By 1985, it was too late. Researchers detected a type of mosquito called Aedes albopictus, native to Southeast Asia, in Harris County, Texas (1). Just 50 miles northwest from the Port of Galveston, they likely arrived a decade earlier in shipping containers stacked with used tires – an ideal refuge for a few mosquito eggs (1, 2). Now, they were a large breeding population, far too many to wipe out. Aedes albopictus, the mosquito that transmits diseases like dengue fever, zika, and malaria, was here (1, 2, 3).
Over the next three decades, the mosquitoes pushed North, riding on the backs of long-haul trucks and in the bellies of airplanes (1). By 2013, the mosquitoes reached Long Island, New York, a mere 42 miles from JFK Airport. One day at the tail-end of summer, a traveler arrived at the international hub, likely returning from a tropical region. Unbeknownst to them, the traveler carried a stowaway in their blood: dengue fever (4).
From there, perhaps the traveler returned home to Long Island in the early evening, the time of day when Aedes albopictus bites. As the traveler unloaded their bags, a mosquito landed on exposed skin. Slurping viral particles from the traveler’s blood, it turned itself into a dengue factory.
Within days, the dengue-laden mosquito hunted, finding the arm, or the neck, or perhaps the ankle – as mosquitoes are so apt to find – of a 50-year-old man. While mosquitoes are one of life’s common nuisances, the viral hitchhiker was an unfamiliar foe, and the man’s immune system stood defenseless. A hospital stay led to recovery, and the oncoming fall ensured the transmission of dengue started and stopped with him. Yet, his case revealed a growing threat: mosquito-borne illnesses will spread in the US, spurred by a climate crisis that’s remapping the livable range of mosquitoes and their human prey.
The world is warming. In burning fossil fuels, we release carbon that turns Earth’s atmosphere into a furnace (8). For some species, a warming Earth is a death knell; for others, like mosquitoes, it’s an invitation to settle in new, sometimes far-flung, locales (9). Since the 1950s, the world grew 7% more hospitable for the disease-transmitting Aedes aegypti, a cousin of Aedes albopictus. In that time, Aedes aegypti colonized 26 US states, a region size expected to double by 2050 (10).
This map shows CDC’s best estimate of the potential range of Ae. albopictus and Ae. aegypti in the United States based on where they are or have been previously found, as of 2017. (Source)
But mosquitoes don’t spread simply because a new location becomes hospitable. Mosquitoes are short-range travelers, with Aedes species flying maximum distances of only a quarter mile (11). Colonization needs more than just opportunity; it needs the push of a more mobile species—humans.
The spread of mosquitoes depends on human movement (1, 5, 6). As we ship goods around the world, mosquitoes hitch a ride. As we return from tropical vacations, mosquito eggs slip into carry-ons. We are the mosquito superhighway, streamlining their arrival to new habitats. Even more alarming, a 2019 study from Oxford suggested that human-mediated mosquito spread will only increase in the coming years (2). Because as the world continues to warm, it’s not only the mosquitoes who are on the move.
Humans migrate in response to changing climates. It pushed ancient humans from Africa (12), and it’s how our ancestors spread in fits and starts throughout the Americas, echoing the Ice Age cycles (13). And while the effects of our current climate crisis are only beginning, humans are already moving.
Seasons of drought pulsed waves of Mexican farmers northward in the late 1990s. Floods in Southeast Asia pushed 8 million people into the Middle East, Europe, and North America in 2015 alone (8, 14). Escaping extreme heat, rural farmers in Nigeria (15) and Pakistan (16) burned trails to swelling cities over a 20-year period starting in the early 1990s. As fires light up the West Coast, people flee, with more than 200,000 people leaving California in 2018 (17). Yet, as we evade one climate catastrophe, we tip the beginnings of another.
People flock first to the cities. The World Bank estimates that by 2050, 68% of the world will live in cities, 2.5 billion more than today (9). Resource insecurity – from drought, floods, or other climatic disruptions– drives people to a city already straining from the deluge. As cities bulge, mosquitoes and disease enter through the seams. Since 2010, rural villages swallowed by flooding rivers and rising seas in Bangladesh pushed 400,000 people each year into the country’s capital, Dhaka (18). In parallel, Dhaka – the fastest growing city globally – faced surging dengue outbreaks, the deadliest occurring just last year (19).
