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

Khamis A Haji12, Bakari O Khatib1, Stephen Smith3, Abdullah S Ali1, Gregor J Devine456, Maureen Coetzee2* and Silas Majambere46

Author Affiliations

1 Zanzibar Malaria Control Program, Mwanakwerekwe, P.O. Box 407, Zanzibar, Tanzania

2 Malaria Entomology Research Unit, School of Pathology, Faculty of Health Sciences, University of the Witwatersrand, 7 York Road, Parktown, Johannesburg, 2193, South Africa

3 Centers for Disease Control and Prevention, 4770 Buford Highway, Atlanta, GA, 30341, USA

4 Ifakara Health Institute, Dar es Salaam, P.O. Box 78373, Dar es Salaam, Tanzania

5 Cairns Public Health Unit, P.O. Box 1103, Cairns, Queensland, 4870, Australia

6 Liverpool School of Tropical Medicine, Pembroke Place, Liverpool, L3 5QA, UK

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Parasites & Vectors 2013, 6:82  doi:10.1186/1756-3305-6-82

Published: 28 March 2013

Abstract

Background

Long-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.

Methods

Mosquitoes 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.

Results

Species 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).

Conclusion

The 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.

Keywords:
Anopheles gambiae; Anopheles arabiensis; Tanzania; LLINs; Insecticide resistance