Early detection of malaria foci for targeted interventions in endemic southern Zambia

Ryan G Davis,Aniset Kamanga,Nnenna Chime,Clive Shiff,Carlos Castillo-Salgado,Sungano Mharakurwa

doi:10.1186/1475-2875-10-260

Abstract

BackgroundZambia has achieved significant reductions in the burden of malaria through a strategy of "scaling-up" effective interventions. Progress toward ultimate malaria elimination will require sustained prevention coverage and further interruption of transmission through active strategies to identify and treat asymptomatic malaria reservoirs. A surveillance system in Zambia's Southern Province has begun to implement such an approach. An early detection system could be an additional tool to identify foci of elevated incidence for targeted intervention.MethodsBased on surveillance data collected weekly from 13 rural health centres (RHCs) divided into three transmission zones, early warning thresholds were created following a technique successfully implemented in Thailand. Alert levels were graphed for all 52 weeks of a year using the mean and 95% confidence interval upper limit of a Poisson distribution of the weekly diagnosed malaria cases for every available week of historic data (beginning in Aug, 2008) at each of the sites within a zone. Annually adjusted population estimates for the RHC catchment areas served as person-time of weekly exposure. The zonal threshold levels were validated against the incidence data from each of the 13 respective RHCs.ResultsGraphed threshold levels for the three zones generally conformed to observed seasonal incidence patterns. Comparing thresholds with historic weekly incidence values, the overall percentage of aberrant weeks ranged from 1.7% in Mbabala to 36.1% in Kamwanu. For most RHCs, the percentage of weeks above threshold was greater during the high transmission season and during the 2009 year compared to 2010. 39% of weeks breaching alert levels were part of a series of three or more consecutive aberrant weeks.ConclusionsThe inconsistent sensitivity of the zonal threshold levels impugns the reliability of the alert system. With more years of surveillance data available, individual thresholds for each RHC could be calculated and compared to the technique outlined here. Until then, "aberrant" weeks during low transmission seasons, and during high transmission seasons at sites where the threshold level is less sensitive, could feasibly be followed up for household screening. Communities with disproportionate numbers of aberrant weeks could be reviewed for defaults in the scaling-up intervention coverage.

Highlights

Zambia has achieved significant reductions in the burden of malaria through a strategy of “scalingup” effective interventions
While Plasmodium falciparum is endemic throughout Zambia, the country has documented significant reductions in the burden of malaria [1,2] through the scale-up of malaria control interventions advocated by the Roll Back Malaria (RBM) Partnership [3] and Zambia’s National Malaria Control Programme [4]
Since August of 2008, a surveillance system has been established based on rapid diagnostic tests (RDTs), for rapid and accurate diagnoses, and mobile telephones to transmit weekly data by SMS text message from the rural health centres (RHCs) to the Malaria Institute at Macha (MIAM), located centrally [5]

Summary

Introduction

Zambia has achieved significant reductions in the burden of malaria through a strategy of “scalingup” effective interventions. Zambia’s implementation of the “scale-up for impact” approach has included ambitious goals as stated in RBM 2008 Global Malaria Action Plan calling for 80% population coverage. To this end, efforts are already underway at a cluster of 13 rural health centres (RHCs) in the Choma and Namwala districts in Zambia’s Southern Province (see Figure 1). The RHC sites farther south are at slightly higher elevations, away from the floodplain, and the water table is generally about 10-40 m below the ground [5]. The discrepancy in the water table impacts the nocturnal humidity and the permanence of surface water, important for the Anopheles breeding and blood feeding behaviours [7,9]

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Conclusion