Richard Cibulskis and colleagues present estimates of the worldwide incidence of malaria in 2009, together with a critique of different estimation methods, including those based on risk maps constructed from surveys of parasite prevalence, and those based on routine case reports compiled by health ministries.
Measuring progress towards Millennium Development Goal 6, including estimates of, and time trends in, the number of malaria cases, has relied on risk maps constructed from surveys of parasite prevalence, and on routine case reports compiled by health ministries. Here we present a critique of both methods, illustrated with national incidence estimates for 2009.
Methods and Findings
We compiled information on the number of cases reported by National Malaria Control Programs in 99 countries with ongoing malaria transmission. For 71 countries we estimated the total incidence of Plasmodium falciparum and P. vivax by adjusting the number of reported cases using data on reporting completeness, the proportion of suspects that are parasite-positive, the proportion of confirmed cases due to each Plasmodium species, and the extent to which patients use public sector health facilities. All four factors varied markedly among countries and regions. For 28 African countries with less reliable routine surveillance data, we estimated the number of cases from model-based methods that link measures of malaria transmission with case incidence. In 2009, 98% of cases were due to P. falciparum in Africa and 65% in other regions. There were an estimated 225 million malaria cases (5th–95th centiles, 146–316 million) worldwide, 176 (110–248) million in the African region, and 49 (36–68) million elsewhere. Our estimates are lower than other published figures, especially survey-based estimates for non-African countries.
Estimates of malaria incidence derived from routine surveillance data were typically lower than those derived from surveys of parasite prevalence. Carefully interpreted surveillance data can be used to monitor malaria trends in response to control efforts, and to highlight areas where malaria programs and health information systems need to be strengthened. As malaria incidence declines around the world, evaluation of control efforts will increasingly rely on robust systems of routine surveillance.
Please see later in the article for the Editors' Summary
Malaria is a life-threatening disease caused by the Plasmodium parasite, which is transmitted to people through the bites of infected mosquitoes. According to latest estimates from the World Health Organization (WHO), in 2009, there were 225 million cases of malaria and an estimated 781,000 deaths worldwide—most deaths occurring among children living in the WHO African Region (mainly sub-Saharan Africa). Knowing the burden of malaria in any country is an essential component of public health planning and accurately estimating the global burden is essential to monitor progress towards the United Nations' Millennium Development Goals.
Currently, there are generally two approaches used to estimate malaria incidence:
One method uses routine surveillance reports of malaria cases compiled by national health ministries, which are analyzed to take into account some deficincies in data collection, such as incomplete reporting by health facilities, the potential for overdiagnosis of malaria among patients with fever, and the use of private health facilities or none at all. The second method uses population-based surveys of Plasmodium prevalence and case incidence from selected locations in malaria endemic areas and then uses this information to generate risk maps and to estimate the case incidence of malaria per 1,000 population, for all of the world's malaria endemic regions. The Malaria Atlas Project—a database of malaria epidemiology based on medical intelligence and satellite-derived climate data—uses this second method.
Why Was This Study Done?
In order for malaria epidemiology to be as accurate as possible, an evaluation of the strengths and weaknesses of both methods is necessary. In this study, the researchers analyzed the merits of the estimates calculated by using the different approaches, to highlight areas in which both methods need to be improved to provide better assessments of malaria control.
What Did the Researchers Do and Find?
The researchers estimated the number of malaria cases in 2009, for each of the 99 countries with ongoing malaria transmission using a combination of the two methods. The researchers used the first method for 56 malaria endemic countries outside the WHO African Region, and for nine African countries which had the quality of data necessary to calculate estimates using the researchers statistical model—which the researchers devised to take the upper and lower limits of case detection into account. The researchers used the second method for 34 countries in the African Region to classify malaria risk into low-transmission and high-transmission categories, and then to derive incidence rates for populations from observational studies conducted in populations in which there were no malaria control activities. For both methods, the researchers conducted a statistical analysis to determine the range of uncertainty.
The researchers found that using a combination of methods there was a combined total of 225 million malaria cases, in the 99 countries malaria endemic countries—the majority of cases (78%) were in the WHO African region, followed by the Southeast Asian (15%) and Eastern Mediterranean regions. In Africa, there were 214 cases per 1,000 population, compared with 23 per 1,000 in the Eastern Mediterranean region, and 19 per 1,000 in the Southeast Asia region. Sixteen countries accounted for 80% of all estimated cases globally—all but two countries were in the African region. The researchers found that despite the differences between methods 1 and 2, the ratio of the upper and lower limit for country estimates was approximately the same.
What Do These Findings Mean?
Using the combined methods, the incidence of malaria was estimated to be lower than previous estimates, particularly outside of Africa. Nevertheless the methods suggest that malaria surveillance systems currently miss the majority of cases, detecting less than 10% of those estimated to occur globally. Although the best assessment of malaria burden and trends should rely on a combination of surveillance and survey data, accurate surveillance is the ultimate goal for malaria control programs, especially as routine surveillance has advantages for estimating case incidence, spatially and through time. However, as the researchers have identified in this study, strengthening surveillance requires a critical evaluation of inherent errors and these errors must be adequately addressed in order to have confidence in estimates of malaria burden and trends, and therefore, the return on investments for malaria control programs.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001142.
This study is further discussed in a PLoS Medicine Perspective by Ivo Mueller and colleagues
The WHO provides information on malaria and produces the World Malaria Report each year, summarizing global progress in malaria control
More information is available on The Malaria Atlas Project