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Story Publication logo September 26, 2023

Feeling the Heat: Malaria Cases Could Ebb in an Even Hotter World. But Other Diseases Will Get Worse

A castor bean tick (ixodes ricinus) on grass, possibly carrying disease

For years researchers have predicted that human health will suffer as the climate warms...

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In early July, on one of the hottest days ever recorded globally, Colin Carlson walked into a radio studio in Washington, D.C., to answer some questions about how a warming climate is affecting infectious diseases. Cases of locally transmitted malaria in Florida and Texas—not seen in decades—had made the national news. The radio host, Jenn White, asked Carlson whether these cases were linked to climate change.

Carlson, a disease ecologist just shy of his 27th birthday and already on the faculty at Georgetown University, was wearing flip-flops, shorts, and a heavy metal style T-shirt demanding “Medicare for all.” On his left hand he had scrawled a reminder in large letters: “Don’t swear.”

The day before, in his office at Georgetown, Carlson’s alarm about the planet’s trajectory had boiled over. He recalled a paper he had read years ago: a modeling study suggesting that if the carbon dioxide levels in the atmosphere rose above 1200 parts per million, Earth’s clouds could start to disappear. “It is not small potatoes, it is not this small, incremental change in the way that the world works,” he said, stabbing the tip of his pencil into his notebook. “It’s what if our planet didn’t have f------ clouds?”

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Infectious diseases, too, could undergo a dramatic shift as the globe warms, Carlson says. The list of plausible catastrophes is long. Mosquitoes and ticks carrying pathogens could multiply faster and spread farther, endangering ever more people. More frequent droughts and floods, a consequence of warming, could expose more people to deadly waterborne microbes such as the cholera-causing bacterium Vibrio cholerae. Migrating animals could mingle with species they have not encountered before, allowing pathogens to spread to new animal hosts and eventually humans. Respiratory infections could change their patterns, and fungi usually warded off by body heat could adapt to the world’s higher temperatures and become more adept at infecting humans.

But in the studio, Carlson explained calmly that the malaria cases in Florida and Texas probably could not be blamed on climate change. If cases of dengue had appeared in Washington, D.C., it would be much easier to make the link, he told White, because that disease is spread by different mosquitoes: “If we see something like that, it’s very easy for us to say, ‘OK, climate brought the mosquitoes, the mosquitoes brought dengue.’” But mosquitoes able to carry malaria have always lived in southern states. A traveler returning from abroad with the parasite in their blood is all it takes to seed local spread. “I think this is one of those weird edge cases,” Carlson says. “This is just something that happens, but it looks a lot like the things that climate change will probably bring.”

It is a position researchers like Carlson constantly find themselves in: caught between the nuance of now and the threat of tomorrow.

Defining that threat is not simple. The challenge is to do for infectious diseases what other researchers can now do for droughts or hurricanes: “attribute” events to climate change. “The very cutting edge right now is actually trying to start doing attribution … to say, for each degree of temperature change, how many more cases do we get?” says Erin Mordecai, an infectious disease ecologist at Stanford University.

Carlson and Mordecai are just two researchers working on this, but the complexity is staggering. Climate change influences temperature, humidity, and rainfall patterns as well as animal and human behavior. Each factor could affect a disease differently—and that effect could vary depending on whether the disease is caused by a virus, bacterium, parasite, or fungus and whether it is spread by mosquitoes, ticks, or another vector; through water; or through the air. One question is what will happen to a disease already prevalent in a region as its climate shifts. A different one is whether diseases will spread into new regions as the climate changes. And those are just the diseases we know about. Will new diseases be more likely to emerge?

"[Climate change] is not small potatoes, it is not this small, incremental change in the way that the world works."

— Colin Carlson, Georgetown University

The answers are important if the world is to prepare for the public health challenges that lie ahead. But linking climate and disease is a scientific problem wrapped in a communications challenge.

Researchers have made the most progress on the disease Carlson was talking about that morning on the radio: malaria, which kills more than half a million people every year. He believes he has made the best case yet that climate changes have increased malaria in some parts of Africa. But the broader story emerging from the work of Carlson, Mordecai, and others isn’t straightforward.

MORDECAI WAS STUDYING grasses when her interest in malaria began. As a graduate student at the University of California, Santa Barbara, passionate about both math and biology, she focused on how plant pathogens spill over from invasive grass species to native ones. Then a paper by her adviser, ecologist Kevin Lafferty, raised an issue that seemed more urgent. He argued that the relationship between human diseases and climate would not be linear. Mordecai was intrigued. “And frankly, I got kind of sick of trying to explain to people why we should care about the coexistence of grass species, and I wanted to do something that was a little more directly relevant to people.”

