Mosquitoes that carry West Nile are becoming resistant to insecticides, CDC says

John VanDenBerg suspects he was gardening when a mosquito got him.

It was September 2018, and VanDenBerg, then 67, had been feeling a little "off" for a few days, he said, like maybe he had the flu.

But one morning, as he was walking out of his Colorado home, he collapsed.

"I just went down," VanDenBerg said. "That's the last I remember for quite some time."

VanDenBerg had a severe form of West Nile virus, caused by a single mosquito bite.

He developed inflammation in his brain. He lost his ability to read and write. His arms and legs stiffened with paralysis.

"I didn't know whether my mobility would ever come back," he said. "It was a pretty scary time."

The rise of West Nile

While this summer saw the first locally acquired cases of another illness linked to mosquitoes, malaria, in two decades, it's West Nile virus — and the mosquitoes that spread it — that most worries federal health officials.

Those bugs, a species of mosquitoes called Culex, are the CDC's "primary concern in the continental U.S. right now," said Roxanne Connelly, a medical entomologist with the Centers for Disease Control and Prevention.

The combination of an unusually wet season from rainfall and melting snow packs and intense heat waves this year appears to be dramatically increasing the mosquito populations.

And, CDC scientists say, those mosquitoes have become increasingly resistant to the insecticides that communities use in mass sprays to try to kill the bugs and their eggs.

"It's not a good sign," Connelly said. "We're losing some of our tools that we normally rely on to control infected mosquitoes."

At CDC's insect lab located in Fort Collins, Colorado — home to tens of thousands of mosquitoes — Connelly's team has found that Culex mosquitoes are living longer when they're exposed to insecticides.

"You want a product that's gonna be able to knock them down, not do this," Connelly said, pointing to a bottle of mosquitoes exposed to the chemicals. Many were still flying.

Experiments at the lab have not shown any resistance to bug sprays people typically use to repel mosquitoes during hikes and other outdoor activities. They continue to work well, Connelly said.

But as the bugs grow stronger than insecticides, they're also increasing in dramatic numbers in certain pockets of the country.

So far in 2023, there have been 69 human cases of West Nile in the U.S., according to the CDC. It's nowhere near a record; there were 9,862 cases in 2003.

But more mosquitoes now two decades later means more chances that people will be bitten and get sick. West Nile cases usually peak in August and September.

"This is just the beginning of when we see West Nile start to take off in the United States," said Dr. Erin Staples, a medical epidemiologist at the CDC's Fort Collins lab. "We expect a steady increase of disease cases to occur over the next several weeks."

In Maricopa County, Arizona, for example, 149 mosquito traps have tested positive for West Nile so far this year, compared with eight in 2022.

Heavy rains that have created pockets of standing water, coupled with extreme heat, appear to be playing a role, said John Townsend, manager of Maricopa County's Vector Control Division of Environmental Services.

"The water that is sitting there is just ripe for mosquitoes to lay eggs in it," Townsend said. Mosquitoes hatch more quickly in warmer water — within three or four days, he said, — compared with up to two weeks when they're in cooler water.

An unusually rainy June in Larimer County, Colorado — where the Fort Collins lab is based — has also led to an "unprecedented abundance" of mosquitoes that can spread West Nile, said Tom Gonzales, the county's public health director.

County data show five times the number of mosquitoes that can spread West Nile this year, compared with last year.

The increases in certain areas of the country are "very concerning," said Connelly. "This is something different than what we've been seeing for the past few years."

What are symptoms of West Nile virus?

Since West Nile was first detected in the United States in 1999, it's become the most common mosquito-borne illness in the country. Each year, several thousands of people are infected, Staples said.

West Nile does not spread from person to person through casual contact. Only Culex mosquitoes spread the virus. The bugs become infected when they bite sick birds, then spread the virus to people through another bite.

There is no treatment or vaccine.

The majority of people never feel a thing. One in five experience a fever, headache, body aches, vomiting and diarrhea according to the CDC. Symptoms typically appear between three and 14 days after being bitten.

One in 150 people with West Nile virus have serious complications, including death. Anyone can become severely ill, but Staples said that people over the age of 60 and those with underlying medical problems are at higher risk.

Five years after his West Nile diagnosis, VanDenBerg has regained many of his abilities, thanks to intense physical therapy. His feet remain numb, however, prompting him to rely on a cane.

"I think I'm functioning really well mentally," he said, "but I have kind of a clunky walk."

While the severity of VanDenBerg's illness is rare, it coincided with another tragic case.

When VanDenBerg collapsed on that September morning in 2018, he'd been on his way to a funeral for a friend who had died from complications of West Nile virus.

The illness, he said, "can be very, very serious, and people need to know that. It can change your life."

Protecting against West Nile

While resistance to insecticides may be on the rise, Connelly's team finds that common repellents people use while outdoors still work well. Bug spray with ingredients like DEET and picaridin are best, according to the CDC.

Other strategies to steer clear of mosquitoes:

• Dress in loose-fitting clothes that cover arms and legs.

• Use air conditioning when possible, or make certain that open windows and doors have screens.

• Regularly empty standing water from common backyard items, such as bird baths, flower pots and toys, where mosquitoes can lay their eggs.

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This article was originally published on NBCNews.com