Climate Change Could Push US Towards ‘Goldilocks Zone’ for West Nile Virus

Michael Keasling of Lakewood, Colorado was an electrician who loved big trucks, fast cars, and Harley-Davidsons. He had been battling diabetes since his teens and needed a kidney transplant from his sister to stay alive. He was already very ill 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 Nov. 11 at age 57 of complications from West Nile virus and diabetes, according to his mother, Karen Freeman. She said she missed him terribly.

“I don’t think I can handle this,” Freeman said shortly after his death.

Spring rains, summer drought and heat created ideal conditions for mosquitoes to spread West Nile virus in 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 number in 18 years.

Rising case numbers may be a sign of what’s to come: As climate change brings more drought and pushes temperatures towards what’s been called the ‘Goldilocks Zone’ for mosquitoes – neither too hot nor too cold – scientists expect West Nile transmission to increase across the country.

“West Nile virus is a really big case study” in the link between climate and health, said Dr. Gaurab Basu, primary care physician and health equity researcher at the Center for Climate, Health, and the Global Environment from the Harvard School of Public Health. .

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 of the brain or spinal cord, paralysis or death, according to the Centers. for Disease Control and Prevention. People over 50 and transplant patients like Keasling are at higher risk.

Over the past decade, the United States has averaged about 1,300 neuroinvasive cases of West Nile each year. Basu saw his first in Massachusetts several years ago, a 71-year-old patient who had brain swelling and severe cognitive impairment.

“It really reminded me of the human toll of mosquito-borne disease and got me thinking a lot about how a warming planet will redistribute infectious disease,” Basu said.

The increase in emerging infectious diseases “is one of our greatest challenges” globally, a result of increased human interaction with wildlife and “climate change creating new patterns of disease transmission”, according to a major United Nations climate report released on February 28. have already been identified as drivers of West Nile infections in southeastern Europe, the report notes.

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 has studied links between climatic factors and West Nile in the United States. United States as a postdoctoral researcher at the University. of California-Santa Cruz.

“The biggest thing across the country was the drought,” she said. As drought intensifies, the percentage of infected mosquitoes increases, she found in a 2017 study.

Why is drought important? It has to do with birds, Paull said, since mosquitoes pick up the virus from infected birds before passing it on to humans. When water supplies are limited, birds congregate in greater numbers around water sources, making them easier targets for mosquitoes. Drought can also reduce bird breeding, increasing the mosquito-to-bird ratio and making each bird more susceptible to bites and infections, Paull said. And research shows that when their stress hormones are elevated, birds are more likely to have infectious West Nile viral loads.

The increase in the number of cases in a single year cannot be attributed to climate change, as cases naturally fluctuate from year to year, in part due to immunity cycles in humans and birds, a said Paul. But we can expect cases to increase with climate change, she found.

Increasing drought could nearly double the number of annual cases of West Nile neuroinvasion across the country by the middle of the 21st century and triple it in areas of low human immunity, according to Paull’s research, compared to the averages from 1999 to 2013.

Drought has become a major problem in the West. The southwest suffered an “unrelenting, unprecedented and costly drought” from January 2020 to August 2021, with the lowest rainfall on record since 1895 and the third hottest daily average temperatures during this period, according to a National Oceanic and Atmospheric Administration report.

Spring rains, summer drought and heat created ideal conditions for mosquitoes to spread West Nile virus throughout Colorado last year. West Nile virus has killed 11 people and caused 101 cases of neuroinvasive infections in Colorado, the highest number in 18 years. (A. Marm Kilpatrick/ Department of Ecology and Evolutionary Biology, University of California-Santa Cruz)

“Unusually warm temperatures due to human-induced warming” have made the southwest more arid, and hot temperatures and drought will continue and increase without serious reductions in greenhouse gas emissions, the report said.

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

“Temperature has a very big impact on how mosquito-borne diseases spread because mosquitoes are cold blooded,” Shocket said. The outside temperature affects their metabolic rate, which “changes how fast they grow, how long they live, how often they bite people to get a meal. And all of those things have an impact on the rate of transmission of the disease,” she said.

In a 2020 paper, Shocket found that 70% of people in the United States 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 expect West Nile transmission to increase in these areas as temperatures rise,” she said. “Overall, the effect of climate change on temperature is expected to increase West Nile transmission across the United States, even as it decreases it in some places and increases it in others. “

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

Birds, mosquitoes, humans and the virus itself can adapt over time, she said. For example, warmer temperatures may encourage humans to spend more time indoors with air conditioning and less time outdoors getting bitten by insects, she said.

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

West Nile is preventable, she noted: The CDC suggests limiting outdoor activities to dusk and dawn, wearing long sleeves and insect repellent, fixing window screens and draining water. standing water from places like birdbaths and discarded tires. Some local authorities also spray larvicides and insecticides.

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

In suburban Denver, Freeman, 75, said she didn’t know where her son got infected.

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

Related Topics

Contact us Submit a tip

Teresa H. Sadler