Mordecai set out to dissect how temperature affects both mosquitoes and the pathogens they carry. Mosquitoes are cold-blooded animals, and so temperature influences almost everything about their lives and potential to spread disease: how long they live, how many eggs a female lays per day, and how likely it is to bite. It also influences mosquito-borne pathogens, determining for instance how likely they are to establish themselves in the insect after it consumes blood from an infected person or animal and how long that takes.

In the lab researchers can study how temperature affects each trait. The relationship generally seems to be hump-shaped: an optimum at a certain temperature and an exponential decline as the temperature increases or decreases from that optimum. The same goes for humans, says Courtney Murdock, an infectious disease ecologist at Cornell University. “There are some temperatures that you experience that are too cold, and some temperatures that are too bloody hot. And there’s the temperature that you’d like to hang out at, usually around 75°[F],” she says. “It’s kind of the same thing for mosquitoes and for the parasites that they transmit.”

Researchers like Mordecai mathematically merge all these measured temperature-dependent traits for a mosquito and a certain pathogen it carries into a single curve that shows how that disease’s transmission changes with temperature (see graphic, below). That curve, too, is hump-shaped—which has some important implications.

Some like it hotter

How fast mosquito-borne diseases spread depends in part on temperature, peaking at an optimum warmth and declining above and below it. Different combinations of diseases and vectors have different peaks. Carried by its usual mosquito vector, the most dangerous form of malaria peaks at 25°C, cool enough that climate change could decrease transmission in some regions. But dengue and Zika could burgeon in a warmer world.

Building a transmission curve

Scientists start by measuring the effect of temperature on many different traits of a specific pathogen and a mosquito species that carries it: how long the mosquito lives, for instance, and how fast the pathogen replicates in the mosquito. The result is a series of curves, which are mathematically combined into a single curve relating temperature to disease transmission.

Graphic courtesy of M. HERSHER/SCIENCE; (DATA) E. MORDECAI ET AL., ECOLOGY LETTERS, 22, (2019).

“The baseline assumption is that warmer climate will make vector-borne disease worse because people observe that most [such] diseases occur in the tropics, and in more temperate places they occur in the summer,” Mordecai says. But studies like hers show, she says, that “the effect of rising temperatures can go either way.”

For many years the temperature optimum for malaria was pegged at about 31°C. But in a 2012 paper in Ecology Letters, Mordecai and others showed that was wrong. They concluded that the temperature optimum was dramatically lower: just 25°C. Above 28°C, transmission decreases rapidly. “If anything, climate warming is actually decreasing malaria transmission in a lot of sub-Saharan Africa,” Mordecai says. In the highlands of South America and southern Africa, on the other hand, temperatures increasingly favor malaria transmission. And in once-malarial regions of Europe and North America that have controlled the disease, keeping it in check may become more challenging.

But zooming in on the relationship between a mosquito, a disease, and temperature leaves out many other climate-related factors. Rainfall patterns are changing, extreme weather events like droughts or floods are more frequent, and people are migrating in response to these global changes. All will put a stamp on disease patterns.

THAT'S WHY SOME scientists trying to tease out climate-disease links are looking to the past, combing historical data for signs that climate change has already affected disease incidence in the real world. One set of data on malaria, from the Kericho tea estates in the eastern highlands of Kenya, has sparked an academic fight that has been raging for decades.

The company running the estates, Brooke Bond Kenya Ltd., provided health care for all employees and their families and kept meticulous records. In the late 1990s, a scientist at the U.S. Army Medical Research Unit in Nairobi named Dennis Shanks “climbed up into a loft in the Kericho tea estate and managed to identify some boxes of malaria admissions and cases going all the way back to 1965,” says Robert Snow, a malaria researcher at the Kenya Medical Research Institute.

Shanks, Snow, and others digitized and analyzed the records, reporting in 2000 that they revealed a dramatic increase in malaria during the 1990s. But the mean monthly temperature in the region had not changed significantly in this time, the researchers noted. “It seems plausible to assume that factors other than climate change would have led to the precipitous rise in malaria warranting inpatient care during the 1990s at the Kericho tea estates,” they wrote, pointing to lapsed mosquito control programs and an “epidemic of drug-resistant malaria” sweeping the region.

The paper kicked off a heated back and forth. In 2006, Mercedes Pascual, a computational ecologist at New York University, published a new look at the estates’ temperature data, showing there was indeed a warming trend there if a longer time period was considered. The dispute was important, she says, because “it was clear that one of the places where we should see the strongest impact of climate change [on malaria] were the highlands.”

