Targeting Malaria Hotspots to Reduce Transmission Incidence in Senegal

Abstract

In central Senegal, malaria incidence declined in response to scaling-up of control measures from 2000 to 2010 and has since remained stable, making elimination unlikely in the short term. Additional control measures are needed to reduce transmission. We simulated chemoprophylaxis interventions targeting malaria hotspots using a metapopulation mathematical model, based on a differential-equation framework and incorporating human mobility. The model was fitted to weekly malaria incidence from 45 villages. Three approaches for selecting intervention targets were compared: (a) villages with malaria cases during the low transmission season of the previous year; (b) villages with highest incidence during the high transmission season of the previous year; (c) villages with highest connectivity with adjacent populations. Our results showed that intervention strategies targeting hotspots would be effective in reducing malaria incidence in both targeted and untargeted areas. Regardless of the intervention strategy used, pre-elimination (1-5 cases per 1000 per year) would not be reached without simultaneously increasing vector control by more than 10%. A cornerstone of malaria control and elimination is the effective targeting of strategic locations. Mathematical tools help to identify those locations and estimate the impact in silico.

Keywords: human mobility; intervention chemotherapy; malaria elimination; mathematical model.

Figure A1

Goodness of fit of malaria metapopulation model according to the assumed lag between rainfall and mosquito bites. Calibrations were undertaken on 2008 and 2010 transmission seasons. Optimization function computing the sum of squared residuals (SSR) were minimized for 6 weeks lag.

Figure A2

Decrease in malaria incidence while targeting one third of HT hotspots in Mbour, Senegal 2008–2012. The y-axis represents intervention efficacy (relative decrease in overall malaria incidence). (a) Unique one-year intervention during rainy season, (b) Repeated interventions on five consecutive rainy seasons, once per year, (c) Unique one-year intervention during the dry season, (d) Repeated interventions on five consecutive dry seasons, once per year.

Figure A3

Mbour zone, Senegal, 2008–2012. Geographical coordinates of the 45 villages circles. Red circles represent connectivity hotspots and gray lines represent main connections between villages.

Figure 1

Mbour zone, Senegal, 2008–2012. The geographical coordinates of the 45 villages are represented by black circles, and moving individuals by gray lines. The thickness of the lines reflects the number of trips.

Figure 2

Malaria transmission diagram at a local village $k$. Letter j stands for remote villages. Human compartments are $S_k$ (susceptible), $P_k$ (premunition), $I_k$ (blood-stage infection), $G{a}_k$ (asymptomatic carriage of gametocytes), $G{m}_k$ (symptomatic carriage of gametocytes) and $R_k$ (resistance due to treatment). Mosquito compartments are $A{i}_k$ (infected mosquitoes) and $A{s}_k$ (susceptible mosquitoes). The arrows represent the transition rates between compartments.

Figure 3

Sensitivity of model parameters in the malaria metapopulation model, Mbour, Senegal, 2008–2012. Right and left correspond to a parameter increase and decrease, respectively. Black and gray bars respectively represent a increase and decrease in total malaria cases, subsequent to parameter variations.

Figure 4

Decrease in malaria incidence while targeting low transmission (LT) hotspots in Mbour, Senegal, 2008–2012. The y-axis represents the relative decrease in malaria incidence for the overall area (45 villages). (a) unique one-year intervention in the rainy season, (b) repeated interventions over five consecutive rainy seasons, once per year, (c) unique one-year intervention in the dry season, (d) repeated interventions over five consecutive dry seasons, once per year. SMC12 corresponds to a theoretical schedule of uninterrupted monthly administration of SMC over 12 months.

Figure 5

Malaria incidence in the year following mass drug administration associated with vector control. Various definitions of hotspots were tested. The x-axis represents the percentage of villages included as hotspots. The y-axis represents the decrease in mosquito bites from baseline.