Global Malaria Estimates Are Reduced

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Benoist Carpentier / WHO

Free, long-lasting insecticidal nets are distributed to children of under 5 yers old in Bohicon, Benin.

The World Health Organization (WHO) has cut its global estimate of yearly malaria cases by more than 100 million, according to a report released Thursday by the health agency. Almost all of that downward revision was attributable to updated surveillance numbers — mostly in Asia, and particularly in India — rather than a measurable reduction of actual malaria cases, agency staff said.

The last World Malaria Report in 2005, tallied the global incidence rate at between 350 million and 500 million new cases of malaria per year. The current report downgrades that figure to 247 million. Likewise, where the last report claimed that the disease kills "more than 1 million" people each year, the 2008 update, which is based on 2006 data (the most recent numbers available), suggests that figure is now closer to 800,000.

The impact of malaria, however, is still massive. "Whether it's 200 million or 500 million [cases], that's a lot of infections with a big health burden and a big economic burden," says Bernard Nahlen, deputy coordinator of the U.S. President's Malaria Initiative — especially, he says, "for something that is treatable and to a large extent preventable. If your child's life can be saved by treatment for 50 cents, you should treat."

But the WHO's correction comes less than a year after the United Nations made a similar announcement, acknowledging last November that its AIDS-battling agency, UNAIDS, had overstated its estimates of the global HIV burden by about 6 million cases; that agency revised its numbers also based on newer and more accurate surveys. At the time, the U.N. came under fire from critics for inflating its estimates in order to exaggerate the urgency of the epidemic — and to spur bigger donations.

There may have been some truth to that argument. Money funneled to HIV and malaria control has soared in the last decade, to almost $9 billion and roughly $1 billion a year, respectively (those figures, too, are hazy estimates), although in the case of HIV especially, money has not always been channeled into disease-control programs based on the best scientific evidence. (That's particularly true for politically sensitive — and, therefore, under-funded or ignored — interventions that aim to prevent high-risk sex and drug abuse.)

In large part, such massive miscalculations have less to do with politics than with the simple fact that epidemiology involves an inordinate amount of guesswork. Routine re-evaluations of existing data often result in data shifts, sometimes huge ones, which global health experts and epidemiologists have come to expect. "If you go up to a little clinic in Africa, first of all, the staff are overwhelmed with patients," says Nahlen, who used to do monitoring work for WHO. The data, if it's properly recorded, then goes up to the officials, who may also be overwhelmed. Those records then get passed along to WHO. "It's sometimes hard to know what these numbers actually mean," he says.

The WHO ascribes most of the current revision to a reassessment of the malaria epidemic in Asia (although the vast majority of malaria cases and deaths still occur in Africa, where the numbers for the continent remain mostly unchanged). Much of the Asian data, which was used in the 2005 WHO report to predict which regions had malaria-carrying mosquitoes — and therefore higher disease incidence — was already 40 years old, says Mac Otten, coordinator of the surveillance, monitoring and evaluation unit at the WHO's Global Malaria Program. Over the past four decades, the situation across Asia has changed dramatically. "With urbanization, deforestation and then malaria control, [the data] is just out of date," he says. Malaria zones in Asia, especially India, where much of the revision took place, have become "patchy," as Otten puts it.

Epidemiologists are often described by the media as "disease detectives," who use statistical tools — carrying out the occasional survey, for example, or, in the case of malaria, using temperature and terrain maps to help predict where disease-carrying mosquitoes may live — to hunt down and eliminate global killers. The comparison is useful for another reason: Disease trackers, like crime solvers, often spend a lot of time sifting through a few, imperfect clues — hunches, really — to piece together a fuller picture. But that picture often ends up being indistinct as well. The WHO says, for example, that the "confidence interval" of its new estimate — the numerical range within which scientists believe the actual malaria incidence most likely lies — is 189 million cases to 327 million cases per year.

It is an enormous but unavoidable margin of error. Unlike in developed countries, such as the U.S., that have the infrastructure to compile more detailed population-wide medical records, disease surveillance in places like the Democratic Republic of the Congo, a large central African country with few doctors, few roads, limited medical infrastructure and a recent history of bloody conflict, is a much more difficult undertaking. Officials have trouble counting births and deaths in some regions, let alone getting a sense of how many people may have suffered from a particular disease.

What's more, even where records exist, there is usually no way to confirm their validity. Doctors in the developing world often lack lab facilities to authenticate cases of suspected malaria. Perhaps more often, they never even get to see patients who have the disease — many patients either cannot afford the time or money to see a doctor or they simply self-diagnose and take cheap over-the-counter medications to battle malaria-like symptoms. The WHO estimates that nationally reported (but often unvalidated) malaria cases account for just 40% of the global estimate; the other 60% comes from "detective work" by epidemiologists.

The fuzzy math aside, the good news is that malaria control efforts are working. The 2008 World Malaria Report singles out Eritrea, Rwanda, Sao Tome and Principe, and the Tanzanian region of Zanzibar for their remarkable improvements in cutting malaria illness and death. Since 2000, all of them have managed a greater than 50% drop in both rates, and all of them have done it through a combination of familiar methods: using long-lasting insecticidal bed nets to prevent mosquito bites; treating the disease with the newer, more effective artemisinin-based combination drug therapy; and spraying homes with insecticide. In these countries, there is little doubt that interventions are working, but the impact doesn't translate to a measurable reduction in global figures because the populations involved are relatively small. Larger countries like Nigeria have been slower to implement prevention and treatment programs.

But similar programs are underway across much of Africa, and so malaria workers are newly optimistic. "When the coverage of the malaria interventions — the nets, the medicines and the spraying — was high, cases came right down and deaths came down as well," says Otten. With luck, for the next World Malaria Report, global figures will be more accurate still — and they'll be falling because real people are really healthier.

See photos of The Global Fund fighting AIDS, Tuberculosis, and Malaria around the world.