When associate professor of biology Erin Mordecai predicted—and then proved—that the ideal temperature for the mosquitoes that spread malaria was 6 degrees Celsius lower than what most scientists had assumed, it was a big deal, and not just for the insects.
Six degrees may not sound like a lot if you’re, say, trying to figure out a celebrity’s professional proximity to Kevin Bacon or what kind of shoes to pack for a warm-weather vacation. But for mosquitoes—which, as the world’s deadliest animal, kill up to 1 million people every year through the diseases they carry—temperature changes in that range can be, well, deadly.
Mordecai (Photo: Harrison Truong)
Mosquitoes are ectotherms, meaning the temperature of their body is the same as the temperature of their surroundings: If it gets too cold or too hot, they’ll die. With the planet warming at unprecedented rates, mosquitoes are on the move to more hospitable climes, and that has implications for those of us who share our backyards with the little buggers. Mordecai understood early on that knowing the temperatures at which these insects survive and thrive, and where they’re likely headed next, are key to tracking the movement and spread of infectious diseases. The deadly diseases malaria and dengue, for example, are transmitted by two different species of mosquito, and their ideal temperatures for transmission differ. As these mosquitoes migrate—enjoying new territories and longer seasons—many of us will confront diseases that had once been comfortably distant. Communities, in turn, will need a whole new set of knowledge to understand and combat the tiny threats in their environments. “There’s a big need for research that can predict when and where disease outbreaks will occur,” Mordecai says.
Mordecai was still a PhD student when she published that groundbreaking malaria research in 2012. Since joining the Stanford faculty in 2015, she has extended her temperature research to West Nile virus, Lyme disease, and eastern equine encephalitis—all of which occur regularly in the United States—as well as to dengue, an emerging virus in the continental United States for which global cases in 2024 were the highest on record. And recent work from the Mordecai Lab shows that mosquitoes may be able to adapt to survive in hotter weather. That means our predators are a moving target in more ways than one.
But mosquitoes aren’t the only ones who can attack from different angles. Mordecai takes a three-pronged approach: Understand the mosquito and its parasite. Connect that knowledge to disease transmission patterns. Predict how those will change going forward. From behind the microscope, she studies the insects’ lifespans and traits. Then she harnesses massive data sets to understand how climate change is affecting the spread of mosquito-borne infectious diseases. Finally, she uses mathematical modeling to predict future outbreaks. Her team is currently working on disease monitoring or mitigation with public health officials in California and in countries such as Brazil, Cambodia, Costa Rica, Guatemala, and Peru.
“The message you might take from reading the general or popular literature is that climate change is going to make everything worse,” says Kevin Lafferty, a U.S. Geological Survey ecologist who advised Mordecai at UCSB. “But that conclusion is not helpful to anybody. It doesn’t help you plan or focus your resources or tell you where more attention is needed. That’s where Erin’s work is impactful.”
‘Impossible to ignore’
There are almost always mosquito eggs in a refrigerator at the Mordecai Lab. They’re stored on pieces of paper in ziplock bags, and whenever someone needs to run an experiment, that person places the paper in some water with a barley mixture. Once the larvae hatch, they are reared to adulthood on a diet of fish flakes, liver powder, and guinea pig chow. Those mosquitoes then lay eggs that can be hatched or stored in the refrigerator, and the cycle begins anew. For one recent experiment, former lab member Lisa Couper, PhD ’23, cared for more than 10,000 eggs, which ultimately turned into 1,736 mosquitoes. “There was one heroic day where everyone in the lab spent the whole day with me counting larvae,” Couper says. “I was probably there for 14 hours.”
The larvae are cute, Mordecai says. “For most of their lifecycle, mosquitoes are completely innocuous.” Still, she likes to remind people that they are the most dangerous animal on the planet. “Even though it’s a misnomer—it’s not actually the mosquitoes but the pathogens they carry—it’s a nice way to get people thinking about the fact that mosquitoes matter.” (In truth, the lab mosquitoes don’t carry diseases dangerous to people, and even bites are rare: “We have ways of feeding them that don’t require human arms,” Mordecai says.)
25°C / 77°F
optimal temperature for malaria transmission via Anopheles gambiae
Mordecai did not start out studying mosquitoes. She explored math, biology, English, and Spanish at the University of Georgia, and became smitten with ecology during a multiweek field program. She earned her undergraduate degree in mathematical biology in 2007. At UCSB, she studied grasses and pathogens, focusing on a fungus called Pyrenophora semeniperda, or black fingers of death. “I found it interesting, but I wanted to start doing work that was more clearly relevant to people,” she says.
