Why Climate Change May Bring More West Nile Virus To The U.S.

11:10 minutes

a fuzzy illustration of a large mosquito on top of north america on a globe
Credit: Lydia Zuraw/KHN

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This story is a collaboration between Science Friday and KHN, written by Melissa Bailey.

Michael Keasling of Lakewood, Colorado, was an electrician who loved big trucks, fast cars, and Harley-Davidsons. He’d struggled with diabetes since he was a teenager, needing a kidney transplant from his sister to stay alive. He was already quite sick in August when he contracted West Nile virus after being bitten by an infected mosquito.

Keasling spent three months in hospitals and rehab, then died on Nov. 11 at age 57 from complications of West Nile virus and diabetes, according to his mother, Karen Freeman. She said she misses him terribly.

“I don’t think I can bear this,” Freeman said shortly after he died.

Spring rain, summer drought, and heat created ideal conditions for mosquitoes to spread the West Nile virus through Colorado last year, experts said. West Nile killed 11 people and caused 101 cases of neuroinvasive infections—those linked to serious illnesses such as meningitis or encephalitis—in Colorado in 2021, the highest numbers in 18 years.

The rise in cases may be a sign of what’s to come: As climate change brings more drought and pushes temperatures toward what is termed the “Goldilocks zone” for mosquitoes—not too hot, not too cold—scientists expect West Nile transmission to increase across the country.

a mosquito on a white human arm
Credit: A. Marm Kilpatrick/ Dept. of Ecology; Evolutionary Biology, University of California-Santa Cruz

“West Nile virus is a really important case study” of the connection between climate and health, said Dr. Gaurab Basu, a primary care physician and health equity fellow at the Center for Climate, Health, and the Global Environment at Harvard’s public health school.

Although most West Nile infections are mild, the virus is neuroinvasive in about 1 in 150 cases, causing serious illness that can lead to swelling in the brain or spinal cord, paralysis, or death, according to the Centers for Disease Control and Prevention. People older than 50 and transplant patients like Keasling are at higher risk of severe outcomes.

Over the past decade, the U.S. has seen an average of about 1,300 neuroinvasive West Nile cases each year. Basu saw his first such patient in Massachusetts several years ago, when a 71-year-old patient came in with swelling in his brain and severe cognitive impairment.

“That really brought home for me the human toll of mosquito-borne illnesses, and made me reflect a lot upon the ways in which a warming planet will redistribute infectious diseases,” Basu said.

A rise in emerging infectious diseases “is one of our greatest challenges” globally, the result of increased human interaction with wildlife and “climatic changes creating new disease transmission patterns,” said a major United Nations climate report released Feb. 28. Changes in climate have already been identified as drivers of West Nile infections in southeastern Europe, the report noted.

The relationship between lack of rainfall and West Nile virus is counterintuitive, said Sara Paull, a disease ecologist at the National Ecological Observatory Network in Boulder, Colorado, who studied connections between climate factors and West Nile in the U.S. as a postdoctoral researcher at the University of California-Santa Cruz.

“The thing that was most important across the nation was drought,” she said. As drought intensifies, the percentage of infected mosquitoes goes up, she found in a 2017 study.

Why does drought matter? It has to do with birds, Paull said, since mosquitoes pick up the virus from infected birds before spreading it to humans. When the water supply is limited, birds congregate in greater numbers around water sources, making them easier targets for mosquitoes. Drought also may reduce bird reproduction, increasing the ratio of mosquitoes to birds and making each bird more vulnerable to bites and infection, Paull said. And research shows that when their stress hormones are elevated, birds are more likely to get infectious viral loads of West Nile.

A single year’s rise in cases can’t be attributed to climate change, since cases naturally fluctuate by year, in part due to cycles of immunity in humans and birds, Paull said. But we can expect cases to rise with climate change, she found.

Increased drought could nearly double the number of annual neuroinvasive West Nile cases across the country by the mid-21st century, and triple it in areas of low human immunity, Paull’s research projected, compared with averages from 1999 to 2013.

Drought has become a major problem in the West. The Southwest endured an “unyielding, unprecedented, and costly drought” from January 2020 through August 2021, with the lowest precipitation on record since 1895, and the third-hottest daily average temperatures in that time period, a National Oceanic and Atmospheric Administration report found.

“Exceptionally warm temperatures from human-caused warming” have made the Southwest more arid, and warm temperatures and drought will continue and increase without serious reduction of greenhouse gas emissions, the report said.

