Trends in Insecticide Resistance in Natural Populations of Malaria Vectors in Burkina Faso, West Africa: 10 Years’ Surveys

K. R. Dabire1, A. Diabate1, M. Namountougou1, L. Djogbenou2, C. Wondji3, F. Chandre4, F. Simard5, J-B. Ouedraogo1, T. Martin6, M. Weill7 and T. Baldet8 1IRSS/Centre Muraz, BP 390 Bobo-Dioulasso, 2IRSP/ Ouidah, 3Liverpool School of Tropical Medicine, 4LIN/Montpellier UMR MIGEVEC 5IRD/IRSS, Bobo-Dioulasso UMR MIGEVEC, 6CIRAD, UR HORTSYS, Montpellier, 7ISEM, CNRS/Universite Montpellier 2, Montpellier, 8IRD/CIRAD/CREC, Cotonou, 1,5Burkina Faso 2,8Benin 3UK 4,6,7France


Introduction
Malaria is a major threat in endemic regions causing at least 1 million deaths each year affecting poor and underserved populations living in tropical and sub-tropical regions. Diseases control ideally entails prevention and treatment of human infections. However, few vaccines are currently available and many pathogens are now resistant to anti-parasitic drugs. Additionally populations from endemic countries have less access to treatments due to economical impediments. Thus, the control of vectors in many instances is the only affordable measure (Beier et al., 2008). Mosquito control is mainly achieved by using insecticides and secondarily bio-larvicides (Bt-H14, B. sphaericus), predators (fish or copepod predators) or parasitic load (fungi), and/or by modifying the physical environment (WHO, 2006). Insecticides target a vital physiological function, leading to mosquito death. Unfortunately, due to their extremely large numbers and short generation span, mosquito populations evolve very rapidly and become resistant to insecticides, leading to repeated field control failures. Resistance results from the selection of mutant individuals able to survive and reproduce in presence of insecticide, the insecticide failing to disrupt the function of its target. In 2007, more than 100 mosquito species were resistant to at least one insecticide, some species being resistant to several compounds (Whalon et al., 2008). Very few classes of synthetic insecticides are available today for vector control, the most recent has been introduced 20 years ago and none are expected in the near future (Nauen, 2007). The low availability of insecticides due to resistance is further reduced in many countries by the removal from the market of compounds for public health because for their toxicity for humans or the lack of specificity in non-target species (Rogan & Chen, 2005). Resistance is a genetic adaptation to the modification of the environment induced by insecticides. It usually appears locally, sometimes independently in different places, but may spread rapidly through migration (Brogdon & McAllister, 1998;Weill et al., 2003). However, mosquito resistance is not only due to the insecticides used for mosquito control, but also to the many pesticide pollutions present in their environment which are generated by a large variety of human activities including insect control for agriculture and other house-hold protections. These pollutions may dramatically affect resistance genes dynamics and threaten vector control strategies. The overall pesticides pressure that select resistance in mosquitoes need to be clarified, both in terms of insecticides usage and quantity. An. gambiae is a complex, with seven sibling species that are closely related and morphologically indistinguishable from each other by routine taxonomic methods (Gillies & Coetzee, 1987). These sibling species are however different with respect to ecological and behavioral characteristics and to vectorial competence. In West Africa, An. gambiae s.s. and An. arabiensis are the two main species of the complex that transmit malaria, with the former being the most efficient vector due to its high anthropophily (White, 1974, Lemasson et al., 1997. Previous study carried out on the species composition in Burkina Faso indicated that An. gambiae s.l. was found to be a mixture of An. gambiae s.s. and An. arabiensis across the Sudan (98.3% vs. 1.7%), Sudan-sahelian (78.6% vs. 21.4%) and the Sahel (91.5% vs. 8.5%) ecotypes (Dabiré et al., 2009a). An. gambiae s.s. contains two molecular forms, M and S, which co-exist in West Africa (della Torre et al., 2005). The M form was predominant in permanent breeding sites such as rice fields, whereas the S form was predominant in temporary habitats notably rain-filled puddles which are productive during the wet season. In Burkina Faso, genes conferring resistance to insecticides display large frequency differences in M and S forms of An. gambiae s.s. and An. arabiensis. Resistance of An. gambiae s.l. to DDT and pyrethroids (PYR) is especially conferred in West Africa by mutation of the sodium channel target site, the L1014F kdr (Chandre et al., 1999;Diabaté et al., 2002;Awolola et al., 2005;Nguessan et al., 2007). Burkina Faso is composed of three agro-climatic zones and the use of insecticides to control agricultural and human health pests varies considerably in the different zones particularly as the main cotton cropping areas are found in the south west of the country. In this last region, the intensive use of insecticides most notably for fighting the cotton Gossypium hirsutum L. pest is thought to have selected insecticide resistance genes in mosquitoes whose breeding sites are exposed to pesticide runoff (Diabaté et al., 2002;Dabiré et al., 2009a & b). The goal of this chapter is to summarise the resistance to insecticides status mainly in An. gambiae s.l. populations throughout these different agro-climatic areas and to discuss how it could limit the efficacy of malaria vector control strategies in short and long terms at the country scale. Such information is vital to determine the suitability of pyrethroids used for bednet impregnation and CX or OP based-combinations for indoor residual spraying (IRS).

