Pyrethroid resistance in Anopheles gambiae from the rubber cultivated area of Niete, South Region of Cameroon

Evolution of insecticide resistance and its mechanisms in Anopheles stephensi in the WHO Eastern Mediterranean Region

BACKGROUND

While Iran is on the path to eliminating malaria, the disease with 4.9 million estimated cases and 9300 estimated deaths in 2018 remains a serious health problem in the World Health Organization (WHO) Eastern Mediterranean Region. Anopheles stephensi is the main malaria vector in Iran and its range extends from Iraq to western China. Recently, the vector invaded new territories in Sri Lanka and countries in the Horn of Africa. Insecticide resistance in An. stephensi is a potential issue in controlling the spread of this vector.

METHODS

Data were collated from national and international databases, including PubMed, Google Scholar, Scopus, ScienceDirect, SID, and IranMedex using appropriate search terms.

RESULTS

Indoor residual spaying (IRS) with DDT was piloted in Iran in 1945 and subsequently used in the malaria eradication programme. Resistance to DDT in An. stephensi was detected in Iran, Iraq, Pakistan, and Saudi Arabia in the late 1960s. Malathion was used for malaria control in Iran in 1967, then propoxur in 1978, followed by pirimiphos-methyl from 1992 to 1994. The pyrethroid insecticide lambda-cyhalothrin was used from 1994 to 2003 followed by deltamethrin IRS and long-lasting insecticidal nets (LLINs). Some of these insecticides with the same sequence were used in other malaria-endemic countries of the region. Pyrethroid resistance was detected in An. stephensi in Afghanistan in 2010, in 2011 in India and in 2012 in Iran. The newly invaded population of An. stephensi in Ethiopia was resistant to insecticides of all four major insecticide classes. Different mechanisms of insecticide resistance, including metabolic and insecticide target site insensitivity, have been developed in An. stephensi. Resistance to DDT was initially glutathione S-transferase based. Target site knockdown resistance was later selected by pyrethroids. Esterases and altered acetylcholinesterase are the underlying cause of organophosphate resistance and cytochrome p450s were involved in pyrethroid metabolic resistance.

CONCLUSIONS

Anopheles stephensi is a major malaria vector in Iran and many countries in the region and beyond. The species is leading in terms of development of insecticide resistance as well as developing a variety of resistance mechanisms. Knowledge of the evolution of insecticide resistance and their underlying mechanisms, in particular, are important to Iran, considering the final steps the country is taking towards malaria elimination, but also to other countries in the region for their battle against malaria. This systematic review may also be of value to countries and territories newly invaded by this species, especially in the Horn of Africa, where the malaria situation is already dire.

Status of Insecticide Resistance and Its Mechanisms in Anopheles gambiae and Anopheles coluzzii Populations from Forest Settings in South Cameroon

A key factor affecting malaria vector control efforts in Cameroon is the rapid expansion of insecticide resistance in Anopheles gambiae s.l (An. gambiae) populations; however, mechanisms involved in insecticide resistance in forest mosquito populations are still not well documented yet. The present study was conducted to screen molecular mechanisms conferring insecticide resistance in An. gambiae s.l. populations from the South Cameroon forest region. WHO bioassays were conducted with F0 An. gambiae females aged three to four days from forest (Sangmelima, Nyabessan, and Mbandjock) and urban sites (Yaoundé (Bastos and Nkolondom)), against pyrethroids (permethrin 0.75% and deltamethrin 0.05%) and carbamates (bendiocarb 0.1%). Members of the An. Gambiae s.l. species complex were identified using molecular diagnostic tools. TaqMan assays were used to screen for target site mutations. The expression profiles of eight genes implicated in insecticide resistance were assessed using RT-qPCR. Cuticle hydrocarbon lipids were measured to assess their potential implication in insecticide resistance. Both An. Gambiae and An. coluzzii were detected. An. gambiae was highly prevalent in Sangmelima, Nyabessan, Mbandjock, and Nkolondom. An. coluzzii was the only species found in the Yaoundé city center (Bastos). Low mortality rate to both pyrethroids and bendiocarb was recorded in all sites. High frequency of L1014F allele (75.32–95.82%) and low frequencies of L1014S (1.71–23.05%) and N1575Y (5.28–12.87%) were recorded. The G119S mutation (14.22–35.5%) was detected for the first time in An. gambiae populations from Cameroon. This mutation was rather absent from An. coluzzii populations. The detoxification genes Cyp6m2, Cyp9k1, Cyp6p4, Cyp6z1, as well as Cyp4g16 which catalyzes epicuticular hydrocarbon biosynthesis, were found to be overexpressed in at least one population. The total cuticular hydrocarvbon content, a proxy of cuticular resistance, did not show a pattern associated with pyrethroid resistance in these populations. The rapid emergence of multiple resistance mechanisms in An. Gambiae s.l. population from the South Cameroon forest region is of big concern and could deeply affect the sustainability of insecticide-based interventions strategies in this region.

