Operationalization of Insecticide Resistance Monitoring Amongst Malaria Vectors in the Philippines

Abstract

Background: Insecticide-based vector control interventions, insecticide-treated nets and indoor residual spraying have been documented to reduce malaria incidence propelling the Philippines towards malaria elimination. These gains should however be sustained by the prevention, retardation and management of insecticide resistance that may cause revulsion in disease and mortality. It is therefore imperative to track development of resistance and monitor their frequencies and distribution. Methods: Three methods were explored to monitor resistance in malaria vectors: dose response assays, use of knockdown time and the WHO standard bioassay method using diagnostic concentrations. Results: Local discriminating dose of 0.01% was established on Anopheles flavirostris for both alphacypermethrin and deltamethrin. Use of knockdown time showed differences in susceptibilities across strains. Results from WHO standard bioassay method across different provincial sites generally showed susceptibility of An. flavirostris strains to pyrethroids (permethrin, deltamethrin, cyfluthrin, lambdacyhalothrin and etofenprox), organophosphates (malathion and fenitrothion), carbamates (propoxur and bendiocarb) and the organochlorine compound (DDT). However, resistance was detected in several strains of An. flavirostris and An. maculatus s.l. to some of the pyrethroids tested. Conclusion: Continuous monitoring of insecticide resistance captures trends that signals for change in vector control options. It is highly recommended that behavioral changes of target vectors be also observed to decipher if the real cause of resistance is physiological rather than avoidance.