As the planet warms, mosquitoes are slowly migrating upward.
The temperature range where malaria-carrying mosquitoes thrive is rising in elevation. Researchers have found evidence of the phenomenon from the tropical highlands of South America to the mountainous, populous regions of eastern Africa.
Scientists now worry people living in areas once inhospitable to the insects, including the slopes of Mount Kilimanjaro and the mountains of eastern Ethiopia, could be newly exposed to the disease.
"As it gets warmer at higher altitudes with climate change and all of these other environmental changes, then mosquitoes can survive higher up the mountain," said Manisha Kulkarni, a professor and researcher studying malaria in sub-Saharan Africa at the University of Ottawa.
Kulkarni led a study published in 2016 that found the habitat for malaria-carrying mosquitoes had expanded in the high-elevation Mount Kilimanjaro region by hundreds of square kilometers in just 10 years. Lower altitudes, in contrast, are becoming too hot for the bugs.
The African region Kulkarni studied, which is growing in population, is close to the border of Tanzania and Kenya. Together, the two countries accounted for 6% of global malaria deaths in 2021.
Deaths decline, still numerous
Global deaths from malaria declined by 29% from 2002 to 2021, as countries have taken more aggressive tactics in fighting the disease. However, the numbers remain high, especially in Africa where children under 5 years old account for 80% of all malaria deaths.
The latest world malaria report from the World Health Organization recorded 247 million cases of malaria in 2021 — Nigeria, the Democratic Republic of the Congo, Uganda and Mozambique alone accounted for almost half of those cases.
"The link between climate change and expansion or change in mosquito distributions is real," said Doug Norris, a specialist in mosquitoes at the Johns Hopkins Bloomberg School of Public Health, who was not involved in the research.
But mosquitoes are picky about their habitat, Norris added, and the various malaria-carrying species have different preferences in temperature, humidity and amount of rainfall. Combined with the use of bed nets, insecticides and other tools, it becomes hard to pin any single trend to climate change, he said.
Jeremy Herren, who studies malaria at the Nairobi-based International Centre of Insect Physiology and Ecology, said there is evidence that climate change is affecting where mosquito populations choose to live. But, he said, it is still difficult to predict how malaria will spread.
For example, in Kenya, Herren said researchers have documented "massive shifts" in malaria in mosquitoes. A species that was once dominant is now almost impossible to find, he said. But those changes are probably not due to climate change, he said, adding that the rollout of insecticide-treated nets is one explanation for that shift.
In general, however, mosquitoes grow faster in warmer conditions, Norris said.
Pamela Martinez, a researcher at the University of Illinois in Urbana-Champaign, said her team's findings on malaria trends in Ethiopia, which were published in 2021 in the journal Nature, lent more confidence to the idea that malaria and temperature — and, therefore, climate change — are linked.
"We see that when temperature goes down, the overall trend of cases also goes down, even in the absence of intervention," Martinez said. "That proves the case that temperature has an impact on transmission."