Materials and methods
In Burkina Faso country-wide surveys associating bioassays and molecular investigations were carried out from 2000 to 2010 through 26 localities and they allow updating the www.intechopen.com Trends in Insecticide Resistance in Natural Populations of Malaria Vectors in Burkina Faso, West Africa: 10 Years' Surveys 481 resistance status to DDT and pyrethroids and the distribution of L1014F kdr among An. gambiae s.l. into different agro climatic zones (table 1). We were also interested more recently from 2007 to perform bioassays with some insecticides among OP and CX and also to detect the ace-1 R mutation 2-3 days aged females of An. gambiae s.l. issued from wild larvae were exposed to several molecules such as DDT 4%, permethrin 0.75%, deltamethrin 0.05%, bendiocarb 0.1%, chlorpyriphos methyl (CM) 0.04%, carbosulfan 0.04% and fenithrotion 0.04% according to the WHO tube protocol (WHO, 1998). These active molecules were chosen as they represented each family of classic insecticides commonly used in public health. Some of these molecules such as permethrin, deltamethrin and bendiocarb are now in use in Burkina Faso through impregnated bednets and IRS application. Study sites: Burkina Faso covers three ecological zones, the Sudan savannah zone in the south and west, the arid savannah zone (Sudan-sahelian) which extends throughout much of the central part of the country and the arid land (Sahel) in the north. The northern part of the country experiences a dry season of 6-8 months with less than 500 mm of rainfall per year. Rainfall is heaviest in the south-west (5-6 months) with a relatively short dry season. The varied ecological conditions are reflected in the different agricultural systems practiced throughout the country, from arable to pastoral lands. The western region constitutes the main cotton belt extending to the south where some new cotton areas have been cultivated. All ecological zones support the existence of Anopheles species that vector malaria and the disease is widespread throughout the country (Figure 1).

Mosquitoes sampling:
To evaluate the status of resistance of An. gambiae s.l. to insecticides in the three ecological zones of Burkina Faso, anopheline larvae were sampled in countrywide collections during the rainy season, from September to October. Larvae were collected at each locality from breeding sites such as gutters, tires, swallow wells and pools of standing water. Larvae were brought back to the insectary and reared to adulthood. When it was not possible to collect larvae because of the distance between the sampling site and the insectary or due to sampling constraints at the site such as excessive rainfall or flooding, alternative collections of adult mosquitoes were made using indoor aerosol insecticide spraying. An. gambiae s.l. were identified morphologically using standard identification keys of Gillies & Coetzee (1987

