Urbanization and the global malaria recession

Abstract

Background: The past century has seen a significant contraction in the global extent of malaria transmission, resulting in over 50 countries being declared malaria free, and many regions of currently endemic countries eliminating the disease. Moreover, substantial reductions in transmission have been seen since 1900 in those areas that remain endemic today. Recent work showed that this malaria recession was unlikely to have been driven by climatic factors, and that control measures likely played a significant role. It has long been considered, however, that economic development, and particularly urbanization, has also been a causal factor. The urbanization process results in profound socio-economic and landscape changes that reduce malaria transmission, but the magnitude and extent of these effects on global endemicity reductions are poorly understood.

Methods: Global data at subnational spatial resolution on changes in malaria transmission intensity and urbanization trends over the past century were combined to examine the relationships seen over a range of spatial and temporal scales.

Results/conclusions: A consistent pattern of increased urbanization coincident with decreasing malaria transmission and elimination over the past century was found. Whilst it remains challenging to untangle whether this increased urbanization resulted in decreased transmission, or that malaria reductions promoted development, the results point to a close relationship between the two, irrespective of national wealth. The continuing rapid urbanization in malaria-endemic regions suggests that such malaria declines are likely to continue, particularly catalyzed by increasing levels of direct malaria control.

Figure 1

Maps showing global changes in urban population size 1900-2000 and change in malaria endemicity. The bar height is proportional to the size of urban population change. Areas that saw no change or an increase in endemicity are coloured dark red, those that saw a reduction by one endemicity class are in red, two classes in orange and three or more classes in yellow.

Figure 2

Box plots of the percentage urban area in 1900 and in 2000 for countries that are still endemic and in the decade that elimination was achieved for those countries that achieved elimination.

Figure 3

Plots showing urban area changes 1900-2000 between areas of countries that remain malaria endemic today, and those that have undergone elimination for (a) the Americas, (b) Africa plus Arabian peninsula and (c) Asia. In each case, scatterplots of urban area increases in endemic versus eliminated areas with one-to-one lines overlaid are shown at the top, and example plots of trends in urban area percentages between areas that eliminated malaria (blue) and that remain endemic (red) are shown at the bottom (the full set of these plots is provided in Additional file 9). The ISO country abbreviation for country name is used on the scatterplots ( http://www.iso.org/iso/english_country_names_and_code_elements).

Figure 4

Plasmodium falciparum basic reproductive number (PfR_C) changes and urbanization. Countries in red show a greater percentage increase in urban area over the past century in areas of the country with the greatest magnitude of PfR_C decreases than areas that showed the smallest changes. Countries in yellow show the reverse. Countries in light grey had either insufficient variation in PfR_C changes, or no transmission. Countries in dark grey have either always been malaria free, or only exhibited unstable transmission.