How the Rise of Cities Helped Mosquitoes Thrive

An explorer who searched for ways to avoid Aedes Aegypti may have hastened its spread.

Andre Penner / AP

I moved into my Tucson, Arizona, apartment at the start of a sputtering monsoon during which the skies binge-rained, hard, about once a week or so. For short bursts the roads, sidewalks and sometimes even the wash flowed with water that broke the summer’s terrible heat, conjuring vegetable plots and garden parties, inspiring me to plant.

After less than a week, I abandoned my ambitions to the mosquitoes. I ran the swamp cooler instead and watched the dawn through dusty windows. But no matter where I went, the mosquitoes followed, even into the shower. A dark mosquito with feathery antennae and silvery markings, Aedes aegypti is delicate, tentative, its long legs striped black and silvery, its black body spotted white. Aedes aegypti is skittish and fast. I never successfully swatted one, although I did smack myself in the face trying. That was why I hadn’t wanted roommates: so that I could write in peace.

In Tucson, an arid place, the ubiquity of backyard mosquitoes can surprise newcomers who have not learned the city’s ecology. Here in the Borderlands, human history—some human history, anyway—can be everywhere and nowhere at once, shaping the land but erased from memory. But current ecology bears witness to the past, even in the form of bloodthirsty insects. Aedes aegypti, those tiny torments, thrive anywhere they find water: in the detritus of pool parties; road trash; stomping puddles; effusive gardening intentions. In Tucson, Aedes aegypti tell stories revealing that the most persistent-seeming boundaries—between urban and wild; trammeled and not; between bodies, countries, continents—are permeable after all.

In May, Kasey Ernst, an infectious disease epidemiologist from the University of Arizona, traveled from Tucson to testify before Congress about Zika. Ernst investigates connections between mosquito-borne diseases and the environment. A clear-eyed researcher of arboviruses, Ernst explains statistical software to jumpy undergraduates in her office as patiently as she summarizes national contagious disease predictions for House Republicans.

Beginning with the mosquito’s appearance in North America—it was here by the 1640s, judging by yellow-fever epidemics—Ernst led members of Congress through the current state of knowledge regarding Aedes aegypti and the potential for Zika outbreaks.

Insect, virus, and human form a complex relationship triangle. Ernst described the unanswered questions residing at the triangle’s center. Was vertical transmission, where infected female mosquitoes pass Zika directly to their offspring, occurring? Scientists had found an infected male mosquito. But males do not eat blood, which suggests vertical transmission. Once the virus showed up in a new place, it potentially could stick around from year to year.

Scientists also lacked basic information about Aedes aegypti, such as the mosquito’s exact range in North America. Ernst explained that different human communities surveyed differently for Aedes. Many communities didn’t have the resources to survey for mosquitoes at all, much less invest in control. In the U.S.-Mexico border region where she lived, the same personnel often handled mosquito control, restaurant inspections, pest abatement and other environmental health hazards.

One thing was clear, though. “This mosquito exploits the ways we have changed our environment,” Ernst said.

For all their slapping, slathering, baiting, scratching, or fleeing, humans beckon Aedes aegypti. The tangled history of human and mosquito cannot be unknotted. Certainly not from the mosquito’s perspective: On ship or on land, in canopy or clearing, the mosquitoes that feed and reproduce pass on their genes. Human behavior has selected for the insects most suited to urban landscapes. To an individual mosquito, jungle, desert, or city are meaningless categories. We, their food sources, inflect these connections with cultural meaning, drawing conclusions about the world from a mosquito’s bite.

An illustration of Alexander von Humboldt in South America
(Prisma Archivo / Alamy / HCN)

The shared lives of insect and mammal are essential to the mosquito’s survival and disease transmission. This lesson has taken centuries to learn. One of the first scientists to study Aedes aegypti in the Americas was Prussian explorer Alexander von Humboldt. In his Personal Narrative of Travels to the Equinoctial Regions of America During the Years 1799-1804, Volume 2, is a section entitled “Plague of Mosquitoes.” Subheadings include “Intensity of the Plague,” “Their Voracity in Certain Places,” “Effects of the Mosquito-Sting” and “Absence of Any Remedy.”

Humboldt did not know that one of the mosquito species plaguing South America while he explored the continent, robbing him of sleep and possibly some measure of sanity, was Aedes aegypti. His travels to the equinoctial regions included significant detours, such as when he crossed into Brazil and found out there was a bounty for his capture by the Portuguese government, which did not want him exploring its colony. Another was when his party skipped a planned stay in a town to avoid a yellow fever outbreak.

A scientist and adventurer with windswept hair and deep blue eyes, Humboldt traveled through Latin America on foot and by boat for five years during his early 20s, ending his trip in 1804, the year Lewis and Clark began their exploration of the Western United States. Humboldt traveled the length of the Orinoco River by boat, something no other European had done. His traveling companions included a French botanist, Aimé Bonpland; their servant, whose name Humboldt recorded as José; five Indigenous crewmembers, whose names Humboldt did not record at all; and Father Zea, who had been living at an isolated mission in the jungle when they passed through.