As cities reach their capacity, resource-rich forests beckon humans. When people breach the tree line, they deliver mosquitos and their pathogenic stowaways. Between 1960 and 1990, the rainforest of Rondônia, Brazil – with its fertile soils, caches of gold, and veins of riverways – attracted farmers, miners, and hydroelectric workers. As people flooded the malaria-free region, the pathogen harbored in hitchhiking mosquitos or loaded in their own veins, they transformed Rondônia into a malaria hot zone (20). Wherever people move, mosquitos and vector-borne diseases are never far behind.
The climate crisis is forcing people to migrate, increasing the density of people – and therefore mosquitoes – in more hospitable areas. Models estimate that 30 million migrants, escaping brutal heat, merciless droughts, and choking floods, will enter the US by 2050 (8). Within the US, 13 million on the coasts will push inward (21), filling cities and carving out forests. The freshwater access of Chicago makes it a likely destination for climate migrants (22), even as its climate grows more inviting to the mosquito-borne diseases that will follow (2). The climate crisis, while expanding the mosquitoes’ livable map, is contracting ours.
In the US, local transmission of mosquito-borne illness has yet to root. There are isolated incidents – dengue in Long Island (4) and a few zika cases in Florida (23). But these are glimmers of an imminent shift, the product of routine events that, in hindsight, have no other outcome. The final dominoes before the push. A shipping container delivers mosquito eggs. A warming climate welcomes the new arrival. A traveler returns from vacation. A virus swims in their veins. A bite from a mosquito. Another bite. A warning: the dominoes are set. We are just waiting for the push.
References
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Kraemer, M.U.G., Reiner, R.C., Brady, O.J. et al. Past and future spread of the arbovirus vectors Aedes aegypti and Aedes albopictus. Nat Microbiol4, 854–863 (2019). https://doi.org/10.1038/s41564-019-0376-y
Powell JR. Mosquitoes on the move. Science. 2016 Nov 25;354(6315):971-972. doi: 10.1126/science.aal1717.
Brown JE, McBride CS, Johnson P, Ritchie S, Paupy C, Bossin H, Lutomiah J, Fernandez-Salas I, Ponlawat A, Cornel AJ, Black WC 4th, Gorrochotegui-Escalante N, Urdaneta-Marquez L, Sylla M, Slotman M, Murray KO, Walker C, Powell JR. Worldwide patterns of genetic differentiation imply multiple ‘domestications’ of Aedes aegypti, a major vector of human diseases. Proc Biol Sci. 2011 Aug 22;278(1717):2446-54. doi: 10.1098/rspb.2010.2469.
Powell JR, Tabachnick WJ. History of domestication and spread of Aedes aegypti–a review. Mem Inst Oswaldo Cruz. 2013;108 Suppl 1(Suppl 1):11-17. doi:10.1590/0074-0276130395
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Iwamura, T., Guzman-Holst, A. & Murray, K.A. Accelerating invasion potential of disease vector Aedes aegypti under climate change. Nat Commun11, 2130 (2020). https://doi.org/10.1038/s41467-020-16010-4
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Jessica E. Tierney, Peter B. deMenocal, Paul D. Zander. A climatic context for the out-of-Africa migration. Geology, 2017; DOI: 10.1130/G39457.1
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Hossain MS, Siddiqee MH, Siddiqi UR, Raheem E, Akter R, Hu W. Dengue in a crowded megacity: Lessons learnt from 2019 outbreak in Dhaka, Bangladesh. PLoS Negl Trop Dis. 2020 Aug 20;14(8):e0008349. doi: 10.1371/journal.pntd.0008349.
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Hauer, M., Evans, J. & Mishra, D. Millions projected to be at risk from sea-level rise in the continental United States. Nature Clim Change6, 691–695 (2016). https://doi.org/10.1038/nclimate2961
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Kristen is an exhibit developer at the Museum of Science and Industry in Chicago Illinois. They hold a Ph.D. in Cellular and Molecular Biology from the University of Chicago. You can find Kristen on Twitter at www.twitter.com/KristenWitte
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