HONG KONG (Reuters) - Nearly 3 billion people, or two-fifths of the world's population, were at risk of contracting malaria in 2009 and closer study of the mosquito's life cycle is needed to combat the disease, researchers said in two reports.
In the first study, scientists mapped out the geographical spread of the Plasmodium vivax -- the most common parasite that causes malaria -- using reported cases of malaria and details on temperature and aridity.
"We estimate that the global population at risk of P. vivax malaria in 2009 was 2.85 billion people. Regionally, the great majority of this population (91 percent) resides in central and southeast Asian countries," wrote Simon Hay, a zoologist at the University of Oxford who co-authored the study.
"P. vivax remains the most widely distributed human malaria parasite even after a century of development and control," he wrote, replying to questions from Reuters.
However, chances of infection by this parasite is low across Africa because of a genetic trait that protects mostly people of African origin.
But transmission of the parasite does occur in the continent and remains a concern for travelers and people who do not carry the trait, the researchers said.
The malaria atlas was published on Wednesday in the journal PLoS Neglected Tropical Diseases.
In 2008, there were 247 million cases of malaria worldwide and nearly one million deaths, mostly among children.
Knowing where the P. vivax thrives is critical so that plans can be made to control it, wrote Carlos Guerra, another author of the atlas and also from the University of Oxford.
Hay said the parasite, which is carried by the female Anopheles mosquito, is sensitive to environmental factors.
"Low temperatures delay the development of the parasite in the mosquito and if this time exceeds the life span of the vector (mosquito), then transmission is not possible," Hay said.
"Aridity acts mainly on the vector by increasing mortality through desiccation and also by limiting the availability of suitable breeding sites (i.e. collections of water)."
In the second paper, another team of researchers said vector control measures such as insecticide-treated nets and sprays have not been able to break the transmission cycle of the Plasmodium falciparum, another parasite that causes malaria in the most endemic parts of Africa and the Pacific.
It is regarded as a more dangerous cause of malaria as it has the highest rates of complications and death.
"Global commitment to malaria eradication necessitates a corresponding long-term commitment to vector ecology," wrote Gerry Killeen from the Ifakara Health Institute in Dar es Salaam, Tanzania, and colleagues in the journal PLoS Medicine.
"Priority areas will include understanding aspects of the mosquito life cycle beyond the blood feeding processes which directly mediate malaria transmission.