Status of insecticide resistance and its biochemical and molecular mechanisms in Anopheles stephensi (Diptera: Culicidae) from Afghanistan

Background

Insecticide resistance of Anopheles stephensi, the main malaria vector in eastern Afghanistan, has been reported previously. This study describes the biochemical and molecular mechanisms of resistance to facilitate effective vector control and insecticide resistance management.

Methods

Mosquito larvae were collected from the provinces of Kunar, Laghman and Nangarhar from 2014 to 2017. The susceptibility of the reared 3–4 days old adults was tested with deltamethrin 0.05%, bendiocarb 0.1%, malathion 5%, permethrin 0.75% and DDT 4%. Cytochrome P450 content and general esterase, glutathione S-transferase (GST) and acetylcholinesterase (AChE) activities were measured in the three field populations and the results were compared with those of the laboratory susceptible An. stephensi Beech strain. Two separate allele-specific PCR assays were used to identify L1014, L1014F and L1014S mutations in the voltage gated sodium channel gene of An. stephensi. Probit analysis, ANOVA and Hardy–Weinberg equilibrium were used to analyse bioassay, biochemical assay and gene frequency data respectively.

Results

The population of An. stephensi from Kunar was susceptible to bendiocarb, apart from this, all populations were resistant to all the other insecticides tested. The differences between all values for cytochrome P450s, general esterases, GSTs and AChE inhibition rates in the Kunar, Laghman and Nangarhar populations were statistically significant when compared to the Beech strain, excluding GST activities between Kunar and Beech due to the high standard deviation in Kunar. The three different sodium channel alleles [L1014 (wild type), L1014F (kdr west) and L1014S (kdr east)] were all segregated in the Afghan populations. The frequencies of kdr east mutation were 22.9%, 32.7% and 35% in Kunar, Laghman and Nangarhar populations respectively. Kdr west was at the lowest frequency of 4.44%.

Conclusions

Resistance to different groups of insecticides in the field populations of An. stephensi from Kunar, Laghman and Nangarhar Provinces of Afghanistan is caused by a range of metabolic and site insensitivity mechanisms, including esterases, cytochrome P450s and GSTs combined with AChE and sodium channel target site insensitivity. The intensity and frequency of these mechanisms are increasing in these populations, calling for urgent reorientation of vector control programmes and implementation of insecticide resistance management strategies.

Insecticide Exposure Triggers a Modulated Expression of ABC Transporter Genes in Larvae of Anopheles gambiae s.s

Insecticides remain a main tool for the control of arthropod vectors. The urgency to prevent the insurgence of insecticide resistance and the perspective to find new target sites, for the development of novel molecules, are fuelling the study of the molecular mechanisms involved in insect defence against xenobiotic compounds. In this study, we have investigated if ATP-binding cassette (ABC) transporters, a major component of the defensome machinery, are involved in defence against the insecticide permethrin, in susceptible larvae of the malaria vector Anopheles gambiae sensu stricto. Bioassays were performed with permethrin alone, or in combination with an ABC transporter inhibitor. Then we have investigated the expression profiles of five ABC transporter genes at different time points following permethrin exposure, to assess their expression patterns across time. The inhibition of ABC transporters increased the larval mortality by about 15-fold. Likewise, three genes were up-regulated after exposure to permethrin, showing different patterns of expression across the 48 h. Our results provide the first evidences of ABC transporters involvement in defence against a toxic in larvae of An. gambiae s.s. and show that the gene expression response is modulated across time, being continuous, but stronger at the earliest and latest times after exposure.

