Malaria is such a problem globally, especially in sub-Saharan Africa, that the World Health Organization (WHO), together with other international AID organizations, launched the Roll Back Malaria campaign with a goal of halving the incidence of the disease by 2010. According to results published today in Nature, that task may be even more daunting than expected. The results indicate that there are nearly 50 percent more clinical cases of malaria worldwide than previous estimates suggested.

Numbers provided by the WHO estimated that there were 273 million cases of malaria worldwide in 1998, with 90 percent occurring in Africa. To calculate the new figures, Robert Snow of the Kenya Medical Research Institute and his colleagues relied on epidemiological, geographical and demographic data. They determined that some 2.2 billion people were exposed to the threat of Plasmodium falciparum, the malaria parasite, in 2002. The conservative estimate they supply for active cases in the same year is 515 million, with about 70 percent occurring in Africa. The authors note that the new distribution maps highlight the fact that almost one third of the global incidence occurs outside Africa. The researchers did not examine how many deaths malaria caused in 2002, but report that the risk of death from an attack is much higher in Africa than South East Asia or the western Pacific.

In addition to the Roll Back Malaria program, the United Nation's Millennium Development Goals include a target of lowering the incidence of malaria by 2015. The scientists hope their results will aid those goals. "Inadequate descriptions of the global distribution of disease risk make it impossible to determine priorities and advise funding agencies appropriately," the team concludes. "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 10 years."