The medical data, one of the few long-term records of malaria, were reanalyzed again and again. “Everyone and their tennis partner have sort of downloaded it and tried to match it to climate data,” says Snow, one of the authors of the original paper. In a 2002 paper reiterating that climate had played no role in the resurgence of malaria, Snow and others wrote that they hoped their work would “help focus attention on the real and immediate causes of these malaria resurgences.”

He and other malaria researchers worry that the growing focus on climate change is a distraction from more pressing questions about how to deliver antimalarial measures to those who most need them. “I don’t for one minute doubt the significance to the planet of global warming,” Snow says. “But in the malaria space, I do feel that it gets more attention than it deserves.”

Pascual disagrees. “The research efforts, the funding, everything related to malaria has been much, much bigger for public health in general than it is for climate change. It is only recently that some of this research on climate and disease is attracting more real attention.”

CARLSON'S RECENT WORK is drawing some of that attention, because he claims to have pinned down the influence of climate on malaria. A child prodigy who started at the University of Connecticut at 12 years old, he originally studied how likely parasites were to go extinct because of climate change, then shifted to how climate change would affect the diseases that parasites cause. “Parasites led me to disease and disease led me here,” Carlson says. There is a personal element in his interest in climate change. Given his age, Carlson says, “I am going to live through more of climate change than most of my colleagues.”

He decided to investigate the role of climate in malaria with methods from a field known as climate econometrics, which has been used, for instance, to estimate how human-caused climate change has affected food production and conflict. The method is essentially an elaborate game of scientific “what if.” Using a new and larger data set covering 115 years of malaria in sub-Saharan Africa that Snow had published in 2017, Carlson and his colleagues developed 1000 plausible models of how temperature shifts, floods, and droughts could have contributed to the changes in the disease’s incidence. With them, the team then simulated thousands of alternative worlds with no climate change, allowing comparisons of malaria incidence in those “what if” worlds with the real world. That let them estimate the odds that climate change has worsened the disease.

In a preprint earlier this year, Carlson’s team reported a 66% likelihood that humanmade climate change has already increased malaria in sub-Saharan Africa as a whole. They estimate that for every 100,000 children there between ages 2 and 10, 84 cases of malaria can be attributed to climate change at any time. That may not sound like a lot, but with about 300 million children in that age range in sub-Saharan Africa, “there are probably right now more than 200,000 children who have malaria, who would not have it without climate change,” Carlson says. “That’s a huge impact, not just in terms of total mortality, but also cumulative impact of that burden of disease.” That impact is highest in eastern and southern Africa, whereas climate change has probably reduced malaria in west and central Africa over time, he and his colleagues concluded.

“Our study resolves a decades-old debate about one of the earliest health impacts of global warming,” Carlson writes in the preprint’s abstract. But the study has not been peer reviewed yet. And there are important caveats. One is that even in the regions with the highest impact of climate change, the malaria increase pales in comparison with the reduction that public health measures such as bed nets, vector control, and treatments have achieved. “We can hold in our head both thoughts,” Carlson says. “Climate change matters. But public health interventions have been 200 times more important.”

Carlson’s study also offers hope, suggesting that as the climate keeps warming, the trend will reverse and actually lead to a net reduction in malaria across the continent by the middle of the century even if global warming is curbed at 2°C.

THAT MAY SOUND reassuring, but the outcome may be very different for other mosquito-borne diseases, Mordecai’s transmission curves show. Some have much higher temperature optimums than the most deadly form of malaria, which is caused by the Plasmodium falciparum parasite carried by the mosquito Anopheles gambiae. “Everything to the right of falciparum malaria [on the temperature diagram]—Rift Valley fever, Ross River virus, dengue, Zika—that is going to have experienced and will experience much greater increase due to climate change,” Carlson says.

Dengue is a particular worry. The virus already causes hundreds of millions of infections a year and an estimated 20,000 deaths, and the threat is only likely to increase as the world warms. “Whereas climate change is driving malaria in different directions in different places, it’s kind of driving dengue in the same direction in every place,” Mordecai says. On top of that, urbanization and a lapse in mosquito control have led to a resurgence of dengue-carrying Aedes aegypti in many places. “Dengue is the one that’s really predicted to get worse and is getting worse.”

Even for malaria the respites may be transient. The mosquito and parasite could adapt to higher temperatures. And another mosquito carrying the same parasite, Anopheles stephensi, could become more important, Murdock says. “Stephensi, which is the urban malaria that’s now expanding throughout Africa, actually tolerates much higher temperatures,” she says.

The complexities are no less daunting for diseases that don’t depend on insects to spread (see sidebar "Disease Forecasting," below). Influenza, for instance, follows a seasonal pattern, surging in winter—and not just because people collect indoors, but also because cold, dry air favors its transmission. Changes in air temperature and humidity will surely affect the spread of the virus, and perhaps even its evolution, but just how isn’t clear. “It’s an unbelievably tortured tangled thing,” says Jessica Metcalf, a disease ecologist at Princeton University.

