Challenges for malaria elimination in Zanzibar: pyrethroid resistance in malaria vectors and poor performance of long-lasting insecticide nets

Khamis A Haji,Silas Majambere,Maureen Coetzee,Stephen Smith,Abdullah S Ali,Gregor J Devine,Bakari O Khatib

doi:10.1186/1756-3305-6-82

Abstract

BackgroundLong-lasting insecticide treated nets (LLINs) and indoor residual house spraying (IRS) are the main interventions for the control of malaria vectors in Zanzibar. The aim of the present study was to assess the susceptibility status of malaria vectors against the insecticides used for LLINs and IRS and to determine the durability and efficacy of LLINs on the island.MethodsMosquitoes were sampled from Pemba and Unguja islands in 2010–2011 for use in WHO susceptibility tests. One hundred and fifty LLINs were collected from households on Unguja, their physical state was recorded and then tested for efficacy as well as total insecticide content.ResultsSpecies identification revealed that over 90% of the Anopheles gambiae complex was An. arabiensis with a small number of An. gambiae s.s. and An. merus being present. Susceptibility tests showed that An. arabiensis on Pemba was resistant to the pyrethroids used for LLINs and IRS. Mosquitoes from Unguja Island, however, were fully susceptible to all pyrethroids tested. A physical examination of 150 LLINs showed that two thirds were damaged after only three years in use. All used nets had a significantly lower (p < 0.001) mean permethrin concentration of 791.6 mg/m2 compared with 944.2 mg/m2 for new ones. Their efficacy decreased significantly against both susceptible An. gambiae s.s. colony mosquitoes and wild-type mosquitoes from Pemba after just six washes (p < 0.001).ConclusionThe sustainability of the gains achieved in malaria control in Zanzibar is seriously threatened by the resistance of malaria vectors to pyrethroids and the short-lived efficacy of LLINs. This study has revealed that even in relatively well-resourced and logistically manageable places like Zanzibar, malaria elimination is going to be difficult to achieve with the current control measures.

Highlights

Long-lasting insecticide treated nets (LLINs) and indoor residual house spraying (IRS) are the main interventions for the control of malaria vectors in Zanzibar
Malaria control Programme, 2009 unpublished report) and, since 2006, six rounds of IRS have been conducted with lambda-cyhalothrin (ICON 10WP/CS) giving greater than 90% coverage of all dwellings (Zanzibar Malaria control Programme, 2011 unpublished report)
Species identification Molecular identification of species tested in 2008 revealed that of the samples that amplified with PCR on Unguja (n = 300), 79% were An. arabiensis, 19% An. merus and 2% An. gambiae s.s

Summary

Introduction

Long-lasting insecticide treated nets (LLINs) and indoor residual house spraying (IRS) are the main interventions for the control of malaria vectors in Zanzibar. Malaria control Programme, 2009 unpublished report) and, since 2006, six rounds of IRS have been conducted with lambda-cyhalothrin (ICON 10WP/CS) giving greater than 90% coverage of all dwellings (Zanzibar Malaria control Programme, 2011 unpublished report). These two vector control interventions target indoor resting and indoor feeding mosquitoes [2]. A major concern for many malaria vector control programmes in sub-Saharan Africa is that malaria mosquitoes are increasingly developing resistance to insecticides [3,4]. Monitoring the susceptibility of vectors to insecticides is essential for predicting the sustainability and efficacy of these control tools [11]

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