Ecologist Marta Shocket has studied how climate change may affect another important factor: the Goldilocks temperature. That’s the sweet spot at which it’s easiest for mosquitoes to spread a virus. For the three species of Culex mosquitoes that spread West Nile in North America, the Goldilocks temperature is 75 degrees Fahrenheit, Shocket found in her postdoctoral research at Stanford University and UCLA. It’s measured by the average temperature over the course of one day.

a woman in a lab wearing a multicolored lab coat holding a dropper over a container
Dr. A. Desiree LaBeaud, an infectious disease physician at Stanford University, travels the world studying mosquito-borne diseases. Credit: Norbert von der Groeben/Stanford School of Medicine

“Temperature has a really big impact on the way that mosquito-transmitted diseases are spread, because mosquitoes are cold-blooded,” Shocket said. The outdoor temperature affects their metabolic rate, which “changes how fast they grow, how long they live, how frequently they bite people to get a meal. And all of those things impact the rate at which the disease is transmitted,” she said.

In a 2020 paper, Shocket found that 70% of people in the U.S. live in places where average summer temperatures are below the Goldilocks temperature, based on averages from 2001 to 2016. Climate change is expected to change that.

“We would expect West Nile transmission to increase in those areas as temperatures rise,” she said. “Overall, the effect of climate change on temperature should increase West Nile transmission across the U.S. even though it’s decreasing it in some places and increasing it in others.”

Janet McAllister, a research entomologist with the CDC’s Division of Vector-Borne Diseases in Fort Collins, Colorado, said climate change-influenced factors like drought could put people at greater risk for West Nile, but she cautioned against making firm predictions, since many factors are at play, including bird immunity.

Birds, mosquitoes, humans, and the virus itself may adapt over time, she said. For instance, hotter temperatures may drive humans to spend more time indoors with air conditioning and less time outside getting bitten by insects, she said.

Climate factors like rainfall are complex, McAllister added: While mosquitoes do need water to breed, heavy rain can flush out breeding sites. And because the Culex mosquitoes that spread the virus live close to humans, they can usually get enough water from humans’ sprinklers and birdbaths to breed, even during a dry spring.

West Nile is preventable, she noted: The CDC suggests limiting outdoor activity during dusk and dawn, wearing long sleeves and bug repellent, repairing window screens, and draining standing water from places like birdbaths and discarded tires. Some local authorities also spray larvicide and insecticide.

“People have a role to play in protecting themselves from West Nile virus,” McAllister said.

In the Denver suburbs, Freeman, 75, said she doesn’t know where her son got infected.

“The only thing I can think of, he has a house, they have a little baby swimming pool for the dogs to drink out of,” she said. “So maybe the mosquitoes were around that, I don’t know.”

KHN (Kaiser Health News) is a national newsroom that produces in-depth journalism about health issues. Together with Policy Analysis and Polling, KHN is one of the three major operating programs at KFF (Kaiser Family Foundation). KFF is an endowed nonprofit organization providing information on health issues to the nation.

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Segment Guests

Melissa Bailey

Melissa Bailey is a contributor to Kaiser Health News and a Scripps Environmental Journalism Fellow based in Boulder, Colorado.

Segment Transcript

JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. We all know the story of Goldilocks and the Three Bears, right? There are three beds. One is two hard. One is too soft. And one is just right.

Now, Goldilocks isn’t the only one who thrives in these just right conditions. We’ve talked about the Goldilocks zone for planets. It’s just the right distance from their sun to support life. Turns out some mosquitoes have their own Goldilocks zone too. At a certain temperature threshold, the mosquitoes that carry West Nile virus thrive.

As climate change makes our world warmer, more places are likely to fall into this Goldilocks zone for mosquitoes who carry the virus. Scientists expect West Nile cases will rise in the future alongside hotter temperatures. Joining me today to talk about this story is my guest, Melissa Bailey. She’s a writer for Kaiser Health News, and an Environmental Journalism Fellow at the University of Colorado Boulder. Welcome to Science Friday, Melissa.

MELISSA BAILEY: Thanks for having me, John.

JOHN DANKOSKY: So Melissa, let’s start by just talking about how many cases we’ve seen of West Nile virus in the state where you are, in Colorado, for instance.