Review of the evolution of insecticide resistance in main malaria vectors in Cameroon from 1990 to 2017

BACKGROUND

Malaria remains a major public health threat in Cameroon and disease prevention is facing strong challenges due to the rapid expansion of insecticide resistance in vector populations. The present review presents an overview of published data on insecticide resistance in the main malaria vectors in Cameroon to assist in the elaboration of future and sustainable resistance management strategies.

METHODS

A systematic search on mosquito susceptibility to insecticides and insecticide resistance in malaria vectors in Cameroon was conducted using online bibliographic databases including PubMed, Google and Google Scholar. From each peer-reviewed paper, information on the year of the study, mosquito species, susceptibility levels, location, insecticides, data source and resistance mechanisms were extracted and inserted in a Microsoft Excel datasheet. The data collected were then analysed for assessing insecticide resistance evolution.

RESULTS

Thirty-three scientific publications were selected for the analysis. The rapid evolution of insecticide resistance across the country was reported from 2000 onward. Insecticide resistance was highly prevalent in both An. gambiae (s.l.) and An. funestus. DDT, permethrin, deltamethrin and bendiocarb appeared as the most affected compounds by resistance. From 2000 to 2017 a steady increase in the prevalence of kdr allele frequency was noted in almost all sites in An. gambiae (s.l.), with the L1014F kdr allele being the most prevalent. Several detoxification genes (particularly P450 monooxygenase) were associated with DDT, pyrethroids and bendiocarb resistance. In An. funestus, resistance to DDT and pyrethroids was mainly attributed to the 119F-GSTe2 metabolic resistance marker and over-expression of P450 genes whereas the 296S-RDL mutation was detected in dieldrin-resistant An. funestus.

CONCLUSIONS

The review provides an update of insecticide resistance status in malaria vector populations in Cameroon and stresses the need for further actions to reinforce malaria control strategies in the coming years.

Protection against mosquito vectors Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus using a novel insect repellent, ethyl anthranilate

Growing concern on the application of synthetic mosquito repellents in the recent years has instigated the identification and development of better alternatives to control different mosquito-borne diseases. In view of above, present investigation evaluates the repellent activity of ethyl anthranilate (EA), a non-toxic, FDA approved volatile food additive against three known mosquito vectors namely, Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus under laboratory conditions following standard protocols. Three concentration levels (2%, 5% and 10% w/v) of EA were tested against all the three selected mosquito species employing K & D module and arm-in-cage method to determine the effective dose (ED₅₀) and complete protection time (CPT), respectively. The repellent activity of EA was further investigated by modified arm-in-cage method to determine the protection over extended spatial ranges against all mosquito species. All behavioural situations were compared with the well-documented repellent N,N-diethylphenyl acetamide (DEPA) as a positive control. The findings demonstrated that EA exhibited significant repellent activity against all the three mosquitoes species. The ED₅₀ values of EA, against Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus were found to be 0.96%, 5.4% and 3.6% w/v, respectively. At the concentration of 10% w/v, it provided CPTs of 60, 60 and 30min, respectively, against Aedes aegypti, Anopheles stephensi and Culex quinquefasciatus mosquitoes. Again in spatial repellency evaluation, EA was found to be extremely effective in repelling all the three tested species of mosquitoes. Ethyl anthranilate provided comparable results to standard repellent DEPA during the study. Results have concluded that the currently evaluated chemical, EA has potential repellent activity against some well established mosquito vectors. The study emphasizes that repellent activity of EA could be exploited for developing effective, eco-friendly, acceptable and safer alternative to the existing harmful repellents for personal protection against different hematophagous mosquito species.

