The global distribution and population at risk of malaria: past, present, and future

Abstract

The aim of this review was to use geographic information systems in combination with historical maps to quantify the anthropogenic impact on the distribution of malaria in the 20th century. The nature of the cartographic record enabled global and regional patterns in the spatial limits of malaria to be investigated at six intervals between 1900 and 2002. Contemporaneous population surfaces also allowed changes in the numbers of people living in areas of malaria risk to be quantified. These data showed that during the past century, despite human activities reducing by half the land area supporting malaria, demographic changes resulted in a 2 billion increase in the total population exposed to malaria risk. Furthermore, stratifying the present day malaria extent by endemicity class and examining regional differences highlighted that nearly 1 billion people are exposed to hypoendemic and mesoendemic malaria in southeast Asia. We further concluded that some distortion in estimates of the regional distribution of malaria burden could have resulted from different methods used to calculate burden in Africa. Crude estimates of the national prevalence of Plasmodium falciparum infection based on endemicity maps corroborate these assertions. Finally, population projections for 2010 were used to investigate the potential effect of future demographic changes. These indicated that although population growth will not substantially change the regional distribution of people at malaria risk, around 400 million births will occur within the boundary of current distribution of malaria by 2010: the date by which the Roll Back Malaria initiative is challenged to halve the world's malaria burden.

Figure 1

The global distribution of malaria since preintervention (~1900–2002). All-cause malaria distribution maps for the preintervention distribution (circa 1900) and for the years 1946, 1965, 1975, 1992, 1994, and 2002^– were georeferenced using ERDAS Imagine 8.5 (Leica Geosystems GIS & Mapping, Atlanta, GA, USA). Maps were then digitised on screen with MapInfo Professional 7.0 (MapInfo Corp, NY, USA). Areas of high and low risk were merged throughout to establish all-cause malaria transmission limits. The only modification of original maps was infilling areas labelled as unknown in China in the 1975 map with the distribution recorded in 1965. Each map was then overlaid to create a single global distribution map of malaria risk which illustrates range changes through time. Note that the 1992 distribution is excluded from the figure for clarity because it was so similar to that of 1994.

Figure 2

The Lysenko map of global malaria endemicity. This map was digitised from the original source using the method outlined in figure 1. Endemicity as used by Lysenko is defined by the parasite rate (PR) in the 2–10-year age cohort (hypoendemic <0·1; mesoendemic 0·11–0·5; hyperendemic 0·51–0·75) except the holoendemic class (0·75) where the PR refers to the 1-year age group. The black line represents the 2002 limit of malaria risk. Note that the “epidemic” class is restricted to the temperate regions in these maps and that this term is used differently today.

Figure 3

A histogram of (A) the global area (km²) and (B) population at risk of all-cause malaria risk subdivided by endemicity class (~1900–2002). Derivations of area and population at risk estimates are described in tables 1–3. The bars in each endemicity class show data for the years (left to right) 1900, 1946, 1965, 1975, 1992, 1994, and 2002.

Figure 4

Histograms of population at all-cause malaria risk (~1900–2010) subdivided by malaria endemicity and WHO regional grouping. (A) AFRO, (B) AMRO, (C) EMRO, (D) EURO, (E) SEARO, (F) WPRO, and (G) a map of WHO regional groupings. Derivations of area and population at risk estimates are described in tables 1–3. The WHO regional grouping map was generated from global administrative boundaries (Environmental Systems Research Institute Inc, Redlands, CA, USA) and county tables in annexes of the 2002 World Health Report. The bars in each endemicity class show data for the years (left to right) 1900, 1946, 1965, 1975, 1992, 1994, and 2002.

Figure 5

The national falciparum prevalence (NfP) cartogram for 2002. The NfP was calculated using the method outlined in table 4. These continuous area cartograms were generated using MAPresso (http://www.mapresso.com), a public domain Java applet. Ten iterations were used.

Figure 6

Pie charts of the national falciparum prevalence (NfP) by WHO region for 2002 (A) and projected to 2010 (B). Data derived as for figure 5 and estimated using population projected for 2010 as described in table 1.