Resistance to pyrethroids and organochlorine
Reports of resistance in mosquito vector populations in Burkina Faso appeared as early as the 1960 s, when An. funestus and An. gambiae s.l. populations that showed resistance to dieldrin and DDT, were described (Hamon et al., 1968a;Hamon et al., 1968b). More recent studies have confirmed that resistance to DDT4% is still prevailing with highest level in An. gambiae s.l. populations in Burkina Faso where also resistance to certain pyrethroids was increasingly reported (Diabaté et al., 2002(Diabaté et al., , 2004aDabiré et al., 2009a). Indeed An. gambiae s.l.
populations were resistant to DDT4% in every part of the country and mortality rates below 60 % were observed at the country scale ( Fig. 2). They were found also resistant to permethrin 0.75% in the Sudan climatic zone in the western region and also in several sites in the central part of Burkina Faso (Fig. 3). Surprisingly, except for the areas with a very long history of cotton cropping, the tested populations of An. gambiae remained susceptible to deltamethrin 0.05%, although decreased mortality values lead to suspect an emergence of resistance in the ongoing years (Dabiré et al., 2009a). However, this result should be interpreted with caution as the resistance is a progressive process and recent data recorded in 2009-2010 showed 5 sites mostly located in the central region remained susceptible foci (Fig. 4). Resistance is the result of a limited number of physiological mechanisms; it is often monogenic and due to point mutations in a structural gene, gene amplification or changes in transcriptional regulation (Hollingworth & Dong, 2008). It results from the selection of mutant individuals able to survive and reproduce in presence of insecticide, the insecticide failing to disrupt the function of its target (Whalon et al., 2008). The resistance phenotype to pyrethroids and DDT 4% observed in natural populations of An. gambiae s.l. was already attributed to a kdr mutation as it is the major mechanism involved in cross resistance to pyrethroids and DDT4% in West Africa (Chandre et al., 1999;Diabaté et al., 2002). Until recently, it was assumed that this mutation was the L1014F substitution in West Africa while the L1014S substitution was found in the East (Ranson et al., 2000). However, we now know that both mutations coexist in some countries and are widely distributed throughout sub-Saharan continent and also in Benin and Burkina Faso In Burkina Faso, the frequency of the L1014F kdr mutation was first described in the S form of An. gambiae s.s. in high frequencies especially in the Western part of the country where the use of insecticides is intensive in agriculture (Chandre et al., 1999, Diabaté et al., 2002. But few years later it had been also found within the M form and was suspected to be the result of an introgression from the S form An. gambiae s.s. (Weill et al., 2000;Diabaté et al., 2003). Up to day the distribution of this mutation at the country scale is variable, ranging from 0.5 to 0.97 for the S form in the Sudanian region and decreasing in the Sudano-sahelian and Sahelian areas with averaged values fluctuating between 0.1 and 0.6. Compared to 2000 data (Diabaté et al., 2004a), the frequency of L1014F kdr mutation increased notably from 2004 to 2006 before getting stable around the fixation level in some localities (Fig. 5). As mentioned above, no kdr was detected in 1999 in the M form (Chandre et al., 1999). But early in 2000, the L1014F mutation was identified from few specimens of M form from rice growing area, peaking maximally at 0.04 (Diabaté et al., 2003). Nowadays the L1014F kdr has increased drastically in the M form with varying frequencies between climatic areas, and reaching high frequencies (0.93) in cotton growing belts with a geographic expansion to the sudanosahelian region where it was formerly absent (Dabiré et al., 2009a). It has also increased in An. arabiensis (0. 28  mean frequency of the kdr allele was the highest in populations from the Sudanian zone where the lowest mortality rates to pyrethroids and DDT 4% were seen in bioassays. By contrast, the kdr allele frequency was lower in An. gambiae s.s. populations is central and eastern sites where cotton cultivation is recent. A majority of susceptible phenotypes were observed in wild populations of An. gambiae from these areas. In addition, the results of this study suggest that the domestic use of insecticides may also exert a selection on An. gambiae populations that is secondary to that from the agricultural insecticides. Indeed all collections made in cities located outside the cotton belt showed high mortality rates and a relative low frequency of kdr compared to those of cotton belt. The correlation between the high frequencies of L1014F kdr mutation and the proportion of surviving individuals after DDT/pyrethroids exposures ( figure 7) suggests that this mutation is the main mechanism of resistance to these insecticides.

