Published online 26 February 2008 | Nature | doi:10.1038/news.2008.621

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Malaria map brings good news

Disease transmission is low throughout large areas of malarial risk.

Malaria risk: areas where malaria is endemic, with moderate to high transmission risk (shown in red), or sporadic and unstable, with low transmission risk (shown in pink). See more maps.S. HAY

Some 2.4 billion people live in places where they risk catching the deadliest form of malaria. But a new study brings some good news: 1 billion of them live in zones where transmission is so low that the disease should be easy to bring under control, or even eradicate.

The study was published in PLoS Medicine today1 by the Malaria Atlas Project, a collaboration between Bob Snow's team at the Kenyan Medical Research Institute in Nairobi, and researchers at the University of Oxford, UK, and University of Florida in Gainesville. They collected data on the risk of malaria transmission worldwide, and generated what they say is the most extensive global map since a now-obsolete one made in 1968.

The maps show that, unsurprisingly, high risk from the most deadly malaria parasite, Plasmodium falciparum, is most prevalent in Africa. This continent hosts almost all the regions worldwide where more than half of the children there are infected with the parasite. But to both the north and south of the belt of high malaria endemicity, transmission was much lower than generally thought, they report.

The researchers hope that their maps will allow donors and health organizations to improve the targeting of their resources for control measures, such as insecticide-treated bed nets, to reduce or eliminate malaria.

Snow’s team plans to improve the maps and compare them against where the money is going to fight the disease. Already the researchers suspect that not enough money is being spent on South East Asia and the Western Pacific region, which the maps show are home to 47% of the world's population at high risk. Nigeria, which has a population of around 150 million and is highly endemic, also seems to be neglected.

Data trawl

The scientists combed through 4,278 reports with data on the number of people with malaria between 2002 and 2006, as well as data on annual parasite numbers from the World Health Organization. Because such data are often patchy and of poor quality, the researchers also factored in climate models, which can be used to help predict the risk of transmission. Below certain temperatures, for example, mosquitoes don't live long enough to transmit P. falciparum. Rainfall and aridity also affect mosquito survival and disease transmission.

They then superimposed the risk estimates on a global population map to generate figures for the number of people at risk of malaria. They classed areas as either endemic, where transmission risk was moderate to high (shown in red), or as sporadic and unstable, where the risk of transmission was low (shown in pink) — fewer than one case per 10,000 people per year. They intend to publish more detailed maps this summer showing gradation within these categories.

For now, the large areas of pink bring some hope for controlling malaria. "We were surprised that these pink areas included such a large percentage of the global population at risk," says Simon Hay, a lead scientist on the Malaria Atlas Project at the University of Oxford.

Broad brush strokes

Scientists are divided over how useful the maps will be. Mark Grabowsky, the malaria coordinator of the Global Fund to Fight AIDS, Tuberculosis and Malaria, says that they are “an extraordinary effort” and “a huge service” for guiding efforts to stomp down on malaria.

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But Judy Omumbo and Madeleine Thomson, risk mappers at the International Research Institute for Climate and Society in New York, describe this edition as “very coarse”. They add that different data sets and climate models can produce very different estimates of risk for the same areas.

Burton Singer, an expert in disease demographics at Princeton University in New Jersey, says that the "broad-stroke picture" may be a useful "first cut" as to where priority targets should lie. But he thinks that it ignores the importance of local details in the risk of transmission.

The map and accompanying paper suggest that eradicating malaria in Latin America should be easy, Singer notes, but this isn't the case, he says. Pockets of high transmission from human migration in the region tend to make it hard to stomp down on the disease.

Hay admits that the new maps are a first step. "Getting to this stage has been a Herculean undertaking," he says. "They are the best we can do with the information we have." The team will continue to work to improve the maps. 

Find more on the mapping project, and see the Nature News malaria special .

  • References

    1. Guerra C.A., et al. PLoS Med. 5, e38 (2008).
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