries striving to create a sustainable insecticide resistance management plan.

Gene expression divergence between malaria vector sibling species Anopheles gambiae and An. coluzzii from rural and urban Yaoundé Cameroon

Divergent selection based on aquatic larval ecology is a likely factor in the recent isolation of two broadly sympatric and morphologically identical African mosquito species, the malaria vectors Anopheles gambiae and An. coluzzii. Population-based genome scans have revealed numerous candidate regions of recent positive selection, but have provided few clues as to the genetic mechanisms underlying behavioural and physiological divergence between the two species, phenotypes which themselves remain obscure. To uncover possible genetic mechanisms, we compared global transcriptional profiles of natural and experimental populations using gene-based microarrays. Larvae were sampled as second and fourth instars from natural populations in and around the city of Yaoundé, capital of Cameroon, where the two species segregate along a gradient of urbanization. Functional enrichment analysis of differentially expressed genes revealed that An. coluzzii--the species that breeds in more stable, biotically complex and potentially polluted urban water bodies--overexpresses genes implicated in detoxification and immunity relative to An. gambiae, which breeds in more ephemeral and relatively depauperate pools and puddles in suburbs and rural areas. Moreover, our data suggest that such overexpression by An. coluzzii is not a transient result of induction by xenobiotics in the larval habitat, but an inherent and presumably adaptive response to repeatedly encountered environmental stressors. Finally, we find no significant overlap between the differentially expressed loci and previously identified genomic regions of recent positive selection, suggesting that transcriptome divergence is regulated by trans-acting factors rather than cis-acting elements.

CYP6 P450 enzymes and ACE-1 duplication produce extreme and multiple insecticide resistance in the malaria mosquito Anopheles gambiae

Malaria control relies heavily on pyrethroid insecticides, to which susceptibility is declining in Anopheles mosquitoes. To combat pyrethroid resistance, application of alternative insecticides is advocated for indoor residual spraying (IRS), and carbamates are increasingly important. Emergence of a very strong carbamate resistance phenotype in Anopheles gambiae from Tiassalé, Côte d'Ivoire, West Africa, is therefore a potentially major operational challenge, particularly because these malaria vectors now exhibit resistance to multiple insecticide classes. We investigated the genetic basis of resistance to the most commonly-applied carbamate, bendiocarb, in An. gambiae from Tiassalé. Geographically-replicated whole genome microarray experiments identified elevated P450 enzyme expression as associated with bendiocarb resistance, most notably genes from the CYP6 subfamily. P450s were further implicated in resistance phenotypes by induction of significantly elevated mortality to bendiocarb by the synergist piperonyl butoxide (PBO), which also enhanced the action of pyrethroids and an organophosphate. CYP6P3 and especially CYP6M2 produced bendiocarb resistance via transgenic expression in Drosophila in addition to pyrethroid resistance for both genes, and DDT resistance for CYP6M2 expression. CYP6M2 can thus cause resistance to three distinct classes of insecticide although the biochemical mechanism for carbamates is unclear because, in contrast to CYP6P3, recombinant CYP6M2 did not metabolise bendiocarb in vitro. Strongly bendiocarb resistant mosquitoes also displayed elevated expression of the acetylcholinesterase ACE-1 gene, arising at least in part from gene duplication, which confers a survival advantage to carriers of additional copies of resistant ACE-1 G119S alleles. Our results are alarming for vector-based malaria control. Extreme carbamate resistance in Tiassalé An. gambiae results from coupling of over-expressed target site allelic variants with heightened CYP6 P450 expression, which also provides resistance across contrasting insecticides. Mosquito populations displaying such a diverse basis of extreme and cross-resistance are likely to be unresponsive to standard insecticide resistance management practices.

Malaria control depends heavily on only four classes of insecticide to which Anopheles mosquitoes are increasingly resistant. It is important to manage insecticide application carefully to minimise increases in resistance, for example by using different compounds in combination or rotation. Recently, mosquitoes resistant to all available insecticides have been found in Tiassalé, West Africa, which could be problematic for resistance management, particularly if common genetic mechanisms are responsible (‘cross-resistance’). Tiassalé mosquitoes also exhibit extreme levels of resistance to the two most important classes, pyrethroids and carbamates. We investigated the genetic basis of extreme carbamate resistance and cross-resistance in Tiassalé, and the applicability of results in an additional population from Togo. We find that specific P450 enzymes are involved in both extreme and cross-resistance, including one, CYP6M2, which can cause resistance to three insecticide classes. However, amplification of a mutated version of the gene which codes for acetycholinesterase, the target site of both the carbamate and organophosphate insecticides, also plays an important role. Mechanisms involved in both extreme resistance and cross resistance are likely to be very resilient to insecticide management practices, and represent an alarming scenario for mosquito-targeted malaria control.

Resistance is not Futile: It Shapes Insecticide Discovery

Conventional chemical control compounds used for the management of insect pests have been much maligned, but still serve a critical role in protecting people and agricultural products from insect pests, as well as conserving biodiversity by eradicating invasive species. Although biological control can be an effective option for area-wide management of established pests, chemical control methods are important for use in integrated pest management (IPM) programs, as well as in export treatments, eradicating recently arrived invasive species, and minimizing population explosions of vectors of human disease. Cogitated research and development programs have continued the innovation of insecticides, with a particular focus on combating insecticide resistance. Recent developments in the fields of human health, protecting the global food supply, and biosecurity will be highlighted.

Bendiocarb resistance in Anopheles gambiae s.l. populations from Atacora department in Benin, West Africa: a threat for malaria vector control

BACKGROUND

Owing to pyrethroid resistance in An. gambiae, the carbamate and organophosphate insecticides are currently regarded as alternatives or supplements to pyrethroids for use on mosquito net treatments. Resistance monitoring is therefore essential to investigate the susceptibility of An. gambiae s.l to these alternative products.

METHODS

Two to three day old adult female Anopheles mosquitoes were reared from larvae collected in the five districts (Kouandé, Natitingou, Matéri, Péhunco, Tanguiéta) of the Atacora department. Mosquitoes were then exposed to WHO impregnated papers. The four treatments consisted of: carbamates (0.1% bendiocarb, 0.1% propoxur) and organophosphates (0.25% pirimiphosmethyl, 1% fenitrothion). PCR assays were run to determine the members of the An. gambiae complex, the molecular forms (M) and (S), as well as phenotypes for insensitive acetylcholinesterase (AChE1) due to ace-1(R) mutation.

RESULTS

Bioassays showed bendiocarb resistance in all populations of An. gambiae s.s. tested. Propoxur resistance was observed in Matéri, Péhunco and Tanguiéta, while it was suspected in Kouandé and Natitingou. As for the organophosphates, susceptibility to pirimiphos-methyl was assessed in all populations. Fenitrothion resistance was detected in Kouandé, Péhunco and Tanguiéta, while it was suspected in Matéri and Natitingou. The S-form was predominant in tested samples (94.44%). M and S molecular forms were sympatric but no M/S hybrids were detected. The ace-1(R) mutation was found in both S and M molecular forms with frequency from 3.6 to 12%. Although the homozygous resistant genotype was the most prevalent genotype among survivors, the genotypes could not entirely explain the bioassay results.

CONCLUSION

Evidence of bendiocarb resistance in An. gambiae populations is a clear indication that calls for the implementation of insecticide resistance management strategies. The ace-1(R) mutation could not entirely explain the resistance to bendiocarb observed and is highly suggestive of involvement of other resistance mechanisms such as metabolic detoxification.