Metabolic resistance
Most studies conducted in Burkina Faso have focused on the modification of target sites by mutation and did investigate the occurrence and the role of metabolic resistance in the observed resistance of An. gambiae s.l. Recent tests performed in Dioulassoba (an old central district of Bobo-Dioulasso crossed by the Houet river) showed that An. arabiensis from urban polluted breeding sites was resistant to DDT 4% but fully susceptible to pyrethroids and OP/CX, suggesting an existence of metabolic resistance probably GST which is more specific to DDT acting as the main resistance mechanism (Dabiré et al., unpubl.). Even more recently, preliminary results gathered only in VK7 (a sample from a rice growing area surrounded by cotton fields) showed an overexpression of detoxifying enzymes such as glutation-S-transferases, cytochrome P450 oxygenases in populations of An. gambiae s.s. with high kdr frequencies suggesting the existence of multi-resistance mechanisms to pyrethroids (Fig. 8A,B&C). But more investigations are needed to better address the role of metabolic components on the expression of resistance phenotypes observed in natural populations of An. gambiae s.l. especially in areas where insecticide pressure is high.

Resistance to organophosphates (OP) and carbamates (CX) and geographic distribution of ace-1 R mutations and duplicated ace-1 D allele
In Burkina Faso the resistance to OP/CX has been monitored since 2002 only in few sites of the Western areas of the country, and lately extended to the country scale since 2006. Although fenithrotion 0.4%, chlorpyriphos methyl (CM) 0.4%, carbosulfan 0.4% and bendiocarb 0.1 % were tested, the monitoring was well sustained only with bendiocarb 0.1% which was expected to be used in Burkina Faso indoor residual spraying to supplement the efficacy of ITNs especially in localities where An. gambiae is resistant to PY. Except for CM 0.4% for which An. gambiae populations were fully susceptible irrespectively of the locality, the other OP/CX mentioned above showed mortality rates ranging from 5% to 100% (Fig. 9). The lowest mortality rates were obtained with carbosulfan 0.4% (5%) and bendiocarb 0.1% (20%) especially in areas located in cotton belt such as Houndé, Orodara, Tiefora and Banfora. The susceptibility to bendiocarb 0.1% was also recorded in the central areas where cotton growing is recent (Fig. 10). From 2005 on, the detection of ace-1 R mutation involved in OP/CX resistance allowed to evaluate the distribution of this allele in field populations of An. gambiae s.l. The characterisation of this allele was based on a PCR-RFLP diagnostic (Weill et al., 2004) that allow the identification of the amino-acid substitution, from a glycine to a serine at the position 119, in the AChE1 catalytic site (G119S). In Culex pipiens, there is direct and indirect evidence that the resistance allele (ace-1 R ) entails a large fitness cost, probably due to the mutated AChE1 having a much lower level of activity. Homozygous ace-1 R mosquitoes survive in the presence of insecticide, but are rapidly outcompeted in the www.intechopen.com  absence of insecticide. There are evidences that the same phenomenon exists also in An. gambiae s.s (Djogbenou et al., 2010). Even though the results of bioassays were more recent (2009) we presented only the ace-1 R frequencies from 2006 to 2008 samples. This mutation was distributed throughout the Sudan and Sudan-Sahelian localities reaching relative high frequencies (0.6) in the South-West, moderate frequencies (<50%) in the central region, and being absent in the Sahel. It was far more frequent in the S form than in the sympatric M mosquitoes (averaging in mean 0.32 for the S form vs. 0.036 for the M form) (Djogbenou et al., 2008a). Even though the ace-1 R mutation was spread across two climatic zones, it was recorded mostly in the cotton growing areas (Dabiré et al., 2009b). Although the ace-1 R mutation was less spread within the An. gambiae s.s. M form, the highest frequency (0.63) was recorded in this form at Houndé located just on the limit of the Sudan region in 2008 (Fig. 11). The observed genotypic frequencies were not significantly different from Hardy-Weinberg expectations at the 95% confidence level in populations from any site except in the An. gambiae s.s. S form population from Orodara, where an excess of heterozygotes was observed. S-form samples from a number of other sites also showed a higher than expected number of heterozygous genotypes including Banfora (expected 10, observed 13), Diebougou (expected 11, observed 16) and VK7 (expected 12, observed 17) although Hardy-Weinberg equilibrium was not rejected. These results suggest that a fitness cost is associated to this mutation (Labbé et al., 2007), but see the next paragraph. No An. arabiensis was detected up today carrying the ace-1 R mutation.   The frequency evolution of this allele during the two years is not regular ( fig.12), but considering only the Sudan area, it seems to decrease within the S form and increase slightly in the M form from 2006 to 2008 (Fig. 12). As no data existed before 2006, we did not know the trait of evolution of this allele in the past. That should greatly contribute to explain the inverse tendency of this allele within the two forms because the resistance pattern is complex in this area where excess of heterozygous for ace-1 R allele should probably co-exist with the duplication allele Ag-ace-1 D (see the next paragraph). However the allele frequencies in the two forms need to be compared statistically from a solid sample sizes. Regular monitoring of the same localities with the same protocols should give a better insight of the evolution of the G119S mutation of the ace-1 gene. Data will have to be analyzed in relation with the possible coexistence of other resistance mechanisms such as kdr mutation or metabolic based resistance as well as with the existence of the duplicated allele Ag-ace-1 D which may decrease the fitness cost of this mutation (Berticat et al., 2008).

