Michael Keasling, from Lakewood, Colorado, was an electrician who liked big trucks, fast cars, and Harley-Davidsons. He had battled diabetes since his teens and needed a kidney transplant from his sister to stay alive. In August he was already quite ill when he contracted West Nile virus after being bitten by an infected mosquito.
Keasling spent three months in hospitals and in rehab. He died Nov. 11, aged 57, of complications from West Nile virus and diabetes, according to his mother, Karen Freeman, who said she missed him dearly.
“I don’t think I can take this,” Freeman said shortly after his death.
Spring rains, summer drought and heat created ideal conditions for mosquitoes to spread West Nile virus across Colorado last year, experts say. In Colorado, the virus has killed 11 people and caused 101 cases of neuroinvasive infections — those linked to serious illnesses like meningitis or encephalitis — in 2021, the highest number in 18 years.
The rise in cases may be a sign of things to come. As climate change brings more drought and pushes temperatures into what’s called the “warm zone” for mosquitoes — not too hot, not too cold — scientists expect virus transmission to increase across the country.
“West Nile virus is a really important case study” of the climate-health connection, said Dr. Gaurab Basu, a primary care physician and health equity fellow at the Center for Climate, Health, and Environment. Global Environment from the Harvard School of Public Health.
Although most of these infections are mild, the virus is neuroinvasive in about 1 in 150 cases, causing severe illness that can lead to brain or spinal cord swelling, paralysis, or death, according to the Centers for Disease Control and Prevention. Disease Prevention (CDC). People over the age of 50 and transplant patients like Keasling are at higher risk.
In the past decade, an annual average of about 1,300 neuroinvasive cases of West Nile virus have been reported in the United States. Basu saw the first in Massachusetts several years ago, a 71-year-old patient with brain swelling and severe cognitive impairment.
“It made me understand the human cost of mosquito-borne diseases and made me think about how global warming will redistribute infectious diseases,” Basu said.
The increase in emerging infectious diseases “is one of our greatest challenges” worldwide, the result of increased human interaction with wildlife and “climate changes that create new patterns of disease transmission,” according to a broad United Nations climate report published on February 28. The report notes that climate change has already been identified as a cause of West Nile virus infections in southeastern Europe.
The relationship between lack of rain and West Nile virus is counterintuitive, said Sara Paull of the National Network of Ecological Observatories in Boulder, Colorado, who has studied the connections between climatic factors and West Nile in the United States. , as a postdoctoral researcher at the University of California-Santa Cruz.
“The biggest factor across the country was the drought,” he said, and as the drought intensifies, the percentage of infected mosquitoes increases, according to a 2017 study.
Why is drought important? It has to do with birds, Paull explained, since mosquitoes pick up the virus from infected birds before transmitting it to humans.
When water supplies are limited, birds gather 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 individual birds more vulnerable to bites and infection, Paull said. And research shows that when their stress hormones are elevated, birds are more likely to contract infectious West Nile viral loads.
The increase in cases in a single year cannot be attributed to climate change, as cases naturally fluctuate each year, in part due to the immunity cycles of humans and birds, according to Paull, adding that you can expect an increase in cases with climate change.
Increased drought could nearly double the number of annual neuroinvasive West Nile cases nationwide by the mid-21st century, and triple it in areas of low human immunity, according to projections from Paull’s research, compared with averages from 1999 to 2013.
Drought has become a major problem in the West. The Southwest endured an “unrelenting, unprecedented and costly drought” from January 2020 to August 2021, with the lowest recorded rainfall since 1895 and the third-highest daily average temperatures in that time period, according to a report from the Administration. National Oceanic and Atmospheric.
“Unusually warm temperatures due to human-induced warming” have made the Southwest more arid, and warm temperatures and drought will continue and increase if greenhouse gas emissions are not seriously reduced, the report concluded.
Ecologist Marta Shocket has studied how climate change can affect another important factor: temperature “Goldilocks”. That’s the sweet spot where it’s easiest for mosquitoes to spread a virus. For the three species of Culex mosquitoes that spread West Nile virus in North America, the “warm” temperature is 75 degrees Fahrenheit, Shocket found in her postdoctoral research at Stanford University and UCLA. It is measured by the average temperature throughout a day.
“Temperature has a really big impact on how mosquito-borne diseases spread, because these insects are cold-blooded,” Shocket said. The outside temperature affects their metabolic rate, which “changes how fast they grow, how long they live, and how often they bite people for food. And all this influences the rate of transmission of the disease, ”he said.
In a 2020 article, Shocket found that 70% of Americans 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.
“West Nile transmission could be expected to increase in those areas as temperatures rise,” he said. “Overall, the effect of climate change on temperature should increase West Nile transmission across the country, even if 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, Colo., said factors influenced by climate change, such as drought, could increase the risk of West Nile virus, but cautioned that no clear predictions could be made, as many factors are at play, such as bird immunity.
Birds, mosquitoes, humans and the virus itself can adapt over time, he said, citing the fact that rising temperatures can cause humans to spend more time indoors, with air conditioning. , and less time outdoors, exposed to insect bites.
Climatic factors, such as rainfall, are complex, McAllister added: Although mosquitoes need water to breed, heavy rains can wipe out breeding sites. But because the Culex mosquitoes, which spread the virus, live close to humans, they often get enough water from sprinklers and birdbaths to breed, even during a dry spring.
West Nile virus is preventable, he noted. The CDC suggests limiting outdoor activity during dusk and dawn, wearing long sleeves and bug spray, repairing window screens, and draining 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 stressed.
In suburban Denver, Freeman, 75, said he doesn’t know where his son got infected.
“The only thing I can think of is that in his house he has a small pool for babies that the dogs drink from,” he said. “So maybe the mosquitoes were out there, I don’t know.”
KHN (Kaiser Health News) is the newsroom of KFF (Kaiser Family Foundation), which produces in-depth health journalism. It is one of three major programs of KFF, a nonprofit organization that analyzes the nation’s health and public health issues.