Reduced Efficacy of Insecticide-treated Nets and Indoor Residual Spraying for Malaria Control in Pyrethroid Resistance Area, Benin

Raphael N’Guessan,Vincent Corbel,Mark Rowland,Martin Akogbéto

doi:10.3201/eid1302.060631

Abstract

The pyrethroid knockdown resistance gene (kdr) has become widespread in Anopheles gambiae in West Africa. A trial to test the continuing efficacy of insecticide-treated nets (ITN) and indoor residual spraying (IRS) was undertaken in experimental huts at 2 sites in Benin, the first where kdr is present at high frequency (Ladji), the second-where An. gambiae is susceptible (Malanville). Holes were made in the nets to mimic worn nets. At Malanville, 96% of susceptible An. gambiae were inhibited from blood-feeding, whereas at Ladji feeding was uninhibited by ITNs. The mortality rate of An. gambiae in ITN huts was 98% in Malanville but only 30% at Ladji. The efficacy of IRS was equally compromised. Mosquitoes at Ladji had higher oxidase and esterase activity than in a laboratory-susceptible strain, but this fact did not seem to contribute to resistance. Pyrethroid resistance in An. gambiae appears to threaten the future of ITN and IRS in Benin.

Highlights

During the last decade, pyrethroid-treated mosquito nets have become the main method of malaria prevention in many malaria-endemic African countries [1,2]
The reduction in efficacy affected indoor residual spraying (IRS) and insecticide-treated nets (ITN) : only 19% of mosquitoes in the ITN hut and only 22% in the IRS hut were killed after correction for natural mortality
98% of mosquitoes entering the ITN hut and 72% entering the IRS hut located in the susceptible north of Benin were killed by the lambdacyhalothin treatments after correction for natural mortality

Summary

Introduction

Pyrethroid-treated mosquito nets have become the main method of malaria prevention in many malaria-endemic African countries [1,2]. The 2 approaches to malaria prevention, insecticide-treated nets (ITNs) and spraying (IRS), are not mutually exclusive, and in malaria-endemic areas where ITN coverage is still limited, the feasibility of introducing IRS to reduce transmission is being considered, for example, by the President’s Malaria Initiative Fund [4]. Trials of IRS and ITNs have shown that in areas with pyrethroid-susceptible Anopheles gambiae the effectiveness of the 2 methods in controlling malaria does not differ [5]. This comparability may not hold true for areas with pyrethroid-resistant populations. To assess the practicability of applying IRS with pyrethroid in West Africa, we need to examine the effectiveness of this approach against a kdr-resistant population of An. gambiae. One compares the impact of IRS and ITN against a pyrethroid- resistant population in the southern part of the country; the other compares IRS and ITNs against a pyrethroid-susceptible population several hundred kilometers to the north

Methods

Results

Conclusion