MELISSA BAILEY: So cases actually tend to fluctuate from year to year. And that’s because humans get West Nile virus from mosquitoes, which get it from birds. So the cases go up and down depending on these cycles of immunity in birds and in humans. But in Colorado last year, we saw 11 deaths and over 100 neuroinvasive infections. Those were the highest numbers in 18 years.

JOHN DANKOSKY: But next door in Arizona, it was a much different story, even more deaths and more cases.

MELISSA BAILEY: Yeah, the cases were through the roof in Arizona. There were over 100 deaths, and over 1,000 neuroinvasive cases, just way higher than previous years.

JOHN DANKOSKY: Now, we talked about this Goldilocks zone for mosquitoes. Explain to us a little bit about what that means. What is the zone the mosquitoes kind of thrive in?

MELISSA BAILEY: Well, the first thing to know is that mosquitoes are cold blooded, so they’re very sensitive to outdoor temperature. It affects every aspect of how they function. The Goldilocks temperature is their favorite temperature, which is easiest to spread disease. So I talked to Ecologist, Marta Shocket, who studies mosquito-borne diseases.

MARTA SHOCKET: If it’s too cold, you won’t have enough mosquitoes. They won’t be growing fast enough. It will take too long for the virus to develop inside the mosquito into an infection that can get to their salivary glands that they can pass on. The other hand, if it’s too hot, they’ll actually die too quickly. So it’s this middle kind of Goldilocks temperature, not too cold, not too hot where mosquito borne diseases are best transmitted.

JOHN DANKOSKY: So how is that zone changing right now, Melissa?

MELISSA BAILEY: The mosquitoes favorite temperature is staying the same, around 75 degrees Fahrenheit. But Shocket found that 70% of people in the US live in places where average summer temperatures are colder than that Goldilocks zone. So as temperatures rise, scientists expect more people to live in climates where it’s easier for mosquitoes to pass on West Nile.

JOHN DANKOSKY: OK. So more people in that zone, more mosquitoes in that zone, so we expect to probably see more of this disease. Now, as we’ve talked on the show in recent years about the state where you are living right now in Colorado, we talk about it a lot, because of drought, the severe drought that you’ve had there. Now, when I think of mosquitoes, I think of insects that thrive in very wet and humid climates. So explain to us the connection between West Nile virus and this drought.

MELISSA BAILEY: So you’re right, John, mosquitoes do need water to breed. But yeah, it’s counterintuitive. Research has shown that when there’s more drought, there’s more West Nile. And one big reason has to do with birds. So birds are important, because that’s where mosquitoes pick up the virus and spread it to humans.

In really dry conditions, birds gather in greater numbers around these limited water supplies, so that brings them closer to the spots where mosquitoes hang out. And mosquitoes have more chances to pick up the virus from birds. So I talked to Dr. Desiree Labeaud. She’s an infectious disease physician at Stanford University. She travels the world studying mosquito borne diseases. And I reached her in Kenya.

DESIREE LABEAUD: Every mosquito has its favorite place to call home. And for Culex mosquitoes, the mosquitoes that spread West Nile virus, they love stinky, standing water. So you can imagine if you have a lot of rain, that can actually wash away some of the developing eggs. But climate change can also promote a lot of drought. And having more drought could actually bring more West Nile, because you have more and more stinky, standing water, which isn’t being washed away.

MELISSA BAILEY: So here, we have two theories, one about the birds, and one about water that are leading to more West Nile when we have drought conditions.

JOHN DANKOSKY: And now, West Nile virus for people who haven’t followed very closely is a pretty serious disease, but it’s not serious in all cases certainly. What exactly does West Nile do to the body?

MELISSA BAILEY: So most people who get West Nile virus, about eight in 10 don’t have symptoms. The others typically get a fever, and other ailments, such as a headache, joint pain, vomiting, or diarrhea. And it’s actually important to note that humans can’t pass West Nile on to someone else. If a mosquito bites a person with a West Nile infection, and then bites another person, the second person won’t get West Nile.

JOHN DANKOSKY: OK, so s doesn’t spread that way, but it is spread through mosquitoes.

MELISSA BAILEY: It spreads from birds, to mosquitoes, to humans.

JOHN DANKOSKY: Tell us about the severe edge of this illness. How exactly severe can West Nile be in people?

MELISSA BAILEY: So some cases are called neuroinvasive. That’s when the infection invades the brain, or spinal column, and can cause serious illness, paralysis, or even death.

JOHN DANKOSKY: But these severe cases of West Nile are pretty rare, right?

