Journal Article

The impact of pyrethroid resistance on the efficacy and effectiveness of bednets for malaria control in Africa

Griffin, Jamie T, Churcher, Thomas S, Worrall, Eve, Ranson, Hilary, Lissenden, Natalie

2016

Overview

Long lasting pyrethroid treated bednets are the most important tool for preventing malaria. Pyrethroid resistant Anopheline mosquitoes are now ubiquitous in Africa, though the public health impact remains unclear, impeding the deployment of more expensive nets. Meta-analyses of bioassay studies and experimental hut trials are used to characterise how pyrethroid resistance changes the efficacy of standard bednets, and those containing the synergist piperonyl butoxide (PBO), and assess its impact on malaria control. New bednets provide substantial personal protection until high levels of resistance, though protection may wane faster against more resistant mosquito populations as nets age. Transmission dynamics models indicate that even low levels of resistance would increase the incidence of malaria due to reduced mosquito mortality and lower overall community protection over the life-time of the net. Switching to PBO bednets could avert up to 0.5 clinical cases per person per year in some resistance scenarios. In recent years, widespread use of insecticide-treated bednets has prevented hundreds of thousands cases of malaria in Africa. Insecticide-treated bednets protect people in two ways: they provide a physical barrier that prevents the insects from biting and the insecticide kills mosquitos that come into contact with the net while trying to bite. Unfortunately, some mosquitoes in Africa are evolving so that they can survive contact with the insecticide currently used on bednets. How this emerging insecticide resistance is changing the number of malaria infections in Africa is not yet clear and it is difficult for scientists to study. To help mitigate the effects of insecticide resistance, scientists are testing new strategies to boost the effects of bednets, such as adding a second chemical that makes the insecticide on bednets more deadly to mosquitoes. In some places, adding this second chemical makes the nets more effective, but in others it does not. Moreover, these doubly treated, or “combination”, nets are more expensive and so it can be hard for health officials to decide whether and where to use them. Now, Churcher et al. have used computer modeling to help predict how insecticide resistance might change malaria infection rates and help determine when it makes sense to switch to the combination net. Insecticide-treated bednets provide good protection for individuals sleeping under them until relatively high levels of resistance are achieved, as measured using a simple test. As more resistant mosquitos survive encounters with the nets, the likelihood of being bitten before bed or while sleeping unprotected by a net increases. This is expected to increase malaria infections. As bednets age and are washed multiple times, they lose some of their insecticide and this problem becomes worse. Churcher et al. also show that the combination bednets may provide some additional protection against resistant mosquitos and reduce the number of malaria infections in some cases. The experiments show a simple test could help health officials determine which type of net would be most beneficial. The experiments and the model Churcher et al. created also may help scientists studying how to prevent increased spread of malaria in communities where mosquitos are becoming resistant to insecticide-treated nets.

• Publisher: eLife Science Publications, Ltd,eLife Sciences Publications Ltd,eLife Sciences Publications, Ltd
• Subject: Africa / Animals / Anopheles - physiology / bednets / Bioassays / Biological Assay - statistics & numerical data / Clinical trials / Culicidae / Epidemiology / Epidemiology and Global Health / Health aspects / Humans / Infection control / Insecticide Resistance - physiology / Insecticide-Treated Bednets - statistics & numerical data / Insecticide-Treated Bednets - supply & distribution / Insecticides / Malaria / Malaria - prevention & control / Malaria - transmission / Mathematical models / Methods / Models, Statistical / Mortality / mosquito / Mosquito Control - methods / Mosquito Vectors - physiology / Mosquitoes / Nets / Nitriles / Observations / Permethrin / Pesticide resistance / Pesticide Synergists - supply & distribution / Piperonyl butoxide / Piperonyl Butoxide - supply & distribution / Population / Prevention / Public health / Pyrethrins / pyrethroid resistance / Pyrethroids / Seasonal variations / Sensitivity analysis