Abstract
BackgroundPlasmodium falciparum transmission has decreased significantly in Zambia in the last decade. The malaria transmission is influenced by environmental variables. Incorporation of environmental variables in models of malaria transmission likely improves model fit and predicts probable trends in malaria disease. This work is based on the hypothesis that remotely-sensed environmental factors, including nocturnal dew point, are associated with malaria transmission and sustain foci of transmission during the low transmission season in the Southern Province of Zambia.MethodsThirty-eight rural health centres in Southern Province, Zambia were divided into three zones based on transmission patterns. Correlations between weekly malaria cases and remotely-sensed nocturnal dew point, nocturnal land surface temperature as well as vegetation indices and rainfall were evaluated in time-series analyses from 2012 week 19 to 2013 week 36. Zonal as well as clinic-based, multivariate, autoregressive, integrated, moving average (ARIMAX) models implementing environmental variables were developed to model transmission in 2011 week 19 to 2012 week 18 and forecast transmission in 2013 week 37 to week 41.ResultsDuring the dry, low transmission season significantly higher vegetation indices, nocturnal land surface temperature and nocturnal dew point were associated with the areas of higher transmission. Environmental variables improved ARIMAX models. Dew point and normalized differentiated vegetation index were significant predictors and improved all zonal transmission models. In the high-transmission zone, this was also seen for land surface temperature. Clinic models were improved by adding dew point and land surface temperature as well as normalized differentiated vegetation index. The mean average error of prediction for ARIMAX models ranged from 0.7 to 33.5%. Forecasts of malaria incidence were valid for three out of five rural health centres; however, with poor results at the zonal level.ConclusionsIn this study, the fit of ARIMAX models improves when environmental variables are included. There is a significant association of remotely-sensed nocturnal dew point with malaria transmission. Interestingly, dew point might be one of the factors sustaining malaria transmission in areas of general aridity during the dry season.
Highlights
Plasmodium falciparum transmission has decreased significantly in Zambia in the last decade
In 2010, roughly 200 million malaria cases and 660,000 deaths related to malaria were reported worldwide and 86% of these deaths were among children under five years of age [1]
Perusal of the literature in general suggests that this is the first time that a condition related to the dew point or moisture in the column of air at or near ground level has been obtained from satellite data reflecting discrete and focal ground conditions and is used for predicting malaria transmission foci
Summary
Plasmodium falciparum transmission has decreased significantly in Zambia in the last decade. Perusal of the literature in general suggests that this is the first time that a condition related to the dew point or moisture in the column of air at or near ground level has been obtained from satellite data reflecting discrete and focal ground conditions and is used for predicting malaria transmission foci. As these data are available nocturnally they relate directly to the environmental conditions affecting the potential flight and feeding behaviour of the vector species. The connection between malarial transmission and environmental factors is evident across the spectrum of parasite and mosquito development, feeding and survival
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