Do holes in long-lasting insecticidal nets compromise their efficacy against pyrethroid resistant Anopheles gambiae and Culex quinquefasciatus? Results from a release–recapture study in experimental huts

Sanjiarizaha Randriamaherijaona,Christophe Rogier,Olivier J T Briët,Sébastien Boyer,Aziz Bouraima,Vincent Corbel,Raphael N’Guessan

doi:10.1186/s12936-015-0836-7

Abstract

BackgroundResistance of malaria vectors to pyrethroids threatens the effectiveness of long-lasting insecticidal nets (LLINs) as a tool for malaria control. Recent experimental hut and observational studies in Benin show that pyrethroid resistance reduces the insecticidal effect and personal protection of LLINs especially when they become torn. The World Health Organization has proposed a threshold for when nets are “too torn” at 1,000 cm2 for rectangular holes and 790 cm2 for round holes. This study examines whether there is a threshold above which LLINs no longer reduce malaria transmission.MethodsIntact and artificially-holed LLINs under three months old and untreated nets were tested by releasing mosquitoes from a susceptible Anopheles gambiae colony, a pyrethroid-resistant An. gambiae population and a resistant Culex quinquefasciatus population in closed experimental huts in Southern Benin, West Africa. The efficacy of LLINs and untreated nets was evaluated in terms of protection against blood feeding, insecticidal effect and potential effect on malaria transmission.ResultsPersonal protection by both LLINs and untreated nets decreased exponentially with increasing holed surface area, without evidence for a specific threshold beyond which LLINs could be considered as ineffective. The insecticidal effect of LLINs was lower in resistant mosquitoes than in susceptible mosquitoes, but holed surface area had little or no impact on the insecticidal effect of LLINs. LLINs with 22,500 cm2 holed surface area and target insecticide content provided a personal protection of 0.60 (95 % CI 0.44–0.73) and a low insecticidal effect of 0.20 (95 % CI 0.12–0.30) against resistant An. gambiae. Nevertheless, mathematical models suggested that if 80 % of the population uses such nets, they could still prevent 94 % (95 % CI 89–97 %) of transmission by pyrethroid-resistant An. gambiae.ConclusionsEven though personal protection by LLINs against feeding mosquitoes is strongly reduced by holes, the insecticidal effect of LLINs is independent of the holed surface area, but strongly dependent on insecticide resistance. Badly torn nets that still contain insecticide have potential to reduce malaria transmission. The relationship between LLIN integrity and efficacy needs to be understood in order to guide LLIN distribution policy.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-015-0836-7) contains supplementary material, which is available to authorized users.

Highlights

Resistance of malaria vectors to pyrethroids threatens the effectiveness of long-lasting insecticidal nets (LLINs) as a tool for malaria control
Mortality was low with 3.8 % (n = 26, 95 % CI 0–20) dead in An. gambiae AKRON, 0 % (n = 25, 95 % CI 0–14) dead in Cx. quinquefasciatus, and 4 % (n = 25, 95 % CI 0–20) dead in An. gambiae KISUMU
Compared to the proportion fed with an intact LLIN, the proportion fed was significantly different (Fisher’s exact test, alpha = 0.05) with LLINs with 750 cm2 or more, with 30 cm2 or more and with 225 cm2 or more holed surface area for An. gambiae KISUMU, An. gambiae AKRON, and Cx. quinquefasciatus, respectively

Summary

Introduction

Resistance of malaria vectors to pyrethroids threatens the effectiveness of long-lasting insecticidal nets (LLINs) as a tool for malaria control. Between 2000 and 2012, the estimated number of malaria cases per 1,000 persons at risk of malaria was reduced by 29 % globally and by 31 % in the African region This has mostly been credited to the distribution of long-lasting insecticidal nets (LLINs), with 443 million nets distributed over 2010–2014 in sub-Saharan Africa, and implementation of indoor spraying of houses with residual insecticides (IRS) [1]. These two vector control methods have become essential components of worldwide malaria control and elimination efforts [2, 3]. The biting nuisance caused by Cx. quinquefasciatus and other non-malaria vectors can have severe implications for malaria control because a positive correlation exists between the biting nuisance and the effective use of LLINs by populations [7, 8]

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