The global distribution of clinical episodes of Plasmodium falciparum malaria

Abstract

Interest in mapping the global distribution of malaria is motivated by a need to define populations at risk for appropriate resource allocation and to provide a robust framework for evaluating its global economic impact. Comparison of older and more recent malaria maps shows how the disease has been geographically restricted, but it remains entrenched in poor areas of the world with climates suitable for transmission. Here we provide an empirical approach to estimating the number of clinical events caused by Plasmodium falciparum worldwide, by using a combination of epidemiological, geographical and demographic data. We estimate that there were 515 (range 300-660) million episodes of clinical P. falciparum malaria in 2002. These global estimates are up to 50% higher than those reported by the World Health Organization (WHO) and 200% higher for areas outside Africa, reflecting the WHO's reliance upon passive national reporting for these countries. Without an informed understanding of the cartography of malaria risk, the global extent of clinical disease caused by P. falciparum will continue to be underestimated.

Figure 1

Annual clinical incidence of P. falciparum per 1,000 population according to hypoendemic (n = 39), mesoendemic (n = 25) and combined hyperendemic and holoendemic (n = 8) conditions. The box indicates the inter-quartile range (25% and 75%) and the thick line within the box represents the median. The whiskers represent the 2.5% and 97.5% centiles and outliers are plotted as circles outside this range. Three studies were excluded because they were undertaken in areas of a recorded zero P. falciparum prevalence, and each reported no clinical attacks due to P. falciparum.

Figure 2

P. falciparum endemicity distribution within the global limits of risk. Endemicity classes: light green, hypoendemic (areas in which childhood infection prevalence is less than 10%); medium green, mesoendemic (areas with infection prevalence between 11% and 50%); dark green, hyperendemic and holoendemic (areas with an infection prevalence of 50% or more). Unclassified areas (yellow) represent only 6% of the global population at risk and are due to discrepancies between the 2002 delineation of risk and the endemicity risk limits developed in refs and . Grey areas are a combined mask of areas outside of the transmission limits and areas of population density less than 1 person km⁻² (ref. 16).