The growing development of insecticide resistance constitutes a serious threat to malaria control programmes and if measures are not taken in time, resistance may compromise control effort in the foreseeable future[4, 22]. Monitoring the development of vector resistance in the field prior to the implementation of any malaria vector control initiative is of paramount importance.
This study documented the susceptibility/resistance status of An. gambiae to three important pyrethroids commonly used to treat mosquito nets currently distributed across sub Saharan Africa, including Côte d’Ivoire (permethrin, deltamethrin and lambda-cyhalothrin). The focus sites of interest were two distinct areas of Adzopé, a city in Eastern Côte d’Ivoire receiving LLINs donated by the Global Fund initiative. The study was carried out prior to the nets distribution to inform the National Malaria Control Programme of the resistance status of vectors and the need for close surveillance of the resistance phenomenon.
The results presented here show that An. gambiae s. s. in the forested areas of Port-Bouët and Tsassodji in Adzopé was exclusively of the M-form. This agrees with a previous study conducted in forest areas of southern Côte d’Ivoire. However, the study was conducted only during May-June and we cannot exclude the occurrence of the S form of An. gambiae s. s. during the rest of the year.
Both vector populations have developed various levels of resistance to the three pyrethroids tested. In the bioassays, An. gambiae s. s. populations sampled in suburban Port-Bouët area of Adzopé city where more controlled agricultural practices with irrigation system exist, showed higher vulnerability to pyrethroid deposits compared to samples that were collected in the heart of the city (Tsassodji) with no such agricultural practice.
This bioassay observation contrasted with the molecular results: the L1014F kdr mutation was detected at both sites but the frequencies of the allele were equivalent (0.67). We do not overlook the implication of the kdr mutation in pyrethroid resistance observed in mosquitoes at both sites as this is also supported by the increased in knockdown time (KDT50) relative to the normal Kisumu strain. Higher KDT50 values in field populations of mosquitoes have been suggested to provide an early indication of the involvement of kdr gene in phenotypic resistance[17, 24]. However, with same kdr rate (67%) but phenotypic difference in expression of resistance by bioassay mortality between both An. gambiae s. s. populations suggests the co-existence of both kdr and other mechanisms, probably enzyme detoxifiers such as esterases, monooxygenases or GSTs. Over-expression of cytochrome P450 genes associated with pyrethroid resistance is most common in An. gambiae s. s., sometimes in association with the L1014F kdr allele[25–28]. No further investigation was conducted to detect additional mechanisms to kdr conferring resistance, but one must not preclude any metabolically mediated mechanisms in the pyrethroid resistance observed at Adzopé. Integrated investigations, which allow detection of target sites mutations and metabolic detoxification agents, should be stressed in order to provide a more comprehensive insight into the genetic basis and the mechanisms responsible for the resistance phenotype in these mosquito populations.
The study confirmed the spread of pyrethroid resistance in An. gambiae s. s. first detected in Côte d’Ivoire, and now in all western African countries investigated[8, 30]. This is the first instance of pyrethroid resistance recorded in An. gambiae s. s. from the eastern part of Côte d’Ivoire, particularly in Adzopé. This resistance may be explained by the selection pressure from both agricultural and domestic usage of insecticides. Farmers in Port-Bouët admitted to use pyrethroids for crop protection. With agricultural practices, the amount of insecticides being applied to the environment is greatly increasing, and may have a pronounced effect on the mosquito ecology and resistance[31–34]. The protection measures against mosquito bites at both sites in Adzopé are mainly domestic aerosols and mosquito coils. Port-Bouët, the suburban area is densely populated with lower educated classes compared to the higher living standard of people at Tsassodji with modern housing and wealth. The household use of products may explain the resistance level observed at Tsassodji as previously reported in rural Côte d’Ivoire.