Lafferty, one of her advisers, had published a paper arguing that the impact of climate change on infectious disease wouldn’t be as simple as warmer temperatures increasing disease transmission across the board, as many scientists had assumed. Mordecai decided to investigate that assertion. She created and led a working group with experts in subjects ranging from geography to thermal physiology. “She wanted to apply mathematical models and empirical treatments to [my] ideas,” Lafferty says. “I was overjoyed.”
First she considered malaria, which infects 263 million people annually and causes roughly half a million deaths. Symptoms are flu-like, including high fever and chills. Up to that point, most malaria models had incorporated just a few temperature-dependent mosquito or parasite traits. Mordecai’s accounted for them all—not just the mosquito’s lifespan but also the number of eggs that produce adults, the mosquito’s development rate, the malaria parasite’s development rate, the mosquito’s bite rate, and so on. The resulting “thermal performance curve” showed that the optimal temperature for malaria transmission via the Anopheles gambiae mosquito was 25 degrees Celsius (77 F), not 31 C (88 F) as previously predicted. “That was the big surprise,” Lafferty says.
Then Mordecai went a step further, comparing her predictions with actual temperature and malaria transmission data from 14 countries in Africa over almost two decades. The comparison showed that her model was correct. “It’s one thing to say, ‘My model does something different than your model,’ because who knows which model is right,” Lafferty says. “It’s quite another thing to say, ‘My model fits the data and yours doesn’t.’ That’s, like, the ultimate thing—to have independent sources of information correspond with and support and validate each other. That’s what made her work impossible to ignore.”
Diving in on dengue
While her early work focused on Anopheles gambiae, commonly known as the African malaria mosquito, in recent collaborations she has trained her gaze on Aedes aegypti, the species once known for spreading yellow fever. Today, it is the major vector for dengue.
Dengue—also called break-bone fever—is a viral disease that affects up to 400 million people per year and can be life-threatening. Mild symptoms include fever or joint pain; one in 20 people experience severe disease, leading to shock, internal bleeding, or death. According to Mordecai’s team, global warming could cause dengue cases to double in areas that are currently cooler than the disease’s optimal temperature for transmission—areas that collectively are home to more than 260 million people. And that’s just in the 21 countries the team studied.
Mordecai and colleagues crunched climate data for 21 countries in Asia and Central and South America. They predict there will be between 68 million and 96 million new dengue infections annually in those places by mid-century.
Aedes aegypti thrives in hotter weather than Anopheles gambiae does; Mordecai has found that peak transmission occurs at 29 degrees Celsius (84 F). In Africa, warmer temperatures might lead to a shift in the location of malaria cases, as well as an overall decrease—and an increase in diseases such as dengue, according to a paper Mordecai published in 2020 with Desiree LaBeaud, a professor of pediatrics.
“The fact that the dengue burden could grow, grow, grow is really important,” LaBeaud says. “Africa is totally unprepared for that.” Currently, there are no antiviral medications approved to treat the disease. Effective pediatric vaccines exist, though one was discontinued by the manufacturer; the World Health Organization promotes the other, Qdenga, although it’s not approved for use in the United States. Taking extra precautions to avoid getting bitten during the day—when Aedes aegypti are most active—is a top public health recommendation. (Think long sleeves and bug spray.)
Global warming has already caused a spike in dengue transmission in Asia and in Central and South America, according to a recent collaboration between the labs of Mordecai and associate professor of environmental social sciences Marshall Burke, ’02, led by their co-advisee Marissa Childs, PhD ’22. Digging into temperature data and dengue case counts in the 21 countries studied, the group found that warming temperatures were responsible for an 18 percent increase in dengue cases on average from preindustrial times to the turn of the 20th century. The researchers predict that by mid-century, climate change will have caused dengue cases to increase between 49 percent and 76 percent on average over what they would have been without it. In certain areas, they expect cases to double. “There is going to be a changing demographic distribution of a lot of these infectious diseases—a lot of them are moving north, away from the equator, as temperature allows,” Burke says. In the Southern hemisphere, researchers expect a similar shift toward the pole.
Aedes aegypti is common throughout many parts of the United States and its territories, including Puerto Rico, American Samoa, and the U.S. Virgin Islands. And mosquito larvae can travel—on planes, trains, and automobiles. “These mosquitoes lay their eggs in containers,” Mordecai says. “It’s really easy for them to get transported around the world in tires or lucky bamboo plants—any little vessel with a tiny amount of water.” Once the mosquitoes are present, people take care of the rest: If someone gets dengue while traveling and then returns to the United States and is bitten by a mosquito of the same breed, that mosquito can spread the virus to another person and unleash further local transmission. While dengue hasn’t yet been a major problem in the mainland United States, “it’s probably coming for us,” Burke says. “Erin’s work is important in understanding that risk.” Locally transmitted dengue cases have been documented in Florida, Texas, Hawaii, Arizona, and California. (In April, the first Aedes aegypti of the year in Stanford’s home county of Santa Clara was found.)
