THE deadliest form of malaria is up to twice as widespread as is generally thought and causes more than half a billion infections every year, the most comprehensive analysis yet of its extent has revealed.

A map drawn up by British scientists shows 515 million infections with the most dangerous strain in 2002, compared with World Health Organisation (WHO) estimates of 270 million to 400 million.

The findings indicate that the impact of malaria has been greatly underestimated, and that it may outstrip HIV-Aids as the world’s deadliest infectious disease.

Previous figures put annual deaths from malaria at between a million and 2.7 million. While the new research did not reassess this, greater prevalence of its most serious form suggests that the toll could easily exceed the three million people killed annually by Aids.

The study found that 2.2 billion people, more than a third of the world’s population, are at risk of contracting malaria. The greatest impact is on children. While most of these live in Africa, South-East Asia and the western Pacific Rim are much more badly affected than had been suspected.

More than a quarter of cases of Plasmodium falciparum, the most dangerous strain of the malaria parasite, occur in this region. This figure of more than 130 million infections is three times greater than previous estimates.

The results, published today in the journal Nature, will help the WHO, governments and charities to target efforts to fight malaria more effectively.

Bob Snow, of Oxford University, led the study. He said: “We have taken a conservative approach to estimating how many attacks occur each year but even so the problem is far bigger than we thought.

“Getting numbers right is important. Not knowing the size of the problem limits our ability to say how much money we need to tackle it. Not knowing where the problem is means you can’t spend wisely.

“This is particularly important for new drugs. These are expensive and difficult to produce and production capacity and financing can be driven by speculation, poor data or simply best guesses.”

Professor Snow, who is based at the Wellcome Trust laboratories in Nairobi, Kenya, said that the findings have important implications for the UN’s Roll Back Malaria programme. It aims to halve mortality from the disease over the next six years.

“World leaders are now seriously focusing on malaria as a problem that can be tackled with tools we know work and are comparatively cheap,” he said.

“Hopefully, these data will provide not only more ammunition as to why they should take it seriously but help them decide where to spend their money to best effect.”

Professor Snow’s team used advanced statistical methods and epidemiological, geographical and demographic data to map the distribution of infections more accurately than has previously been possible.

Evidence for the numbers likely to be infected was taken from studies of particular regions in which cases were sought out by researchers. These data were extrapolated using detailed information on populations, geographical and environmental conditions that affect transmission rates.

Richard Feachem, executive director of the Global Fund to Fight Aids, Tuberculosis and Malaria, said: “The work is important. Many have believed that existing data grossly underestimate malaria, morbidity and mortality in Africa and Asia. We now have confirmation of this.”

Nick White, director of the Wellcome Trust’s South-East Asia unit, said: “Falciparum malaria has increasingly been thought of as an African problem. These estimates challenge that notion and suggest there is a lot more in South-East Asia than previously thought.”

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