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In July 1957, a newly created federal agency launched an aggressive and advanced program to eradicate a parasitic disease that had vexed state and local health officials across the United States for decades: malaria.

Within five years, the federal government had proudly announced that malaria had been eradicated in the U.S. And the fledgling agency it had created, now known as the Centers for Disease Control and Prevention, would end up outlasting the disease it had been founded to defeat.

But in June, the CDC announced that locally acquired malaria has been detected in the U.S. for the first time in 20 years. While experts say there is no cause for alarm—just eight cases combined were reported in Florida and Texas—some experts warn that this may be a canary-in-the-coal mine moment signaling that climate change could be altering the way malaria spreads across the planet.

In recent decades, most malaria cases reported in the United States involved people who had traveled from parts of the world where such transmission is relatively common. Sub-Saharan Africa and parts of Oceania such as Papua New Guinea have some of the highest transmission rates, according to the CDC.

Experts say that more detailed studies are needed to predict how the spread of the disease might play out across the nation, particularly in connection with climate change. But they note that the combination of warmer temperatures, changing rainfall levels and other factors could easily create the conditions needed for disease-bearing insects to thrive. Humans generally contract malaria when bitten by a female mosquito infected with a parasite.

Shauna H. Gunaratne, an infectious disease specialist, said the myriad effects of climate change would undoubtedly affect mosquito populations. First, because of the planet’s warming temperatures, mosquito larvae tend to mature faster, said Gunaratne, an assistant professor of medicine in the Division of Infectious Diseases at Columbia University Medical Center.

“That means that there’s more mosquitoes—an increased mosquito population with warming temperatures,” she said. “That means there’s more chances to infect humans and then also become infected.”

Gunaratne said that higher temperatures shorten the incubation period for some mosquito-borne infections, including malaria, dengue and chikungunya, meaning that mosquitoes become infected more rapidly than was typical in the past. Warmer temperatures can also prolong the disease season, she added. 

And climate change can increase precipitation, said Gunaratne, who directs Columbia’s Infectious Diseases Fellowship Program. “That increases vegetation, and that sort of can increase the chances for breeding sites for mosquitoes.” 

Malaria differs from illnesses like COVID in that it is not spread directly from human to human. Malaria is transmitted when a mosquito bites an infected person, absorbs microscopic parasites and then bites someone else. 

Globally, a Quarter-Billion Cases

The symptoms of the disease include fever, flu-like illness, nausea, vomiting and diarrhea, and, if not properly treated, it can be fatal. According to the World Health Organization, there were 241 million cases of malaria globally in 2020, resulting in 627,000 deaths.

Immunologists are researching the potential effects of an increase in mosquito numbers on future transmission patterns.

“Conditions that increase mosquito populations have an impact,” said Dyann F. Wirth, a professor of infectious diseases at Harvard’s T.H. Chan School of Public Health.

Based on projections of how climate change will affect temperatures, rainfall levels, and other environmental factors, Wirth said there was a possibility of transmission at higher elevations and in more widely ranging geographic areas.

“Remember, mosquitoes breed in freshwater, right?” she said. “And so the amount of fresh standing water that’s available affects the mosquito population and the numbers.”

She cautioned that the altered rainfall patterns associated with climate change could yield widely different results, depending on location. 

“You can have situations of long drought, and there you might predict that actually the number of mosquitoes would go down and malaria transmission would go down,” Wirth said. “So I think it’s not a unique directional change. It’s not necessarily going to get worse, but there is going to be a dynamic shift as the ecology where the mosquitoes breed changes.”

Gunaratne said the heat factor could be similarly tricky. 

“We expect that as the world gets warmer, we will likely see increased geographic range of many of the vectors for malaria,” as well as for mosquitoes and for other insects such as ticks, she said. “They might be able to exist at certain altitudes in latitude where they previously did not.”

But above a certain temperature, some of those effects can be detrimental to mosquitoes, Gunaratne pointed out. “So you might see in some areas where the temperature rises and it becomes drier, that might be less favorable for those mosquitoes,” she said. “You might see malaria go away in certain parts of the world, but then emerge in other parts of the world where it previously was not found and transmitted.”

Experts say the malaria parasite can go to sleep in the liver and come out years later. 

Rob McCann, a medical entomologist and assistant professor at the University of Maryland School of Medicine, said he felt it was unlikely that the cases identified in Florida and Texas were directly related to climate change. “But whether or not climate change might somehow change our overall risk of transmission is slightly harder to say because we don’t have really specific studies that have looked at that,” he said.

With global climate change driving up temperatures in the southeastern U.S., “we might expect that window of opportunity for transmission to expand a little bit,” he said. McCann also sees a possibility that mosquito populations could expand in Northern states, even reaching some areas where they do not exist now.

The eight people diagnosed in Florida and Texas have all recovered and received anti-malaria treatment to prevent future relapses. Officials said there is no evidence to suggest that any of the cases were related to one another.

Researchers and health officials stressed the importance of preventing the spread. Wirth said that with most viruses, one case can lead to one or two subsequent cases, but with malaria, “one person can lead to a hundred infections” as mosquitoes pick up the parasite in a blood meal and make their way to new humans. 

Gunaratne said that Americans traveling outside the United States should be particularly vigilant. People visiting places where transmission rates are high can take a prescription medication for malaria, sleep under mosquito nets and use insect repellent.

“I think with our increasingly globalized world, we’re seeing obviously infections can spread very rapidly, and we can all sort of do our part and be responsible and talk to our health care providers to make sure that we protect ourselves and also protect other people,” she said.

“Climate change is unfortunately happening,” with attendant risks, Gunaratne added. “But I think the most important thing about malaria is just being aware of it.”