Last year, a team of researchers made a surprising discovery: Aedis aegypti mosquitoes—the species that spreads West Nile Virus, dengue, chickungunya and, most recently, Zika—were living year-round in the Capitol Hill neighborhood of Washington, D.C. In a paper published in the American Journal of Tropical Medicine and Hygiene, the researchers wrote that the mosquitoes had been living in the area since at least 2011, biting and reproducing in the summer months and likely riding out the winter underground. Previously, scientists had believed that the mosquitoes couldn’t survive year-round anyplace north of South Carolina.
While the D.C. population of A. aegypti isn’t believed to carry Zika, its presence nevertheless came as a shock. And as the Zika epidemic continues to spread through Brazil, Central America, and now the United States, scientists will continue to wrestle with how environmental factors like climate change are affecting the creatures that spread infectious diseases.
Before last year, Zika outbreaks had appeared in sporadic small outbreaks near the equator in Africa, Southeast Asia, and the Pacific Islands. In May 2015, the Pan American Health Organization confirmed the first cases of Zika in Brazil; since then, the virus has spread rapidly throughout the Americas, including the first cases of Zika in the U.S. A student in Virginia contracted the virus while traveling in Central America, and public-health officials identified a case of sexual transmission in Texas.
The World Health Organization recently declared Zika a public-health emergency, which means that controlling the epidemic “requires a coordinated international response,” said Kathryn Jacobsen, a professor of global health at George Mason University. The designation of “public-health emergency,” she explained, allows the WHO to designate additional resources for researching the virus, tracking its spread, and developing programs to prevent and control the disease.
Zika, named for a forest in Uganda, is a flavivirus, a type of virus that mutates frequently. Scientists believe a new mutation to the Zika virus may explain the emergence of birth defects and other side effects not seen in past outbreaks. (Researchers strongly suspect a link between Zika infection in pregnant mothers and babies born with unusually small heads and neurological disorders like Guillain-Barré syndrome.)
Flavivurses are also typically spread through animal vectors. Zika’s vector, the A. aegypti mosquito, is found in warm, humid areas in Central and South America, Mexico, and the Southeastern United States (among others). Most people have a rough sketch of these mosquitos’ life cycles and dietary habits: They are known to reproduce in standing water. Bites from the female, which feeds on human and animal blood, are the primary mechanism of Zika’s spread (although the Texas case, so far the only case to be communicated in the United States, has made sexual transmission a concern as well).
And like other viruses spread by mosquitos and ticks, Zika could soon enjoy a greater reach, thanks to climate change. Last year, a team of researchers mapped the global distribution of Aedes mosquitos to better understand the global human-health risk, noting that the mosquitos are more widely distributed than ever before.
In 2005, Paul Epstein of Harvard Medical School published an influential paper on climate change and human health, outlining mosquitoes’ sensitivity to temperature changes. “Warming of their environment—within their viable range—boosts their rates of reproduction and the number of blood meals they take, prolongs their breeding season, and shortens the maturation period for the microbes they disperse,” he wrote. In other words, they bite more, breed more, and spread more disease.
But exactly how climate change drives the spread of Zika and other diseases is hard to define. In 2013, researchers at the University of Arizona published a paper examining the effect of climate factors on dengue and its Aedes vectors. Their conclusion highlighted just how far scientists still have to go in understanding the climate-disease link: “Climate influences dengue ecology by affecting vector dynamics, agent development, and mosquito/human interactions,” they wrote, but “although these relationships are known, the impact climate change will have on transmission is unclear.”
Climate change introduces additional complications into an already complex system, the study authors explained: It’s difficult enough to understand how weather, climate, human interaction, or mosquito behavior contribute to the spread of a virus. Assembling those pieces to create even the vaguest picture of how a virus will act weeks, years, or decades from now is a herculean analytical task.
The WHO identifies environmental changes—climate change chief among them—as hazards to human health, and the populations most vulnerable to climate-related health issues tend to live in populations that lack a robust public-health infrastructure. One WHO publication predicts that warming of two to three degrees Celsius would put up to 7 percent more people—several hundred million globally—at risk of malaria, another mosquito-borne disease.
“Zika showing up in the Americas is probably more a function of international travel and trade than of climate change. But now that the virus has started circulating in the Americas, both climate factors and human behavior will play a role in where it spreads,” Jacobsen said. “Changes in temperature, precipitation, and humidity can alter how long the mosquitoes live, how often they bite, how many offspring they have, and how quickly a virus reproduces inside an infected mosquito, and each of those changes can mean more humans are exposed to mosquitoes.”
The concern extends beyond public-health organizations—on a more immediate level, primary-care physicians are also taking notice of climate change as a health concern.
“I’ve done several surveys of physicians and asked them whether they think climate change is relevant to direct care of patients,” Mona Sarfaty, the director of George Mason University’s Center for Climate Change Communication, told me.“Two-thirds say yes. It’s definitely relevant to patient care.” In 2011, the American Medical Association published an editorial arguing the same thing.
As WHO devotes more resources to understanding Zika, therefore, it may make sense to incorporate research on climate. “There is a general need for more studies of the linkages between ecological health, animal health, and human health,” Serfaty said, “in addition to the very specific and urgent need to learn more about Zika virus.”