29°C / 84°F
optimal temperature for dengue transmission via Aedes aegypti
Climate change doesn’t just involve temperatures but also precipitation. And because mosquitoes breed in water, precipitation is likely to have a huge impact on disease transmission, although researchers have found that difficult to document and predict because people’s water usage and storage habits vary widely across countries and cultures. An opportunity presented itself, however, in March 2023, when Cyclone Yaku hit northern Peru and was followed by the worst dengue outbreak in the country’s history. Cases exceeded the five-year average by a factor of 10, and nearly 400 deaths were reported by the end of July 2023.
Mordecai connected then-graduate student Mallory Harris, PhD ’24, with collaborators in Peru to tease out the relationship between the cyclone and the outbreak. In a forthcoming paper, co-authored with Mordecai, professor of Earth system science Noah Diffenbaugh, ’96, MS ’97, and others, Harris estimates that nearly 60 percent of the dengue cases in the areas affected by the cyclone were caused by the cyclone—particularly its vast increase in mosquito habitat due to standing water, its displacement of people from their homes, and its washing away of mosquito control efforts.
Mordecai is now pursuing research that looks at how tropical cyclones might be affecting dengue transmission globally. “A lot of health work in the vector-borne disease space focuses on future impacts,” Harris says. “But we are already experiencing climate change, and we wanted to know, can you detect the impacts of existing climate change on vector-borne disease? That’s pretty new.”
An evolving threat
Mordecai is always looking for novel and relevant questions to answer, but one query has continued to come up: To what degree can mosquitoes and the pathogens they transport adapt to higher temperatures? How else might they respond to global warming?
“We’ve never had a good answer for that,” she says.
Couper—the mosquito-counting hero who now studies emerging fungal diseases at UC Berkeley—decided to find out. Several years ago, she and Mordecai chose to study the western tree hole mosquito, which is harmless to humans (although an “aggressive biter,” according to Couper). The species is found across western North America and lays its eggs in the water-filled nooks and crannies of trees.
Finding them is “kind of fun,” Mordecai says (she and Couper once collected the eggs together at Stanford’s Jasper Ridge Biological Preserve), but the reasons for studying them are serious: They come from the same genus as Aedes aegypti, one of dengue’s vectors. If mosquitoes can adapt or evolve to survive in higher heat, one endemic disease may persist even as temperatures rise and another disease moves into the same area, meaning countries may have to balance multiple public health crises at once.
Raising western tree hole mosquito larvae at either a high temperature of 30 C (86 F) or a control temperature of 22 C (72 F), Couper, Mordecai, and their collaborators found “pervasive evidence” that the species has the potential to adapt: For instance, the larvae reared at the higher temperature grew to have smaller bodies as adults than the control group. But curiously, even though they survived “stressfully high” temperatures for nearly two weeks as larvae, they had lower tolerance for temperature spikes as adults. The team isn’t sure but suspects the mosquitoes may be making “trade-offs” to survive different things at different stages of their life. Nevertheless, judging by their genetic variation, responses to selection (traits furthered through mating preferences), and relatively short generation times, Couper says, “they have the potential to evolve heat tolerance on pace with climate warming.”
Good as gold
Giulio De Leo, a professor of oceans and of Earth system science, co-founded Stanford’s Disease Ecology in a Changing World initiative with Mordecai last year. One of the initiative’s cornerstone efforts, he says, is her project studying the risk factors for dengue transmission in rural areas in Costa Rica. He says it feels like Mordecai has a golden touch. “She is so talented and uses her time and energy and lab so efficiently that, when she decides to invest in something, something really good is going to come out of it,” he says. (Mordecai does have a connection to actual gold: A former postdoc from her lab is studying how illegal gold mining has led to an increase in malaria among the Yanomami people in Brazil.)
For Mordecai, data is the only currency that counts, and what it can buy is more awareness of what she considers the underappreciated effects of climate change. “Right now, people are just thinking about sea-level rise and storms, but infectious disease is a huge part of this,” she says. She feels fortunate to be working at the intersection of climate change and infectious disease, even if she thinks there’s a ways to go before work like hers is fully integrated into policy. “The kinds of topics I study are constantly in the news, whether it’s another record-breaking dengue outbreak linked to warming and extreme storms, or a resurgence of malaria in the jungle due to land use change,” she says. “When global policymakers are thinking about the costs and benefits of climate mitigation, this should count.”
Rebecca Beyer is a freelance writer in the Boston area. Email her at stanford.magazine@stanford.edu.
