Can the performance of pyrethroid-chlorfenapyr nets be reduced when combined with pyrethroid-piperonyl butoxide (PBO) nets?

Abstract

Background: Pyrethroid-chlorfenapyr (CFP) and pyrethroid-piperonyl butoxide (PBO) nets are being scaled across endemic countries to improve control of malaria transmitted by pyrethroid-resistant mosquitoes. CFP is a pro-insecticide requiring activation by mosquito cytochrome P450 monooxygenase enzymes (P450s) while PBO improves pyrethroid potency by inhibiting the action of these enzymes in pyrethroid-resistant mosquitoes. The inhibitory action of PBO against P450s may thus reduce the efficacy of pyrethroid-CFP nets when applied inside the same household as pyrethroid-PBO nets.

Methods: Two experimental hut trials were performed to evaluate the entomological impact of two different types of pyrethroid-CFP ITN (Interceptor^® G2, PermaNet^® Dual) when applied alone and in combination with pyrethroid-PBO ITNs (DuraNet^® Plus, PermaNet^® 3.0) against a pyrethroid-resistant vector population in southern Benin. In both trials, all net types were tested as single and double net treatments. Bioassays were also performed to assess the resistance profile of the vector population at the hut site and investigate interactions between CFP and PBO.

Results: The vector population was susceptible to CFP but exhibited a high intensity of pyrethroid resistance that was overcame by PBO pre-exposure. Vector mortality was significantly lower in huts with combinations of pyrethroid-CFP nets plus pyrethroid-PBO nets compared to huts with two pyrethroid-CFP nets (74% vs. 85% for Interceptor^® G2 and 57% vs. 83% for PermaNet^® Dual, p < 0.001). PBO pre-exposure reduced the toxicity of CFP in bottle bioassays suggesting this effect may be partly attributable to antagonism between CFP and PBO. Higher levels of vector mortality were observed in huts with net combinations that included pyrethroid-CFP nets compared to those that did not and highest mortality was achieved when pyrethroid-CFP nets were applied alone as two nets together (83-85%).

Conclusions: This study shows evidence of a reduced performance of pyrethroid-CFP nets when combined with pyrethroid-PBO ITNs compared to when applied alone and higher efficacy with net combinations that included pyrethroid-CFP nets. These findings suggest that in similar contexts, prioritizing distribution of pyrethroid-CFP nets over other net types would maximize vector control impact.

Keywords: Anopheles gambiae; Antagonism; Chlorfenapyr; Experimental huts; Insecticide-treated nets; Malaria; Mosquitoes; Piperonyl butoxide; Pro-insecticide; Pyrethroid; Vector control.

Fig. 1

Mortality of F1 progeny of Anopheles gambiae s.l. collected from the experimental hut site in Covè in World Health Organization tube tests and bottle bioassays. Panel a presents results from trial 1 and panel b presents results from trial 2. In trial 1, all insecticides were tested in bottle bioassays while in trial 2, only chlorfenapyr was tested in bottle bioassays. Red dashed line represents standard 98% susceptibility cut-off while grey dashed line represents provisional 90% susceptibility cut-off used to confirm chlorfenapyr resistance. Error bars represent 95% CIs. Panel a presents results with single nets and panel b presents results with double net combinations

Fig. 2

Blood-feeding rates of wild, pyrethroid-resistant Anopheles gambiae s.l. collected in experimental huts containing alpha-cypermethrin-chlorfenapyr nets (Interceptor^® G2) and alpha-cypermethrin-piperonyl butoxide nets (DuraNet^® Plus) applied alone and in combination in Covè, Benin (Trial 1). Panel a presents results with single net treatments and panel b presents results with double net combinations. Bars bearing the same letter do not differ significantly at 5% level (p > 0.05) according to logistic regression analysis. Error bars represent 95% confidence intervals

Fig. 3

Blood-feeding rates of wild, pyrethroid-resistant Anopheles gambiae s.l. collected in experimental huts containing deltamethrin-chlorfenapyr nets (PermaNet^® Dual) and deltamethrin-piperonyl butoxide nets (PermaNet^® 3.0) applied alone and in combination in Covè, Benin (Trial 2). Panel a presents results with single net treatments and panel b presents results with double net combinations. Bars bearing the same letter do not differ significantly at 5% level (p > 0.05) according to logistic regression analysis. Error bars represent 95% confidence intervals

Fig. 4

Mortality rates of wild, pyrethroid-resistant Anopheles gambiae s.l. collected in experimental huts containing alpha-cypermethrin-chlorfenapyr nets (Interceptor^® G2) and alpha-cypermethrin-piperonyl butoxide nets (DuraNet^® Plus) applied alone and in combination in Covè, Benin (Trial 1). Panel a presents results with single net treatments and panel b presents results with double net combinations. Bars bearing the same letter do not differ significantly at 5% level (p > 0.05) according to logistic regression analysis. Error bars represent 95% confidence intervals

Fig. 5

Mortality rates of wild, pyrethroid-resistant Anopheles gambiae s.l. collected in experimental huts containing deltamethrin-chlorfenapyr nets (PermaNet^® Dual) and deltamethrin-piperonyl butoxide nets (PermaNet^® 3.0) applied alone and in combination in Covè, Benin (Trial 2). Panel a presents results with single net treatments and panel b presents results with double net combinations. Bars bearing the same letter do not differ significantly at 5% level (p > 0.05) according to logistic regression analysis. Error bars represent 95% confidence intervals

Fig. 6

Mortality of pyrethroid-resistant Anopheles gambiae s.l. Covè strain exposed to bottles coated with chlorfenapyr with and without pre-exposure to piperonyl butoxide. Cohorts of approximately 150 mosquitoes were exposed to each treatment arm in six batches of 20–25

Fig. 7

Mortality (a) and blood-feeding inhibition (b) of susceptible Anopheles gambiae s.s. Kisumu strain and pyrethroid-resistant An. gambiae s.l. Covè strain exposed to net pieces cut from whole nets before and after experimental hut trial 1 in supplementary tunnel tests. Approximately 100 mosquitoes were exposed to each of two randomly selected net pieces from each treatment arm in one replicate tunnel test. Error bars represent 95% confidence intervals

Fig. 8

Mortality (a) and blood-feeding inhibition (b) of susceptible Anopheles gambiae s.s. Kisumu strain and pyrethroid-resistant An. gambiae s.l. Covè strain exposed to net pieces cut from whole nets before and after experimental hut trial 2 in supplementary tunnel tests. Approximately 100 mosquitoes were exposed to each of two randomly selected net pieces from each treatment arm in one replicate tunnel test. Error bars represent 95% confidence intervals