Humboldt had an astonishing breadth of interests—on the Orinoco, he recorded everything from the elevation of equatorial mountain ranges to the taste of curare, an unbelievably toxic poison Indigenous peoples used on their arrow tips—and a masochistic dedication to the pursuit of data. According to one biographer, while studying electric eels—which can give shocks of up to 600 volts—Humboldt and Bonpland “subjected themselves to every imaginable abuse in the name of science.”

Unlike his contemporaries, Humboldt was a big thinker, looking for unifying ideas to explain the natural world. One of Personal Narrative’s greatest fans was a teenaged Charles Darwin. Humboldt’s physical feats matched his scientific endeavors: Humboldt, Bonpland, and Ecuadorian scientist Carlos de Montúfar y Larrea-Zurbano—who may have been Humboldt’s love interest—climbed an extinct volcano, Chimborazo, to a height above 18,000 feet, setting a world ascent record that stood for 30 years. Humboldt collected plant, insect, and geological data during the ascent.

All of which is to say, Humboldt was a bit of a badass. He was fearlessly eager to explore the world, its physical patterns and biogeographical revelations.

Unfortunately for him, mosquitoes were a part of his world, their patterns a mystery he could not solve, their revelations a misery. In his Personal Narrative, Humboldt seems self-conscious about how his mosquito descriptions might come off back home. “Plague of Mosquitoes,” which includes a reference to Dante, does read like it’s been written by someone whose voice is getting higher and higher.

“Whatever fortitude be exercised to endure pain without complaint,” Humboldt writes, “whatever interest may be felt in the objects of scientific research, it is impossible not to be constantly disturbed by the mosquitos, zancudos, jejens, and tempraneros.” This litany included mosquitoes, gnats, and biting flies that swarmed hands and faces, bit through clothes, flew up noses and into mouths, and caused coughing and sneezing fits when people tried to speak.

In the late 1700s and early 1800s—before mosquito netting, insect repellent, screen doors or solid walls—there was no sealed-off inside where humans anywhere in the world could avoid pests. The missionaries and villagers Humboldt met along rivers barely slept at all, though not for lack of trying.

As described by Humboldt, mosquito-evading techniques included: burying oneself in sand; sleeping surrounded by a herd of cows; building a tree house; sleeping in the middle of waterfalls; using mosquito netting so stifling it required periodically getting up and walking around in the mosquito-infested air; and filling with smoke a room so small the occupant had to crawl into it then seal himself inside for the night. These rooms were called hornitos, or “little ovens.” Humboldt helpfully informs the reader that the smell of crocodiles does not, in fact, ward off mosquitoes, as the whole time he and his party were dissecting a large crocodile the mosquitoes were relentless.

Driven perhaps by his desperation to get a good night’s sleep, Humboldt sought patterns in the presence or absence of mosquitoes and other insects, suggesting everything from the color of river water to insect exhaustion as possible explanations. Naturally, Humboldt correlated an abundance of mosquitoes with a decline in quality of life for humans, describing unrelenting flying insects that made the places he explored “almost uninhabitable.”

If mosquitoes made a place uninhabitable for humans, then perhaps building towns and homes would make a place uninhabitable to mosquitoes. Humboldt hypothesized that forest destruction reduced mosquito populations, and that towns with overgrown lawns and squares had worse mosquito problems. Orderliness was the solution, Humboldt concluded: “The insects will diminish when the old trees of the forest have disappeared; when in those countries now desert, the rivers are seen bordered with cottages, and the plains covered with pastures and harvests.”

What Humboldt didn’t realize was that changes of the kind he envisioned—from uncultivated jungle to towns and farms, from tribal land to European missions—brought unanticipated dangers of their own. That what appeared to be the creation of order from chaos—carving European towns from South American tropical forest—was actually a disruption of social and ecological systems that would cause new health threats, including the spread of insects and the diseases they carried. Not that he knew what diseases were or that insects could carry them. Humboldt explored the Orinoco five years before Charles Darwin was born. Without the framework of evolution, he had no concept that human behavior could shape an insect species. Like his contemporaries, Humboldt thought mosquitoes and diseases came from “miasmas”—bad air.

Humboldt and his contemporaries didn’t see the connection between global travel, colonialism, and mosquito spread—much less disease spread. Humboldt chalked up tropical fevers to heat, vapors, bad air and other inanimate, locally occurring causes: to things that were part of the land, not to something that might have been brought to the land at the same time as millions of enslaved humans. Not to something new. And certainly not to something alive.

Potted plants in a Tuscon garden—where Aedes aegypti and other mosquitoes can thrive
(Arizona News Service)

Aedes aegypti evolved in West Africa as a canopy species feeding on monkeys. When humans felled trees, mosquitoes came down to ground level and found a new host—us. Aedes aegypti flew out of the woods and into the garden, out of the ahistorical wild, a place expected never to change, and into the city, a place expected never to evolve. Urban Aedes aegypti have evolved into a separate subspecies.

