High efficacy of chlorfenapyr-based net Interceptor® G2 against pyrethroid-resistant malaria vectors from Cameroon

Abstract

Background: The increasing reports of resistance to pyrethroid insecticides associated with reduced efficacy of pyrethroid-only interventions highlight the urgency of introducing new non-pyrethroid-only control tools. Here, we investigated the performance of piperonyl-butoxide (PBO)-pyrethroid [Permanet 3.0 (P3.0)] and dual active ingredients (AI) nets [Interceptor G2 (IG2): containing pyrethroids and chlorfenapyr and Royal Guard (RG): containing pyrethroids and pyriproxyfen] compared to pyrethroid-only net Royal Sentry (RS) against pyrethroid-resistant malaria vectors in Cameroon.

Methods: The efficacy of these tools was firstly evaluated on Anopheles gambiae s.l. and Anopheles funestus s.l. from Gounougou, Mibellon, Mangoum, Nkolondom, and Elende using cone/tunnel assays. In addition, experimental hut trials (EHT) were performed to evaluate the performance of unwashed and 20 times washed nets in semi-field conditions. Furthermore, pyrethroid-resistant markers were genotyped in dead vs alive, blood-fed vs unfed mosquitoes after exposure to the nets to evaluate the impact of these markers on net performance. The XLSTAT software was used to calculate the various entomological outcomes and the Chi-square test was used to compare the efficacy of various nets. The odds ratio and Fisher exact test were then used to establish the statistical significance of any association between insecticide resistance markers and bed net efficacy.

Results: Interceptor G2 was the most effective net against wild pyrethroid-resistant An. funestus followed by Permanet 3.0. In EHT, this net induced up to 87.8% mortality [95% confidence interval (CI): 83.5-92.1%) and 55.6% (95% CI: 48.5-62.7%) after 20 washes whilst unwashed pyrethroid-only net (Royal Sentry) killed just 18.2% (95% CI: 13.4-22.9%) of host-seeking An. funestus. The unwashed Permanet 3.0 killed up to 53.8% (95% CI: 44.3-63.4%) of field-resistant mosquitoes and 47.2% (95% CI: 37.7-56.7%) when washed 20 times, and the Royal Guard 13.2% (95% CI: 9.0-17.3%) for unwashed net and 8.5% (95% CI: 5.7-11.4%) for the 20 washed net. Interceptor G2, Permanet 3.0, and Royal Guard provided better personal protection (blood-feeding inhibition 66.2%, 77.8%, and 92.8%, respectively) compared to pyrethroid-only net Royal Sentry (8.4%). Interestingly, a negative association was found between kdrw and the chlorfenapyr-based net Interceptor G2 (χ² = 138; P < 0.0001) with homozygote-resistant mosquitoes predominantly found in the dead ones.

Conclusions: The high mortality recorded with Interceptor G2 against pyrethroid-resistant malaria vectors in this study provides first semi-field evidence of high efficacy against these major malaria vectors in Cameroon encouraging the implementation of this novel net for malaria control in the country. However, the performance of this net should be established in other locations and on other major malaria vectors before implementation at a large scale.

Keywords: Anopheles; Dual active ingredient nets; Insecticide resistance; Interceptor G2; Malaria.

Fig. 1

Performance of PBO-based and new generation nets compared to pyrethroid-only nets against Anopheles gambiae s.l. and Anopheles funestus s.l. in cone test. Mortality rate 24 h after 3-min cone test for various nets against wild Anopheles gambiae s.l. (A), and wild Anopheles funestus s.l. (B); Red dot line represents minimal efficacy (50% mortality) and the green dot represents optimal efficacy (80% mortality) of the nets. Error bars represent the standard error on the mean (SEM)

Fig. 2

Performance of the dual AI net interceptor G2 compared to pyrethroid-only net interceptor against Anopheles gambiae s.l. and Anopheles funestus s.l. in tunnel test. Mortality rate 72 h after tunnel assay with 3 replicates of 100 mosquitoes (n = 300) for various nets against. Error bars represent the confidence interval on the percentage mortality

Fig. 3

Influence of the L1014F-kdrw resistant marker on the efficacy of bed nets after cone assays with hybrid Anopheles gambiae s.l. Genotype (A) and allele (B) distribution of L1014F mutation between alive and dead An. gambiae s.l. after exposure to Interceptor G2, Royal Guard, Permanet 3.0, and the standard net Royal Sentry. IG2: interceptor G2, RG: Royal Guard, P3.0: Permanet 3.0, RS: Royal Sentry. For genotype, RR: homozygote resistant, RS: heterozygote, SS: homozygote susceptible. In B, R: resistant allele, S: susceptible allele

Fig. 4

Composition of mosquitoes collected during the experimental hut trials in Elende from December 2021 to January 2022

Fig. 5

Exophilic rate (A) and blood-feeding inhibition with personal protection rate (B) during the experimental huts trials in Elende. Error bars represent the 95% confidence interval and * represents the level of significance for each LLIN relative to the control net and personal protection rate (PPR). IG2: interceptor G2, RG: Royal guard, P3.0: Permanet 3.0, RS: Royal Sentry, 0w: unwashed nets, 20w: 20 times-washed nets

Fig. 6

Percentage mortality of Anopheles funestus s.l. corrected for control mortality 24 h and 72 h after exposure during the experimental hut trials in Elende. The red dot line represents minimal efficacy and the green dot represents the optimal efficacy of the nets. *represents the level of significance (*P < 0.05 and **P < 0.01 and ***P < 0.001). 0w: unwashed nets, 20w: 20 times-washed nets

Fig. 7

Correlation between L119F-GSTe2 resistant marker and the efficacy of bed nets in experimental hut trial. Genotype distribution of L119F mutation between alive and dead mosquitoes after exposure to Interceptor G2 (A), Royal Guard (B), Permanet 3.0 (C), and the pyrethroid-only net Royal Sentry (D). IG2: interceptor G2, RG: Royal guard, P3.0: Permanet 3.0 and RS: Royal Sentry. For genotype, RR: homozygote resistant, RS: heterozygote, SS: homozygote susceptible

Fig. 8

Correlation between L119F-GSTe2 resistant marker and the blood-feeding ability of field Anopheles funestus s.l. from Elende in the presence of various bed nets in the experimental hut trial. Genotype distribution of L119F mutation between Blood-fed and unfed mosquitoes after exposure to Interceptor G2 (A), Royal Guard (B), Permanet 3.0 (C), and the pyrethroid-only net Royal Sentry (D). IG2: interceptor G2, RG: Royal guard, P3.0: Permanet 3.0, and RS: Royal Sentry. For genotype, RR: homozygote resistant, RS: heterozygote, SS: homozygote susceptible

Fig. 9

Correlation between L119F-GSTe2 resistant marker and the exiting ability of field Anopheles funestus s.l. from Elende in the presence of various bed nets in experimental hut trial. Genotype distribution of L119F mutation between mosquitoes collected in the room, under bednets, and in the verandah from the huts with Interceptor G2 (A), Royal Guard (B), Permanet 3.0 (C), and the pyrethroid-only net Royal Sentry (D). IG2: Interceptor G2, RG: Royal guard, P3.0: Permanet 3.0, RS: Royal Sentry. For genotype, RR: homozygote resistant, RS: heterozygote, SS: homozygote susceptible