Duplication of ace-1 D allele in An. gambiae s.s. from Burkina Faso
The G119S mutation conferring resistance to organophosphates and carbamates was distributed throughout the Sudan and Sudan-sahelian correlated with the cotton growing areas. This mutation has been identified in the ace-1 R allele, and recently in a "duplicated allele" (ace-1 D ), putting in tandem a susceptible and a resistant copy of ace-1 on the same chromosome. The ace-1 D has been recorded in field populations of An. gambiae M and S forms and was shown to have come from the same duplication event in both forms (Djogbenou et al., 2008b). A unique ace-1 D allele has been observed in Côte d'Ivoire and Burkina Faso, with an estimated frequency >50% in some populations (Djogbenou et al., 2009). In Burkina Faso, the ace-1 D allele frequency could reach 50% and is mainly present in the S form principally in the old cotton belt in the South West. The duplicated allele was also www.intechopen.com  (Fig. 13). If, as suspected by Labbé et al. (2007), ace-1 D allele has a lower fitness cost than ace-1 R , it would increase dramatically the diffusion of OP and CX resistance in An. gambiae s.s. natural populations.
Presently, there is no simple test to characterized the duplicated allele ace-1 D as diagnostic tests do not discriminate between heterozygotes ace-1 S /ace-1 R and genotypes with the ace-1 D allele. Formal identification of ace-1 D in a field female thus necessitate to cross this female with a susceptible male, and screen its offspring for CX resistance (see Labbé et al. 2007 for detailed procedure). This is clearly not possible on a large number of specimens and to better address the role and the impact of the ace-1 duplication in resistance schemes, it appears urgent to build an ace-1 D homozygous laboratory strain to investigate how the duplication modifies the fitness of its carriers. Fig. 13. Agace-1 D frequency in Western Africa. The frequency is given for each An. gambiae molecular form: S (red) and M blue). Samples in which ace-1 D was detected by molecular analysis are bolded and underlined. Significant presence of the duplicated allele (before Bonferroni correction) is given with * for P < 0.05, ** for P < 0.01 and *** for P < 0.001.  (Dabiré et al., 2008). This mutation was found together with kdr mutation within the same populations.
That suggests the existence of multiresistance mechanisms occurring in the same populations. The same individuals were found carrying the two genes but the kdr always appeared as homozygous (kdr R /kdr R ). The functional links of the two genes needed to be further investigated (Dabire et al., 2008). Indeed as indicated by the results of bioassays, the occurrence of such multiresistance mechanisms should explain why An. gambiae s.s. populations are becoming resistant to all classes of insecticides especially in the South west parts of the country. The individuals carrying the two genes appeared to be phenotypically more resistant to pyrethroids and bendiocarb than those carrying only kdr or ace-1 R , respectively. Such a synergy between kdr and ace-1 R has been observed in Culex pipiens (Berticat et al., 2008). Assuming that the ace-1 R is associated to a high fitness cost (Djogbenou et al., 2010), it should be interesting to investigate how the kdr mutation influences the fitness cost related to ace-1 R . Also, it was previously shown in Culex pipiens that mosquitoes carrying both kdr and ace-1 R mutations suffer less cost than the one carrying only ace-1 R (Berticat et al., 2008). Both this synergy between pyrethroids and OP/CX resistance mechanisms and the spread of the ace-1 D allele in natural populations of An. gambiae s.s. could largely hamper the expected results of using OP/CX as alternative insecticides to the PYR becoming ineffective by the presence of kdr. It is crucial to build laboratory colonies carrying the two mutations from which the benefit of changing insecticides could be properly tested. More recently, in 2011, we also recorded metabolic based resistance in An. gambiae from Burkina Faso. Even though these results are preliminary, they further complicate the pattern of resistance in this country, and may represent a dramatic threat for malaria vector control in the near future.
In conclusion, all these aspects need to be properly addressed by fine fundamental research to decrypt the link between the resistance schemes and to give sense in vector control point of view.

