The impact of targeted IRS, vector dynamics, and population movement on malaria in a high-transmission setting in northern Zambia
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Background: The scale-up of malaria control efforts has led to a global decline in malaria burden, but progress has stalled or reversed in high-transmission regions. In Zambia, malaria cases increased annually since 2009 despite extensive malaria control activities, and malaria remains the most common cause of child mortality. To achieve the goal of malaria elimination in Zambia by 2021, drivers of transmission in high-burden areas must be identified and new intervention strategies must be developed and evaluated. Methods: The study was conducted in Nchelenge District, a high-transmission area in northern Zambia. The region has two main malaria vectors, Anopheles funestus s.s. and An. gambiae s.s., whose distribution varies spatially and temporally. Household surveys were conducted bimonthly from April 2012 to July 2017. Parasite prevalence was measured using rapid diagnostic tests (RDTs), and malaria vectors were collected with indoor light traps. Correlates of parasite prevalence and household vector abundance were identified, and the relationships between vector abundance and prevalence were defined. An evaluation was conducted to determine the impact of three years of targeted indoor residual spraying (IRS) with pirimiphos-methyl on vector abundance and parasite prevalence. Individual movement patterns were characterized using global positioning systems (GPS) data loggers and linked to malaria risk. Results: Parasite prevalence was approximately 50% across all participants, and an average of 7.0 An. funestus and 0.8 An. gambiae were collected per household. An. funestus counts were positively correlated with both rainy- and dry-season malaria transmission, and An. gambiae counts were positively associated with rainy-season transmission only. Within the area targeted for IRS, there was a 28% decline in parasite prevalence in the rainy season, and a 51% and 36% decline in An. funestus and An. gambiae counts. Three-quarters of participants spent time in both sprayed and unsprayed areas, and half spent at least an hour away from home per night during times of peak vector biting activity. Conclusions: Malaria transmission in Nchelenge District remains high with many barriers to control. Novel intervention strategies are needed to successfully reduce and interrupt transmission in high-burden areas, including year-round comprehensive vector control. Population movement patterns have the potential to increase malaria risk and must be considered in malaria control activities.