MELISSA BAILEY: Yeah, about one case in every 150 is neuroinvasive. It is rare. But Doctor LaBeaud told me once you see a case, you never forget it. And the risk of serious illness is higher for people with medical complications, and who are over 60-years-old.

And so that’s people like Betsy Marston’s husband, Ed, who died of West Nile a few years ago. Ed was 78. He loved to hike near their small town in Western Colorado. Betsy tells me that might be how he got infected, because he was older and recovering from heart surgery, he was at higher risk.

BETSY MARSTON: He had uncontrollable shaking. And there’s nothing you can do about that. I mean, this disease was just rampaging through him. And he found it very, very difficult to talk. And when he did talk, he said, I’m just suffering through this. That was one of the last things he said. And then it went to his brain.

So it was hopeless. It’s a brutal disease. It has no mercy. It just rips through your body and takes you away.

JOHN DANKOSKY: Wow, that’s quite a story, and very sad. We’ve been focused here on Colorado where your reporting is, Melissa. But let’s zoom out a little bit. We first saw this virus in the US back in the late 1990’s in the New York City area.

And over the course of the last few decades, it’s spread to other parts of the country. We probably haven’t heard about it quite as much as other diseases in recent years. Did the researchers that you spoke with say if other parts of the US can now become vulnerable to West Nile?

MELISSA BAILEY: Yes, actually, every state in the mainland US has seen West Nile. These mosquitoes are everywhere. And some places in the US may get so hot that mosquitoes will die off quicker and spread less virus. But experts say overall, they expect West Nile infections to increase across the country due to temperature and drought.

JOHN DANKOSKY: Are there preventive measures that people can take to avoid getting this disease?

MELISSA BAILEY: Yes, so these mosquitoes love to come out in the summer at dawn and dusk. Some communities spray pesticide to kill the larva. And Betsy points out that there’s a lot individuals can do to avoid getting bitten.

BETSY MARSTON: I can see we needed to be more vigilant. You need long sleeves. And you need to be aware of when you’re outside. I don’t go out at dusk anymore, and I used to never care.

It’s unpredictable what’s happening in our climate now. But we do know that West Nile is here. It’s dangerous. And we have to protect ourselves and be aware of it.

JOHN DANKOSKY: Yeah, so just staying away from mosquito bites is something I remember from the first West Nile outbreak. Melissa, as the world gets warmer, I assume that this isn’t the only disease that we’re following that could migrate and wind up in places that it hasn’t been, because the climate is changing.

MELISSA BAILEY: Yes, John, it’s already happening. Lyme disease is a great example. There’s evidence that the ticks that spread the disease have already expanded their range due to climate change. And scientists expects ticks to keep marching north and coming out earlier in the year with hotter temperatures.

There’s also Dengue fever, a tropical mosquito borne virus. And scientists expect Dengue to expand its reach in the United States. And then there are all these waterborne diseases that spread more easily with heavy rainfall, or warmer water. One of those is called Ciguatera fish poisoning. It gives people gastrointestinal trouble, and this weird symptom where your sense of hot and cold can be reversed.

It’s kind of wild. It’s been popping up in the Gulf of Mexico. And it’s expected to spread further as oceans get hotter.

So there are many examples of how human-caused climate change is already making people sicker. And scientists are warning us that as we keep heating the planet, many diseases will spread to new locations, and get more people sick. And just one thing to keep in mind is that these diseases hit hardest on vulnerable groups, which often includes children, the elderly, communities of color, and people with low income.

JOHN DANKOSKY: Yeah, and that’s something especially to watch. In areas in the West, there’s Indigenous communities, other places where health outcomes have already been relatively poor, and more disease spreading across some of these areas could really have a huge impact.

MELISSA BAILEY: Yeah, and it’s often the people who are least culpable for climate change that are feeling the largest impacts on their health.

JOHN DANKOSKY: Melissa Bailey is a writer for Kaiser Health News, and an Environmental Journalism Fellow at the University of Colorado Boulder. Melissa, thank you so much for sharing this story with us.

MELISSA BAILEY: Thanks so much, John.

JOHN DANKOSKY: And you can read the full story on our website sciencefriday.com/westnile.

Copyright © 2022 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/

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John Dankosky works with the radio team to create our weekly show, and is helping to build our State of Science Reporting Network. He’s also been a long-time guest host on Science Friday. He and his wife have three cats, thousands of bees, and a yoga studio in the sleepy Northwest hills of Connecticut. 

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