Abstract
The effectiveness of insecticide-based malaria vector control interventions in Africa is threatened by the spread and intensification of pyrethroid resistance in targeted mosquito populations. The present study aimed at investigating the temporal and spatial dynamics of deltamethrin resistance in An. gambiae s.l. populations from North Cameroon. Mosquito larvae were collected from 24 settings of the Garoua, Pitoa and Mayo Oulo Health Districts (HDs) from 2011 to 2015. Two to five days old female An. gambiae s.l. emerging from larval collections were tested for deltamethrin resistance using the World Health Organization’s (WHO) standard protocol. Sub samples of test mosquitoes were identified to species using PCR-RFLP and genotyped for knockdown resistance alleles (Kdr 1014F and 1014S) using Hot Ligation Oligonucleotide Assay (HOLA). All the tested mosquitoes were identified as belonging to the An. gambiae complex, including 3 sibling species mostly represented by Anopheles arabiensis (67.6%), followed by Anopheles coluzzii (25.4%) and Anopheles gambiae (7%). Deltamethrin resistance frequencies increased significantly between 2011 and 2015, with mosquito mortality rates declining from 70–85% to 49–73% in the three HDs (Jonckheere-Terstra test statistic (JT) = 5638, P< 0.001), although a temporary increase of mortality rates (91–97%) was seen in the Pitoa and Mayo Oulo HDs in 2012. Overall, confirmed resistance emerged in 10 An. gambiae s.l. populations over the 24 field populations monitored during the study period, from 2011 to 2015. Phenotypic resistance was mostly found in urban settings compared with semi-urban and rural settings (JT = 5282, P< 0.0001), with a spatial autocorrelation between neighboring localities. The Kdr 1014F allelic frequencies in study HDs increased from 0–30% in 2011 to 18–61% in 2014–2015 (JT = 620, P <0.001), especially in An. coluzzii samples. The overall frequency of the Kdr 1014S allele was 0.1%. This study revealed a rapid increase and widespread deltamethrin resistance frequency as well as Kdr 1014F allelic frequencies in An. gambiae s.l. populations over time, emphasizing the urgent need for vector surveillance and insecticide resistance management strategies in Cameroon.
Highlights
Malaria is one of the most dangerous parasitic diseases of the current decade [1] impacting on the health and standard of living of populations in endemic areas
Considering the fact that pyrethroid insecticides have been commonly used in a number of the study sites and long-lasting insecticidal nets (LLINs) were distributed two times across the Health Districts (HDs) at universal coverage in 2010 and 2015 in the study HDs, field populations of An. gambiae s.l. might be exposed to different patterns of selection pressure from one locality to another and from one year to another
Several other factors might influence the status of insecticide resistance, including the number of genes interacting to produce the phenotype of resistance, the dominance relationship of the alleles as well as the size and proportion of the population affected by insecticide treatments as suggested by Chareonviriyaphap et al [40]
Summary
Malaria is one of the most dangerous parasitic diseases of the current decade [1] impacting on the health and standard of living of populations in endemic areas. Since no antimalarial vaccine is commercially available, efforts to prevent the disease rely mainly on vector control tools such as long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) [3]. These measures are threatened by rapid expansion of insecticide resistance in vector populations [4, 5]. Two main physiological mechanisms confer insecticide resistance in malaria vectors: target site modification including mutations at the voltage gate sodium channel (Kdr 1014F or 1014S mutations) conferring resistance to DDT and pyrethroids, and cetylcholinesterase (G119S Ace-1 mutation) conferring resistance organophosphates and carbamates [13, 14, 15], and detoxification enzymes involved in insecticide degradation [16, 17]. It is common to find mosquitoes displaying multiple insecticide resistance mechanisms [19]
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