More than half a billion people, nearly double previous estimates, were affected by the deadliest form of Malaria in 2002, scientists said on Wednesday.

Most were in sub-Saharan Africa but nearly 25 percent occurred in southeast Asia and the Western Pacific.

“The disease burden is 515 million clinical attacks a year on the planet. That is quite substantial,” said Professor Bob Snow of the Kenyan Medical Research Institute in Nairobi, Kenya.

“We have taken a conservative approach to estimating how many attacks occur globally each year but even so the problem is far bigger than we previously thought,” he told Reuters.

The figures, which are reported in the science journal Nature, are almost twice those of the World Health Organisation (WHO), which estimated the global incidence of Malaria at 273 million cases in 1998 with 90 percent of cases in Africa.

“It is quite substantially higher than the WHO estimate,” said Snow who calculated there were 365 million cases of malaria in Africa alone in 2002.

Malaria is transmitted by the bite of an infected mosquito. It occurs in more than 100 countries and kills more than a million people each year - mostly young children in sub-Saharan Africa. Most deaths are caused by the Plasmodium falciparum parasite.

Snow and his colleagues used epidemiological data, studies, demographic information and data from satellites to pinpoint areas where the disease is most prevalent.

“For the first time we have provided a framework for estimating how many clinical attacks there are each year due to Plasmodium falciparum - the most lethal of the malaria parasites that affect man,” said Snow.

The research suggests that 2.2 billion people are at risk of malaria. Although the scientists did not estimate deaths from the disease, the risk of severe life-threatening complications is about 10 times higher in Africa than in southeast Asia and the western Pacific.

“Getting the numbers right is important,” said Snow. “Not knowing the size of the problem limits our ability to articulate how much money we need to tackle the problem - not knowing where the problem is located means you can’t spend wisely.”

The Roll Back Malaria campaign, organised by the WHO, the United Nations Children’s Fund (UNICEF), the United Nations Development Programme (UNDP) and the World Bank, aims to halve malaria deaths by the year 2010.

SOURCE: Nature, March 10, 2005.