A new global malaria estimate dramatically increases the number of cases worldwide, especially in Southeast Asia.

An international team of researchers from Kenya, Thailand, and Britain says the number of malaria cases is much higher than previous estimates by the World Health Organization (WHO).

Their new map of malaria distribution, published in the journal Nature, estimates that about a half a billion cases occurred in 2002.  This is about 50 percent more than WHO estimates globally and double for areas outside Africa.

The new numbers show Africa still bears the brunt of malaria. But one of the mappers, Bob Snow of the Kenya Medical Research Institute in Nairobi, says they lower Africa's share of the global burden from 90 percent.

"It does demonstrate that there is about a quarter of all the attacks that occur on the planet occur outside of Africa, whereas previously the dogma has been, I suppose, that falciparum malaria, the most deadly of the parasites, is peculiar to Africa, whereas it is quite clear there is a huge problem outside of Africa as well," said Mr. Snow.

The revisions show that malaria is three times more prevalent in Brazil than previously thought and 1000 times more in Pakistan. In fact, south and southeast Asia account for most of the newly-estimated rise in the world's malaria prevalence.  The region is now believed to have one-fourth of the world's cases.

The discrepancy between the new and old estimates reflects different counting methods. For Africa, the World Health Organization has relied on data from independent research studies to make nationwide estimates. In countries outside Africa presumed to have better health reporting, the WHO has accepted government figures based on data from clinics.

Mr. Snow says such government figures are inaccurate.

"You can't rely on national statistics to tell you the size of your disease burden. Clearly, not everyone who develops malaria will go to a clinic," he noted.

Mr. Snow's team combined several sets of data to come up with its updated malaria estimates for 22 countries. It included published research showing areas of prevalence and advice given to travelers who visit the tropics. The researchers also rated areas of the world for risk based on geography, with risk highest in humid rural areas where the mosquito that carries the malaria parasite can survive. They then compared this geographical risk map with one showing how population is distributed.

At the World Health Organization's Roll Back Malaria program, Eline Korenromp says the agency is in the process of adopting similar methods in its effort to reduce the number of cases by half by 2010.

"It is quite important to know the total burden of malaria, especially for allocating resources for fighting the disease,? she added.  ?It is also important to know how the disease burden is changing over time to see if countries are making progress and having an impact on the disease."

In Nairobi, Bob Snow says he hopes the new malaria numbers raise more awareness about a disease that kills at least one million people annually so more money can be raised to fight it.