Why the mosquito that spreads West Nile virus is becoming resistant to insecticides

West Nile virus causing illness in thousands of Americans each year, leading to flu-like illness in about a fifth of people who get it, and less commonly serious problems affecting the brain and spinal cord. Ever since the infection appeared in the United States in the late 90s, there has been a fairly successful means of controlling its spread: insecticides that target the mosquitoes that carry the virus from wild birds to humans.

But it’s getting harder to kill these specific mosquitoes using common insecticides, according to a recent report from NBC News. Researchers at the Centers for Disease Control and Prevention say that in laboratory studies, these mosquitoes live longer when exposed to these products — rather than dying. This raises their concern that it will become more difficult to control these mosquito populations in the wild, increasing the risk of West Nile virus transmission.

West Nile virus transmission currently does not appear to be higher than expected for this time of year – although most transmission occurs in the early fall. In the meantime, here’s what all this means right now and how you can keep yourself safe.

What is West Nile virus and what are its symptoms?

West Nile virus first appeared in the United States in the fall of 1999. Since the virus first appeared in the state, it has led to more than 55,000 total cases and killed nearly 2,800 Americans. But more of it happened in the years after its emergence in the United States than in recent years: At its peak in 2003, it sickened nearly 10,000 people and killed 264 people. More recently, in 2022, the virus caused about a tenth as many infections and 90 deaths — a third as many as 20 years ago.

About 4 out of 5 people who become infected with West Nile virus have no symptoms at all – but for the unlucky, the infection is often involving fever, along with headache, body aches, joint pain, vomiting, diarrhea or rash.

A much smaller number – about 1 in every 150 infected people; people over 60 are more at risk — have serious illness involving inflammation of the brain or the membranes surrounding the brain and spinal cord. It is these more serious symptoms that can lead to death in those affected.

What type of mosquito transmits West Nile virus?

It takes a specific type of mosquito to transmit West Nile virus. Mosquitoes in the Culex genus are generally capable of spreading the infection, and historically they have lived all over the world, including many parts of the United States. They are among most common mosquitoes in the United States, and they bite a lot at dawn, dusk, and overnight: If you wake up with a fresh bite, chances are good that you gave a snack to a Culex mosquito while you were sleeping.

Culex mosquito bites look no different than other mosquito bites. But different people react differently to bites from the same mosquito because their immune systems are not identical.

How is West Nile virus spread?

The virus is transmitted by mosquitoes that become infected when they bite infected wild birds. Controlling West Nile virus transmission generally means controlling mosquito populations—and controlling mosquito populations usually means spraying mosquito-killing insecticides in the warm, wet places where they live.

Recently, warmer and wetter weather across North America means Culex mosquitoes have been on the rise: While they used to be found only as far north as the American Midwest, they’ve recently been found in Canada. Scientists project that the entire United States and much of southern Canada is now home to some Culex species.

And because of the widespread use of insecticides for various reasons, many of them are harder to kill now than they were in years past.

How do mosquitoes become resistant to insecticides?

Mosquitoes develop resistance to insecticides in a number of ways, said Sadie Ryan, a medical geographer at the University of Florida who studies the ecology of emerging pathogens. In some cases, insecticides used in agricultural settings “off-target” accidentally hit mosquitoes. In other cases, groups of mosquitoes are exposed to insecticides that would kill them if they were used in high enough concentrations or amounts – but haven’t been.

Both situations can lead to a “survival of the fittest” phenomenon, which can result in mosquitoes that are not really harmed, or not killed quickly enough by the insecticides commonly used to kill them. The mosquitoes that have a higher resistance to insecticides are the ones that continue to form families, creating new generations of more resistant mosquitoes.

The problem, Ryan said, is that lots of people are spraying pesticides — farm workers trying to produce crops, vector control workers trying to prevent the spread of disease, landscape workers trying to reduce nuisances. And no one knows how much insecticide a single population of mosquitoes is exposed to.

“It’s not like people are cutting corners or anything,” Ryan said. “It’s literally that you might be doing the perfect job in light of not knowing what else happened with exposure.”

As a result, mosquitoes infected with West Nile virus may not die immediately when sprayed with an insecticide, allowing them to continue spreading the infection. That can make outbreaks last much longer and affect far more people than they would if the mosquitoes were easier to kill.

Why are the mosquitoes that carry West Nile becoming insecticide resistant now?

It is not entirely clear why Culex mosquitoes show more resistance to insecticides now. Scientists may be looking for it more as more mosquito-borne infections affect Americans. (After Zika, funding for mosquito control increased meaningfully, leading to efforts to target mosquito populations more broadly. This could mean more spraying for mosquitoes where there was no spraying before – which could lead to more resistance.)

What seems clear is that across the United States, researchers looking for Culex insecticide resistance are finding it, whether in Great Lakes Region of the Midwest, i Mississippior i Florida.

This is part of a worldwide pattern: Culex has developed resistance everywhere else where insecticides are used, including south of Sahara Africa, Chinaand Europe. While that may make Culex’s increasing insecticide resistance predictable, it doesn’t make it any less worrisome.

How does climate change affect West Nile virus?

The number of people infected during each year’s West Nile virus season varies unpredictably, likely due to weather patterns and changes in human activity. But our warming climate is likely increasing the population of the mosquitoes that spread the disease.

Mosquitoes breed in warm water, and Culex in particular like to breed in it fresh or stagnant water near people or animals. In addition, they like the air temperature warm, but not too hot – above 80 degrees Fahrenheit, they become less active and breed less efficiently. Meanwhile, West Nile virus prefers to replicate at temperatures around 75 degrees Fahrenheit.

What this means is that while the warmest parts of the United States are not breeding grounds for the virus, other regions, like parts of the upper Midwest, are becoming increasingly appealing to Culex mosquitoes (and increasingly at risk for West Nile virus transmission) as they get warmer and wetter.

How can I protect myself from West Nile virus?

There is much people can do to protect themselves from Culex mosquitoes and the diseases they can carry.

It’s important for people to do their best to avoid mosquito bites, said Michael Drennon, an epidemiologist with the Sarasota County, Florida, health department when I interviewed him in late June, during the early days of the county’s malaria outbreak. He advises people to wear long sleeves and cover their legs – which he admits is challenging in the summer heat.

It is also very important to remove mosquito sites: Culex mosquitoes often congregate and breed near people and animals and near standing water, so it is important to remove places where water can collect. These insects don’t require a lot of water to form a family, so fill in any shallow divots that create puddles and old tree holes where water can collect. It also helps to regularly drain water from tires, buckets, flower pots and toys that can collect rainwater.

And yes, bug spray still helps. These mosquitoes are resistant to insecticides sprayed in the environment, such as around swamps and in ditches — not to consumer mosquito repellents, such as DEET, which people can spray on themselves to reduce bites. So these products are still effective to use on skin and clothing. The Danish Environmental Protection Agency has one website to help people find the insecticide that is right for them.