Mosquitoes that can transmit malaria have moved southward in Africa by more than 310 miles, on average, since the late 19th century, a rapid expansion of their range that is consistent with predictions of climate change altering where the disease-carrying insects can live.
Scientists expect that climate change will upend the range of many tropical disease-carrying insects as warming temperatures make more of the globe hospitable. But a study published Tuesday finds that forecast future is already happening for mosquitoes that carry malaria, a disease that killed nearly 600,000 people in Africa in 2021 alone.
Since 1898, African Anopheles mosquitoes have expanded southward by an average of nearly 3 miles a year, according to the study, published in Biology Letters. The mosquitoes also climbed higher, gaining over 2,200 feet in elevation, suggesting that rising temperatures may already be pushing these disease vectors to new places and doing so faster than some other land species.
“These guys are trucking across the continent,” said Colin Carlson, a global change biologist at Georgetown University’s Center for Global Health Science and Security who led the study. Generally, scientists are finding land species gain about 3 feet of elevation and roughly a mile of latitude a year, he said. “The mosquitoes that spread malaria are moving pretty significantly faster than that,” he said.
The pace of movement is consistent with climate change, he said, and underlines how a warming planet is already posing serious threats to human health.
Sadie Ryan, a medical geographer at the University of Florida, said that the results were in line with predictions of climate change’s effects on insects’ geographic ranges, although more data is needed to directly tie this movement to warming temperatures. Still, “the fact that this huge data set really shows such dramatic shifts does underscore what we suspect is the impact of climate on vector-borne disease transmission,” said Ryan, who wasn’t involved in the study. “The shifts are stark and, yes, dramatic, but not surprising given what we know.”
Shifting climate, shifting ranges?
Malaria is an enormous — and deadly — problem in Africa. Scientists have long expected that climate change could introduce the disease to new parts of the continent, which has warmed by 0.9 degrees Fahrenheit (0.5 degrees Celsius) or more over the last 50 to 100 years. But definitively tying this increase to more malaria transmission has proved challenging for scientists.
“It speaks to a broader thing in climate and health where, weirdly, we know a lot more about the future than the present day,” said Carlson. “It’s hard to get data to actually see these things as they happen.”
But a massive data set documenting the ranges of over 20 malaria-carrying Anopheles mosquitoes each year from 1898 to 2016 offered an opportunity. “It’s hard to understate how many millions of hours of entomologists’ time went into the data we’re working with,” said Carlson. But “it’s a real simple study. We look at where they are, and we see if it changes over time.”
That simplicity is powerful, said Carlson, and clearly shows mosquitoes showing up where they weren’t before. But the study can’t say whether these shifts led to more malaria cases or rule out other potential factors, including differences in how mosquitoes were sampled over time or changes in human activity that may have facilitated their spread. The study also couldn’t assess changes in the rate of range expansion. In Africa, “a lot of temperature changes aren’t really detectable until somewhere between the 1950s and 1980s,” said Carlson, meaning the mosquitoes may have moved much more recently.
“We still have to dig into these questions, but knowing that mosquitoes are showing up in new places gives us something actionable,” he added. “We need to try to eradicate mosquitoes before they gain a foothold.”
Improved monitoring and vector control in places on the front lines of this expansion could nip the disease in the bud.
Malaria control strategies, like insecticide-treated mosquito nets or treatment for breeding areas, can greatly reduce the diseases’ impact. “In the 1900s, something like 40 percent of children under 5 years old had malaria in any given survey,” said Carlson. “That number dropped to 24 percent between 2000 and 2015.” Applying these strategies in places where malaria isn’t yet a problem, but could be, is crucial, he said.
Malaria is not the only disease carried by mosquitoes, and climate change will likely continue introducing diseases to new areas on the backs of many insect vectors.
“There are many diseases besides malaria that will be impacted by the effects of climate change,” said Felipe Colón, technology lead at Wellcome, a charitable foundation that funds climate research.
“As the world continues to warm, it will become more vulnerable to dengue fever, Zika virus and cholera — climate-sensitive diseases that thrive in those conditions. Many regions are already being impacted and are consequently facing catastrophic health impacts,” he said. “It is imperative that there is greater focus on this issue to ensure that we can mitigate and adapt against these effects.”
Thanks to Lillian Barkley for copy editing this article.
