Background
In sub-Saharan Africa, the major vector of
Plasmodium falciparum, the parasite responsible for the most severe form of human malaria, is
Anopheles gambiae sensu lato (
s.l.) [
1]. This highly anthropophilic mosquito comprises 8 sub-species among which
An. gambiae sensu stricto,
Anopheles coluzzii and
Anopheles. arabiensis are the major malaria vectors in Africa [
2]. The vector control strategy of Benin’s National Malaria Control Programme (NMCP) relies mainly on the distribution of LLINs and indoor residual spraying (IRS). With the financial support of the President’s Malaria Initiative (PMI) of the US government and the World Bank, the NMCP launched a national mass LLIN distribution campaign in 2011 to ensure universal coverage of the population (1 LLIN for every 1.8 people) [
3]. LLINs are an excellent means of providing personal and community protection from malaria [
4,
5]. Until very recently, pyrethroids were the only insecticide class used for impregnation of LLINs, owing to their rapid action, excito-repellent effects, effectiveness at low doses and low toxicity to humans [
6]. Unfortunately, pyrethroid resistance in malaria vectors has emerged and spread rapidly in several parts of Africa, including Benin [
7‐
11], Burkina Faso [
12], Cameroon [
13], Côte d’Ivoire [
14,
15] and, Kenya [
16]. In Benin,
kdr L1014F mutation as well as metabolic enzymes such as CYP450s, CYP6P3 and CYP6M2 [
10,
11,
17,
18] are implicated in malaria vector resistance to pyrethroids.
Distribution of Olyset
® nets, a polyethylene 150D LLIN impregnated with permethrin (2%), was carried out by the NMCP in Benin in 2011 with the aims of achieving universal coverage of populations-at risk. It is possible that this expansion of pyrethroid-based vector control may have eliminated susceptible mosquitoes in favour of resistant ones, thus increasing levels of pyrethroid resistance in malaria vector populations [
19]. Considering this, the aim of the current study was to monitor changes in insecticide susceptibility and the frequency of the
kdr L1014F mutation in the natural populations of
An. gambiae s.l. before (2009) and after (2012–2013) the distribution of LLINs in 4 districts of the Plateau department, South-East Benin.
Discussion
The current study evaluated the evolution of insecticide resistance of malaria vectors in the Plateau department following mass distribution of Olyset nets in 2011 by the NMCP. Overall,
An. gambiae s.l. was the main malaria vector in the Plateau department as previously showed by Padonou et al. [
19] in Ouémé, a bordering department of Plateau. The molecular characterization revealed the simultaneous presence of
An. gambiae and
An. coluzzii post-intervention, whereas pre-intervention, only
An. coluzzii was found. The detection of
An. coluzzii only, over the pre-intervention period could presumably be due to the fact that, the data collection covered a shorter period as compared to the post-intervention period which spanned 2 years. This fully justified the low number of vector specimens sampled for PCR analysis pre-intervention. Thus, it is possible that
An. gambiae were present at a very low frequency during the pre-intervention period and that a greater sample size of vectors may have demonstrated their presence. Moreover, the post-intervention data might have been generated with adult mosquitoes having emerged from larvae collected in highly diverse breeding sites as compared to the pre-intervention period. Hence, data collected during the post-intervention period may have provided a more representative capture of vector diversity and the species present in the study sites.
Post-intervention, the decrease in susceptibility to permethrin, deltamethrin and DDT combined with the significant increase in
kdr L1014F frequency could be due to increased use of pyrethroid LLINs following the mass distribution by the NMCP in 2011. Similar observations have been made in Kenya and Niger, respectively by Stump et al. [
26] and Czeher et al. [
27]. Indeed LLINs might have killed susceptible mosquitoes within natural populations, thus selecting for resistant ones that will mate and produce more resistant offspring. Domestic use of aerosol insecticides [
28] as well as the uncontrolled use of insecticides in agriculture [
29] observed in Southern Benin, might have also been causal factors of increased pyrethroid resistance levels.
The decrease susceptibility to pyrethroid insecticides as well as the continued susceptibility to bendiocarb observed post-intervention in
An. gambiae s.l. suggest that IRS with carbamate insecticides could effectively control
An. gambiae s.l. in the Plateau Department. A combined intervention of pyrethroid LLINs and IRS with bendiocarb could be particularly effective in improving the impact of control whilst delaying the onset of resistance. However, the emergence of carbamate resistance in Atacora, a department in Northern Benin [
30], emphasizes the importance of judicious insecticide application. Rotational use of IRS insecticides such as bendiocarb, pirimiphos-methyl and clothianidin, could prevent the establishment of resistance and preserve the effectiveness of the non-pyrethroid insecticide classes.
The logistic regression performed reveals that the
kdr L1014F mutation as well as the molecular species were non-significantly correlated with the mortality rate to deltamethrin, which suggests that if they had an impact, it was in a very low way. The results are similar to those from Reimer et al. [
31] in Cameroon. Thus, apart from the
kdr L1014F mutation, a combination of other resistance mechanisms might explained the pyrethroid resistance observed in
An. gambiae s.l. This is confirmed by the similarity of the
kdr L1014F frequency in dead and live mosquitoes of the 4 surveyed districts. A non significant association between the
kdr L1014F mutation and the mortality rate to deltamethrin was also observed in
An. coluzzii while, the opposite result was obtained by Ibrahim et al. [
32] with lambacyhalothrin.
The presence of mono-oxygenase mediated pyrethroid resistance has been demonstrated in
An. gambiae s.l. collected in Missérété, a neighbouring site of the Plateau Department [
33]. In addition, the presence of the N1575Y mutation was demonstrated in the natural populations of
An. gambiae s.l. in Covè, another neighbouring district of the Plateau department [
18]. It is, therefore, possible that the detoxification enzymes as well as the N1575Y mutation are also implicated in the resistance of the vectors to pyrethroids in the Plateau department.
Conclusion
The data of the current study provide important information on vector resistance to insecticides in the Plateau Department, following mass deployment of LLINs. The decrease of the susceptibility of An. gambiae s.l. to pyrethroids and DDT, as well as the increase of the frequency of the kdr L1014F mutation constituted an alert to the NMCP which should at the time, consider development and implementation of an effective resistance management strategy. At the molecular level, it would have been of interest to perform insecticide resistance intensity tests, and Taqman PCR assays to evaluate the contribution of metabolic enzymes and N1575Y mutation to vector resistance to pyrethroids. At present, the effectiveness of a strategy for combatting malaria vectors in the area requires an update of their resistance status.
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