**8. Distribution of insecticide resistance**

The emergence and spread of insecticide resistance to all four classes of insecticides useful in public health invariably threatens the effectiveness of malaria vector control as most pro‐ grammes rely heavily on insecticide usage [60]. Resistance has been observed in more than 500 insect species worldwide, among which over 50 *Anopheles* species (Diptera: Culicidae) are responsible for the transmission of malaria parasites to humans [13, 58]. Globally, resistance to at least one insecticide has been identified in 64 countries with on-going malaria transmis‐ sion [7]. Currently, 27 countries in sub-Saharan Africa have reported pyrethroid resistance in *Anopheles* vectors [102]. The real figure could very well be higher, as a lack of in-country resistance monitoring prevents accurate assessment [60]. Insecticide resistance is a focal phenomenon and as such is not evenly distributed among vector species and varies markedly from one place to the other. Several platforms are available online with vast information on the distribution of insecticide resistance in malaria vectors such as: Anobase (http:// anobase.vector‐base.org/ir/), Arthropod Pesticide Resistance Database (http://www.pestici‐ deresistance.org), MARA (http://www.mara.org.za), and IR mapper (http://www.irmap‐ per.com). Persuasive evidence for the presence of resistance in primary vector species to all available insecticides has been presented from Africa, Southeast Asia and India, and South and Central America. Corbel and N'Guessan present a detailed description of the country by country situation analysis of resistance in these regions [86]. A summary is presented below:

In Africa, target-site and metabolic-mediated resistance has been detected in *An. gambiae s.l.* malaria vectors across the continent south of the Sahara. Most of the documented evidence comes from west Africa where pyrethroid resistance is predominant in *An. gambiae s.s.* compared to *An. arabiensis* [102, 103]. High levels of resistance have also been extensively reported in the two major vectors in Central, East, Austral, and South African countries [104, 105]. Two kdr alleles exist in *An. gambiae s.s*. and *An. arabiensis*, the L1014S gene originally from East Africa [90] and the 1014F gene of west African origin alleles [91]. Both mutations have also been reported to co-exist in Gabon and Cameroon [106] and in Uganda [107]. The western kdr was also detected in Kenya [108], Tanzania [109], and also in Zambia [110, 111]. In pyrethroid-resistant *An. gambiae* s.l. metabolic resistance involving increased levels of P450 has been reported in several countries with CYP6P3 and CYP6M2 genes over-expressed [112, 113]. Most data on resistance in *An. funestus* comes from South African countries where the species seems to be the predominant malaria vector [101]. In *An. funestus*, pyrethroid resistance involving increased activity of P450 monooxygenase and/or GST was demonstrated in Southern Africa [114–116] with over-expressed CYP6M7, CYP6P9a, and CYP6P9b genes [117]. Pyrethroid resistance in *An. funestus* has also been detected in East [118] and West Africa [119, 120]. Recently, the spatial scale of the problem in sub-Saharan Africa was brought to the fore through the IRMapper [105].

Behavioural resistance does not have the same "importance" as physiological resistance but may be considered to be a contributing factor [96, 97]. It remains unclear whether adaptation of malaria vectors species to insecticidal-based vector control interventions may result from a phenotypic plasticity or from selected behavioural traits [98]. Notably, behavioural resistance

*Penetration resistance*: Reduced penetration involves changes that decrease the rate of penetra‐ tion or absorption of insecticide through the insect cuticle or digestive tract linings and confers low levels of resistance [68, 100]. This resistance mechanism is not specific and can affect a broad range of insecticides. Reduced uptake of insecticide, often referred to as cuticular resistance, is frequently described as a minor resistance mechanism. More effort is required to

The understanding of the development of resistance and the design of novel strategies to manage it and to effectively control disease vectors is greatly owed to the characterisation of genes and the molecular mechanisms involved in insecticide resistance [101]. However, the mechanisms of insecticide resistance are generally far less well-understood. Particularly, the contribution these enzymes make towards pyrethroid resistance and their biochemical

