Evaluating the impact of indoor residual spraying on malaria transmission in Madagascar using routine health data

ABSTRACT Introduction Indoor residual spraying (IRS) and insecticide-treated bed-nets (ITNs) are cornerstone malaria prevention methods in Madagascar. This retrospective observational study uses routine data to evaluate the impacts of IRS overall, sustained IRS exposure over multiple years, and level of spray coverage (structures sprayed/found) in nine districts where non-pyrethroid IRS was deployed to complement standard pyrethroid ITNs from 2017 to 2020. Methods Multilevel negative-binomial generalized linear models were fit to estimate the effects of IRS exposure overall; consecutive years of IRS exposure; and spray coverage level on monthly all-ages population-adjusted malaria cases confirmed by rapid diagnostic test at the health facility level. The study period extended from July 2016 to June 2017. Facilities missing data and non-geolocated communes were excluded. Facilities in IRS districts were matched with control facilities by propensity score analysis. Models controlled for ITN survivorship, mass drug administration coverage, precipitation, enhanced vegetation index, seasonal effects, and district. Predicted cases under a counterfactual no IRS scenario and number of cases averted by IRS were estimated using the fitted models. Results Exposure to IRS overall reduced case incidence by an estimated 30.3% from 165.8 cases per 1,000 population (95%CI=139.7-196.7) under a counterfactual no IRS scenario, to 114.3 (95%CI=96.5-135.3), over 12 months post-IRS campaign in 9 districts. A third year of IRS reduced malaria cases 30.9% more than a first year (IRR=0.578, 95%CI=0.578-0.825, P0.05). Coverage of 86%-90% was associated with a 19.7% reduction in incidence (IRR=0.803, 95%CI=0.690-0.934, P=0.005) compared to coverage ≤85%, although these results were not robust to sensitivity analysis. Conclusion This study demonstrates that non-pyrethroid IRS appears to substantially reduce malaria incidence in Madagascar and that sustained implementation of IRS over 3 years confers additional benefits. KEY MESSAGES What is already known on this topic The use of non-pyrethroid insecticides for indoor residual spraying (IRS) in communities using pyrethroid-based insecticide treated nets (ITNs) is associated with reduced malaria prevalence, and with reduced malaria incidence in some settings.[1] To date, few studies have investigated the impact of high spray coverage and sustained IRS implementation over multiple years.[2,3] What this study adds This study estimates the impact of IRS in a setting with heterogenous malaria transmission and variation in intervention impact at the subnational level. Additionally, this study presents evidence of a benefit to continuing IRS implementation over multiple consecutive years. How this study might affect research, practice, or policy The results reported here bolster the evidence around the effectiveness of non-pyrethroid insecticides for IRS while suggesting that policymakers should consider the benefits of sustaining IRS implementation over multiple years when undertaking decisions to move locations and/or withdraw IRS.