Gene expression modulation of ABC transporter genes in response to permethrin in adults of the mosquito malaria vector Anopheles stephensi

Living organisms have evolved an array of genes coding for detoxifying enzymes and efflux protein pumps, to cope with endogenous and xenobiotic toxic compounds. The study of the genes activated during toxic exposure is relevant to the area of arthropod vector control, since these genes are one of the targets upon which natural selection acts for the evolution of insecticide resistance. ATP-binding cassette (ABC) transporters participate to insecticide detoxification acting as efflux pumps, that reduce the intracellular concentration of toxic compounds, or of their metabolic derivatives. Here we analyzed the modulation of the expression of six genes coding for ABC transporters, after the exposure of adult females and males of the mosquito Anopheles stephensi, a major malaria vector in Asia, to permethrin. Male and female mosquitoes were exposed to insecticide for one hour, then the expression profiles of the ABC transporter genes AnstABCB2, AnstABCB3, AnstABCB4, AnstABCBmember6, AnstABCC11, and AnstABCG4 were analysed after one and 24h. Our results showed that three genes (AnstABCB2, AnstABCBmember6, AnstABCG4) were up-regulated in both sexes; two of these (AnstABCBmember6 and AnstABCG4) have previously been shown to be up-regulated also in larval stages of An. stephensi, supporting a role for these genes in permethrin defence in larvae as well as in adults. Finally, the same ABC transporter genes were activated both in females and males; however, the timing of gene induction was different, with a prompter induction in females than in males.

Mosquito repellents: An insight into the chronological perspectives and novel discoveries

Mosquito being the major medically important arthropod vector; requires utmost attention to reduce the sufferings and economic consequences of those living in the endemic regions. This is only possible by minimising the human-mosquito contact by an absolute preventing measure. However, unfortunately, such absolute measures are yet to be developed despite enormous efforts and huge investments worldwide. In the absence of vaccines for number of mosquito-borne diseases, repellents could be an attractive option for both military personal and civilians to minimise the risk of contacting different mosquito-borne diseases. However, to achieve this golden goal, the detailed knowledge of a particular repellent is must, including its mode of repellency and other relevant informations. Here, in the present article, an effort has been made to convey the best and latest information on repellents in order to enhance the knowledge of scientific community. The review offers an overview on mosquito repellents, the novel discoveries, and areas in need of attention such as novel repellent formulations and their future prospective.

Knockdown resistance of Anopheles sinensis in Henan province, China

Background

Vivax malaria was historically epidemic in Henan Province of China and Anopheles sinensis was the main vectors and poor farming communities bare the greatest burden of disease. Knockdown resistance in An. sinensis is one of the mechanisms of resistance against pyrethroids. In the present study, the frequency of mutations from An. sinensis was examined in Henan province, China.

Methods

Anopheles was collected from Kaifeng, Tongbai, Tanghe, Pingqiao, Shihe, and Yongcheng counties of Henan province in 2013. Molecular identification of Anopheles species was conducted by polymerase chain reaction (PCR) amplifying the internal transcribed spacer 2 (ITS2). Part of the IIS6 domain of the para-type sodium channel protein gene was polymerase chain reaction-amplified and directly sequenced. Frequency and geographic difference of kdr gene mutant types were analysed.

Results

208 Anopheles were received molecular identification, of which 169 (81.25%) were An. sinensis, 25 (12.02%) were Anopheles yatsushiroensis, and 12 (5.77%) were Anopheles lesteri. A 325 bp fragment of the para-type sodium channel gene including position 1014 was successfully sequenced from 139 Anopheles, of which 125 (89.93%) were An. sinensis, 12 (8.63%) were An. yatsushiroensis, 2 (1.44%) were An. lesteri. The molecular analyses revealed that mutations existed at codon 1014 in An. sinensis but not in An. yatsushiroensis and An. lesteri. Frequency of kdr mutation was 73.60% (92/125) from population of An. sinensis in Henan province, of which L1014F (TTT + TTC) allele frequencies accounted for 46.40% (58/125), and was higher than that of L1014C(TGT) which accounted for 27.20% (34/125) ( χ2 = 55.423, P < 0.001). The frequency of kdr mutation in Kaifeng county was 100% (49/49), and was higher than that of 37.93% (11/29) in Tongbai, 54.55% (6/11) in Pingqiao, 50.00% (3/3) in Shihe, and 62.50% (10/16) in Yongcheng county, respectively (χ2 = 39.538, P < 0.001; χ2 = 24.298, P < 0.001; χ2 = 25.913, P < 0.001; χ2 = 20.244, P < 0.001). While 92.86% (13/14) frequency of kdr mutation was found in Tanghe county, which was higher than that in Tongbai county (χ2 = 11.550, P = 0.0018).

