The US is on the verge of a mosquito-borne disease crisis

BROWNSVILLE, Texas — How do you capture the deadliest animal on Earth, one that has been responsible for the death of more people than any other species in history? Here at America’s southern border, it’s not with high-tech weaponry, but with a black plastic tub of stinky water tucked under a bush.

Surrounded by tall, yellowing grass near a sports complex, the bait water reeks of damp soil and decaying vegetation. And on top of the basin is the trap: a gray toolbox with a small battery-powered fan sucking air into a mesh net.

This simple, humble device is called a gravid trap and its fetid smell is meant to mimic the ideal breeding site for mosquitoes. On a recent, humid Texas summer day with triple-digits on the thermometer, it proved to be very effective: Ensnared inside are dozens of mosquitoes, many ready to lay eggs.

Collecting mosquitoes from traps and looking for larvae in roadside ditches isn’t an activity you’ll find in South Texas tourism brochures. But for Brownsville officials like Yaziri Gonzalez, 32, it’s one of the more important — and enjoyable — parts of the job of understanding the extent of mosquito-borne threats. To lay eggs, mosquitoes have to drink blood, so if they can be captured right before they breed, she can find out what they’ve been biting, and potentially, what diseases they’re spreading.

The diseases these insects may carry have left an indelible mark on history and culture, here in the US and in the world. Malaria has been around for as long as people walked on two feet, and it still infects around 250 million people per year and kills more than 600,000. Yellow fever infects about 200,000 people around the world annually, causing around 30,000 deaths.

These diseases prompted the creation (and popularity) of gin and tonic cocktails, shaped the construction of the Panama Canal, and even influenced the outcome of the Civil War. Mosquito-borne diseases like malaria used to infect tens of thousands of people in the US each year, and in regions like the Tennessee River Valley, almost one-third of residents were infected. Mosquitoes are humanity’s blood tax collectors.

Right now, most of the devastation from infections transmitted by mosquitoes is in developing countries. In much of the US, mosquitoes are mostly just a buzzy, itchy nuisance.

It took decades of painstaking public health work in the US to control disease-carrying insects, reshaping the landscape and using blunt tactics like draining swamps, cutting down forests, and recklessly spraying pesticides like DDT, whose effects we’re still dealing with.

Continuing to keep these diseases away demands constant vigilance, but the country’s success in diminishing mosquito-borne disease has pushed surveillance down the list of public health priorities, and few health departments are investing the resources to keep tabs on these bloodsuckers. Laying traps, collecting them a day or two later, cataloging the insects, running genetic tests to identify pathogens, collating the data, and preparing a counteroffensive is difficult, tedious work, and Gonzalez is one of just a handful of experts in the whole country doing this job full time, and Brownsville is one of the few cities willing to invest in her work.

That’s because Brownsville, a city of 190,000 people on the US-Mexico border, can’t afford to be complacent.

Every day, thousands of people — potential hosts — cross back and forth by car and on foot from Brownsville and the Mexican city of Matamoros to go to school, to work, or to the doctor, in addition to unauthorized border crossings. For some residents, the towering border fence is their backyard fence.

Brownsville’s location has made it a sentinel for the US-wide rise in vector-borne disease driven by travel, urbanization, and a warming climate — and the city’s efforts to stay ahead of the threats are uncommonly robust because it’s where all these factors collide. “We usually get the first of everything and anything,” Gonzalez said.

It’s good for the rest of the US that a city like Brownsville is leading the resistance. Vector-borne illnesses are starting to erupt further inland as changes in the environment and human movement overwhelm our past efforts to contain mosquitoes. Over the past 20 years, the number of reported vector-borne disease cases in the US has doubled and shows no signs of slowing down, according to the Centers for Disease Control and Prevention.

Mosquitoes are a fact of nature, but the evolving threats they pose are a consequence of how humanity has reshaped the environment, and how that environment in turn is changing us. “We’re blaming the mosquito, but honestly, it’s the humans who are bringing the diseases from other countries,” said Arturo Rodriguez, Brownsville’s director of health, wellness and animal services. “Now you have a greater presence of mosquitoes because of the warmer temperatures, and you’re going to end up with an outbreak in parts of the world that probably would never have seen it before.”

Why is this border city investing so much to counter mosquitoes?

In an overgrown lot in a residential neighborhood dotted with illegally dumped mattresses and furniture, Gonzalez checks another type of mosquito trap. This one is a bucket-sized collapsible cylinder, like a laundry hamper, and is meant to look for biters.

