Study suggests the disease may afflict twice as many people as thought.
The number of malaria cases worldwide may be close to double that previously estimated, according to a new tally of the killer disease.
The study, which is one of the most comprehensive efforts to map the prevalence of malaria, shows that over half a billion people could have the disease. This elevated count could increase pressure on governments to pump money into prevention efforts.
Malaria, which is caused by a mosquito-borne parasite, is one of the world's biggest infectious killers. But it has been tough to figure out exactly how many people succumb to the disease. Earlier estimates relied to some extent on health clinics that voluntarily report their cases. But this can be inaccurate because many people with malaria do not go to clinics - and many clinics do not submit figures.
“It's known to be a terrible burden and now it appears it's even more terrible than we thought. Andrew Spielman , Harvard School of Public Health”
In the new study, published in Nature1, Robert Snow at the Wellcome Trust Research Laboratories in Nairobi, Kenya, and his colleagues created a computer model to build a detailed world map showing how many people are likely to be experiencing malaria symptoms.
To do this, they first divided the world into regions in which the disease is present, based on medical advice to travellers. They excluded areas above a certain altitude, where the parasite is less able to survive, and very built-up areas where there is less clean water in which mosquitoes can breed.
The team refined this map by adding estimates of the population density in each area, the risk of picking up an infection from a mosquito bite, and medical reports on the likelihood that an infection would blossom into the full-blown fever and other symptoms. "Then we pressed a button and the number came out," Snow says.
The researchers reckon that there were around 515 million clinical cases of malaria in 2002, although the actual figure could lie anywhere between 300 and 660 million. This is not far off double the estimate of 273 million cases produced by the World Health Organisation (WHO) in 1998. And in areas outside Africa, the new figures are at least three times as high as those previously estimated by the WHO.
Snow's numbers are still only a rough approximation of malaria's prevalence. But "it's probably the best estimate we have", says Andrew Spielman, a specialist in tropical infectious diseases at Harvard School of Public Health in Boston. "It's known to be a terrible burden and now it appears it's even more terrible than we thought."
Snow hopes that the new study will help work out the amount of money, medicines and other resources needed to combat malaria. Without it, for example, pharmaceutical companies struggle to know how many drugs to make and public health agencies find it hard to gauge how well they are curbing the disease. The map could also help guide where in the world money and aid should be targeted.
The report comes at a time when international momentum to tackle malaria is building. For example, the Global Fund to Fight AIDS, Tuberculosis and Malaria, which coordinates government and private funding, has channelled $3.1 billion into these diseases since its establishment in 2002.
Even so, public health researchers and advocates say that they need more. They point out that unlike other infectious diseases such as HIV and tuberculosis, we already know how to defeat malaria using simple measures such as insecticide-treated bed nets and drugs. Spielman predicts that some groups will use the study as "ammunition" to ask for more funding.
Experts at the WHO say they are now working with Snow's team to refine their own estimates of malaria's incidence. The WHO's new figures, which are due to be published later this year, lie between 350 and 500 million and largely overlap with Snow's, says Eline Korenromp, who works for the organization's malaria-monitoring unit in Geneva, Switzerland.
SnowR. W., GuerraC. A., NoorA. M., MyintH. Y. & HayS. I. Nature doi:10.1038/nature03342 (2005).
Harvard School of Public Health