Conclusions

A high frequency of kdr gene mutations from population of An. sinensis in Henan province was found.

Nationwide assessment of insecticide susceptibility in Anopheles gambiae populations from Zimbabwe

Background

The scale-up of malaria interventions in sub-Saharan Africa has been accompanied by a dramatic increase in insecticide resistance in Anopheles spp. In Zimbabwe resistance to pyrethroid insecticides was reported in Gokwe District in 2008. This study reports results of the first nation-wide assessment of insecticide susceptibility in wild populations of Anopheles gambiae sensu lato (s.l.) in Zimbabwe, and provides a comprehensive review of the insecticide resistance status of An. gambiae s.l. in southern African countries.

Methods

World Health Organization (WHO) insecticide susceptibility tests were performed on 2,568 field collected mosquitoes originating from 13 sentinel sites covering all endemic regions in Zimbabwe in 2011–2012. At each site, 24-hour mortality and knock-down values for 50% and 90% of exposed mosquitoes (KD₅₀ and KD₉₀, respectively) were calculated for pools of 20–84 (mean, 54) mosquitoes exposed to 4% DDT, 0.1% bendiocarb, 0.05% λ-cyhalothrin or 5% malathion. Susceptibility results from Zimbabwe were compiled with results published during 2002–2012 for all southern African countries to investigate the resistance status of An. gambiae s.l. in the region.

Results

Using WHO criteria, insecticide resistance was not detected at any site sampled and for any of the insecticide formulations tested during the malaria transmission season in 2012. Knock-down within 1 hr post-insecticide exposure ranged from 95% to 100%; mortality 24 hours post-insecticide exposure ranged from 98% to 100%. Despite the lack of insecticide resistance, high variability was found across sites in KD₅₀ and KD₉₀ values. A total of 24 out of 64 (37.5%) sites in southern Africa with reported data had evidence of phenotypic insecticide resistance in An. gambiae s.l. to at least one insecticide.

Conclusion

Despite a long history of indoor residual spraying of households with insecticide, up to 2012 there was no evidence of phenotypic resistance to any of the four insecticide classes in An. gambiae s.l. collected across different eco-epidemiological areas in Zimbabwe. Results reinforce the need for careful monitoring over time in sentinel sites in order to detect the potential emergence and propagation of insecticide resistance as insecticidal vector control interventions in Zimbabwe continue to be implemented.

Electronic supplementary material

The online version of this article (doi:10.1186/1475-2875-13-408) contains supplementary material, which is available to authorized users.

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.

Re-visiting insecticide resistance status in Anopheles gambiae from Côte d'Ivoire: a nation-wide informative survey

Insecticide resistance constitutes a major threat that may undermine current gain in malaria control in most endemic countries. National Malaria Control Programmes (NMCPs) need as much information as possible on the resistance status of malaria vectors and underlying mechanisms in order to implement the most relevant and efficient control strategy. Bioassays, biochemical and molecular analysis were performed on An. gambiae collected in six sentinel sites in Côte d'Ivoire. The sites were selected on the basis of their bioclimatic status and agricultural practices. An. gambiae populations across sites showed high levels of resistance to organochloride, pyrethroid and carbamate insecticides. The kdr and ace-1(R) mutations were detected in almost all sentinel sites with mosquitoes on the coastal and cotton growing areas mostly affected by these mutations. At almost all sites, the levels of detoxifying enzymes (mixed-function oxidases (MFOs), non-specific esterases (NSE) and glutathione-S-transferases (GSTs)) in An. gambiae populations were significantly higher than the levels found in the susceptible strain Kisumu. Pre-exposure of mosquitoes to PBO, an inhibitor of MFOs and NSEs, significantly increased mortality rates to pyrethroids and carbamates in mosquitoes but resistance in most cases was not fully synergised by PBO, inferring a residual role of additional mechanisms, including kdr and ace-1 site insensitivity. The large distribution of resistance in Côte d'Ivoire raises an important question of whether to continue to deploy pyrethroid-based long-lasting insecticidal nets (LLINs) and insecticide residual spraying (IRS) towards which resistance continues to rise with no guarantee that the level of resistance would not compromise their efficacy. Innovative strategies that combine insecticide and synergists in LLINs or spatially LLIN and an effective non-pyrethroid insecticide for IRS could be in the short term the best practice for the NMCP to manage insecticide resistance in malaria vectors in Côte d'Ivoire and other endemic countries facing resistance.

