Mosquitoes that carry West Nile are becoming resistant to insecticides, the 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 he might have the flu.

But one morning, as he walked out of his home in Colorado, he collapsed.

“I just went down,” VanDenBerg said. “That’s the last thing I remember for quite some time.”

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

John VanDenBerg lies paralyzed in bed with his dog in September 2018.
John VanDenBerg lies paralyzed in bed with his dog, Joe, in September 2018.Courtesy John VanDenBerg

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

“I didn’t know if 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 mosquito-linked disease, malaria, in two decades, it’s the West Nile virus — and the mosquitoes that spread it — that worries federal health officials.

These errors, a species of mosquito called Culexis the CDC’s “primary concern in the continental United States right now,” said Roxanne Connellya medical entomologist with the Centers for Disease Control and Prevention.

The CDC's Roxanne Connelly studies mosquitoes that can carry West Nile virus.
Roxanne Connelly of the CDC studies mosquitoes that can carry West Nile virus.Lauren Dunn/NBC News

The combination of an unusually wet season from rainfall and melting snowpack and intense heat waves this year appears to have increased the mosquito population dramatically.

And, CDC researchers say, these mosquitoes have become increasingly resistant to the insecticides that communities use in mass sprays to try to kill the insects 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 the CDC’s insect laboratory in Fort Collins, Colorado—home to tens of thousands of mosquitoes—Connelly’s team has found that Culex mosquitoes live longer when exposed to insecticides.

“You want a product that’s going to be able to knock them down, don’t do this,” Connelly said, pointing to a bottle of mosquitoes exposed to the chemicals. Many were still flying.

Experiments in the laboratory have shown no resistance to bug sprays, which people typically use to repel mosquitoes during hiking and other outdoor activities. They continue to work well, Connelly said.

But as the insects outgrow insecticides, they are also increasing in dramatic numbers in certain pockets of the country.

CDC employee Juan De Rivera checks to see if mosquitoes that have been exposed to insecticide have died.
CDC employee Juan De Rivera checks to see if mosquitoes that have been exposed to insecticide have died.Lauren Dunn/NBC News

So far in 2023, there have been 69 human cases of West Nile in the United States, 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 for people to be bitten and get sick. West Nile cases usually peak in August and September.

“This is just the beginning as we see West Nile begin to take off in the United States,” said Dr. Erin Staples, a medical epidemiologist at the CDC’s Fort Collins laboratory. “We expect a steady increase in cases over the next few weeks.”

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

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

“The water that’s sitting there is just ripe for mosquitoes to lay eggs in it,” Townsend said. Mosquitoes hatch faster in warmer water — within three or four days, he said — compared to up to two weeks when 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 to last year.

The increase in certain areas of the country is “very concerning,” Connelly said. “This is something different than what we’ve seen in the last few years.”

What are the symptoms of West Nile virus?

Since West Nile was first discovered in the United States in 1999, it has become the most common mosquito-borne disease in the country. Every year, several thousand people become infected, Staples said.

West Nile is 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.

Most people never feel anything. One in five experiences fever, headache, body aches, vomiting and diarrhoea 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 seriously ill, but Staples said people over 60 and those with underlying medical problems are at higher risk.

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

“I think I function very well mentally,” he said, “but I have a bit of a clumsy gait.”

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

John VanDenBerg on his farm walking in May 2019.
John VanDenBerg at his Colorado farm in May 2019.Courtesy John VanDenBerg

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

The disease, he said, “can be very, very serious, and people need to know that. It can change your life.”

Protection against West Nile

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

Other strategies to avoid mosquitoes:

  • Dress in loose clothing that covers arms and legs.
  • Use air conditioning when possible or make sure open windows and doors have screens.
  • Regularly drain standing water from common backyard items, such as birdbaths, flower pots and toys, where mosquitoes can lay their eggs.

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