The Global Burden of Diseases programme of the WHO has attempted to enumerate the health consequences of malaria infection8,9
. Because the African region has a notoriously weak system of reporting infectious diseases, epidemiological evidence from carefully conducted prospective, ‘active’ case-detection studies of malaria morbidity, disability and mortality in populations living under different transmission intensity risks have been compiled to estimate the disease burden10
. A different approach was adopted for WHO regions outside Africa, where the burden was computed from ‘passive’ national disease and mortality notifications to WHO regional offices without precisely defining the populations exposed to varied malaria infection risks9,11,12
. This use of national disease registration systems to provide accurate reflections of disease rests on three assumptions: that there is complete temporal coverage (every month is reported by a facility), that there is complete spatial coverage (every health facility reports nationwide), and that all disease events present to, and are reported by, health facilities. In reality, passive detection of disease events in most resource-poor countries is incomplete, even outside Africa.
Here we provide a standard global approach to deriving clinical malaria burden by using evidence of the epidemiological risks of disease outcome from active case-detection studies in combination with estimates of populations at risk of various P. falciparum
transmission conditions. A comprehensive outline of these procedures is given in Methods. A conservative approach is defined to further account for the confounding of malaria diagnosis efficiency by endemicity (see Supplementary Information A
for more detail and original data) and the modifying influence on endemicity of current levels of control and urbanization (see Supplementary Information B
Our global model suggests that, in 2002, 2.2 billion people were exposed to the threat of P. falciparum
malaria, resulting in a conservative estimate of 515 (range 300–660) million clinical attacks attributable to this parasite during that year ( and ). At a regional level, most clinical events attributable to P. falciparum
were concentrated in the African region (70%), but the highly populated South East Asia region contributed 25% of the world’s clinical attacks in 2002 ( and ). The WHO suggests that there were 273 million clinical attacks of malaria worldwide in 1998 and that 90% of the global disease incidence is borne by Africa9
. Other WHO estimates report that in 1990 the global incidence of malaria was 213 million cases13
. Neither of these sources provides sufficient detail on how the estimates were derived. Our models, by contrast, are both data-driven and reproducible. They also indicate that the number of clinical attacks due to P. falciparum
might be 50% higher than WHO estimates, and highlight the fact that almost one-third of the global incidence occurs outside Africa.
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 (more ...)
Populations (millions) at risk in 2002
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% (more ...)
Estimated data for P. falciparum clinical malaria cases in 2002 (millions)
We have not examined mortality attributed directly to P. falciparum
, because of the paucity of prospective epidemiological descriptions of cause-specific mortality outside Africa14
. The risk of death after a clinical attack of P. falciparum
seems much higher in Africa than in South East Asia and the western Pacific. The incidence of severe, life-threatening complications of P. falciparum
malaria in Africa15
is at least tenfold that in similar malaria endemic areas in India16
. Reasons for this are unclear but might include better access to prompt treatment18
and some cross-Plasmodium
species protection against severe disease outcomes19
We had estimated previously from epidemiological data that there were 221 million P. falciparum
attacks in Africa in 1995 (ref. 10
). Our 2002 estimate for Africa of 365 million clinical cases derives from a more specific, urban-adjusted endemicity map than that developed specifically for Africa in 1995, which was not structured according to levels of parasite prevalence. It was estimated12
from national statistics that there were 51.2 million P. falciparum
cases outside Africa in 1995; our estimate of 150 million cases is considerably higher. There are several possible explanations for this disparity, including our assigning populations at risk of different transmission conditions on the basis of an endemicity map constructed in 1968. We have used this map in its original form because there is no modern equivalent but have taken a very conservative approach to reclassifying areas at risk in 2002 by stepping down endemicity risks in all areas outside Africa and allowing for the rapid increases in urbanization since 1968. Furthermore, the clinical data on active detection of cases were derived from a wide range of malaria endemicities (see Supplementary Information A
) to create plausible endemicity-specific median estimates of disease. It seems unlikely that we have overestimated the clinical risks when reapplied to the global distributions of the three broad endemicity classes.
The most obvious explanation is the dependence on national statistics derived from passive detection of cases for the WHO’s present global disease estimates outside Africa. In our analysis we were able to compare WHO reports of clinical incidence from 12 administrative units with survey reports of data on active case detection in the same areas. These limited comparisons demonstrated the scale of under-reporting by passive detection, varying from a threefold difference in Brazil to a 1,000-fold difference in Pakistan.
The global Roll Back Malaria (RBM) initiative aims at halving the burden of malaria within the next six years9
. The Millennium Development Goal’s target is to halt the rising incidence of malaria by 2015 (ref. 20
). To achieve this, international priorities and resources must be targeted using different information sources, including national economic capacities, evidence-based cost-effective strategies and disease burdens. Inadequate descriptions of the global distribution of disease risk make it impossible to determine priorities and advise funding agencies appropriately. Redressing these deficiencies with robust data must be a priority if international agencies are to understand the size of the challenge set by their targets over the next ten years.