A preliminary investigation of the relationship between water quality and Anopheles gambiae larval habitats in Western Cameroon

Background

Water quality and anopheline habitat have received increasing attention due to the possibility that challenges during larval life may translate into adult susceptibility to malaria parasite infection and/or insecticide resistance.

Methods

A preliminary study of Anopheles gambiae s.s. larval habitats in the north-west and south-west regions of Cameroon was conducted in order to detect associations between An. gambiae s.s. molecular form and 2La inversion distributions with basic water quality parameters. Water quality was measured by temperature, pH, conductivity, total dissolved solids (TDS) at seven sites in Cameroon and one site in Selinkenyi, Mali.

Results

Principal components and correlation analyses indicated a complex relationship between 2La polymorphism, temperature, conductivity and TDS. Cooler water sites at more inland locations yielded more S form larvae with higher 2La inversion polymorphism while warmer water sites yielded more M form larvae with rare observations of the 2La inversion.

Discussion

More detailed studies that take into account the population genetics but also multiple life stages, environmental data relative to these life stages and interactions with both humans and the malaria parasite may help us to understand more about how and why this successful mosquito is able to adapt and diverge, and how it can be successfully managed.

Insecticide resistance status of Anopheles gambiae s.s population from M'Bé: a WHOPES-labelled experimental hut station, 10 years after the political crisis in Côte d'Ivoire

BACKGROUND

An experimental hut station built at M'Bé in 1998 was used for many years for the evaluation of insecticidal product for public health until the civil war broke out in 2002. Breeding sites of mosquitoes and selection pressure in the area were maintained by local farming practices and the West African Rice Development Association (WARDA, actually AfricaRice) in a large rice growing area. Ten years after the crisis, bioassays, molecular and biochemical analyses were conducted to update the resistance status and study the evolution of resistance mechanisms of Anopheles gambiae s.s population.

METHODS

Anopheles gambiae s.s larvae from M'Bé were collected in breeding sites and reared until emergence. Resistance status of this population to conventional insecticides was assessed using WHO bioassay test kits for adult mosquitoes, with 10 insecticides belonging to pyrethroids, pseudo-pyrethroid, organochlorides, carbamates and organophosphates with and without the inhibitor piperonyl butoxyde (PBO). Molecular and biochemical assays were carried out to identify the L1014F kdr, L1014S kdr and ace-1(R) alleles in individual mosquitoes and to detect potential increase in mixed function oxidases (MFO) level, non-specific esterases (NSE) and glutathione S-transferases (GST) activities.

RESULTS AND DISCUSSION

Anopheles gambiae s.s from M'Bé exerted high resistance levels to organochlorides, pyrethroids, and carbamates. Mortalities ranged from 3% to 21% for organochlorides, from 50% to 75% for pyrethroids, 34% for etofenprox, the pseudo-pyrethroid, and from 7% to 80% for carbamates. Tolerance to organophosphates was observed with mortalities ranging from 95% to 98%. Bioassays run with a pre-exposition of mosquitoes to PBO induced very high levels of mortalities compared to the bioassays without PBO, suggesting that the resistance to pyrethroid and carbamate relied largely on detoxifying enzymes' activities. The L1014F kdr allelic frequency was 0.33 in 2012 compared to 0.05 before the crisis in 2002. Neither the L1014S kdr nor ace-1(R) mutations were detected. An increased activity of NSE and level of MFO was found relative to the reference strain Kisumu. This was the first evidence of metabolic resistance based resistance in An. gambiae s.s from M'Bé.

CONCLUSION

The An. gambiae s.s population showed very high resistance to organochlorides, pyrethroids and carbamates. This resistance level relied largely on two major types of resistance: metabolic and target-site mutation. This multifactorial resistance offers a unique opportunity to evaluate the impact of both mechanisms and their interaction with the vector control tools currently used or in development.

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.