Malaria transmission and insecticide resistance in Anopheles gambiae in Ellibou, Côte d'Ivoire

Background
Malaria is a major public health problem in Côte d'Ivoire with a countrywide prevalence exceeding 60%. The main strategies to fight the disease include early diagnosis, prompt and effective treatment as well as individual and community effective preventive measures, such as the use of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS). The deployment of any of these preventive measures must take into account the local conditions of malaria transmission, in particular the level of endemicity, the vectors involved in the transmission and the factors which influence the level of transmission. Unfortunately, resistance of malaria vectors to pyrethroids (PYRs), the main approved class of insecticides for treatment of bednets is widespread and constitutes a problem in disease control by the National Malaria Control Programme (NMCP). Ellibou is located in the southern part of Côte d'Ivoire and apart from the LLIN distribution campaign by the NMCP in 2014, no entomological study has been conducted in this locality. However, knowledge of the local malaria vectors and their resistance status to insecticides are essential to implement vector control strategies.
Aim and objectives
The aim of this PhD study was to determine key entomological parameters of malaria transmission and to assess the insecticide susceptibility of malaria vectors in Elibou, South Côte d'Ivoire. Specifically, we (i) determined the species composition of mosquitoes in Elibou, the sporozoite rate and the seasonal variation of malaria vector density over an entire year; (ii) assessed the insecticide susceptibility of the main malaria vector to the four conventional classes of insecticides used in public health and characterise the mechanisms involved in resistance to these insecticides; and (iii) investigated the knowledge, attitudes, practices and believes (KAPB) of the population in relation to the use of insecticides and insecticide resistance in malaria vectors.
Methods
We carried out an entomological survey, including larvae and adult mosquito collections in Elibou village over a 1-year period. To catch adult mosquitoes we performed pyrethroid spray catches (PSCs) and deployed CDC light traps (LTs). Adults mosquitoes were identified morphologically and diagnosted by PCR to genus and, where possible, to species level.
The susceptibilty of female Anopheles moquitoes emerged from larvae to insecticides was assessed by World Health Organization (WHO) insecticide susceptibility assays, while we characterised the mechanisms involved in insecticide resistance by real-time reverse-transcription polymerase chain reaction (RT-PCR). Finally, a KAPB survey, using questionaires, focus group discussions (FGDs) and interviews, was carried out.
Results
We caught 2,383 adult mosquitoes, 884 specimens by CDC light traps and 1,499 by PSCs. We morphologically identified 10 different taxa, including three Anopheles taxa. Anopheles gambiae s.l. represented the predominant malaria vector. The other two Anopheles taxa were Anopheles funestus and Anopheles pharoensis representing less than 1% each. The molecular identification showed that An. gambiae s.s. (38.3%), Anopheles coluzzii (39.0%) and Anopheles arabiensis (19.5%) as the main malaria vectors in Elibou. To our knowledge, this is the first report of An. arababiensis in Côte d'Ivoire. We also found An. gambiae/coluzzii hybrids (4.7%). The other mosquito species included Aedes sp., Culex sp. and Mansonia sp. and represented 57.8% of the total of mosquitoes collected. The sporozoite rates were 5.3%. The numbers of adult An. gambiae s.l. mosquitoes increased considerably during the rainy seasons mainly between June and July. The main malaria vector was resistant to deltamethrin, dichloro-diphenyl-trichloroethane (DDT) and bendiocarb while susceptible to malathion. In the An. gambiae s.l. specimens we found the insecticide target site mutations kdr L1014F, L1014S and N1575Y, and the Ace-1 G119S at both the homozygous and heterozygous stage. While the L1014S was only found in An. arabiensis, nine individuals harboured simultaniousely the L1014F and L1014S alleles. The detoxification genes CYP6P3, CYP6M2, CYP6Z1, CYP6P4, CYP6P1, CYP9K1, CYP4G16 and GSTE2 were found to be overexpressed in all Anopheles tested from Ellibou. CYP6P4 and CYP6M2 were the most upregulated genes. The KAPB survey revealed that people have poor knowledge about insecticide resistance, while they felt that preventing measures were ineffective. The main reason given was that insecticides were diluted by the manufacturers as a marketing strategy to sell larger quantities of their products. More than a third of the farmers used agricultural pesticides for domestic purposes to kill weeds or mosquitoes.
Conclusion
The results of this study contribute to a better understanding of the main local malaria vectors and their resistance status to insecticides used in public health and agriculture as well as factors that lead to insecticide resistance and persistence of malaria affecting the NMCP activities in the fight against malaria. Here, we report for the first time the presence An. arabiensis among malaria vectors in Côte d’Ivoire. This finding is an early warning sign because An. arabiensis shows a different behaviour and ecology than An. coluzzii and An. gambiae s.s. Thus, different methods for vector control are required.
Resistance to insecticides as observed in Ellibou is a great concern. Our results show that several resistance mechanisms are involved in insecticide resistance. In addition, the KAPB results show that the pesticides used for agriculture are also used in households for domestic purposes and might be a cause of insecticide resistance development in mosquitoes in Elibou. Given the misunderstanding of resistance by the local population, it is important to inform them about the current situation for awareness and sensitise them for a change in behaviour on the best use of insecticides and the environmental sanitation.
For future studies, we recommend to investigate on the history of the onset of An. arabiensis in Côte d’Ivoire, its distribution, its resistance status and its implication in malaria transmission. For An. gambiae s.l., the use of an alternative insecticide such as pyrimiphos-methyl, an organophosphate for IRS, is recommended. However, An. arabiensis is primarily an outdoor biting mosquito, and hence IRS and LLINs may be less effective against this malaria vector. To this end, we recommend to implement an effective control using a transdisciplinary approach while involving the local population because it plays an important role in the spread of resistance. The conclusions of our study will guide decision makers to design and implement an effective vector control strategy in the Ellibou region and other localities with the same characteristics.
Key words : Anopheles arabiensis, Anopheles gambiae, Ellibou, Côte d’Ivoire, insecticide resistance, metabolic resistance, transmission, malaria.