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Millions of people living at higher altitudes in the tropics are at risk of contracting malaria as global warming and higher temperatures have caused the disease to spread.
Researchers have found the mosquito-born disease, which affects 300 million people each year, moves to higher during warmer years and falls to lower altitudes when temperatures cool.
Scientists analysed two mountainous regions in Africa and South America over two decades, which showed the risk of malaria rose in populations living above 1,200 metres above sea level.
Menno Bouma, who was part of the research team from the London School of Hygiene and Tropical Medicine, told the Independent: "We have identified that malaria does indeed move up and down and that the movement is temperature dependent. It's been difficult to prove and people have been questioning it now for 20 years."
Teams from London and the University of Michigan searched through regional records in Ethiopia and Colombia, from 1990 to 2005 and 1993 to 2005.
It is estimated the disease kills 600,000 people annually in warmer tropical areas dubbed the "malaria-belt".
Dr Bouma said: "The implications are that if this is true, and that a global warming is occurring leading to an increase in temperatures, then malaria will increase at higher altitudes where many people live. The high altitude areas in the tropics are particularly highly populated.
"They are more populated than the lowlands partly because there is more rainfall but also partly because there has been less malaria. They have been privileged places to live because of that, and many of these people have not been exposed to malaria and so will not be naturally protected against it."
According to Bouma, millions of people in the tropics live at higher altitudes because they are historically at lower risk of malaria and other tropical diseases.
Around 43% of Ethiopia's population live in the Debre Zeit region, between 1,600 metres and 2,410 metres.
Mercedes Pascual, a disease ecologist at the University of Michigan and a co-author on the study, said: "This is indisputable evidence of a climate effect. Our findings here underscore the size of the problem and emphasize the need for sustained intervention efforts in these regions, especially in Africa."
Speaking to Newsweek, Pascual said: "We showed that highland malaria in both Ethiopia and Colombia basically moves up in altitude in warmer years. The importance of this is that climate change will, without mitigation, result in an increase of malaria burden in these densely populated highlands of Africa and South America."
Pascual told AFP: "Our findings here underscore the size of the problem and emphasize the need for sustained intervention efforts in these regions, especially in Africa."
The study was published in the journal Science. Previous research in the Debre Zeit region of Ethiopia, where the recent research was conducted, estimated that an extra three million children would contract malaria each year with a 1C increase in average local temperatures.
The disease is caused by a microbe called Plasmodium that is transmitted by the bite of a mosquito. The Plasmodium and the mosquitoes carrying the disease thrive in warmer temperatures and can survive at increasingly warmer higher altitudes that have traditionally been malaria-free. The disease can be prevented by using nets, insecticides and medicines.
Dr Bouma added: "Traditionally, we think of malaria as a disease with limited prevalence in highland regions, but we are now seeing a shift due to climate change. Our latest research suggests that with progressive global warming, malaria will creep up the mountains and spread to new, high-altitude areas. And because these populations lack protective immunity, they will be particularly vulnerable for severe morbidity and mortality."