**1. Introduction**

*Anopheles gambiae* s.l. is a primary vector of *Plasmodium falciparum* across sub-Saharan African countries causing malaria that continues to be a leading cause of morbidity and mortality across the continent [1]. Malaria is the world's number one parasitic disease, threatening more than 1 billion people [2]. According to the World Health Organisation (WHO), 241 million cases and 62,700 deaths due to malaria were reported in 2020 [3]. Of these cases, 95% occur in sub-Saharan Africa. Globally, nearly 95% of deaths were recorded in 31 countries, of which 3 in Africa accounted for nearly 39% (Nigeria 23%; DRC 11%, Tanzania 5% [2]. More than 200 million cases of malaria are recorded annually in DRC [3]. This high prevalence of malaria reflects a situation of poverty and inadequate health services [4–6]. Moreover, malaria itself and its consequences contribute to keeping populations in a state of poverty. Malaria costs sub-Saharan African countries more than US\$ 12 billion each year through lost income, foreign investment and tourism resources [4–6]. International agencies (WHO, UNDP, UNICEF and the World Bank) launched the Roll Back Malaria (RBM) programme, which set out to combat malaria with the aim of reducing malaria mortality by 50% in 2010, 30% in 2015 and 20% in 2025. In its technical strategy for malaria control, WHO has set the goal of eliminating malaria by 2030 [7]. Thus, by the year 2030, malaria should cease to be a major cause of morbidity, mortality, and socio-economic loss [8].

To achieve its objectives, the DRC has developed malaria control strategies based on preventive measures centred on targeted chemoprophylaxis offered to high-risk or vulnerable populations such as infants, pregnant women and migrants; early case management, which implies diagnosis and rapid recourse to appropriate care; and finally, vector control which must remain accessible to malaria-endemic populations [9]. Regarding preventive measures, the WHO gives an important place to vector control. To respond favourably to both international and national strategies, approaches and recommendations aimed at eliminating malaria, the DRC has been committed for at least 7 years through the NMCP and its partners to scale up high-impact malaria control interventions [10]. These interventions include universal distribution of ITNs, chemoprevention of malaria in pregnant women, administration of artemisininbased combination therapies (ACTs) and strengthening of epidemiological surveillance [11, 12].

Following the evaluation of malaria control interventions, with reference to the implementation framework of the Global Technical Strategy for Malaria Control 2016–2030 in the African Region, WHO has launched the High Burden High Impact (HBHI) approach [13, 14] aiming to reduce malaria morbidity and mortality in countries with a high burden of malaria, including the DRC. It is based on four main pillars: i) political will in favour of the fight against malaria, ii) strategic information that can increase the impact of malaria tenfold, iii) improved support for policies and strategies and iv) coordination of the national response [13, 14].

The significant progress made in malaria control over the past decade in endemic countries is largely attributable to the mass coverage of insecticide-based vector control interventions, such as long-lasting insecticidal nets (ITNs) and indoor residual spraying (IRS) [7, 13, 14].

Long-lasting insecticidal nets are one of the main malaria control tools recommended by WHO and adopted by DRC NMCP through mass and routine distributions channels [15, 16]. Several field studies have demonstrated the effectiveness of its large-scale use [15, 16] showing they can reduce morbidity by 50% and overall mortality by 20–30% in children under 5 years of age [10, 16, 17].

Based on these observations, the promotion of ITNs among the population is an essential component of the NMCP in DRC. Several studies have shown that the combined action of insecticides and the physical barrier is an effective means of controlling

#### *Effect of the Mass Distribution of ITNs in an Endemic Area with a High Entomological Index… DOI: http://dx.doi.org/10.5772/intechopen.105021*

malaria vectors [15–17]. Nevertheless, the disease remains endemic in the country, with a worrying prevalence of 32%, despite this large-scale promotion [3, 10]. Counteracting the effectiveness of ITNs are two major handicaps: the increasingly widespread mosquito resistance to pyrethroids used for impregnation of ITNs; and the low recorded durability of ITNs compared to the expected duration of 3 years [10, 13]. The number of *Anopheles* species resistance to insecticides has increased and affects nearly two-thirds of the countries where transmission persists. Resistance to pyrethroids in *An. gambiae* s.l. was first observed in Côte d'Ivoire in followed by other West African countries (Benin and Burkina Faso) and first reported in the DRC in 2012 [12, 18]. Pyrethroid resistance in *An. gambiae* has now been reported in all the DRC NMCP's sentinel sites, where it has the potential to cause the operational failure of the vector control tools put in place by the malaria control programmes [12, 18].

The DRC, like all the countries of sub-Saharan Africa, pays a heavy price for malaria. However, its geographical location and the diversity of its climates make it unique among its neighbours [2, 10, 19]. Spread over an area of approximately 2,345,000 km2 , this country has almost all the epidemiological facies found in Africa, from the Sahelian savannahs to the equatorial forests [10, 19, 20]. Moreover, 97% of the population lives in the stable malaria zones characterised by the equatorial and tropical facies [13, 14]. Three plasmodial species are present (*Plasmodium falciparium* (*P.f*) 95%, *Plasmodium ovale* (*P.o*) and *Plasmodium malariae* (*P.m*)). Currently, *Plasmodium vivax* infections are present in some areas of the country, but in very small numbers [9, 19, 20]. The most common vectors in the DRC are *An. gambiae*, 92%, *An. funestus*, *An. nili*, *An. moucheti* and *An. paludis* [9, 21, 22]. These *Anopheles* can have highly variable vectorial capacities and behaviours [23, 24].

However, this heterogeneity does not allow us to understand malaria in the DRC in its entirety. Knowledge of the different Congolese geographical areas (territories, districts, etc.) and the description of the local epidemiology of malaria in these different environments are initial conditions for a good understanding of the malaria endemicity in this country [13, 14, 18].

Work carried out in a few sites in the DRC, such as Kingasani, Bolenge, Kimpese and Katana by Watsenga and collaborators, has shown that pyrethroid resistance was associated with the presence of the kdr mutation [25]. However, the problem of *Anopheles* resistance to insecticides seems to be growing in the DRC, according to the PMI entomological surveillance report and the DRC Malaria Control Strategic Plan 2013–2015 [26, 27]. Therefore, it is not only necessary to regularly monitor the susceptibility of vectors to these insecticides and to understand the mechanisms of this resistance, but also to consider the actual impact of this resistance on vector control [27, 28].

Our study focused on Kwilu province (ex-Bandundu) where the prevalence of malaria was 18%, sporozoite index 5.6%. *A. gambiae* s.l. is the major malaria vector with 8.86 anopheles per house, 1.55 bites per man per night and entomological stability index 6.512 [29]. On this basis, it was necessary to assess and determine the response of *An. gambiae* s.l. to insecticides and test the hypothesis that the selection of a resistance gene in malaria vectors is associated with the widespread use of insecticides [12, 15].

The aim of this study was to examine the status of *An. gambiae* s.l. resistance to pyrethroids and to determine the mechanism of this resistance in several locations in Kwilu province, once the country's agricultural heartland where different farming systems were used [2, 30, 31]. In addition, this province has long been the focus of vector control interventions through the distribution of ITNs [32]. The results from

this study will serve as a basis for decision-makers to properly orientate resistance management policy in the DRC particularly considering the recrudescence and outbreak of malaria cases noted in 2019, and the lack of documentation of *Anopheles* resistance in this part of the country.
