MARY KATHERINE WILDMAN (Associated Press)
As the planet warms, mosquitoes slowly migrate upwards.
The temperature range in which malaria mosquitoes thrive is increasing. Researchers have found evidence of this phenomenon from the tropical mountains of South America to the mountainous densely populated areas of East Africa.
Now scientists fear that people living in areas once hostile to insects, including on the slopes of Mount Kilimanjaro and in the mountains of eastern Ethiopia, may be re-infected with the disease.
“Because it gets warmer at higher altitudes due to climate change and all these other environmental changes, mosquitoes can survive higher in the mountains,” said Manisha Kulkarni, a professor and researcher who studies malaria in sub-Saharan Africa at the University of Ottawa.
EDITOR’S NOTE: This story is part of a collaboration between The Associated Press and Grist exploring the intersection of climate change and infectious disease.
Kulkarni led a study published in 2016 that found that malaria mosquito habitat in the high mountain region of Mount Kilimanjaro expanded hundreds of square kilometers in just 10 years. On the contrary, it becomes too hot for the beetles at lower altitudes.
Similar phenomena have been found in other places. For example, researchers in 2015 also noticed that native Hawaiian birds were being pushed out of lower habitats as mosquitoes carrying avian malaria slowly migrated up into their territory. But given that 96% of malaria deaths in 2021 occurred in Africa, most of the research on this trend is being done there.
The growing population region of Kulkarni is close to the border of Tanzania and Kenya. Together, these two countries accounted for 6% of global malaria deaths in 2021.
Global malaria deaths fell by 29% between 2002 and 2021 as countries became more aggressive in their fight against the disease. However, numbers remain high, especially in Africa, where children under 5 years of age account for 80% of all malaria deaths. The latest WHO world malaria report recorded 247 million cases of malaria in 2021 – Nigeria, the Democratic Republic of the Congo, Uganda and Mozambique alone account for almost half of these cases.
“The link between climate change and the spread or change in mosquito distribution is real,” said Doug Norris, a mosquito specialist at the Johns Hopkins Bloomberg School of Public Health, who was not involved in the study.
Despite this, uncertainty remains about how changing mosquito populations will affect humans in the future. A recent Georgetown University study examining the movement of mosquitoes across sub-Saharan Africa also found that vectors move up in altitude at a rate of 6.5 meters (approximately 21 feet) each year.
Mosquitoes are picky about their habitat, Norris added, and different malaria-carrying species have different preferences for temperature, humidity and rainfall. Add to that the fact that people are fighting malaria with bed nets, insecticides and other tools, and it becomes difficult to link any one trend to climate change, he said.
Jeremy Herren, who studies malaria at the International Center for Insect Physiology and Ecology in Nairobi, said there is evidence that climate change is already affecting where mosquito populations live. But, he said, it is still difficult to predict how malaria will spread.
In Kenya, for example, Herren says researchers have seen “massive shifts” in the incidence of malaria in mosquitoes. The once dominant species is now almost impossible to find, he says. But these changes are likely unrelated to climate change, he said, adding that the introduction of insecticide-treated nets is one explanation for the change.
In general, however, mosquitoes grow faster in warmer conditions, Norris said.
Rising temperatures are also not the only way climate change is giving mosquitoes an advantage. Insects tend to thrive in extreme environments, which are more likely to occur due to anthropogenic climate change.
Longer rainy seasons can create a better habitat for mosquitoes that breed in the water. But conversely, while droughts can dry out these habitats, they also encourage people to store water in containers, creating ideal breeding grounds. For these reasons, an outbreak of chikungunya, another mosquito-borne disease, between 2004 and 2005 was associated with drought in coastal areas of Kenya.
The researchers were also able to link the decline in malaria cases in the highlands of Ethiopia in the early 2000s to a drop in temperatures occurring at the same time.
Weather conditions in previous years have stopped the effects of global warming.
Pamela Martinez, a researcher at the University of Illinois at Urbana-Champaign, said her group’s findings on malaria trends in Ethiopia, published in 2021 in the journal Nature, gave more credibility to the idea that malaria and temperature — and thus climate change — are linked.
“We see that when the temperature drops, the overall trend of the disease also decreases, even in the absence of intervention,” Martinez said. “This proves that temperature affects transmission.”
The researchers also noticed that mosquito populations creep up during warmer years.
Temperatures began to rise again in the mid-2000s, but around the same time, public health officials also stepped up efforts to control malaria in the highlands of Ethiopia, resulting in a steady decline in cases.
But even as the Ethiopian Ministry of Health came up with a plan to eliminate malaria by 2030, its authors outlined the threats to that goal: population migration, lack of funding, a new species of mosquito and climate change.
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