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

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Scientists are reporting increasing insecticide resistance among American Culex mosquitoes that spread West Nile virus to humans.

West Nile virus causes disease in thousands of Americans each year, leading to flu-like illness in about one-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’s been a fairly successful means to control its spread: insecticides that target the mosquito that ferries 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. Scientists at the Centers for Disease Control and Prevention say that in laboratory studies, these mosquitoes are living longer when exposed to these products — rather than dying. This increases their concern it’ll get harder to control these mosquito populations in the wild, raising the risk of West Nile virus transmission.

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

West Nile virus first appeared in the US in the fall of 1999. Since the virus first emerged stateside, it has led to more than 55,000 total cases and has killed nearly 2,800 Americans. But more of that happened in the years following its US emergence than in more 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 one-tenth as many infections and 90 deaths — a third as many as it did 20 years ago.

About 4 out of 5 people who get infected with West Nile virus have no symptoms at all — but for that unlucky one, the infection often involves fever, along with headache, body aches, joint pains, vomiting, diarrhea, or rash.

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

It takes a specific type of mosquito to transmit West Nile virus. Mosquitoes in the Culex genus are generally able to spread the infection, and historically, they have lived all over the world, including many parts of the US. They’re among the most common mosquitoes in the US, and they do a lot of their biting at dawn, dusk, and overnight: If you wake up with a fresh bite, odds are good you provided a snack to a Culex mosquito while you were asleep.

Culex mosquito bites don’t look any different from other mosquito bites. However, different people react differently to bites from the same mosquito because their immune systems aren’t identical.

The virus is transmitted by mosquitoes who get 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.

Lately, warmer, wetter weather throughout North America means Culex mosquitoes have been on the move: While they used to be found only as far north as the American Midwest, they have recently been found in Canada. Scientists project that the entire US and much of southern Canada are now home to one Culex species or another.

And due to the widespread use of insecticides for a variety of reasons, many of them are harder to kill now than they were in previous years.

Mosquitoes evolve 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 hit “off-target” mosquitoes by accident. In other cases, groups of mosquitoes get exposed to insecticides that would kill them if they were applied 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 aren’t really harmed, or aren’t killed fast enough, by the insecticides commonly used to kill them. Those mosquitos that have a higher resistance to insecticides are the ones that go on to make families, creating new generations of more resistant mosquitoes.

The problem, said Ryan, is that lots of people are spraying pesticides — agricultural workers trying to produce crops, vector control workers trying to prevent disease spread, landscaping workers trying to reduce nuisance. And nobody knows how much insecticide any one population of mosquitoes is getting exposed to.

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

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

It’s not entirely clear why Culex mosquitoes are showing more resistance to insecticides now. It might be that scientists are looking for it more as more mosquito-borne infections affect Americans. (After Zika, funding for mosquito control rose meaningfully, leading to efforts targeting mosquito populations more broadly. That might mean more spraying for mosquitoes where there wasn’t spraying before — which could lead to more resistance.)

What does seem clear is that throughout the US, scientists looking for Culex insecticide resistance are finding it, whether in the Great Lakes region of the Midwest, in Mississippi, or in Florida.

This is part of a worldwide pattern: Culex has evolved resistance everywhere else that insecticides get used, including sub-Saharan Africa, China, and Europe. While that might make Culex’s increasing insecticide resistance predictable, it doesn’t make it less concerning.

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 populations of the mosquitoes that spread the disease.

Mosquitoes breed in warm water, and Culex in particular like to breed in fresh or stagnant water near people or animals. Additionally, they like the air temperature hot, 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 hottest parts of the US are not breeding grounds for the virus, other regions, like parts of the upper Midwest, are becoming increasingly appealing to Culex mosquitoes (and increasingly risky for West Nile virus transmission) as they become warmer and wetter.

There’s a lot 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, a Sarasota County, Florida, health department epidemiologist, 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 acknowledges is challenging in the summer heat.

Eliminating mosquito hangout spots is also very important: Culex mosquitoes often congregate and breed near humans and animals and near standing water, so it’s important to eliminate places where water can accumulate. These insects don’t require a lot of water to make a family, so fill in any shallow divots that create puddles, and old tree holes where water can pool. It also helps to regularly empty water out of tires, buckets, flower pots, and toys that might collect rainwater.

And yes, bug spray still helps. These mosquitoes are resistant to insecticides sprayed in the environment, like around swamps and in ditches — not to consumer mosquito repellents, such as DEET, that people can spray on themselves to reduce biting. So those products are still effective to use on skin and clothing. The Environmental Protection Agency has a website to help people find the insect repellent that’s right for them.

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