Its lure is meant to well, smell like us.

It essentially imitates the scent of a person after a hard workout on a hot day, emitting carbon dioxide and the smell of lactic acid. Our most dangerous adversaries, it turns out, see us merely as carbon-spewing bowls of sweat. “Everyone smells different, and that’s why some people get attacked more than others,” Gonzalez said with a shrug. (The lure’s manufacturer offers a line of scents designed to attract even the most discerning mosquitoes.)

Back in 2016, Brownsville reported some of the earliest known cases of local transmission of the Zika virus in the US, meaning it wasn’t found in people coming back from a trip, but in residents who caught it locally. Zika virus disease is a heartbreaking infection spread by Aedes mosquitoes, a collection of species that thrives in tiny pools of water near where people live. In pregnant people, the disease can cause serious birth defects, including infants born with shrunken heads. In total, there were 26 Zika cases reported in 2016 in Cameron County, where Brownsville is located, and 315 cases across Texas.

The prospect of the virus taking root in Brownsville alarmed residents and city officials and shocked the community into action. City health officials began to build up a mosquito surveillance program to try to find dangerous diseases before they start infecting people. “Zika, for us, was the big moment,” said Rodriguez, Brownsville’s public health director.

Gonzalez joined the team in October 2020, fresh out of grad school, and just in time to put her skills to the test. “I literally had just defended my thesis that Wednesday, and by the next Thursday, I was already dealing with four cases of suspected dengue,” Gonzalez said.

Dengue continues to lurk in the Rio Grande Valley. Last year, Cameron Country reported a locally transmitted case of dengue.

Gonzalez now regularly deploys and checks a network of traps across neighborhoods, industrial areas, and roadsides. The city has a hotline where residents can report mosquito infestations and request spraying, especially if they know which species is buzzing around. The health department also tracks potential vector-borne infections. That data then feeds into mapping software that can identify potential hot spots and coordinate mosquito control measures across the city.

Alexis de la Cruz, a public health analyst for the city of Brownsville, told me that officials can see where citizens have asked for insecticide treatments, identified potential breeding sites, or found mosquito larvae. Using GPS, de la Cruz can track the routes of spraying trucks and where anti-vector treatments like mosquito dunks have been deployed.

The data tracking has revealed some surprises. “People think neighborhoods that are low income are going to have the worst mosquitoes, but people in really nice neighborhoods also have really bad mosquito problems because they water their lawns a lot,” de la Cruz said. “All of that water goes into the storm drains and that water is just easier to breed in.”

Applying mosquito controls is already a complicated and expensive process. Hard as it may be to believe, most mosquitoes aren’t harmful — even if they do leave us pretty itchy. In the US, there are more than 200 mosquito species, but only a dozen are known to spread disease. And of those, there are just three main genuses to worry about: Anopheles, Culex, and Aedes — and seven key species among them.

Many of the chemicals used to spray for mosquitoes aren’t precise and can kill off valuable insects like bees and butterflies, or harm humans and their pets. Mosquitoes can also become desensitized to certain pesticides over time. So city workers have to be very precise with spraying to stay within their budget and limit collateral damage.

Plus, the most dangerous mosquito species are cagey, hiding away during the day and creeping back out at dusk to bite. While the temperatures can get too hot even for mosquitoes to get on our nerves, they know how to lay low. Unfortunately, “they don’t spontaneously combust,” Gonzalez lamented.

All of this is taking place against an environment that is becoming more hospitable to mosquitoes. Brownsville, like much of the US, is getting warmer, and mosquitoes prosper in the heat, breeding faster and for a longer duration. The region is also poised to suffer more bouts of severe rainfall thanks to climate change. Sudden deluges can create pools and puddles that make ideal breeding sites for mosquitoes; some mosquitoes, like those from the Aedes genus, don’t need much and can breed in a bottle cap filled with standing water. Expanding suburban sprawl plays a role as well. Losing natural habitats means there are fewer natural mosquito predators like bats and birds around — allowing mosquitoes to creep in closer and in greater numbers.

New neighborhoods are going up around Brownsville, which means lots more paved surfaces and retention ponds — essentially creating little island chains where water can stagnate and mosquitoes can jump from one region to another. “Brownsville is growing so fast,” Gonzalez said. “In the past year, we have about 48 miles of new subdivision, which is insane. That is very hard to keep up when you are only nine people trying to cover the entire city.”