The emergence and spread of insecticide resistance to all four classes of insecticides useful in public health invariably threatens the effectiveness of malaria vector control as most pro‐ grammes rely heavily on insecticide usage [60]. Resistance has been observed in more than 500 insect species worldwide, among which over 50 *Anopheles* species (Diptera: Culicidae) are responsible for the transmission of malaria parasites to humans [13, 58]. Globally, resistance to at least one insecticide has been identified in 64 countries with on-going malaria transmis‐ sion [7]. Currently, 27 countries in sub-Saharan Africa have reported pyrethroid resistance in *Anopheles* vectors [102]. The real figure could very well be higher, as a lack of in-country resistance monitoring prevents accurate assessment [60]. Insecticide resistance is a focal phenomenon and as such is not evenly distributed among vector species and varies markedly from one place to the other. Several platforms are available online with vast information on the distribution of insecticide resistance in malaria vectors such as: Anobase (http:// anobase.vector‐base.org/ir/), Arthropod Pesticide Resistance Database (http://www.pestici‐ deresistance.org), MARA (http://www.mara.org.za), and IR mapper (http://www.irmap‐ per.com). Persuasive evidence for the presence of resistance in primary vector species to all available insecticides has been presented from Africa, Southeast Asia and India, and South and Central America. Corbel and N'Guessan present a detailed description of the country by country situation analysis of resistance in these regions [86]. A summary is presented below: In Africa, target-site and metabolic-mediated resistance has been detected in *An. gambiae s.l.* malaria vectors across the continent south of the Sahara. Most of the documented evidence comes from west Africa where pyrethroid resistance is predominant in *An. gambiae s.s.*

identify the significance of cuticular resistance in phenotypic resistance [86].

relationships with P450-mediated resistance is still unclear [84].

**8. Distribution of insecticide resistance**

is characteristically difficult to quantify [99].

164 Insecticides Resistance

Southeast Asia and India insecticide resistance has been detected in the main malaria vector species [87]. In the Mekong region, *Anopheles dirus* s.s. [121] and *Anopheles minimus* s.l. [122]. No kdr mutation has been observed so far in these species [123] and pyrethroid resistance seems to result from increased detoxification by esterases and/or P450 monooxygenases [124]. Esterase-mediated pyrethroid detoxification in both *An. epiroticus* and *An. subpictus* and GSTmediated DDT resistance in *An. subpictus* have been reported [121]. *An. vagus* and *An sinen‐ sis* are resistant to pyrethroids with high 1014S kdr alleles [124–127]. The presence of the 1014F allele has been revealed in *An. sundaicus, An. aconitus, An. subpictus*, and *An. vagus* [128] (http:// www.itg.be/malvecasia/). In India, *An. culicifacies* s.l. has developed strong resistance to pyrethroids [129], DDT [130, 131], dieldrin/HCH [132], and malathion [131]. Both 1014F and 1014S kdr phenotypes have been detected in pyrethroid and DDT-resistant *An. culicifacies* s.l. [130] and *An. stephensi* [134] with elevated activities of GST in DDT resistance in this mosquito species [135]. *An. annularis, An. subpictus*, and *An. philippinensis* are resistant to pyrethroid, DDT, and/or dieldrin/HCH [135]. In Sri Lanka, metabolic resistance involving carboxylester‐ ases (malathion) or monooxygenases and GSTs (DDT) has been detected in *An. culicifacies* s.l. and *An. subpictus* [136, 137]. In Bangladesh, *An. philippinensis, An. maculatus* s.l., and *An. aconitus* have all developed resistance to DDT [138]. *An. stephensi* and *An. sacharovi* in Iran and Turkey are resistant to DDT and dieldrin [139–141]. While *An. maculatus* s.l. and *An. Aconi‐ tus* have developed resistance to DDT in Nepal, *An. stephensi* is resistant to malathion in Pakistan [142].

In Central and South America, the primary malaria vectors are *An. darlingi* and *An. albima‐ nus*. In Mexico, *An. albimanus* exhibits high levels of DDT and pyrethroid-resistance with elevated levels of GST, P450, and esterases, and iAChE-mediated carbamate and organophos‐ phate resistance [143, 144]. In Peru, *An. Albimanus* is resistant to pyrethroids [145]. In Colombia, DDT resistance has been reported in *An. darlingi* [146, 147] and pyrethroid resistance in both *An. darlingi* and *An. albimanus* [148]. In *An. darlingi*, both multi-function oxidase (MFO)- and non-specific esterase (NSE)-based metabolic resistance were reported in a deltamethrin and DDT-resistant population [149]. *An. Nuneztovari*, a secondary malaria vector, is resistant to organophosphate and pyrethroids [150].
