Estimating the Global Clinical Burden of Plasmodium falciparum Malaria in 2007

Abstract

Simon Hay and colleagues derive contemporary estimates of the global clinical burden of Plasmodium falciparum malaria (the deadliest form of malaria) using cartography-based techniques.

Abstract

Background

The epidemiology of malaria makes surveillance-based methods of estimating its disease burden problematic. Cartographic approaches have provided alternative malaria burden estimates, but there remains widespread misunderstanding about their derivation and fidelity. The aims of this study are to present a new cartographic technique and its application for deriving global clinical burden estimates of Plasmodium falciparum malaria for 2007, and to compare these estimates and their likely precision with those derived under existing surveillance-based approaches.

Methods and Findings

In seven of the 87 countries endemic for P. falciparum malaria, the health reporting infrastructure was deemed sufficiently rigorous for case reports to be used verbatim. In the remaining countries, the mapped extent of unstable and stable P. falciparum malaria transmission was first determined. Estimates of the plausible incidence range of clinical cases were then calculated within the spatial limits of unstable transmission. A modelled relationship between clinical incidence and prevalence was used, together with new maps of P. falciparum malaria endemicity, to estimate incidence in areas of stable transmission, and geostatistical joint simulation was used to quantify uncertainty in these estimates at national, regional, and global scales.

Combining these estimates for all areas of transmission risk resulted in 451 million (95% credible interval 349–552 million) clinical cases of P. falciparum malaria in 2007. Almost all of this burden of morbidity occurred in areas of stable transmission. More than half of all estimated P. falciparum clinical cases and associated uncertainty occurred in India, Nigeria, the Democratic Republic of the Congo (DRC), and Myanmar (Burma), where 1.405 billion people are at risk.

Recent surveillance-based methods of burden estimation were then reviewed and discrepancies in national estimates explored. When these cartographically derived national estimates were ranked according to their relative uncertainty and replaced by surveillance-based estimates in the least certain half, 98% of the global clinical burden continued to be estimated by cartographic techniques.

Conclusions and Significance

Cartographic approaches to burden estimation provide a globally consistent measure of malaria morbidity of known fidelity, and they represent the only plausible method in those malaria-endemic countries with nonfunctional national surveillance. Unacceptable uncertainty in the clinical burden of malaria in only four countries confounds our ability to evaluate needs and monitor progress toward international targets for malaria control at the global scale. National prevalence surveys in each nation would reduce this uncertainty profoundly. Opportunities for further reducing uncertainty in clinical burden estimates by hybridizing alternative burden estimation procedures are also evaluated.

Please see later in the article for the Editors' Summary

Editors' Summary

Background

Malaria is a major global public-health problem. Nearly half the world's population is at risk of malaria, and Plasmodium falciparum malaria—the deadliest form of the disease—causes about one million deaths each year. Malaria is a parasitic disease that is transmitted to people through the bite of an infected mosquito. These insects inject a parasitic form known as sporozoites into people, where they replicate briefly inside liver cells. The liver cells then release merozoites (another parasitic form), which invade red blood cells. Here, the merozoites replicate rapidly before bursting out and infecting more red blood cells. This increase in the parasitic burden causes malaria's characteristic symptoms—debilitating and recurring fevers and chills. Infected red blood cells also release gametocytes, which infect mosquitoes when they take a blood meal. In the mosquito, the gametocytes multiply and develop into sporozoites, thus completing the parasite's life cycle. Malaria can be prevented by controlling the mosquitoes that spread the parasite and by avoiding mosquito bites. Effective treatment with antimalarial drugs also helps to reduce malaria transmission.

Why Was This Study Done?

In 1998, the World Health Organization (WHO) and several other international agencies launched Roll Back Malaria, a global partnership that aims to provide a coordinated, global approach to fighting malaria. For this or any other malaria control initiative to be effective, however, an accurate picture of the global clinical burden of malaria (how many people become ill because of malaria and where they live) is needed so that resources can be concentrated where they will have the most impact. Estimates of the global burden of many infectious diseases are obtained using data collected by national surveillance systems. Unfortunately, this approach does not work very well for malaria because in places where malaria is endemic (always present), diagnosis is often inaccurate and national reporting is incomplete. In this study, therefore, the researchers use an alternative, “cartographic” method for estimating the global clinical burden of P. falciparum malaria.

What Did the Researchers Do and Find?

The researchers identified seven P. falciparum malaria-endemic countries that had sufficiently reliable health information systems to determine the national clinical malaria burden in 2007 directly. They divided the other 80 malaria endemic countries into countries with a low risk of transmission (unstable transmission) and countries with a moderate or high risk of transmission (stable transmission). In countries with unstable transmission, the researchers assumed a uniform annual clinical incidence rate of 0.1 cases per 1,000 people and multiplied this by population sizes to get disease burden estimates. In countries with stable transmission, they used a modeled relationship between clinical incidence (number of new cases in a population per year) and prevalence (the proportion of a population infected with malaria parasites) and a global malaria endemicity map (a map that indicates the risk of malaria infection in different countries) to estimate malaria incidences. Finally, they used a technique called “joint simulation” to quantify the uncertainty in these estimates. Together, these disease burden estimates gave an estimated global burden of 451 million clinical cases of P. falciparum in 2007. Most of these cases occurred in areas of stable transmission and more than half occurred in India, Nigeria, the Democratic Republic of the Congo, and Myanmar. Importantly, these four nations alone contributed nearly half of the uncertainty in the global incidence estimates.

What Do These Findings Mean?

These findings are extremely valuable because they provide a global map of malaria cases that should facilitate the implementation and evaluation of malaria control programs. However, the estimate of the global clinical burden of P. falciparum malaria reported here is higher than the WHO estimate of 247 million cases each year that was obtained using surveillance-based methods. The discrepancy between the estimates obtained using the cartographic and the surveillance-based approach is particularly marked for India. The researchers discuss possible reasons for these discrepancies and suggest improvements that could be made to both methods to increase the validity and precision of estimates. Finally, they note that improvements in the national prevalence surveys in India, Nigeria, the Democratic Republic of the Congo, and Myanmar would greatly reduce the uncertainty associated with their estimate of the global clinical burden of malaria, an observation that should encourage efforts to improve malaria surveillance in these countries.

Additional Information

Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000261.

• A PLoS Medicine Health in Action article by Hay and colleagues, a Research Article by Guerra and colleagues http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.0050038, and a Research Article by Hay and colleagues provide further details about the global mapping of malaria risk

• Additional national and regional level maps and more information on the global mapping of malaria are available at the Malaria Atlas Project

• Information is available from the World Health Organization on malaria (in several languages)

• The US Centers for Disease Control and Prevention provide information on malaria (in English and Spanish)

• Information is available from the Roll Back Malaria Partnership on its approach to the global control of malaria (in English and French)

• MedlinePlus provides links to additional information on malaria (in English and Spanish)