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News / Health / Health Wire

WA’s mosquito trackers are seeing a spike of West Nile virus

By Conrad Swanson, The Seattle Times
Published: September 23, 2023, 6:03am
2 Photos
Benton County Mosquito Control District surveillance technician Kylie Morgan inserts dead mosquitoes into vials so that they can be tested for West Nile virus, among other diseases, Thursday, Sept. 7, 2023, in Richland, Washington.
Benton County Mosquito Control District surveillance technician Kylie Morgan inserts dead mosquitoes into vials so that they can be tested for West Nile virus, among other diseases, Thursday, Sept. 7, 2023, in Richland, Washington. (Luke Johnson/The Seattle Times/TNS) Photo Gallery

BYRON PONDS, Yakima County — The traps dangle from shepherd’s hooks or tree branches and Kylie Morgan keeps an eye out for them as she drives her truck carefully over the rough, pitted back roads in south central Washington.

She stops at each trap site and inspects her loot, gently tapping the catch bags with her fingertips to nudge a black swarm of mosquitoes off the cloth sides. She pulls the draw string taught. Everything goes in a cooler in the truck bed until she returns to the lab in Richland where the insects will be tested for deadly pathogens, like West Nile virus.

So far this year the Benton County Mosquito Control District (which also covers portions of Yakima County) has had 42 batches of mosquitoes test positive for West Nile virus, more cases than they’ve seen in over a decade. Other counties east of the Cascades are also seeing upticks in mosquitoes carrying the virus.

The Benton County team, to which Morgan belongs, and a patchwork of others like them stand as Washington’s early detection system for serious and potentially deadly diseases spread by mosquitoes.

People in Washington fall ill with West Nile virus most years, peaking at several dozen cases. The virus isn’t threatening to overwhelm communities and hospitals but the risk is increasing and scientists are seeing similar trends with other insect-borne illnesses like malaria, Lyme disease and anaplasmosis.

One person in King County contracted the virus this year, and while additional information about that case is sparse, it underscores the need for mosquito surveillance and testing, epidemiologists and entomologists say.

Meanwhile, warming trends are transforming the Pacific Northwest into a more hospitable place for insects that spread disease.

Tracking West Nile virus and other fatal pathogens through human cases is costly and inefficient, so public health officials must rely on these other methods.

“The virus is always present,” said Angela Beehler, director of the Benton County district. “And really your only way to get a sense of what’s happening in the area is through mosquito data.”

Most counties in Washington don’t find any mosquitoes with West Nile virus in a given year but there’s a catch to that data: Most counties aren’t looking.

The state has 19 mosquito control districts to track the insects and tamp down massive swarms. Most of those districts sit in rural areas, far away from the state’s biggest population centers, leaving an information gap for epidemiologists keeping watch for rising diseases.

“I’m definitely concerned about the sparse surveillance that we’re able to do,” said Hanna Oltean, head of the state Department of Health’s Zoonotic and Vector-borne Disease Program. “Detections in mosquitoes are our early warning system, to raise the alert level.”

Even some places that do have districts are severely limited in what they can do.

Jay Lawrence, head and sole employee of the Camano Island Mosquito Control District, said he’s never had a batch of mosquitoes test positive for West Nile virus. He’s never gathered enough of the right species needed to run a proper test.

Lawrence’s district is the nearest to Seattle and he only has time to set and check perhaps five mosquito traps a week.

“And I’m just covering a tiny little area,” Lawrence said.

Genus and species

Perhaps 21 different mosquito species have historically been found throughout Washington, Shoemaker said. About 14 of them have been found around the Tri-Cities area. But really mosquito control districts are most worried about just two: the Culex tarsalis and Culex pipiens.

Those two species are the most likely to carry West Nile virus.

The virus was first seen in the United States in 1999 and the majority of people (up to 80%) who contract it don’t show symptoms. Of those who do, most develop flu-like symptoms, including fever, body aches, vomiting, diarrhea and potentially a rash. Less than 1% of cases lead to neuroinvasive diseases like encephalitis or meningitis. About 10% of those severe cases are fatal.

Horses can also contract the virus, Shoemaker said, and about 1 in 3 of them dies.

The mosquitoes pick up the virus after biting birds that act as “reservoir hosts,” said Dr. Lawrence Stanberry, an expert on viral diseases and associate dean for international programs at Columbia University. After that, the insects can transmit the virus to humans. West Nile doesn’t spread from one person to another.

The greatest risk for West Nile to spread comes in areas where mosquitoes, birds and humans live in close contact.

Trapping, counting and testing

To trap mosquitoes, Morgan and the district’s other technicians scoop a few pounds of dry ice into a royal blue, cylindrical Coleman cooler with holes drilled into the bottom. The district goes through about 550 pounds of dry ice a week, Shoemaker said.

Under the cooler hangs a small fan kept running by a bank of D batteries, which blows into a catch bag. The whole trap hangs vertically and technicians set them up across an area of 350 square miles.

Dry ice turns to carbon dioxide (in a process called sublimation) and attracts female mosquitoes much the same way a person breathing heavily would, Shoemaker said. The insects fly into the gas stream, the fan pulls them into the trap and keeps them there until the next morning.

A given trap can yield a few dozen to a few thousand mosquitoes, Shoemaker said. The district’s record might be as high as 22,000 for a single trap.