Other malaria vectors
Anopheles funestus belongs to a group of no less than nine species that are difficult to distinguish based solely on morphological characters of a single life stage (Gillies andCoetzee 1987, Harbach 1994). Species identification difficulties have been recently addressed by molecular techniques based on the polymerase chain reaction (PCR) by using a cocktail of species-specific primers permitting the identification of the six most common species of the group (Koekemoer et al., 2002). Recent analyses of rDNA sequences (Cohuet et al., 2003)  An. funestus like other malaria vectors is controlled by the use of insecticides such as insecticide treated materials or as indoor residual spraying (IRS). Unfortunately, An. funestus is increasingly developing resistance across Africa to different classes of insecticides used in public health, such as PYR, CX and DDT (Brooke et al., 2001;Casimiro et al., 2006;Cuamba et al., 2010;Morgan et al., 2010). There are alternative agrochemicals, such as fipronil that could be introduced but the potential for cross-resistance from existing mechanisms segregating in field populations needs to be more investigated.
The insecticide resistance in An. funestus populations was early recorded from Burkina Faso, where resistance was found to dieldrin, a cyclodiene abundantly used in Africa in the 1960s for cotton crop protection but also for malaria vector control (Hamon et al., 1968b). Dieldrin resistance was also reported in An. funestus from Cameroon, Benin, Nigeria and Mali (Service, 1960;Toure, 1982;Brown, 1986

Pesticide pressures on disease vectors are from multiple origins
The question that remains to be clearly identified is the origin of insecticide pressures that select the resistances observed in mosquitoes from sub-Saharan Africa. In Burkina Faso, the emergence of the ace-1 R mutation in An. gambiae s.s. populations is also associated with the insecticide treatment history with OP and CX of cotton. Since the mid 1990s and until recently a pest management strategy including four windows of treatment per cropping cycle using pyrethroids, OP/CX (such as chlorpyrifos, profenofos and trizophos) and organochlorines had been adopted in order to manage the pyrethroid-resistance of Helicoverpa armigera and Bemisia tabaci that emerged throughout the cotton belt. Some bioassays performed in 2003 on An. gambiae populations from four sites located in the cotton belt of western Burkina Faso revealed early resistance against CX and OP insecticides pre-empting the discovery of the genetic resistance mechanism revealed in further studies. In a previous study the agricultural use of insecticides was already implicated in the development of resistance to pyrethroids in An. gambiae s.l. populations. Then the geographical distribution of resistance decreased in An. gambiae s.l. populations from the Sudan savannah to Sahelian areas and the highest levels of resistance were found in sites of cotton cultivation. The areas under cotton cultivation have expanded dramatically in the last ten years (210,000 ha in 1996 to more than 520,000 ha in 2005). A corresponding increase in the level of insecticide use has also been reported reaching more than 3x10 6 litres of pesticide per cropping campaign. Furthermore, a clear knowledge concerning the practices of populations regarding the uses of insecticides in Africa is required.

