Millions of people living at higher altitudes in the tropics will be at risk of malaria as a result of rising temperatures and climate change, according to an analysis of the disease over the past two decades in two mountainous regions in South America and Africa.
Scientists found that when temperatures increased, malaria affected people at higher altitudes than when temperatures were cooler. It is the first study on the ground to show how temperature changes can affect the risk of malaria in populations living 1,200 metres above sea level.
Although researchers have long debated whether malaria will move as a result of climate change, this is one of the first studies showing that the disease does migrate higher when temperatures rise, which threatens the many millions of people who live in historically malaria-free regions of the high-altitude tropics.
“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,” said Menno Bouma of the London School of Hygiene and Tropical Medicine, who was part of the research team.
“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,” Dr Bouma said.
“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,” he said.
The study, published in the journal Science, looked at malaria cases in the Antioquia region of western Colombia from 1990 to 2005 and the Debre Zeit area of central Ethiopia from 1993 to 2005. The researchers matched malaria outbreaks at various altitudes with temperature records to show how the disease rises and falls with warmer or cooler years.
Previous research in the Debre Zeit region of Ethiopia – where 37 million people live in rural areas at risk of higher malaria exposure under a warming climate – estimated that a 1C increase in average local temperatures will result in an extra three million children contracting malaria every year.
“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,” Dr Bouma said.
Mercedes Pascual of the University of Michigan, a senior author of the study, said: “We saw an upward expansion of malaria cases to higher altitudes in warmer years, which is a clear signal of a response by highland malaria to changes in climate. This is indisputable evidence of a climate effect. The main implication is that with warmer temperatures, we expect to see a higher number of people exposed to the risk of malaria in tropical highland areas like these.”
Malaria, which infects more than 300 million people each year, is caused by a single-celled microbe called Plasmodium that is transmitted by the bite of a mosquito. Both the Plasmodium and the malaria-carrying mosquitoes thrive in warmer temperatures and so with rising temperatures can survive at higher altitudes that have traditionally been malaria-free.
Dr Bouma said that many millions of people in the tropics live at higher altitudes because they have been historically at lower risk of malaria and other tropical diseases. For instance, 43 per cent of Ethiopia’s population live in the Debre Zeit region, at an elevation of between 1,600 metres and 2,410 metres.