Morgan’s traps held hundreds. She returned to the lab and set each of them inside the dry ice container. Sixty seconds inside is enough to kill them but she usually waits about 10 minutes just to be sure.

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The mosquitoes need to be dead for the tests, as fresh from the traps as possible.

Then she dumps the insects, one bag at a time, onto a cold metal tray and counts them out.

Culex tarselis bear stripes on their legs and proboscis, while Culex pipiens are lighter in color and a bit more boring to look at, Morgan said, cupping a dozen or so of the dead insects in her gloved hand.

Anopheles freeborni (which can carry malaria) are her favorite to see. They’re fairly dainty and have spotted wings, Morgan said.

“When all you do is count mosquitoes, you’ve got to choose a favorite,” she said, smiling.

Morgan separates the pile of insects by species, stuffing up to 50 of a given species into small vials, which then go to Jasmine Che, the district’s vector biologist.

Che drops a few chemicals into the vials, which also contain a small metal bead, and place them into a countertop device that shakes the containers, pulverizing the contents.

“We call it a mosquito smoothie,” Che said.

A couple of minutes in the centrifuge separates the contents before Che carries the batches under a fume hood, measuring more chemicals into each vial and preparing them for the polymerase chain reaction test machine.

The testing process takes about an hour and a half, Che said, and the machine then sends a report to her lab computer showing whether the mosquitoes carried West Nile virus, St. Louis encephalitis or Western equine encephalitis.

When an area tests positive, Shoemaker said, the district will head back and drop compounds in water to kill larvae or spray insecticides in the area to kill the adults. They hope to either stop the spread of the virus entirely or reduce the number of mosquitoes that might spread it.

The district also responds to complaints from community members noticing large swarms in their neighborhoods. The operation is in such high demand across the area that the county sometimes hires crop dusting planes to spread the compounds and the district owns four drones that can be used to remotely spray areas hard to reach on foot.

Spiking cases and spiking temperatures

If it seems as if there are more mosquitoes swarming through the Seattle area these days, that’s probably because there are, said Jen Brady, a data analyst for the nonprofit Climate Central.

The insects prefer a certain set of conditions, typically daily temperatures between 50 and 95 degrees Fahrenheit and 42% humidity or higher, Brady said. Those are called “mosquito days.”

And shifting climate patterns mean that Seattle has more mosquito days now than in recent decades. In 1979, the area averaged about 105 mosquito days a year, Brady said. Now, it sees closer to 140.

“That’s an additional month every year of suitable days for mosquitoes,” Brady said.

And the high temperatures can coincide with spikes of West Nile virus.

Cases in Washington hit their highest mark in 2009 with 38 human cases of the virus. A heat wave ran through the state that July.

Cases spiked again in 2015 with 24 people falling ill with West Nile virus. Statewide, that year is Washington’s warmest on record, according to the office of the state climatologist.

So far this year one woman has contracted West Nile virus, Public Health — Seattle & King County officials said. She wasn’t hospitalized and has since recovered. She was likely bitten by an infected mosquito in another state.

Other areas are reporting an increase in human cases. Colorado is in the midst of the worst West Nile virus outbreak in the country, with 11 deaths and many more hospitalizations, according to the University of Colorado Health System. Additional deaths have been reported in California, Illinois, Maryland, Nebraska and New Mexico.

Drought, which is becoming more frequent in Washington, also heightens the risk of disease, Shoemaker said.

Fewer available water sources mean that mosquitoes are carrying West Nile virus and can then transfer the disease to people.

These climate trends also mean species not typically found in Washington could migrate north from California, Lawrence said.

Or invasive species could arrive in Seattle’s ports and establish populations here, Shoemaker noted.

Those new species could bring with them new diseases, said Alan Goodman, a molecular biosciences professor at Washington State University. Mosquito districts are well-positioned to notice new species and raise the alarm and take action by treating breeding pools.

Potential diseases that could arrive include chikungunya, dengue fever and Zika virus, Oltean said.

Gaps in the data

Mosquito surveillance is worth the investment for the insight and early warnings it provides, district officials and scientists agree. Proactively understanding an area’s disease risk is safer than waiting for people to fall ill and then react.

But launching new districts can be expensive, and ballot measures to increase taxes aren’t always popular decisions, Shoemaker said.

Nearby cities and counties reach out to Shoemaker intermittently to ask how best to open their own district. But interest usually wanes when winter hits and the insects die off. Lawrence has had similar experiences with neighboring governments.

King County opened a West Nile Virus Prevention Program in the mid-2000s but it was dissolved due to a lack of funding, Public Health — Seattle & King County spokesperson Kate Cole said. County officials aren’t considering a revival because case numbers remain low.

That’s the catch, Shoemaker said.

Without additional testing, public health officials can’t grasp their level of risk, he said. And without knowing their risk level, they have a difficult time justifying the cost of the districts that conduct the tests.

After decades of studying mosquitoes, Shoemaker said he still can’t find a benefit to the insects. They don’t make up a sizable portion of local food chains. Nor do they pollinate any plants.

“They’re blood suckers,” he said. “They’re uncomfortable. They’re miserable. They spread disease. They cause a lot of problems.”

If given the choice and the ability, Shoemaker said, he’d happily wipe all mosquitoes off the face of the earth. But he can’t. Nobody can. And so we must continue to keep a close watch on them.