Comparative field evaluation of combinations of long-lasting insecticide treated nets and indoor residual spraying, relative to either method alone, for malaria prevention in an area where the main vector is Anopheles arabiensis

Abstract

BackgroundLong-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are commonly used together in the same households to improve malaria control despite inconsistent evidence on whether such combinations actually offer better protection than nets alone or IRS alone.MethodsComparative tests were conducted using experimental huts fitted with LLINs, untreated nets, IRS plus untreated nets, or combinations of LLINs and IRS, in an area where Anopheles arabiensis is the predominant malaria vector species. Three LLIN types, Olyset®, PermaNet 2.0® and Icon Life® nets and three IRS treatments, pirimiphos-methyl, DDT, and lambda cyhalothrin, were used singly or in combinations. We compared, number of mosquitoes entering huts, proportion and number killed, proportions prevented from blood-feeding, time when mosquitoes exited the huts, and proportions caught exiting. The tests were done for four months in dry season and another six months in wet season, each time using new intact nets.ResultsAll the net types, used with or without IRS, prevented >99% of indoor mosquito bites. Adding PermaNet 2.0® and Icon Life®, but not Olyset® nets into huts with any IRS increased mortality of malaria vectors relative to IRS alone. However, of all IRS treatments, only pirimiphos-methyl significantly increased vector mortality relative to LLINs alone, though this increase was modest. Overall, median mortality of An. arabiensis caught in huts with any of the treatments did not exceed 29%. No treatment reduced entry of the vectors into huts, except for marginal reductions due to PermaNet 2.0® nets and DDT. More than 95% of all mosquitoes were caught in exit traps rather than inside huts.ConclusionsWhere the main malaria vector is An. arabiensis, adding IRS into houses with intact pyrethroid LLINs does not enhance house-hold level protection except where the IRS employs non-pyrethroid insecticides such as pirimiphos-methyl, which can confer modest enhancements. In contrast, adding intact bednets onto IRS enhances protection by preventing mosquito blood-feeding (even if the nets are non-insecticidal) and by slightly increasing mosquito mortality (in case of LLINs). The primary mode of action of intact LLINs against An. arabiensis is clearly bite prevention rather than insecticidal activity. Therefore, where resources are limited, priority should be to ensure that everyone at risk consistently uses LLINs and that the nets are regularly replaced before being excessively torn. Measures that maximize bite prevention (e.g. proper net sizes to effectively cover sleeping spaces, stronger net fibres that resist tears and burns and net use practices that preserve net longevity), should be emphasized.

Pilot study on the combination of an organophosphate-based insecticide paint and pyrethroid-treated long lasting nets against pyrethroid resistant malaria vectors in Burkina Faso

“…A study performed in experimental huts in Tanzania that tested several IRS compounds used concomitantly with LLINs showed IRS with DDT or pyrethroids did not confer additional value to LLINs alone, but showed IRS with OPs could be effective in preventing blood feeding and increasing vector mortality when combined with LLINs (Okumu et al, 2013). These studies suggest there may be value in adding a nonpyrethroid insecticide paint, insecticide treated plastic sheetings or IRS to LLINs.…”

Efficacy of indoor residual spraying with dichlorodiphenyltrichloroethane against malaria in Gambian communities with high usage of long-lasting insecticidal mosquito nets: a cluster-randomised controlled trial

“…Results from experimental huts indicate that there is no additional benefit of using IRS if LLIN are in use 14 except where pyrethroid-resistant vectors occur, in which case a non-pyrethroid insecticide sprayed on the walls provides additional protection 15,16 . However, a recent analysis of survey data from 17 African countries indicated these concerns might be unwarranted since using LLIN and IRS together was associated with lower malaria prevalence than LLIN alone 17 and a review of non-randomized studies indicated that addition of LLIN to IRS was associated with lower parasite rates than IRS alone 6 .…”

A second chance to tackle African malaria vector mosquitoes that avoid houses and don't take drugs

“…1 Recent studies of residual transmission systems where highly endophagic mosquitoes have been effectively tackled by IRS or LLINs remind us that behavioral avoidance of indoor insecticide exposure, by such a naturally evasive species as An. arabiensis, 6,7,34,35 does not necessarily represent failure, or even waning impact, of these measures but rather their inherent limitations from the outset. 18,[31][32][33][36][37][38] Recent reports of surprising 39 and previously undescribed 40 primary vectors most probably does not reflect the emergence of new vector systems but rather that their role in transmission is now more obvious because they are no longer outnumbered by An.…”