When the mosquito now infamous for spreading the Zika virus suddenly showed up thousands of miles from anywhere it would usually call home, a California insect abatement officer was confounded.
Steve Mulligan and his equally puzzled colleagues first encountered the Aedes aegypti mosquito in 2013, in their work for the Consolidated Mosquito Abatement District in California's Central Valley. Until then, no one had ever reported seeing the mosquito in the area.
Most puzzling to Mulligan, the agency's manager: why had the mosquito popped up around Fresno?
"It was way out of its range," he said.
As Mulligan searched for an explanation, he kept coming back to the warmer temperatures blanketing the state's prime agricultural region year after year.
"We started to realize climate change was probably a part of the reason why we were seeing this mosquito," he said.
As world health officials scramble to combat the spreading virus, which could infect as many as 4 million people this year, scientists and public health officials see the outbreak as an omen in a world steadily warming under the effects of climate change.
"[Zika] is the virus of the moment but can be taken as an indicator of a future where changes in temperature provide a more hospitable environment for viruses to replicate and be transmitted," said Colin Parrish, a professor of virology at Cornell University.
The World Health Organization has designated the Zika virus a global public health emergency, assigning it the highest level of urgency.
The most alarming outbreak of the Zika virus erupted in Brazil in May, and has since spread to 25 countries and territories in Latin America and the Caribbean.
In the United States, at least 48 cases of Zika have been reported in nearly a dozen states, including Florida, where a health emergency has been declared in four counties. The infections in the U.S. have been reported in people sickened while traveling outside of the country. (The CDC has confirmed one case of the virus being transmitted sexually in Texas.)
Although the virus generally causes mild to moderate symptoms in infected adults, it is generating worry over its possible link to microcephaly, a condition that causes babies to be born with unusually small heads and often with brain damage.
Warm, Wet, Worrisome
The Aedes aegypti mosquito that is responsible for spreading the Zika virus — and a related mosquito considered a potential host for the virus, the Aedes albopictus — thrive in warm, wet environments. Researchers worry that the mosquitoes will find a more nurturing environment in the years ahead as temperatures continue to rise steadily, with the unrelenting increase of greenhouse gases.
"The range of the mosquitoes has been expanding and is predicted to continue to expand due to climate change," said Matthew DeGennaro, the principal investigator at Florida International University's laboratory of Mosquito Genetics and Behavior. "The most important drivers of their expansion are increased temperature and precipitation."
According to Andy Monaghan, a scientist specializing in climate change at the National Center for Atmospheric Research and one of the authors of a paper examining the mosquito’s habitat, "climate overall will become more suitable in a larger range for these mosquitoes."
Monaghan said in an interview that a warming climate will allow longer periods of reproduction that result in larger numbers of the insects, which thrive in small containers of water, such as barrels or discarded tires or even in the saucers under flowerpots.
This scenario has sparked anxiety that climate change may help virus-carrying mosquitoes multiply and spread deeper in the United States, and perhaps into Canada.
A study last year of the aegypti and albopictus mosquitoes done in connection with their role in the worldwide spread of the dengue and chikungunya viruses shows both have become established in the United States, especially in the warmer southern regions of the country.
The Zika-carrying aegypti mosquitoes are found in Florida, along the Gulf Coast, and in Hawaii; they have also been detected as far north as Washington, D.C., in hot weather. The albopictus mosquito has become much more entrenched in the U.S. from the Gulf Coast, up the Atlantic seaboard, and into the heart of the South.
"Both species' distributions are highly dependent on the limiting factor temperature places on survival of the adult mosquitoes," according to the study.
Although climate change cannot be singled out as the lone factor in the spread of Zika, it nonetheless has to be calculated into the overall equation.
"It's way too early to know climate's contribution, but in parallel with record hot temperatures, it raises the question for further investigation," Jonathan Patz, director of the Global Health Institute at the University of Wisconsin–Madison, said during a news conference hosted by Climate Nexus.
Human travel, socioeconomics, sanitation, mosquito control, and even air conditioning have to be taken into account, Patz said.
Yet the connection between Zika and other diseases and climate change seems undeniable: "It is not a surprise that you see major epidemics following extreme climate events," he said.
How Far and How Fast
There is little question that climate change will continue to contribute to the spread of Zika and other viruses, said Thomas J. Daniels, an associate research scientist at Fordham University's Louis Calder Center Biological Field Station and the director of the center’s Vector Ecology Lab.
"The question becomes how far and how fast," he said. "As temperatures start to creep up, it allows a foothold in new territory."
Perfect examples of climate change influencing the spread of disease can be seen in the increasing number of reported cases of Lyme disease and West Nile virus, he said. Both Lyme and West Nile are carried by host insects whose environments expand as temperatures rise.
The number of confirmed cases of Lyme disease, which is carried by ticks, has more than doubled — from nearly 11,000 to about 25,000 — since 1995, according to figures from the Centers for Disease Control and Prevention. West Nile virus has exploded since 1999, when 62 cases were reported nationwide, to 2,205 in 2014, according to CDC figures.
"A contributing factor in the spread of those diseases can be attributed to increasing temperatures and altered precipitation patterns that accompany climate change," Daniels said.
Average annual temperatures worldwide have been rising for decades, and last year was the hottest on record, according to data collected by NASA and the National Oceanic and Atmospheric Administration.
The Zika virus was identified in the late 1940s in monkeys in the Zika Forest in Uganda but wasn't found in humans until 1952. It has been limited mostly to the tropical regions of Africa and Asia but spread to Central and South America last year.
The virus uses mosquitoes as hosts where it reproduces and then is transmitted to humans when the mosquito bites. There is no treatment; the World Health Organization has said it would likely take more than a year to develop a vaccine.
Given an expanded environment in which the Zika-carrying mosquitoes can thrive, scientists warn of the potential for a significant public health risk.
Warmer temperatures make the aggressive mosquitoes, which have a special taste for human blood, bite more often and allow the Zika virus to incubate more quickly, so the insects are infectious longer, scientists and public health officials say.
The South may be the most immediately vulnerable because of its warm, humid climate, according to Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, in Houston. Other areas, including the East and West coasts and much of the Midwest, also could become temperate hosts to the mosquitoes.
With millions of people in those regions, Hotez said there is the potential for a serious outbreak.
"We don't yet know all of the connections between climate change and the spread of this virus, but we have to believe it is a component that has the potential for expanding host regions for the mosquitoes," he said. "That larger region means more people being exposed to possible infection and that is where we have the potential for a public health crisis."
Heidi Brown, a professor of epidemiology at the University of Arizona, said that in addition to the climate, there is also a human factor involved in the spread of the virus.
"You have to take into account how climate change alters human behavior in ways that may influence the interaction of the humans and the virus-carrying mosquitoes," she said, including people spending more time outside and keeping windows and doors open.
While Zika has captured the world's attention because of its frightening threat, many vital questions remain to be answered, said Alex Perkins, biological sciences professor at the University of Notre Dame.
But one important fact is being reinforced by the Zika outbreak, he said: climate change must become part of the calculus when confronting new world threats from disease.
"The main takeaway here is: due to climate change we could be facing more of these events then we have in the past," he said.