An urbanized mosquito, Aedes aegypti breeds best in rainwater collected in human-made containers: garden embellishments, bits of trash, empty cans, rainwater-harvesting systems. It doesn’t live much further than a third of a mile from human homes. In addition to being hard for humans to hear, the species feeds in places hard to swat—inside elbows, around ankles, backs of knees. Given a choice between cow and person, Aedes aegypti will feed on the person.

The optimistically named Mosquitoes and Their Control explains that mosquitoes “are found throughout the world, except in deserts and permanently frozen areas.” Geographer Melinda Butterworth pointed out to me that, on a yard-by-yard scale, Tucson is not a desert. In a well-shaded yard, where temperatures drop by as much as 10 degrees and rainwater lingers in old flowerpots, mosquitoes thrive.

Without a concept of virology, Humboldt could not understand acquired immunity, much less arboviruses. Europeans who came to South America developed terrible inflammations from insect bites, while the resident “copper-colored man,” Humboldt wrote, did not. Humboldt ascribed the difference in health to some inherent racial difference. He suggested that mosquito venom could be used as a litmus test to identify “creole whites” who looked European. Based on his observations, Humboldt believed that “Indians, and in general all the people of color, at the moment of being stung, suffer like the whites, although perhaps with less intensity of pain.”

My first response to these thoughts was to mutter something along the lines of maybe-you-just-complain-more-about-the-intensity-of-pain, as though I could pick a race fight with the dead father of the field of biogeography. I reminded myself that he lived in a time when scientists—white men of the leisure class, primarily—tried to make sense of the world by measuring and mapping skulls. It pains me to report that Humboldt and Bonpland raided Native graves, stealing entire skeletons of children as well as assorted skulls to help with these studies, despite the anger of their Indian guides.

Many of the adult “copper-colored” people Humboldt met in South America must have fallen sick with yellow fever and other tropical illnesses during childhood, and so they had acquired immunity: It was vaccination the hard way. Later infections for survivors really were milder. As more members of a community acquired immunity, fewer unexposed people were likely to fall ill because the reservoir of disease carriers shrank, a concept we call herd immunity these days. Humboldt’s European acquaintances’ first experiences of tropical illnesses were sure to be severe. Humboldt didn’t realize that countless Indigenous peoples died when mosquitoes, including Aedes aegypti, first made landfall.

The intertwined human-mosquito ecology first went global four centuries ago when humans set off in tall ships, taking casks of water infested with mosquito larvae and cargoes of enslaved humans. Since then, primate and insect have remained steadfast—if unintentional—travel companions. As humans traveled the globe, Aedes aegypti traveled with them, and centers for human settlement with warm-enough winters became centers for Aedes aegypti, as well. Today, Aedes aegypti continues to be a great traveling companion: It thrives in the tropics, but its eggs can survive drying out for up to a year. Genetic studies show that Aedes aegypti in the Southwest Borderlands has close relatives both in coastal Mexico and on the East Coast of the United States. Researchers believe that mosquitoes travel in the Southwest as passengers along highways.

There were several Aedes-free decades in the Borderlands: A brief DDT-derived respite, after which an entomologist observed what turned out to be Aedes aegypti snacking on her ankles in a Tucson backyard in the mid-1990s. The mosquitoes that feast on me in my garden most likely descended from ancestors who rode into an overland port of entry from Mexico in a shipping truck.

In the Borderlands today, the frame for arboviruses is globalization, with the twist of climate change. Ernst expects chikungunya, an arbovirus that has appeared in Florida, to arrive in Arizona eventually. According to Ernst’s research, while the arbovirus dengue has not yet been a community-wide concern in Tucson, climate change may make it one—if not by affecting mosquitoes or disease survival directly, then perhaps by influencing the ways that humans store and use water, altering the world from a mosquito’s perspective. When Ernst conducted interviews in the U.S.-Mexico border region to study residents’ understanding of dengue and its spread, some respondents mentioned miasmas.

When I thought I might be approaching Humboldtian levels of mosquito misery, unable to sleep through the night because of waking up to probiscidal poking at my eyebrows in the morning, I did what Humboldt never would have: I ran. More precisely, I flew, to visit family back east.

Sometimes, I believe in Humboldt’s answer to mosquitoes: the right way of living. If only my apartment had tight window screens and air conditioning; if I had neighbors who xeriscaped instead of growing exuberantly watered vegetable gardens; if Tucson’s roads drained rainwater instead of flooding—then I would be free of my tiny pests. And on a very small local scale, for at least a short while, I probably would.

But then I catch myself. Like Humboldt, I’ve imagined that I could build away from something that loves built environments; that I could disrupt something that thrives on disturbance. Meanwhile, I’d given no thought to living lushly in a desert—with swimming pools, long flights, globally shipped goods—no thought at all to the rhythms, structures, aspirations of a life holding every door open to mosquitoes.

This article appears courtesy of High Country News.