Spatial and temporal development of deltamethrin resistance in malaria vectors of the Anopheles gambiae complex from North Cameroon

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

Knockdown and mortality rates of susceptible <i>Anopheles gambiae</i> s.s. post contact with community used Alpha-cypermethrin long lasting nets.

<p>M0, M1, M2, M3, M4, M5: month 0, month 1, month 2, month 3, month 4, month 5; KD60: knockdown rate of mosquitoes 60 minutes post contact to treated material. KD60 and mortality rates for interceptor nets remained very during the 5 month trial except on month 3when tested nets were found very dirty. With Fendona SC long lasting treated nets, the KD60 and the mortality rates were slightly but significantly lower than those recorded with Interceptor nets (p<0.05).</p

Knockdown and mortality rates of susceptible <i>Anopheles gambiae</i> s.s. post contact with different insecticide treatments.

<p>A: Indoor Residual Spraying; B: Long lasting insecticidal nets; KD60: knockdown rate of mosquitoes 60 minutes post contact to treated material. All tested IRS insecticide formulations were found effective although a decrease of insecticide activity was seen with lambda cyhalothrin 10 WP (P<0.05). All tested insecticidal nets were found effective although a decrease of insecticide activity was seen with alpha cypermethrin conventional or long lasting treated nets (P<0.05).</p

Knockdown and mortality rates of susceptible <i>Anopheles gambiae</i> s.s. post contact with Alpha-cypermethrin long lasting washed nets.

<p>A: Nets used and washed in community; B: Nets washed in laboratory conditions, M0, M1, M2, M3, M4, M5: month 0, month 1, month 2, month 3, month 4, month 5; KD60: knockdown rate of mosquitoes 60 minutes post contact to treated material; washed CM: nets washed in the community; washed Lb: nets washed in the laboratory. Interceptor Nets remained highly effective after washing up to 25 times in the laboratory or 9 times at community level, although a slight but significant decrease of KD60 and mortality rates was seen after each washing cycle (p<0.05). With Fendona, the KD60 and mortality rates remained high until 3washes either in the lab or at community level. No significant difference of wash resistance was seen between laboratory and community washing procedures (p<0.05).</p

Record of insecticide formulations and long lasting insecticidal nets tested between 2006 and 2011.

<p>(1) CS: capsule suspension; EC  =  emulsifiable concentrate; SC  =  suspension concentrate; WG  =  water dispersible granule;</p><p>WP  =  wettable powder; WDP =  wettable dispersible powder.</p><p>(2) OC =  Organochlorines; OP =  Organophosphates; C =  Carbamates; PY =  Pyrethroids.</p><p>(3) g a.i/m² : grams of active ingredient per meter square.</p><p>(4) CTN: Conventional treated net; LLIN; long lasting insecticidal net; PY: pyrethroid; OP: organophosphate; C: carbamates; IRS: indoor residual spraying.</p

Two level proportional odds regression models for the efficacy of LLINs determined from kd60 (model 1) and mortality (model 2).

<p>Two level proportional odds regression models for the efficacy of LLINs determined from kd60 (model 1) and mortality (model 2).</p

Number of enumeration areas visited, number of children examined, sex ratio and prevalence of ICT by evaluation unit.

<p>Number of enumeration areas visited, number of children examined, sex ratio and prevalence of ICT by evaluation unit.</p

Comprehensive timeline of the programmatic steps taken in the lymphatic filariasis (LF) elimination process in Cameroon.

<p>Green boxes indicate surveys (both mapping and impact assessment) and whitish boxes or arrows indicate interventions. ICT: immunochromatographic test; CDTI: community directed treatment with ivermectin; MTC: mobile team campaign; mf: microfilaria; IVM: ivermectin; ALB: albendazole; MDA: mass drug administration; LLINs: long lasting insecticidal nets; TAS: transmission assessment survey.</p

Map showing implementation (IUs) surveyed in the North and Far North Region of Cameroon.

<p>The grey zones indicate the IUs surveyed and the blue lines indicate the delineation of evaluation units (EUs).</p