COMMUNICATING ALL THIS complexity and uncertainty is challenging. Broad and easily understood statements such as “mosquito-borne pathogens will thrive in a warming world” can be seductive but misleading, Carlson acknowledges. Yet conveying the nuances—that climate change may end up causing more malaria in some places and less in others—risks blurring the urgent message that climate change is a threat to public health. “I don’t think the science is getting walked back,” Carlson says. “I think that there is this impossibly high rhetorical bar that we’ve created for what the science is supposed to look like for it to be a dramatic health impact.”

Just a few weeks after Carlson’s radio interview, the next communication challenge arose: Another locally transmitted malaria case was reported in the United States, this time in Maryland, which had not seen such a case in 40 years. “I mean that is just really weird,” Murdock says. Still, climate change probably isn’t behind this malarial threat.

At least not yet.

Disease forecasting

Will flu outbreaks ease in a warming world?

From cold viruses to influenza to respiratory syncytial virus, viruses that spread through the air cause billions of infections each year. That makes it important to understand how they will respond to climate change. But little is known so far, except that different viruses will react differently. Measles, for instance, spreads efficiently in all climates, suggesting global warming will make little difference to its transmission.

For influenza, animal studies point to a different outcome. “We can place guinea pigs in environmental chambers and unpick the dependencies,” says Jessica Metcalf, a disease ecologist at Princeton University. Humidity has emerged as a crucial factor: The virus transmits best in cold and dry environments, which may help explain winter flu outbreaks. As the climate warms, allowing the air to take up more water vapor, seasonal influenza epidemics are likely to become milder in most places.

But there is a catch. Today, transmission of influenza often falls to near zero between epidemics. As epidemics become less severe in a warming climate, the virus is more likely to circulate year-round in some places. That could have an effect on its evolution, says Brown University epidemiologist Rachel Baker. Without the periodic pauses, flu evolution may speed up, meaning for instance that vaccines might need to be updated more frequently.

As for the respiratory virus that has turned the world upside down for more than 3 years, SARS-CoV-2, so far scientists have no clue how it will respond.

A time of cholera

When the World Health Organization called attention in 2022 to a surge in cholera outbreaks around the globe, it listed the usual factors that favor waterborne diseases: lack of sanitation, humanitarian crises, and conflict. But it also said climate change was worsening the situation.

Cholera is caused by Vibrio cholerae, a bacterium that can produce a toxin and spreads when the stool of infected people contaminates drinking water and food. Microbiologist Rita Colwell has long argued that warmer surface waters can favor the emergence of the bacterium. Whether this plays any role in the large outbreaks around the globe is contested. But another climate link is clear. Floods can aid spread by causing latrines to overflow into water sources, for instance, and droughts can boost the concentration of the bacterium in shrinking ponds and streams and force people to use unsafe water.

Climate change is making such extreme weather events more frequent, so cholera is likely to surge in a warming world, says Andrew Azman, an epidemiologist at Johns Hopkins University. But there is little consensus on the magnitude of the effect. “It is really, really hard to attribute any of the current cholera cases to climate change,” Azman says—in large part because there are few good long-term data on cholera. The same caveat applies to other waterborne diarrheal diseases, which some researchers suspect will also increase in a warmer world.

Will fungal diseases come in from the cold?

It came as a nasty surprise when Candida auris suddenly started to cause deadly infections in 2011. The yeast was resistant to major classes of drugs and killed more than one-third of known patients who developed invasive infections. Even more surprising, C. auris infections had apparently emerged simultaneously around the world. “There was just no good explanation,” says Arturo Casadevall, a microbiologist at the Johns Hopkins Bloomberg School of Public Health. “Why should a fungus that is not known to medicine all of a sudden appear in three continents at the same time?”

Casadevall had a hypothesis ready to go: climate change. In a 2010 paper in mBio he and Mónica García-Solache, then at Yeshiva University, laid out a simple argument: Humans are largely protected from fungal infections by their high body temperature, which slows fungal growth. But as the climate heats up, this “thermal barrier” could erode as fungi in the environment adapt to higher temperatures. That happened with C. auris, Casadevall now says—and the same thing might be expected of other fungal diseases.

For now, he says, it is just a hypothesis. But in 2021 C. auris was found in the environment, on an island in the Indian Ocean. The discovery suggested the infectious strain could have originated in the soil, not an animal host. These environmental samples grew poorly at body temperature, suggesting it took an evolutionary leap—perhaps propelled by climate change—for the fungus to turn deadly.


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