Insecticide resistance and malaria vector control
Although A concern for the potential use of OP and CX as alternative for PYR is that target and metabolic resistances to these insecticides are already present in some An. gambiae s.s. populations in West Africa especially in Burkina Faso. Several studies have suggested that the use of agricultural pesticides, especially for cotton but also for vegetable crops, favored the emergence and facilitated the spread of insecticide resistance within mosquito populations. Other studies have given evidence for the selection of kdr alleles associated with the use of pyrethroids in ITNs and other domestic strategies of personal protection, especially in Kenya and Niger (Czeher et al., 2008). In countries supported by PMI (President Initiative against Malaria) such as Ghana, Senegal, Mali, Benin and Liberia in West Africa the large-scale pilot interventions implemented with bendiocarb 400mg/m 2 in IRS could also contribute to select the OP/CX resistance. However, as no global health control program in Africa used OP and CX for mosquito control, it is necessary to clearly identify the origin of insecticide pressure that select these resistances.

Conclusions
Reports of insecticide resistance in malaria vectors in West Africa especially in Burkina Faso indicate that insecticide resistance increases year after year and highlight the threat to the effectiveness of vector control strategies. In fact An. gambiae s.l. populations in Burkina Faso, and more broadly in West Africa, have evolved resistance to many of the insecticides classes used for vector control. Resistance may be conferred by target-site insensitivity such as kdr and ace-1 R , other metabolic mechanisms or a combination of all as kdr and ace-1 R resistance mechanisms occur concomitantly in the same populations of An. gambiae s.s. in the South-Western region of the country. In conclusion the geographical distribution of insecticide resistance in An. gambiae s.l. populations was found in sites of cotton cultivation and vegetable in urban settlement that has expanded dramatically in the last ten years. But the role of agriculture in the selection of resistance in natural mosquito's populations needs to be clarified, both in terms of insecticides usage and quantity in order to devise strategies that may help to reduce the extension of resistance. Until the discovery of new insecticides or using new formulations of existing insecticides and also the use of genetically modified mosquitoes (GMM) and sterilised males techniques (SIT), it is crucial to integrate the regional vector resistance status www.intechopen.com in the implementation of control interventions that will preserve a long term efficacy of these vector control tools. Unfortunately reports of insecticide resistance in vector populations increase year by year and could jeopardize malaria vector control based on the use of insecticides. The use of insecticides for bednets impregnation or for IRS represents the primary means for malaria prevention worldwide. However the efficacy of such tools has been evaluated in areas where vectors are susceptible to insecticides. Moreover, mosquito resistance is not only due to the insecticides used for mosquito control, but to the many pesticide pollutions present in their environment which are generated by a large variety of human activities necessitating insect control for agriculture (large cultures, fruits and vegetables), animal and other household protections. These pollutions may dramatically affect resistance genes dynamics and threaten these strategies. The overall pesticide pressures that select resistance in mosquitoes need to be clarified, both in terms of insecticides usage and quantity. It is also crucial to improve our knowledge on the practices of people regarding the use of insecticides and the reasons underlying their decision process based on social and cultural contexts. Malaria vector control programs require up-to-date information on the distribution and composition of mosquito vector populations and the susceptibility of these populations to the insecticides used for control.

Acknowlegements
Authors are grateful to Corus 6015 and National Malaria Control Programme of Burkina Faso which supported financially this study on resistance monitoring. We thank Nicole Pasteur for critical reading of the manuscript. We thank also