Assessing indoor residual spraying for malaria control in Chikhwawa, Malawi, using exit traps in houses

Rationale Indoor residual spraying (IRS), using lambda-cyhalothrin, was piloted in Malawi in 2007 by the Presidents Malaria Initiative (PMI) in Nkhota-kota district. The Ministry of Health scaled up IRS to six additional districts across Malawi including Chikhwawa, in 2011. This study was designed to assess the impact of IRS against a background of high malaria prevalence and possible insecticide resistance on the major malaria vectors of Malawi, Anopheles gambiae and An. funestus in Chikhwawa; and to measure the impact of IRS on entomological indices and malaria prevalence in children of under 5 years of age. Methodology Three sentinel sites (Mwingama, Namila and Tsekera) were established in Chikhwawa and 6 window exit traps installed at each site. IRS was conducted in February 2011. Mosquitoes were captured daily, from October 2010 to April 2012, and analysed for species abundance and sporozoites. Separate mosquito collections were carried out using standard WHO insecticide susceptibility assays on An. gambiae and An. funestus from the sentinel sites. Insecticide quantification of IRS was determined by colorimetric analysis of the wall pads placed on selected houses within the sentinel sites. Anaemia and parasitaemia were determined in children of less than 5 years old from a 50 villages catchment area including the three sentinel sites, through rolling malaria indicator surveys (rMIS). Results and Conclusion The study has shown large heterogeneity in mosquito abundance between sentinel sites. Suspected cross resistance found was found in both An. gambiae and An. funestus to carbamates, organophosphates and pyrethroids suggesting a metabolic based resistance mechanism. Clear resistance (77% mortality) was only found at Namila to deltamethrin in An. funestus. There was significant change in resistance pattern at Namila in An. funestus to lambda-cyhalothrin between 2011 and 2012 (X2 = 6.011, p = 0.014). No statistically significant change was observed in An. gambiae and An. funestus abundance differences pre-post IRS suggesting programmatic IRS challenges in Chikhwawa. There was a decline on parasitaemia prevalence from an average of 41% to 29% post IRS. While entomological surveillance is important for the vector control programme in Malawi, there is a need to utilise this data to improve the actual IRS activities, especially when combined with the results of malaria burden as seen here