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The Aedes aegypti mosquito, a black and white-spotted insect no longer than the width of a human fingernail, sickens more people every year than the novel coronavirus, influenza and cancer combined. It lands lightly on an infected host and carries its potentially deadly payload – in the form of any number of viruses – to its next victim without a sound. Unlike other, less lethal mosquitoes, Aedes aegypti doesn’t buzz around in your bedroom. You won’t even know what bit you.
The mosquito and its less lethal counterpart, Aedes albopictus, are just two members of the family Culicidae that spread a growing number of what are known as “vector-borne” diseases: bacteria, viruses and parasites that blood-feeding insects and ticks pass among humans. The Zika virus is vector-borne, as is dengue fever, malaria, yellow fever, West Nile and chikungunya, a viral illness that causes severe joint pain (the name means “to become contorted” in the Kimakonde language of southeast Tanzania and northern Mozambique).
“Vector-borne diseases,” says the World Health Organization, “account for more than 17 percent of all infectious diseases.” Another two-thirds of human illnesses are “zoonotic” – incubated in animals, they later become contagions in the human sphere. Which means the more space we share with wild animals and insects, the sicker we get. And as temperatures rise around the globe, space is getting tight. Climate change means humans are increasingly treading on animal territory to grow food and to live. Insects, meanwhile, are expanding their ranges everywhere.
In 2011, Aedes albopictus turned up in the eastern Los Angeles County city of El Monte. Since then, both Aedes species have been found in California, from the northern Sierra Nevada mountains to the Mexican border, and on the East Coast as far north as New York City. So far, there hasn’t been sufficient density of mosquitoes and people to cause severe disease outbreaks in the U.S. (the northernmost outbreak of dengue fever was at the Texas-Mexico border in 2005). But that could change, says Maria Diuk-Wasser, an associate professor of ecology, evolution, and biology at Columbia University.
Warmer temperatures, Diuk-Wasser says, accelerate mosquito development, making the insects both more prevalent and more lethal. “They develop faster, they bite more frequently, the virus develops faster, they digest the blood faster.” At some point, certain places in the world might get too hot for some of them, but it follows that those places will also be too hot for humans.
Mosquitoes aren’t the only disease vector thriving in the changing climate. Ticks are doing alarmingly well, too. “Rising temperatures and warmer winters mean more ticks will be able to complete the full cycle” from larva to nymph to adult, says Igor Dumic, a researcher and practicing doctor at the Mayo Clinic in Rochester, Minnesota, and co-author of a 2018 study on climate and Lyme disease. In fact, the relationship between climate and the black-legged tick, which infects as many as 300,000 people per year with its Lyme-causing spirochete, is so well known that the U.S. Environmental Protection Agency regards Lyme disease as a climate indicator. The tick needs two to three years of favorable weather to grow into an effective vector. Dumic estimates that a 2-degree Celsius increase in annual average temperature could increase the adult population by more than 20 percent.
Making the leap from bird or bat to cat or pig—and up to humans
The same tick can also pass on other pathogens: babesiosis, anaplasmosis and the Powassan virus, which causes a severe brain infection with a 10 percent mortality rate. “We need to do a better job from a public health perspective to understand how prevalent these diseases are,” Dumic says. Diagnosed early, Lyme can be treated and most people who contract it recover. Without treatment, its long-term complications can kill. By some estimates, the disease costs the U.S. $1.3 billion every year.
There’s less evidence that the zoonotic coronaviruses on all of our minds right now—SARS, MERS, and COVID-19—have any direct connection to warmer temperatures. What we do know, however, is that changes in the climate imperil wildlife habitat – if not with unbearable heat, then by making more frequent and severe the floods and droughts and wildfires that push animals out of the wild and into the human sphere. Any time humans and wildlife crowd together, there exists the possibility that a virus will make the leap from bird or bat to cat or pig. From there, it’s a short hop up the evolutionary tree to humans.
A pair of Malaysian scientists in 2007 hypothesized that a 1998 outbreak of Nipah virus, which kills roughly a third of its human hosts through severe swelling of the brain, resulted from a combination of drought and human-caused fires that drove wild fruit bats out of the forests and into farms, where they infected pigs that spread the virus to humans – swine, as the common name of the H1N1 flu virus suggests, are a common intermediary host in the transfer of viruses to humans. If they’re right (and not everyone thinks they are), we can expect more such lethal events as forests fall to extreme weather and fire
Colin J. Carlson, assistant research professor at Georgetown University
“We live in a world where the global health infrastructure we’ve been building since World War II is in jeopardy.””
The unfortunate news is that governments can’t do much in the near term to arrest the spread of vector-borne diseases fueled by climate change. Even if China bans its wildlife trade and wet markets, forests will still burn and mosquitoes will continue migrating north. But there’s still a lot to be done proactively to guard human health while we work to restore the ecosystems that once kept diseases contained.
“There are two ways we respond to climate change: mitigation and adaptation,” says Colin J. Carlson, an assistant research professor at Georgetown University who studies climate change and emerging diseases. Usually when we talk about climate, we focus on mitigation–how to slow the warming of the earth enough to keep global temperature rise under the two-degrees Celsius the Intergovernmental Panel on Climate Change recommends for a livable future.
“But that’s only half the picture,” says Carlson. Adaptation – preparing ourselves for the threats that hotter temperatures are already visiting upon us – is the other, and presently more achievable, half.
High on the list of adaptive measures to alleviate suffering from vector-borne diseases: vaccines. “They’re a tool that we have to make the most unlivable scenarios somewhat better,” Carlson says. If we can limit global warming to two or three degrees, “We can theoretically prevent most of the excess death if we can just get vaccines where they need to be.”
A vaccine does exist for at least one of the diseases transmitted by those most prolific vectors, the mosquitoes of the Aedes family. Yellow fever, the disease that in 1793 killed one-tenth of Philadelphia’s 50,000-some residents, for the last 80 years has been preventable with a single vaccine that confers immunity for life.
Dengue is a different story. It comes in four different subtypes, none of which provide immunity against the others. If you get dengue fever once, you can get it again, and again, until you’ve built up immunity to all four strains. Only one vaccine is available at the moment, and while it covers all four strains, if you haven’t already had some type of dengue, an infected bite from an Aedes aegypti could end up making you sicker.
“It works well in the tropics, where most of the burden of dengue is,” Carlson says. “But it doesn’t help us on the leading edges of dengue expansion” in North American and Europe.
Another vaccine is in the offing – the Japanese pharmaceutical firm Takeda has already put it through initial clinical trials. “It could be a revolutionary change for disease control,” Carlson says. But that’s only if it gets to the people who need it, regardless of where they live or how much money they have. As repeated yellow fever outbreaks in Nigeria have tragically demonstrated – the disease rate tripled between 2018 and 2019 – that isn’t happening.
“We live in a world,” Carlson says, “where the global health infrastructure we’ve been building since World War II is in jeopardy — where even something as simple as vaccination is complicated.” He suspects that tens of thousands of mammalian viruses have the potential to make the phylogenetic leap to human beings. Without global coordination, we’re at a loss to stop any of them.
“We can’t build a health system around any one of those threats,” Carlson says. “We have to build a health system that’s strong and prepared for everything.”