The city has also seen a change in its demographics as migrants pass through and new residents move in. There have been more asylum-seekers coming through the city from regions like Central America or even further in recent years. Just 30 minutes from the Gulf Coast, Brownsville is also a popular staging area for vacationers. It’s also just inland from Starbase, SpaceX’s rocket launch facility. Brownsville has a rising profile as a birdwatching destination, particularly for migratory birds like the Redhead duck and the Blue-winged Teal.

With the city’s changing landscape and with more people moving in and passing through, Brownville health officials are staying on their toes about new infections taking root and spreading. But there’s only so much the city can do on its own when surveillance is so spotty in neighboring regions, across the border, and throughout the country. “We have a limited view,” Gonzalez said.

Containing vector-borne disease doesn’t start or end with the mosquitoes

Surveillance is only the first step in limiting the spread of vector-borne disease. Brownsville used to have to send suspected infected mosquitoes to the Texas Department of State Health Services in Austin for testing, but now they have a modular laboratory where they can do the work in-house, speeding up the turnaround time. In a white shipping container kitted out with a PCR machine, a microscope, a computer, a super cold freezer, and a super cold AC, Gonzalez documents the mosquitoes she’s collected and grinds a few of them up to be tested for diseases.

Once a mosquito tests positive or a resident falls ill, city health officials work to trace the geographic epicenter of the outbreak. They begin spraying insecticides in the area tailored to a specific vector species. Workers comb the area, dumping small pools of standing water and treating larger water pockets with larvacides. They also talk to local residents and advise them on how to keep mosquitoes out. This whole process of spaying, dumping water, treatment, and communication can last weeks. During that time, city workers like Gonzalez continue to set up traps and patrol the city with F-150 pickup trucks outfitted with noisy insecticide sprayers and air conditioners powerful enough to keep up with the South Texas sun.

It’s a time-consuming, labor-intensive process. And it’s expensive. Each mosquito trap can cost up to $300, a PCR machine to confirm the presence of a disease can cost $15,000, and the salary of a specialist like Gonzalez adds even more to the bill, on top of the other city workers roped in to help with a mosquito outbreak response. Gonzalez is one of three full-time staffers on the vector control team, and the program has an annual operations budget of $400,000. Three-quarters of the budget goes toward buying chemicals and maintaining equipment, and these costs are rising.

That’s why few cities have followed Brownsville’s lead in developing their own in-house mosquito surveillance and response system — it’s a big, ongoing expense to deal with a threat that may not materialize for a long time. But that means help may not be readily available when dengue or West Nile cases suddenly start piling up.

“We have one state entomologist and everyone thinks because she’s available that she’s going to be able to respond to an outbreak, and it’s not the case,” Gonzalez said. “She’s going to be spread super thin.”

Truly containing vector-borne disease requires a multipronged strategy, and there’s only so much city officials can do on their own. Jose Campo Maldonado, an infectious disease physician at the University of Texas Health Rio Grande Valley, said he’s trying to educate his fellow doctors and patients about their role in dealing with infections spread by mosquitoes. Health workers are getting better at recognizing potential symptoms of vector-borne disease, but “there is room for improvement,” Campo Maldonado said. One of the challenges is that vector-borne diseases don’t always present on their own; they often occur in people who are facing other health problems like heart disease or diabetes, so they can be tricky to recognize and treat.

Many patients, on the other hand, have unstable living situations that leave them more vulnerable to getting bitten, or they can’t afford treatments. “It’s really hot outside, and sometimes it’s hard to follow some of the recommendations like using long sleeves,” Campo Maldonado said. Fixing this requires more community outreach as well as efforts to improve housing and access to healthcare.

Containing diseases like yellow fever and chikungunya in the US also requires coordination across the country. “I really see this as a national security matter,” said Christopher Romero, a health researcher in Brownsville.

The US can’t take its historical victories against vector-borne disease for granted, Romero said. It takes active, sustained work to keep these illnesses out and prevent new ones from coming in. Old foes like yellow fever and malaria can still come roaring back. “When you’re dealing with combating pathogens whose evolutionary clock works at a faster pace than humanity, we always will be dealing with an evolving landscape of threats,” Romero said.

Though some of the old tactics that helped the US push vector-borne disease to the margins in the 20th century aren’t viable now, there’s a whole new suite of tools available that Brownsville is trying out: mosquito surveillance, less harmful and more targeted insecticides, disease risk forecasting models, and more.

“We don’t look away from the problem; we realize it’s here and we try to address it,” Rodriguez said.

Now, it’s time for the rest of the country to take notice.