Revisions Sharply Cut Estimates on Malaria

Donald G. McNeil Jr. | 22 Sep 2008
New York Times
The world has many fewer cases of malaria than previously thought, the World Health Organization is reporting. But the agency says the apparent drop is not a result of mosquito nets, miracle drugs and DDT spraying — just better statistical techniques.

The war on the disease still needs to be prosecuted with vigor, the health organization said last week, because malaria is as bad as ever in its rural African epicenter. And although experts agree that the new estimates are probably closer to the truth, they are still not very accurate.

Issuing its annual malaria report, the W.H.O. said there were about 250 million cases of malaria in the world each year, and about 880,000 deaths. Previous official estimates were 350 million to 500 million cases and more than a million deaths.

"Almost all the drop is in the methodology," said Dr. Mac W. Otten Jr., a W.H.O. epidemiologist. Cases in Africa, where 91 percent of malaria deaths occur, remained nearly steady. Virtually the entire drop was in India, Indonesia, Pakistan and other Asian countries, where many cases are a less deadly strain of malaria called vivax.

Cases in Asia were overestimated because those figures relied on population and vegetation maps dating to the 1960s.

Since then, millions of Asians have migrated to cities, and millions of acres have been deforested, reducing mosquito habitat. Also, as countries like India have grown richer, they have gotten better at health care and mosquito control.

But the new estimate revealed a simple truth about malaria: all global estimates are imprecise guesses. The change comes on the heels of last year's downward revision by Unaids, the United Nations' AIDS-fighting agency, of its estimates of the global AIDS burden: 33.2 million cases, down from 39.5 million. In that case, too, most of the drop was due to an earlier assumption that India's caseload was worse than it really was.

Malaria cases may be even harder to count than AIDS cases, experts say. Most poor people with fevers that might be from the parasites never see a doctor or get a malaria test. They simply buy whatever malaria pills are on sale in local shops. No record is kept and no case reported to a national health service — if the country even has one capable of collecting such records.

In fact, doctors say, most fevers are not malaria. In some Asian studies, as few as 10 percent of those seeking malaria treatment actually had malaria. And whether patients recover or die may have little to do with pill purchases; in some Southeast Asian countries, tests on over-the-counter drugs found that more than half were fake.

Given these limitations, Dr. Christopher J. L. Murray, director of the University of Washington's new Institute for Health Metrics and Evaluation, said he found the W.H.O. report "a pretty impressive improvement in the rigor and quality of the surveillance work."

But Amir Attaran, a health policy expert at the University of Ottawa and a frequent critic of various participants in the war on malaria, was dismissive.

"It's better fudging, not better reporting," he said. "It's still assumption built on assumption built on assumption."

And Bob Snow, a University of Oxford epidemiologist working in Kenya, with whom the health agency consulted before issuing its report, said, "Improving on what the W.H.O. did for their 2005 report does not equate to getting it right."

He noted that the new estimate, like old ones, counted case reports from people who went to clinics in countries with broken-down health care systems and then tried to make statistical adjustments for those who never sought care in those countries.

Dr. Snow called that practice "ludicrous" and added that "even a back-of-the-envelope calculation" would come up with higher caseloads through most of Africa, as well as in Indonesia, which is the fourth-most-populous country and has not recently been cooperative at sharing data with the W.H.O.

Eline L. Korenromp, an epidemiologist at the Global Fund to Fight AIDS, Tuberculosis and Malaria who was a chief author of the 2005 report, agreed that its estimates were "quite uncertain," particularly since so few cases were recorded by doctors. For example, she said, she believed that 75 percent of all cases in Myanmar were never recorded.

Inside Africa, a big change in the way caseloads are estimated is the use of satellite imagery, Dr. Otten said. Satellites can track vegetation, temperature and humidity, improving estimates of how many people are at risk of mosquito bites. That is combined with spot studies of selected rural villages to estimate how many villagers are infected and how many die.

Outside Africa, where malaria is focused in patches, national health records and general health surveys of thousands of households — many of which are new, and paid for by the United States — are used to estimate how many of the sick sought treatment and whether they died.

Dr. Otten said the new estimates were almost entirely unaffected by the recent surge in donations of long-lasting insecticide-treated nets, spraying of houses with DDT, and purchases of medicines that include the new malaria drug artemisinin.

Most of the data was gathered in or before 2006, when only 25 percent of people needing mosquito nets and only 3 percent of those needing the new drugs were getting them.

Big improvements will be needed before precise data can be gathered in Africa, Dr. Otten said. For example, hundreds of millions of rapid diagnostic kits that can detect malaria in a finger-stick drop of blood will have to be distributed.

Right now, most diagnosis is done by a technician looking through a microscope. "Microscopy is hard," Dr. Otten said. "You have to look at the slide carefully and for a long time."

There are some bright spots in malaria control in the W.H.O. report. São Tomé and Principe, Rwanda and Eritrea cut their caseloads by more than half, and Zambia, Madagascar and the Tanzanian island of Zanzibar also did well. But those regions have so few people that they are a mere blip in the global figures.