Almost half of the world's population is at risk from the disease known as the 'other malaria', according to the first global map of the distribution of its cause, the parasite Plasmodium vivax.
The map, produced by a team at the University of Oxford, United Kingdom, shows that it is more widespread than its better known and more severe counterpart, P. falciparum, with up to 2.85 billion people at risk of infection, mostly in Central and South-East Asia.
The same team mapped the more widely studied P. falciparum malaria parasite last year. But this is the first time that such a map has been developed for P. vivax, said Carlos Guerra, lead author of the study, published last week (3 August) in PLOS Neglected Tropical Diseases.
"We hope that our estimates will bring more attention to P. vivax as a major public health problem," Guerra told SciDev.Net.
P. vivax causes a severe disease that is rarely fatal, but can be debilitating and lead to economic loses, and is responsible for the majority of malaria cases outside Africa.
Peter Zimmerman of the Center for Global Health and Diseases, in Cleveland, United States, said the study was important.
He said several features of P. vivax biology will likely make it "the most persistent troublemaker for current malaria elimination efforts", and added that "P. vivax is poised to become the face of malaria in the developing world."
The researchers collected data from the malaria control community and national control programmes in almost 100 countries but the information was not always up to date. "We had to resort to medical intelligence and informed assumptions to estimate risk in some countries," Guerra said.
Despite this, Ric Price of the Menzies School of Health Research, Darwin, Australia, who has also highlighted the 'neglected burden' of P. vivax, told SciDev.Net that "the authors present, for the first time, a reproducible methodology that quantifies areas of greatest risk ... and thus where malaria control activities should be prioritised".
But Richard Cibulskis, at the WHO's Global Malaria Programme, questioned whether the methods used were sound. He raised concerns about the size of the geographical units used in the study and the classification of some areas as 'at risk' even though they are free of malaria.
"Ultimately these factors lead to an overestimate of populations at risk," he said. "Moreover, because of the lack of resolution in many areas, the map would provide misleading guidance to those involved in control."
The next steps will be to improve the models and methodologies and to provide estimates of the social and economic burden of P. vivax malaria on countries, said Guerra.
The P. vivax and P. falciparum maps are part of the Malaria Atlas Project (MAP), based at six sites internationally, which aims to create a database of information on malaria transmission and parasite prevalence.
PLoS Neglected Tropical Diseases doi: 10.1371/journal.pntd.0000774 (2010)